This PDF file contains the front matter associated with SPIE Proceedings Volume 11090, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Antiferromagnetic (AFM) spintronics is an emerging field of research, which exploits the Néel vector to control spin- and orbital-dependent transport properties. This talk will address two approaches, involving new material platforms, which can be exploited in AFM spintronics. The first approach comprises the subfield of AFM spintronics known as topological AFM spintronics, where the Néel vector is used to electrically manipulate the symmetry-related topological states. Based on density-functional theory calculations, we demonstrate that room temperature AFM metal MnPd2 allows the electrical control of the Dirac nodal line by the Néel spin-orbit torque. The reorientation of the Néel vector leads to switching between the symmetry-protected degenerate state and the gapped state associated with the dispersive Dirac nodal line at the Fermi energy. The calculated spin Hall conductivity strongly depends on the Néel vector orientation and thus can be used to experimentally detect the predicted effect. The second approach involves Néel vector switching in non-collinear antiferromagnets ANMn3 (A = Ga, Ni, Zn, etc.) with an antiperovskite structure. These compounds are characterized by the competing non-collinear AFM Γ5g and Γ4g phases. We demonstrate from first-principles that by stoichiometry engineering ANMn3 can be tuned close to the critical transition point between the Γ5g and Γ4g phases, at which the Néel vector switching can be achieved by strain or spin-transfer torque. The switching can be detected through the anomalous Hall effect being zero or finite for the Γ5g and Γ4g phases, respectively, due to the symmetry of the Berry curvature. Our results broaden the scope of materials and approaches, which can be exploited in AFM spintronics.

Antiferromagnets host strong quantum fluctuations that may result in a disordered spin liquid ground state, instead of the classical Néel ordered configuration. We show that the same quantum fluctuations imbue the spin-1 excitations, typically called magnons, in easy-axis antiferromagnets with remarkable quantum properties such as entanglement. We find that these spin-1 excitations are comprised by a superposition of a large number of states with spin N+1 on one sublattice and spin -N on the other, such that the net spin remains 1. The ensuing many-body and massively-entangled nature of the antiferromagnet magnons abets a strong coupling to their environment, and thus an enhanced damping. Hence, this new understanding of the eigenmodes provides a physically intuitive picture for the well known exchange enhancement in the damping of antiferromagnetic dynamics. The same strong coupling to the environment leads to a significantly enhanced spin pumping current into an adjacent normal metal when the antiferromagnet interface is uncompensated.

We present a theoretical mapping to show that a ferromagnet with gain (loss) is equivalent to an antiferromagnet with an equal amount of loss (gain). Our findings indicate a novel first-order ferromagnet-antiferromagnet phase transition by tuning the gain-loss parameter. As an appealing application, we demonstrate the realization as well as the manipulation of the antiferromagnetic Skyrmion, a stable topological quasiparticle not yet observed experimentally, in a chiral ferromagnetic thin film with gain. We also consider ferromagnetic bilayers with balanced gain and loss and show that the antiferromagnetic Skyrmion can be found only in cases with a broken parity-time symmetry phase. We predict a spectral region in the first Brillouin zone, in which the parity-time symmetry is never broken in the framework of linear spin wave theory. Our results pave the way for investigating the emerging antiferromagnetic spintronics and parity-time symmetric magnonics in ferromagnets.

Low power control of magnetic properties in nanoscale is of great significance and has attracted lots of interests in magnetic data processing and novel spintronic applications. So far, there have been strenuous efforts on controlling the Curie temperature (Tc) and magnetization (M) by a variety of means. A representative example which has generated much excitement is using electrostatic field-effect gating, which has been shown to alter the magnetic properties of (III, Mn)V films in a certain extent. In this work, we utilize the surface molecular decoration as a new route to achieve large manipulation of Tc, M magnitude and direction in the (Ga,Mn)As films. Carrier densities of the films are vastly changed by two kinds of molecules acting as the electron donors and acceptors, resulting in a prominent variation of Tc up to 36 K and M direction up to 27˚. Charge transfer mechanism through the interface is established, indicating much larger extent of the molecular modulation than electrical gating. Moreover, arbitrary self-assembled monolayer (SAM) nanopatterns on the (Ga,Mn)As film surface could be created via dip-pen nanolithography (DPN), which offers a convenient method for generating magnetic nanostructures. This work opens a more efficient and functional pathway of manipulating magnetic properties in a greater range, and realizes nano-patterns of molecule/semiconductor hybrid structure with marked prospects in disparate areas of spintronic research and nanotechnology in future application.

Antiferromagnets are enriching spintronics research by many favorable properties that include insensitivity to magnetic fields, neuromorphic memory characteristics, and ultra-fast spin dynamics. Designing memory devices with electrical writing and reading is one of the central topics of antiferromagnetic spintronics. So far, such a combined functionality has been demonstrated via 90° reorientations of the Néel vector generated by the current-induced spin-orbit torque and sensed by the linear-response anisotropic magnetoresistance. [1, 2] In my presentation I will show electrically control and detection of 180° Néel vector reversals in a collinear antiferromagnet with broken time reversal and spatial inversion symmetries. [3] [1] Železný, J. et al., Phys. Rev. Lett. 113, 157201 (2014). [2] Wadley, P. et al., Science 351,587 (2016). [3] J. Godhino et. al, Nat. Com. 9, 4686 (2018)

Antiferromagnets have recently attracted considerable attention in the spintronics community because they have some unique advantages over ferromagnetic materials and spintronics provides means of accessing and utilizing the antiferromagnetic order. Compared to ferromagnetic materials, however, the range of spintronic functionalities available in the most studied simple collinear antiferromagnets, is limited. In contrast, in more complex antiferromagnets, such as non-collinear antiferromagnets, symmetry is lower and more spintronics effects can be present. The non-collinear antiferromagnets could thus combine the advantages of collinear antiferromagnets such as fast magnetic dynamics with the useful functionalities of ferromagnets. Here, we discuss spin currents in various antiferrromagnets, primarily in the non-collinear antiferromagnets of the Mn3X type. Our work [1] shows that in the Mn3X antiferromagnets, spin currents have several unusal properties. We have shown that, unlike in simple antiferromagnets, charge current in the non-collinear antiferromagnets is spin-polarized. In addition, we have show that the same mechanism that leads to the spin-polarized current, also leads to a novel type of a spin Hall effect. This “magnetic” spin Hall effect, differs from the conventional spin Hall effect, in that it originates from the magnetic order. Another intriguing result concerning the Mn3X antiferromagnets is that in these antiferromagnets the conventional spin Hall effect exist even in absence of spin-orbit coupling [2]. Apart from the non-collinear antiferromagnets we will also discuss collinear antiferromagnets with low symmetry in which similar spin currents can also occur. [1] J. Železný et al., Spin-polarized current in non-collinear antiferromagnets, Phys. Rev. Lett. 119, 187204 (2017) [2] Y. Zhang et al., New J. Phys. 20 ( 2018 ) 073028

Magnetoresistive random access memory (MRAM) based on spin transfer torque (STT) is entering volume production in the semiconductor industry. While STT-MRAM offers nonvolatile embedded memory operation with high endurance, its ultimate energy efficiency, speed and scalability are limited by its current-controlled write mechanism. In this talk we discuss novel device candidates, physics, and materials which may enable approaching the fundamental limits of speed and energy efficiency in spintronics. Building on the success of STT-MRAM, these emerging device candidates may not only address a broader cross-section of the memory hierarchy, but also enable new computing architectures with simultaneously ultralow-power and high-performance attributes, which are important for machine intelligence on both edge and cloud platforms. We first review the recent progress of voltage-controlled ferromagnetic memory devices, which use interfacial magnetoelectric effects arising from spin-orbit interaction, to switch the magnetization in memory bits. We discuss progress in the development of magnetic tunnel junctions using voltage-controlled magnetic anisotropy (VCMA) for switching, which exhibit the lowest-energy MRAM cells to date (~ 5 fJ/bit with precessional switching times ~1 ns [1-6]). The device and materials-level challenges and opportunities are discussed, including biasing, VCMA coefficients, and write error rates. As a strategy to further reduce switching time, and improve energy efficiency of VCMA-based MRAM towards picosecond and atto-Joules respectively, we then examine the VCMA effect in new free layer structures consisting of antiferromagnetic materials. We propose a method for switching the Néel vector of an antiferromagnetic thin film, by the application of an ultrashort electric field pulse [7]. The electric field induces a reorientation of the antiferromagnetic order parameter, due to the voltage-induced modification of the magnetic anisotropy. When the electric field pulse is timed to half the oscillation period of the Terahertz antiferromagnetic dynamics, it induces a picosecond time-scale reversal of the Néel vector. Importantly, the electric field required to induce this reversal is as small as ~ 100 mV/nm, comparable to fields used for switching of ferromagnetic tunnel junctions in earlier works. This electric field is determined by the anisotropy of the antiferromagnet, while the much larger exchange field determines the frequency of the resulting dynamics (and hence the switching time). Our simulation results indicate the possibility to switch a 50 nm circular antiferromagnetic element with an energy dissipation of 250 aJ, in less than 30 ps, and in the absence of any current-induced torque. The electric-field-induced switching of the Néel vector opens a new route towards energy-efficient and ultrafast magnetic memories and computing devices based on antiferromagnets. 1) P. Khalili Amiri et al., "Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling", IEEE Transactions on Magnetics, 51, 3401507, (2015). 2) C. Grezes et al., "Ultra-low switching energy and scaling in electric-field-controlled nanoscale magnetic tunnel junctions with high resistance-area product", Applied Physics Letters, 108, 012403 (2016). 3) W.-G. Wang et al., "Electric-field-assisted switching in magnetic tunnel junctions", Nature Materials, 11, 64–68 (2012). 4) Y. Shiota et al., "Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses", Nature Materials, 11, 39–43 (2012). 5) Y. Shiota et al., "Reduction in write error rate of voltage-driven dynamic magnetization switching by improving thermal stability factor", Applied Physics Letters, 111, 022408 (2017). 6) S. Kanai et al., "Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction", Applied Physics Letters, 101, 122403 (2012). 7) V. Lopez-Dominguez, H. Almasi, P. Khalili Amiri, "Picosecond electric-field-induced switching of antiferromagnets", Physical Review Applied, 11, in press (2019).

Spin-orbit torque (SOT) offers promising approaches to developing energy-efficient memory devices by electric switching of magnetization. Comparing to other SOT materials such as heavy metal and topological insulator, metallic antiferromagnet (AFM) potentially influences the generation of SOT through its magnetic structure. Here, combining the results from neutron diffraction and spin-torque ferromagnetic resonance experiments, we show that the room-temperature magnetic structure of epitaxially grown L10-IrMn (a collinear AFM) is distinct from the widely presumed bulk one. It consists of two types of domains with the spin axes orienting towards [111] and [-111], respectively. We find that this unconventional magnetic structure is responsible for a much larger SOT efficiency up to 0.60±0.04, comparing to 0.083±0.002 for the polycrystalline face-centered-cubic IrMn. Furthermore, we reveal that the magnetic structure of L10-IrMn induces a large isotropic bulk contribution to the SOT efficiency and an anisotropic interfacial contribution of comparable magnitude, where the latter depends strongly on the electric current direction in the film plane. Our findings shed light on the critical roles of bulk and interfacial antiferromagnetism to SOT generated by metallic AFM.

Theory of spin-charge conversion effects in spintronics are presented in terms of correlation functions of physical observables, spin and electric current. Direct and inverse spin Hall effects and spin pumping effect are studied considering metallic systems with random spin-orbit interaction and spatially nonuniform Rashba interaction. The theory is free from ambiguity associated with spin current, and provides a clear physical picture of the spin-charge conversion effects. In the present approach, the spin current transmission efficiency turns out essentially to be the nonuniform component of magnetic susceptibility.

Spin-momentum locking provides a basic concept to control electron’s spin and charge flow in variety of material systems such as topological insulators, semiconductors, and heavy metals. Induced helical spin texture at the Fermi surface allows us to efficiently generate and detect spin polarization without an external magnetic field or magnetic materials. While spin generation and spin detection using spin-momentum locking have been intensively explored at Rashba interfaces and topological surface states, spin manipulation has yet to be demonstrated: it remains the missing ingredient towards full set of spin control by spin-momentum locking. Here, we experimentally manifested spin manipulation by spin-momentum locking in a magnetic focusing device [1]. We employ an InGaAs/InGaAsP heterostructure which shows strong SO-induced effective magnetic field. Such a large SO field generates and detects in-plane spin-polarized electrons by combining with lateral quantum point contacts (QPCs). By employing the lateral magnetic focusing device, weak out-of-plane external magnetic field focuses ballistic electrons from emitter to collector QPC by Lorentz force. Two QPCs allow us to polarize and detect electron spin along in-plane orientation due to strong SO interaction. We observed the enhanced focusing signal in collector QPC under spin polarized magnetic focusing regime. This indicates that the spin orientation in focused electrons have taken the opposite direction from that of the emitted spins. Electron spin rotates in a circular orbital motion while spin orientation is locked towards the SO field due to the spin-momentum locking. The findings of this study highlight the significance of the electron trajectory in controlling the spin phase through spin-momentum locking. The ability to control the spin phase by the orbital motion opens the door to the development of new concept on spintronic devices as well as topological electronics. [1] M. Kohda et al., Scientific Reports in press (2019).

Several types of spin-based logic devices have been proposed in recent years. Almost all of them are operated by employing the drift transport of electron spins under in-plane electric fields. However, it is still unclear how an in-plane electric field influences the spin dynamics of drifting electrons. Here, we demonstrated long-distance spin transport in a GaAs quantum well, where the spin-orbit interactions (SOI) were set at exactly the SU(2) symmetry generating a persistent spin helix (PSH) state. The spatial distribution and temporal development of optically-injected electron spins in a modulation-doped GaAs single quantum well were detected with Kerr rotation microscopy based on the pump-probe technique at T = 8 K. We found that the transport length of drifting spins in a PSH condition exceeds 100 micrometers due to the suppression of D’yakonov-Perel spin relaxations [1]. The spin precession period, spin decay length and transport path of drifting spins can be controlled by electrical means in a single device. We also observed a drift-induced change in the spin precession period for drifting electron spins by visualizing the spatial distribution of electron spins. This phenomenon results from the cubic term of the Dresselhaus SOI enhanced by the application of in-plane electric fields [2]. Our finding will be of great importance for a deeper understanding of spin transport dynamics as well as for relevant spintronics applications using an in-plane electric field. [1] Y. Kunihashi et al., Nat. Commun. 7, 10722 (2016). [2] Y. Kunihashi et al., Phys. Rev. Lett. 119, 187703 (2017).

We report on advanced metal organic spin architectures constituted by metal (manganese, iron, copper) phthalocyanine molecules magnetically coupled with ferromagnetic layer(s) through a graphene (Gr) spacer. The highly corrugated moiré superstructure of Co-intercalated Gr/Ir(111) drives the assembly of evenly-spaced molecular bits, providing preferential adsorption regions for the phthalocyanine molecules. Our X-ray absorption and photoemission results show that the Gr layer shields the electronic/magnetic state of the molecules, screening the electronic interaction with the metallic surface while allowing for magnetic coupling, as proved by X-ray magnetic circular dichroism. Mn-, Fe- and Cu-phthalocyanines assemble on Gr/Co with identical structural configurations, but MnPc and FePc are strongly antiferromagnetically coupled with Co up to room temperature, while CuPc couples ferromagnetically with weaker thermal stability. The magnetic alignment is induced by a superexchange interaction, driven by the molecular orbitals responsible of the magnetic ground state and electronically decoupled from the underlying metal via the Gr layer, as confirmed by ab initio theoretical predictions. The tunable parameters to further optimize the system are the magnetic moment and anisotropy energy of both MPc molecules and intercalated Gr substrate, to reach room temperature remanence. We will show that, by exploring molecular building blocks with diverse metallic centers and different structure/magnetic state of the substrate, the best configuration can be identified. These archetypal spin interfaces can be prototypes to demonstrate how antiferromagnetic/ ferromagnetic coupling can be optimized by selecting the molecular orbital symmetry, paradigmatic examples to exploit in surface-supported molecular spin electronics.

The Aharonov-Casher (AC) effect where spin phase is modified by an electric field is a fundamental concept of quantum mechanics. The AC spin interferometer is an invaluable to probe spin phases information. We demonstrate a gate-controlled anisotropy in AC spin interferometry using InGaAs mesoscopic rings. The coexistence of Rashba and Dresselhaus spin-orbit interactions (SOIs) manifests as an AC resistance amplitude modulated by the direction of in-plane magnetic field as a probe. We establish ways to manipulate spin dynamic and geometric phases freely by introducing electric and magnetic tuning. Our experimental results are well explained by perturbation theory and numerical analysis performed at constant Dresselhaus spin-orbit interaction (SOI) strength except for one additional anisotropic reversal by tuning Rashba SOI. The observed additional AC resistance anisotropy reversal is explained by sign change in linear Dresselhaus SOI [1]. [1] F. Nagasawa, et al., Phys. Rev. B 98, 245301 (2018)

Ferromagnetic semiconductors (FMSs) with high Curie temperature (TC) are highly required for spintronics device applications. So far, however, the mainly studied FMSs, Mn-doped III-V FMSs, are only p-type and their TC values were much lower than 300 K. To search for new FMSs with high TC, most efforts have been concentrated on wide-gap materials [1], which yields no reliable and systematic result. We present a new class of FMSs with high TC, Fe-doped narrow-gap III-V FMSs. Because Fe atoms replace the cation (group-III) sites in the isoelectronic Fe3+ state, the carrier type can be controlled independently and thus both n-type and p-type FMSs are obtained. Using low-temperature molecular beam epitaxy, we have successfully grown zinc-blende-type thin-film crystals of both p-type FMSs [(Ga,Fe)Sb [2], (Al,Fe)Sb [3]] and n-type FMSs [(In,Fe)As [4], (In,Fe)Sb [5]]. The most notable feature in these Fe-based FMSs is that the TC value increases monotonically as the Fe content increases; and there is a tendency that TC is higher as the bandgap is narrower, which contradicts the prediction of the conventional mean-field Zener model. Intrinsic room-temperature ferromagnetism has been observed in (Ga1-x,Fex)Sb with x > 23% [2] and (In1-x,Fex)Sb with x > 16% [5]. These findings indicate that the Fe-doped III-V FMSs are promising for spintronics devices operating at room temperature. References [1] T. Dielt et al., Phys. Rev. B 63, 195205 (2001). [2] N. T. Tu et al., Appl. Phys. Lett. 108, 192401 (2016) [Featured Article]. [3] L. D. Anh et al., Appl. Phys. Let 107, 232405 (2015). [4] P. N. Hai et al., Appl. Phys. Lett. 101, 182403 (2012). [5] N. T. Tu et al., APEX 11 (6), 063005 (2018).

Combining magnetism and semiconductor enables the development of magnetic semiconductors, a promising way to realize spintronic applications based on use of both charge and spin degrees of freedom. The highest Curie temperature of the most extensively studied diluted magnetic semiconductor (DMS) (Ga,Mn)As has been Tc = 200 K, still far below room temperature. Owing to simultaneous doping of charge and spin induced by Mn substitution, it is difficult to individually optimize charge and spin densities. To overcome these difficulties, recently a new type of DMS (Ba,K)(Zn,Mn)2As2 was observed in experiments with Tc up to 230K [1]. Motivated by the high Tc, density functional theory calculations [2] and photoemission spectroscopy experiments [3] were conducted to understand the microscopic mechanism of ferromagnetism of p-type DMS (Ba,K)(Zn,Mn)2As2. In addition, the n-type DMS Ba(Zn,Mn,Co)2As2 was also reported in the recent experiments with Tc = 45 K[4]. In Mn-doped BaZn2As2, why is the ferromagnetic coupling? Why is Tc much lower in the n-type case than that in p-type case? In general, can p- and n-type DMS be realized? Here, we propose a method to realize DMS with p- and n-type carriers by choosing host semiconductors with a narrow band gap [5, 6]. By density function theory and quantum Monte Carlo simulation, we demonstrate such semiconductors using Mn-doped BaZn2As2, which has a band gap of 0.2 eV. In addition, we found a new non-toxic DMS Mn-doped BaZn2Sb2, of which the Curie temperature Tc is predicted to be higher than that of Mn-doped BaZn2As2, the Tc of which was up to 230 K in the recent experiment [5]. We also predict the DMS Cr-doped BaZn2As2 with stable ferromagnetism in p- and n-type carriers [6]. In addition, by density functional theory calculations we predict the strain-induced room-temperature intrinsic ferromagnetic semiconductors in two-dimensional Cr2Ge2Se6 [7]. We find that Tc = 144 K in Cr2Ge2Se6, which can be enhanced to Tc = 326 K by applying 3% strain, and Tc = 421 K by 5% strain. The enhanced Tc in Cr2Ge2Se6 by strain can be understood by superexchange interaction. References: [1] K. Zhao et al., Nat. Commun. 4, 1442 (2013); Chin. Sci. Bull. 59, 2524 (2014). [2] J. K. Glasbrenner, I. Zutic, and I. I. Mazin, Phys. Rev. B 90, 140403(R) (2014). [3] H. Suzuki et al., Phys. Rev. B 91, 140401(R) (2015); Phys. Rev. B 92, 235120 (2015). [4] H. Y. Man et al., arXiv:1403.4019 (2014); S. Guo et al., arXiv:1712.06764 (2017). [5] B. Gu, and S. Maekawa, Phys. Rev. B 94, 155202 (2016). [6] B. Gu, and S. Maekawa, AIP Advances 7, 055805 (2017). [7] X. J. Dong, J. Y. You, B. Gu, and G. Su, arXiv:1901.09306 (2019).

We systematically study the spin-orbit torque (SOT) and the thermoelectric effects in sputtered BiSb/CoFeB bilayers using the second harmonic Hall resistance (R2𝜔) technique. The R2𝜔 signal was measured as a function of the azimuthal angle and the magnitude of an external field. We develop a general protocol to unambiguously separate the SOT-induced R2𝜔 signal from that generated by the thermoelectric effects (anomalous Nernst effect, spin Seebeck effect and ordinary Nernst effect). At room temperature, we find a damping-like spin Hall efficiency of ~+0.65 in BiSb/CoFeB heterostructures, which is significantly higher than that found in heavy metal/ferromagnet bilayers. The BiSb thickness dependence, temperature and BiSb alloy composition dependence of R2𝜔 will be discussed in the presentation.

In this paper we describe a non-equilibrium thermodynamic approach to spin Seebeck and spin Peltier effects in bilayers composed by a ferromagnetic insulator and a spin Hall metal. In particular we will show the existence and the experimental verification of the reciprocal relation between the two effects, in which both coefficients turn out to be related to two main intrinsic parameters: the absolute thermo-magnetic power coefficient of the ferromagnetic insulator Ε_M and the spin Hall angle of the metal θ_SH . It is also shown how the ratio between the thickness and the intrinsic diffusion length of each layer can influence the measured effects.

Spin current can be used to control the magnetization dynamics of a nano-magnet. There are various ways to generate spin current. Electrical current or heat current passing through a ferromagnet can generate spin current due to the spin dependent conductivity or spin-dependent Seebeck effect. Electrical current passing through a non-magnetic material with high spin-orbit coupling can generate spin current via the spin-Hall effect. Heat current passing through a non-magnet can also generate spin current due the spin-orbit coupling. This effect known as the spin-Nernst effect (SNE), has been shown recently by measuring thermally driven spin-Hall magneto-resistance [1-3]. We have shown that the spin-Nernst effect can be measured directly via a multi-terminal device with ferromagnetic Ni contacts on Pt [4]. We generated heat current in the Pt and the resultant spin current was then detected by Ni contacts. The same multi-terminal device was used to measure the spin-Hall angle for comparison with spin-Nernst angle. In a further work, we have shown that the spin current generated via spin-Nernst effect can be injected into an adjacent FM layer to exert spin-transfer torque (spin-Nernst torque) [5]. We could change the effective damping of the ferromagnet via spin-Nernst torque. [1] S. Meyer et.al., Nat. Mater. 16, 977 (2017). [2] P. Sheng et.al., Sci. Adv. 3, e1701503 (2017). [3] D. J. Kim, et.al., Nat. Commun. 8, 1400 (2017). [4] A. Bose et.al., Applied Physics Letters, 112, 162401 (2018). [5] A. Bose et.al., Phys. Rev. B 98, 184412 (2018).

Skyrmions and chiral domain walls are two- and one-dimensional magnetic solitons, respectively. Both can be realized at room temperature using thin layers of simple materials, such as Co, sandwiched between dissimilar spin-orbit materials [1-4]. Both solitons can be described as quasi-particles with intriguing dynamics. Skyrmion dynamics, in particular, exhibit fascinating topological signatures, such as gyration [1], inertia [1], the skyrmion Hall effect [2], and topological damping [3]. Here, I will discuss the ultrafast dynamics of skyrmions and chiral domain walls in response to electrical and optical stimuli. I will present a quantitative analytical model to describe why and how these dynamics differ between in ferromagnets and antiferromagnets [3]. The model predicts ultrasmall skyrmions and ultrafast solitons in chiral antiferromagnets, which is in excellent agreement with our experimental observation of these physics ferrimagnetic chiral Pt/GdCo/TaOx near compensation [5]. I will conclude with a discussion of very recent data on chiral dynamics in the non-equilibrium state of matter after high intensity femtosecond optical excitation. [1] Büttner et al., Nat. Phys. 11, 225 (2015). [2] Litzius et al., Nat. Phys. 13, 170 (2017). [3] Büttner et al., Sci. Rep. 8, 4464 (2018). [4] Büttner et al., Nat. Nanotech. 12, 1040 (2017). [5] Caretta, et al., Nat. Nanotech. 13, 1154 (2018).

In this work, we study the motion of skyrmions in a ferromagnetic waveguide using a time-dependent non-equilibrium Green’s function formalism implemented on a real space tight-binding model. We analyze the perturbation induced by this motion and look at the time-dependent distribution of the non-equilibrium charge, spin densities and then discuss the topological Hall effect. The current pumped during this motion is analyzed and shown to have an influence on the torque applied on the whole texture with a signature unveiled in the renormalization of the damping parameter. A cooperative effect due to the collective motion of skyrmions is proposed in order to enhance the skyrmion's velocity. The stationary regime is analyzed as a function of the different parameters of the system and explained using the formalism of electronic pumping. A simple formula for the current is provided. We propose this cooperative electronic pumping to enhance the collective skyrmion mobility in racetracks.

A great interest is now focused on physics of magnetic skyrmions and other chiral spin textures. These objects combine a promising applicability (e.g. via race-track memory devices), with a rich fundamental physics driven by the chiral nature of spatial spin pattern. The spin chirality opens up a number of novel solid state phenomena, such as the topological Hall effect. In this work we have theoretically demonstrated that a chiral spin order of the electron gas in magnetic systems is universally induced by an inhomogeneity of the electrostatic potential. We argue that various structural defects, such as ionized impurities, surface imperfections, or built-in electric fields act as a local source of electron chiral spin ordering. Using the linear-response theory we have calculated the spin-density correlation functions and have shown the emergence of chiral spin response upon the variation of the electrostatic potential. For instance, a single short-range impurity appears to be surrounded by an excessive electron spin density of chiral skyrmion-like character. The characteristic size of the emerging texture is determined by the Fermi wavevector, while going away from the center the spin density decays exhibiting Friedel oscillations. The considered mechanism is more effective the stronger is the spin-orbit interaction in carrier spectrum. Thus we believe that the rise of chiral spin pattern due to structural disorder is of high relevance for the intensively studied magnetic Rashba layers, surface of 3D Topological Insulators and dilute magnetic semiconductor quantum wells.

Reliable storage and manipulation of spin coherence is at the heart of quantum information processors. Isolated singly-charged semiconductor quantum dots (QDs) are typically characterized by bright ground state emission and hold promise as efficient spin-to-photon interfaces. Ultrafast interactions in condensed phase could enable high-bandwidth information processors, with an additional possibility to engineer quantum properties of light on sub-picosecond timescales. Toward these goals, we employ multi-color femtosecond write and readout pulses to analyze intrinsic spin dynamics in isolated singly-charged epitaxial CdSe/ZnSe QDs. With this we demonstrate control over single-photon-added amplification as well trion quatnum beats, arising from long intraband coherence.

Terahertz (THz) time-domain spectroscopy and optical pump-THz probe spectroscopy are today widely used within the THz community to study material properties and carrier dynamics in semiconductors and 2D materials. Along with these methods, Laser THz Emission Microscopy (LTEM) is an alternative and somewhat simpler technique where THz pulses are collected from a sample after it is directly photo-excited by a femtosecond (fs) laser. Since most semiconductors intrinsically are able to emit THz pulses when carriers are accelerated on a fs time scale, this method is directly able to provide information about carrier mobility. At the same time, LTEM can image a sample with a much higher spatial resolution than conventional THz systems due to the much lower diffraction-limit of the fs laser pulses. Recently, it has been demonstrated that spintronic THz emitters have the potential of out-competing conventional THz sources showing an impressive efficiency and a bandwidth up to 10s of THz, which is normally only achieved in much more sophisticated THz sources such as air-plasmas generated with amplified fs laser systems. While some work has already been done in optimizing the spintronic structures to generate maximum THz signal and bandwidth, understanding the carrier dynamics from the measured THz pulses is still a challenge. In this presentation we discuss the ability to understand the carrier dynamics spintronic samples with LTEM and THz time-domain spectroscopy, and present an experimental platform where samples can be studied with these methods simultaneously.

In recent years terahertz (THz) technology has been an emerging research field with a broad range of applications. THz radiation falls between the infrared and microwave radiation in the electromagnetic spectrum. Most THz sources to date are not related to the spin degree of freedom; however, recent research efforts in spintronics and ferromagnetism demonstrated that the electron spin offers completely new opportunities for the generation of ultrafast photocurrents. For instance, magnetic heterostructures are very easy to pattern and potentially allow to tailor THz emission characteristics by design. Here, we demonstrate that an ultrafast spin-current pulse driven by a femtosecond laser pulse can create THz transients in microstructured magnetic heterostructures due to the inverse spin Hall effect. We compare the THz electric field and the THz spectrum of a control CoFeB/Pt film with microstructured CoFeB/Pt wires as well as microstructured CoFeB/MgO wires patterned on an extended Pt film. We find that the THz electric field amplitude is proportional to the coverage of the CoFeB/Pt heterostructure on top of the MgO substrate. Furthermore, we analyze the magnetization direction dependence of the THz transients with respect to the easy axis of the ferromagnetic wire. The presented results are the first steps towards shaping and controlling the THz properties by microstructuring of spintronics-based THz emitters.

Spin-Hall Effects and spin-charge conversion phenomena at a short lengthscale either using heavy metals like Pt, W [1], Au:W [2] , Inverse-Edelstein Effect at Rashba [3] or topological interface [4] are the basis of new spintronics functionalities. Combined with FMR spin- pumping, spin-orbit interactions in these systems and heterostructures also enable to probe the interfacial properties possibly involving spin-orbit coupling effects like local spin-loss (SML) at 3d/5d interfaces [5-6]. Beyond, it was recently reported that THz emission may be realized via dynamical spin-to charge conversion in the picosecond scale in the same kind of heterostructures, either metallic [7] or Rashba system [8-9], composed of ferromagnetic (FM) and spin-orbital active materials using time- dependent spectroscopy (TDS) techniques. Here, experiments consist in exciting magnetization and spin-currents within the FM layer via femtosecond laser excitation and measuring, in the picosecond timescale, the relaxation of the correlated spin and charge currents responsible for THz dipolar emission. The spintronics THz emitters typically reaches the power of ZnTe electro-optic semiconductor technology or even higher. In that mind, we will present our last results of THz emission provided by optimized growth bilayers composed of a high-spin orbit material in contact with a ferromagnetic layer like Co/Pt, Co/Ti/Pt, Co/Au/Pt, NiFe/Au:W, Co/Pt/Au:W) including Ti or Au monolayers (MLs) insertion and Au:W spin-sink materials. Those state-of-the art bilayers are analyzed using complementary experiments combining RF-spin-pumping-ISHE measurements and TDS –THZ spectroscopy. Our results show, that in some conditions, the insertion of Ti or AuW based materials in Co/Pt structures may enhance either THz or ISHE signals depending on the layer sequence. In particular, the ML insertion in Co/Ti(ML)/Pt systems evidences a decrease of the spin-orbit strength at the interface, responsible for an increase of the ISHE signal by decrease of SML, conjugated to a decrease of FMR-linewidth and THz emission. We will demonstrate the strict correlation between THz signals and related spin-mixing conductances as well as ISHE signals and will discuss the connection between THz and spin loss phenomena. In particular, the THz signal can be viewed as a straightforward and non-destructive method to probe the Rashba spin-orbit strength and spin-orbit coupling of Interface Shockley states between transitions metals References: [1] J. Sinova et al.. 87, 1213 (2015). [2] P. Laczowski et al., Applied Physics Letters 104, 142403, (2014). [3] J. C. Rojas Sánchez, L. Vila, G. Desfonds, S. Gambarelli et al., Nat. Comm. 4, 2944 (2013). [4] J.-C. Rojas-Sánchez, S. Oyarzún, Y. Fu, A. Marty et al., Phys. Rev.Lett. 116, 096602 (2016). [5] J.C Rojas Sanchez et al., Phys Rev Lett. 112, 106602 (2014). [6] A.J. Berger et al., Phys. Rev. B98, 024402 (2018). [7] T.Seifert et al., Nature photonics 10:483488, (2016) [8] C. Zhou et al., Phys. Rev. Lett. 121, 086801 (2018). [9] M. B. Jungfleisch et al., Phys. Rev. Lett. 120, 207207 (2018). [10] D. Yang et al., Adv. Opt. Mat.(2016). [11] Y. Wu et al., Adv. Materials 29, 1603031 (2017).

Chiral magnetic systems gain more and more relevance for future information technology. Aside from the now famous proposal of skyrmionic information bits, recent publications suggest possible applications in the field of neuromorphic computing in the form of skyrmionic reservoirs which rely on their non-linear magnetoresistance. Despite its practical importance, a general physical understanding of the electrical transport through chiral magnets is yet to be reached. Experimental studies of the Hall conductivity in chiral magnets commonly face the challenge of how to decompose the measured signal into several parts with a clear physical interpretation. Theoretical investigations on the other hand struggle with the complex electronic structure, especially in the case of isolated topological solitons. Inspired by our recent work on the orbital magnetism of chiral magnets, we demonstrate how a semiclassical gradient expansion can shed some light on the classification of Hall effects in chiral magnets. These results may open up new perspectives for the all-electrical detection of non-collinear magnetic structures such as skyrmions, hopfions and chiral bobbers.

Spin-orbit coupling (SOC) and the spin diffusion length in condensed matter are crucial parameters for spintronics applications. In order to study these, we have succeeded in employing a pulsed spin-pumping method based on ferromagnetic resonance (FMR) to generate pure spin currents from ferromagnetic (FM) substrates into non-FM semiconductor layers1 which can then be detected through the inverse spin-Hall effect (ISHE). When the FM is in FMR with a pulsed microwave (MW) excitation, a pure spin-current is generated in the non-FM layer which can circumvent potential impedance mismatches between the FM and the non-FM layer and, therefore generate a strong pulsed ISHE signal. Due to the low duty cycle of the pulsed excitation, MW excitation powers can be used that are strong enough to generate pronounced ISHE signals even in materials with weak SOC such as carbon-based materials. This sensitivity allows for the study of the quantitative nature of the ISHE and thus, to apply scrutiny to a number of questions about the ISHE effect in general, including how strongly FMR-driving field inhomogeneities affect a measured ISHE current, the relationship of ISHE voltages to the ISHE current in devices consisting of layers with different conductivities, as well the experimental conditions which have to be monitored during an ISHE experiment in order to ensure reproducibility. This work was supported by the National Science Foundation (DMR-1404634 – Sample preparation and Experiments) and the NSF-Material Science & Engineering Center (DMR-1121252- Polymer Synthesis and Facilities) at the University of Utah. 1. D. Sun et al., Nature Materials 15, 863–869 (2016). doi:10.1038/nmat4618

Spin-pumping is the phenomenon by which a pure spin current can be injected into a non-magnetic metal from an adjacent ferromagnet. Spin-pumping into Pt is of great interest since Pt exhibits large inverse spin Hall effect. However, Pt limits the efficiency of spin-pumping due to spin-memory loss at the ferromagnet/Pt interface. By alloying Pt with Co we saw a large enhancement in magnetic damping suggesting a significant increase in spin-pumping efficiency. Analyzing the damping results of ferromagnet/CoPt using spin-pumping model yields to an increase in the spin-mixing parameter by a factor of 4 as compared to ferromagnet/Pt. The enhancement in damping cannot be explained by an increase in the interface spin-memory effect since the spin-diffusion length in CoPt is increased by a factor of 2 as compared to pure Pt. An additional potential source of spin-pumping-like interface damping could be originating form the proximity effect. It is found that proximity effect at the ferromagnet/CoPt interface induces a long-range magnetic moment in CoPt, up to 6nm. Proximity effect can also lead to an increase in fluctuating moment in CoPt, which can then enhance the transfer of magnetic momentum from the ferromagnet to CoPt. In this case, effective spin-pumping parameter will depend on the interaction of spin-correlated clusters in CoPt with the adjacent ferromagnet. This is different from the typically used spin-pumping model where the loss of magnetic moment is mediated by the independent normal-metal electrons reflected at the spin-dependent electron potential at the ferromagnet/normal metal interface.

Plasmons play a large role in the optical properties of metals. The rapidly developing field of magneto-plasmonics merges the concepts from plasmonics and magnetism to realize novel and unexpected phenomena and functionalities for the manipulation of light at the nanoscale. Owing to the intertwined optical and magneto-optical properties, magneto-plasmonics may offer a smart toolbox for actively tunable optical ultrathin surfaces and metasurfaces. Here we review fundamentals aspects of the underlying physics [1-3] and recent advances in this emerging research field. A survey of applications to a variety of emerging technologies are presented as an example of their broad scientific and technological perspectives, namely: - magneto-plasmonic nanoantennas for ultra-sensitive and label-free molecular detection [4-8]; - ultra-thin 2D chiroptical surfaces, built on magneto-plasmonic bimetallic meta-atoms where chiral light transmission is modulated by the externally applied magnetic field [9]; - 2D magneto-plasmonic crystals, which support collective modes (surface lattice resonances) or surface plasmon polariton modes displaying a two-dimensional photonic band structure that can be engineered to obtain tailored and enhanced magneto-optical response [11-14]. - thermo-plasmonics based on bimetallic magneto-plasmonic nanoantennas, for harvesting electromagnetic radiation energy and convert it into heat, which can be used to finely tune the magnetization reversal in networks of interacting nanomagnets [15]. References [1] J. Chen et al. Small 7, 2341 (2011) [2] N. Maccaferri et al., Phys. Rev. Lett. 111, 167401 (2013) [3] N. Maccaferri et al. Opt. Express 21, 9875 (2013) [4] V. Bonanni et al. Nano Lett. 11, 5333 (2011) [5] N. Maccaferri et al., Nature Commun. 6, 6150 (2015) [6] I. Zubritskaya et al. Nano Lett. 15, 3204 (2015) [7] R. Verre et al. Nanoscale 8, 10576 (2016) [8] B. Sepulveda et al., Small 14, 1800868 (2018). [9] I. Zubritskaya et al., Nano Lett. 18, 302 (2018) [10] K. Lodewijks et al., Nano Lett. 14, 7207 (2014) [11] N. Maccaferri et al., Nano Lett. 16, 2533 (2016) [12] M. Kataia et al., Opt. Express 24, 3652 (2016) [13] N. Maccaferri et al., ACS Photonics 2, 1769 (2015) [14] A. López et al., Nanoscale 10, 18672 (2018). [15] P. Vavassori and M. Pancaldi, Naemi Leo, and P. Vavassori, submitted to ACS Nano.

In magnetic heterostructures, an in-plane electrical current can manipulate the magnetization direction of the free ferromagnetic layers through spin-orbit torques. Spin-orbit torques are transfers of angular momentum from the crystal lattice to the magnetization using conduction electrons as the medium for transfer. Traditionally, spin-orbit torques have been attributed to two causes: the spin Hall effect and the Rashba-Edelstein effect. However, this framework is incomplete because 1) experiments cannot reproducibly distinguish these mechanisms and 2) theory predicts other torques of comparable strength. Using the Boltzmann equation and first principles calculations, we have investigated the allowed interfacial contributions to spin-orbit torque. These investigations led to the discovery of a novel effect, interface-generated spin currents, which first principles calculations reveal are strong in relevant heavy metal/ferromagnet bilayers (such as Pt/Co). This work has also resulted in simple drift-diffusion models that can capture interfacial spin-orbit effects through the appropriate boundary conditions. Following this work, our group has performed first principles calculations showing that ferromagnets generate spin currents via the intrinsic mechanism that could exert spin-orbit torques at interfaces. In this talk, we discuss how both novel interfacial spin-orbit effects and spin current generation in ferromagnets could play an important role in spin-orbit torque. Recent experiments done in heterostructures, including those with a single ferromagnetic layer, provide evidence that ferromagnetic layers and interfaces cause the spin-orbit torques discussed here. Shedding light on these mechanisms will help clarify the nature of spin-orbit torque, which is crucial for realizing its potential for magnetic memories.

The engineering of topological superconducting correlations in quantum devices holds promises for new schemes of quantum computation. A wide class of systems expected to exhibit such exotic correlations are based on conductors with strong spinorbit interaction subject to a strong external magnetic field. Here, we show how these features can be autonomously induced by using a magnetic texture coupled to any quasi one dimensional conductor. We study a carbon nanotube with superconducting contacts in close proximity to a magnetic texture. Transport spectroscopy reveals a spectrum in which superconducting correlations in the carbon nanotube are deeply modified by a large spin orbit coupling. We extract a value of about 1.1 meV for this synthetic spin-orbi, larger than the intrinsic spin orbit energy in many other platforms. Furthermore, a robust zero energy state, the hallmark of devices hosting localized Majorana modes, emerges at zero magnetic field. Our findings could be used for advanced experiments, including microwave spectroscopy and braiding operations.

The angular dependence of spin-orbit torque in disordered ferromagnet/heavy-metal bilayers is calculated using a first-principles nonequilibrium Green's function formalism with an explicit supercell disorder averaging. We consider Co/Pt, Co/Au, and Co/Pd bilayers with varying thicknesses and disorder strengths. In addition to the usual dampinglike and fieldlike terms, the odd torque contains a sizable planar Hall-like term (m⋅E)m×(z×m) whose contribution to current-induced damping is consistent with experimental observations. The dampinglike torquance depends weakly on disorder strength, suggesting that it is dominated by the intrinsic mechanism. The fieldlike torquance declines with increasing disorder, consistent with the inverse spin-galvanic effect being dominant. It is found that the torques that contribute to damping are almost entirely due to spin-orbit coupling on the Pt atoms, but the fieldlike torque does not require it. The calculated thickness dependence suggests that the dampinglike torque has a bulk-like contribution due to the spin-Hall effect and an interfacial contribution of a comparable magnitude.

Magnetically doped topological insulators enable the quantum anomalous Hall effect which provides quantized edge states for lossless charge transport applications as well as for potential qubits based on chiral Majorana fermions. The magnetic gap at the Dirac point of topological insulators which hosts the chiral edge states has so far never been observed directly. Here, we use low temperature ARPES to reveal the magnetic gap of Mn-doped Bi2Te3 films which is present only below the Curie temperature of 10-12 K. We discuss the size of the gap which is 5 times larger than predicted by density functional theory. We show that this enhancement is due to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, it forms an alternating sequence of septuple and quintuple layer blocks, with Mn in the center of the septuple layers. Mn-doped Bi Se forms a similar heterostructure, however, only a large, albeit nonmagnetic gap is formed. We explain both differences based on the higher spin-orbit interaction in Bi2Te3 and discuss them in the context of the nonmagnetic system of In-doped Bi2Se3. [1] E. D. L. Rienks et al., Large magnetic gap at the Dirac point in a Mn-induced Bi2Te3 heterostructure, arXiv:1810.06238 [2] J. Sánchez-Barriga et al., Anomalous behavior of the electronic structure of (Bi1-xInx )2Se3 across the quantum phase transition from topological to trivial insulator, Phys. Rev. B 98, 235110 (2018)

Widespread applications of spintronic devices require an efficient method to manipulate the local magnetization. Topological insulators, such as Bi2Se3, are an emerging state of quantum matter, expected to exhibit more efficient charge-to-spin conversion and thus spin-obit torques (SOTs) compared to that of heavy metals. Here, we investigate the SOTs in the Bi2Se3/ferromagnet hetrostructures at different temperatures by the spin torque ferromagnetic resonance (ST-FMR) technique. We find that the SOTs efficiency abruptly increases from 0.047 at 300 K to 0.42 below 50 K. Moreover, we observe a significant out-of-plane SOT efficiency in the low temperature range. We identify that the spin-momentum-locked topological surface states (TSS) play an important role in the strong SOTs. By decreasing the Bi2Se3 thickness, we achieve TSS dominant SOT. The SOT efficiency is enhanced by more than 5 times below 8 quintuple layers and reaches a maximum value of ~1.75 at 5 quintuple layers at room temperature. In addition, the electron-mediated spin torque induced magnetization switching in the Bi2Se3/NiFe heterostructure is successfully achieved at room temperature. The switching current density, JC, is extremely low (~6 × 10^5 A/cm^2), which is much smaller than that in heavy metals. Our results illustrate that the thinner topological insulators possess a strong capability for generating SOTs. We also report the magnetization reorientation induced by magnon-mediated spin torque in topological insulator devices. Much less Joule heating is expected by taking advantage of magnons, which can carrier spin angular momentum without involving moving charges. Our observation is of importance for magnon-based future spin-devices.

The manipulation of magnetization by spin transfer torque is one of the most active field of spintronics due to its use for nonvolatile memory and programmable logic applications. In this work, we use a three-terminal spin-torque oscillator to compare the spin-transfer efficiency of two mechanisms used to induce spin transfer torque: the spin orbit torque (i.e., the torque induced by spin currents produced by spin-orbit scattering in a heavy metal) and the spin filtering torque (i.e., the spin transfer torque obtained via the flow of a spin polarized current through a spin valve (SV) or a magnetic tunnel junction (MTJ)). The device used for this study consists of a SV on top of a Pt wire. The device can be excited either by the spin filtering torque (SFT) or by the spin orbit torque (SOT) depending on whether the current is applied through the SV or through the Pt wire. By varying the Pt width and the dimensions of the SV, we tune the SOT and STT and compare their efficiencies. We show that, for the devices that we study, the SFT is more efficient in terms of current density, whereas the SOT can be more efficient than the SFT in terms of current when the Pt wire is narrower than 700 nm. Finally, we use three control samples (where the Pt wire is replaced by a Cu wire or where the SV is replaced by an MTJ) to discuss the limits of the method. [1] Jué, Pufall, and Rippard, Journal of Applied Physics 124, 043904 (2018)

In the last decade, two revolutionary concepts in nano magnetism emerged from research for advanced information processing and storage technologies. The first suggests the use of magnetic domain walls (DWs) in ferromagnetic nanowires to permanently store information in DW race-track memories. The second proposes a hard- ware realisation of neuromorphic computing in nanomagnets using nonlinear magnetic oscillations in the GHz range. Both ideas originate from the transfer of angular momentum from conduction electrons to localised spins in ferromagnets, either to push data encoded in DWs along nanowires or to sustain magnetic oscillations in artificial neurones. Even though both concepts share a common ground, they live on very different time scales which rendered them incompatible so far. Here, we bridge both ideas by demonstrating the excitation of magnetic auto-oscillations inside nano-scale DWs using pure spin currents.

Apart from classical and anomalous Hall effects, an appearance of transverse current in chiral systems has been recently discovered named as topological Hall effect (THE) as it was thought to be possible solely for topologically charged spin textures such as magnetic skyrmions. In this talk we focus on asymmetric electron scattering on chiral spin textures and tunnel transport across a semiconductor interface with account for exchange and spin-orbit interactions. We distinguish between chiral Hall effect (CHE) which requires local chirality of an effective magnetic field and the THE which requires also a non-zero topological charge. THE emerges in adiabatic regime of scattering, which can be described via geometrical Berry phase. With semi-classical approach we clarify why the spin Hall current in this case is proportional to the topological charge of a spin texture and does not depend on its size. For non-adiabatic regime we expand the scattering T-matrix in Born series and show that the scattering cross section acquires an asymmetric part, which depends on the local chirality of the spin texture leading to CHE independently of the topological charge. The analysis allowed us to clarify the reason for the spin Hall effect to vanish in this regime while preserving the charge Hall effect. We have also found theoretically the spin swapping effect for the exchange scattering on chiral spin textures. Further, with the T-matrix analysis we explain the asymmetry in the tunneling across a semiconductor interface emerging due to chiral configuration of exchange and spin-orbit fields leading to generation of transverse current.

Spin-orbitronics, which takes advantage of spin-orbit coupling (SOC), has expanded the research objects of spintronics to nonmagnetic materials. Here, we report the emerging nonlinear spintronic phenomena in the inversion-asymmetric nonmagnetic materials with SOC. For instance, the surface state of three-dimensional topological insulator (TI) owns helical spin textures with the spin and momentum perpendicularly locked. We show the observation of a nonlinear magnetoresistance (called bilinear magneto-electric resistance, BMER) and nonlinear Hall effect in a prototypical TI Bi2Se3, which scale linearly with both the applied electric and magnetic fields. We further reveal that these effects are originated from the conversion of a nonlinear spin current to charge current under the application of an external magnetic field. A close link between the BMER and the spin texture was established in TI surface states, which enables a novel transport probe of spin textures. We further extended the observation of BMER effect to the d-orbital two-dimensional electron gas (2DEG) at a SrTiO3 (STO) (111) surface. The BMER probes a three-fold out-of-plane spin texture, in addition to an in-plane one at the STO(111) surface 2DEG. This novel spin texture is in contrast to the conventional one induced by the Rashba effect. By performing tight-binding supercell calculations, we find that this 3D spin texture is fully described by the confinement effects of the STO t2g conduction band in the (111) plane. These findings open a new branch in spintronics, which discusses the nonlinear transport effects in spin-polarized nonmagnetic materials, and is therefore referred to as nonlinear spintronics.

Due to the spin-momentum locking of their topological surface states (TSS), topological insulators are expected to host spin-orbit coupling that is considerably stronger than in heavy metals. Such spin-orbit coupling can give rise to a strong spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is electrically conductive because it can completely suppress the TSS, but it should be present if the ferromagnet is insulating. This talk reports TSS-induced switching in a bi-layered structure consisting of a topological insulator Bi2Se3 and an insulating ferromagnet BaFe12O19. A charge current in Bi2Se3 can switch the magnetization in BaFe12O19 up and down. When the magnetization is switched by a field, a current in Bi2Se3 can reduce the switching field by about 4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is about 30 times higher than in Pt/BaFe12O19 structures. Such strong effects originate from the presence of more pronounced TSS at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.

Spin-charge coupled dynamics is the subject of fundamental and applied research for its relevance in spintronics. In this context the understanding of the interplay between disorder and spin-orbit coupling (SOC) plays a key role. The nonlinear sigma model for disordered electron systems is a powerful effective field theory for transport phenomena in diffusive conductors and superconductors. At the tree level the nonlinear sigma model is equivalent to the Eilenberger and Usadel equations for the quasiclassical Keldysh Green function. The latter, which were originally developed for superconductors, have been extended in order to include SOC. Here we derive the nonlinear sigma model with Rashba SOC for the disordered two-dimensional electron gas (2DEG). By starting from a functional integral representation of the partition function for disordered fermions in the presence of Rashba SOC, we introduce the Q matrix (in Keldysh and spin space) field to decouple the effective fermion-fermion interaction introduced by the disorder average. After integrating out the fermion degrees of freedom, we get the effective action for the Q field. The saddle-point approximation reproduces the self-consistent Born approximation, in the presence of Rashba SOC. By restricting to the manifold QQ=1, we derive the nonlinear sigma model for diffusive spin and charge density modes. The Rashba SOC appears as a SU(2) gauge field coupled to the Q field. Nonuniform and time-dependent saddle-point solutions for the Q field obey the Usadel equation in the presence of Rashba SOC. Finally, extensions of the theory beyond the 2DEG are briefly discussed.

Bohr’s model of the hydrogen atom can be extended to account for the observed spin-orbit interaction, either with the introduction of the Thomas precession,1 or with the stipulation that, during a spin-flip transition, the orbital radius remains intact. In other words, if there is a desire to extend Bohr’s model to accommodate the spin of the electron, then experimental observations mandate the existence of the Thomas precession, which is a questionable hypothesis, or the existence of artificially robust orbits during spin-flip transitions. This is tantamount to admitting that Bohr’s model, which is a poor man’s way of understanding the hydrogen atom, is of limited value, and that one should really rely on Dirac’s equation for the physical meaning of spin, for the mechanism that gives rise to the gyromagnetic coefficient g = 2, for Zeeman splitting, for relativistic corrections to Schrödinger’s equation, for Darwin’s term, and for the correct 1/2 factor in the spin-orbit coupling energy.

With great potential in spintronic applications due to the long spin diffusion length, graphene shows limitations in the manipulation of spins due to the small spin-orbit coupling (SOC). Recently, spin Hall effect (SHE) or Rashba-Edelstein effect (REE) has been predicted in a SOC-enhanced graphene by proximity to transition metal dichalcogenides. Here, we report the first observation of REE and inverse REE (IREE) in graphene proximity-coupled to 2H-TaS2. Gate voltage and magnetic field dependence of the nonlocal measurements exclude SHEs both in TaS2 and in graphene as the origins of our observation. At room temperature, we have demonstrated large gate-tunable inverse Rashba-Edelstein length λ_IREE from zero near the charge neutral point of graphene/TaS2 to > 10nm at increased Fermi levels. Our findings open up new opportunities in spintronics towards the efficient generation, manipulation and detection of spins by graphene at room temperature.

While classical spintronics has traditionally relied on ferromagnetic metals as spin generators and spin detectors, spin-orbitronics exploits the interplay between charge and spin currents enabled by the spin-orbit coupling (SOC) in non-magnetic systems. An efficient spin-charge interconversion can be obtained through Spin Hall Effect and Inverse Spin Hall Effect in heavy metals such as Pt or Ta. Yet a more efficient conversion can be obtained by exploiting the direct and inverse Edelstein effects at interfaces where broken inversion symmetry induces a Rashba SOC. Although the simple Rashba picture of split parabolic bands is usually used to interpret such experiments, it fails to explain the largest conversion effects and their relation to the actual band-structure. Here, we demonstrate a giant spin-to-charge conversion effect by means of Spin Pumping FMR in an interface-engineered high-carrier-density SrTiO3 two-dimensional electron gas. We use angle-resolved photoemission and Boltzmann calculations to map its peculiar gate dependence. We show that the conversion process is amplified by enhanced Rashba-like splitting due to orbital mixing, and in the vicinity of avoided band crossings with topologically non-trivial order. Our results indicate that oxide 2DEGs are strong candidates for spin-based information readout in novel memory and transistor designs. In parallel, they confirm the promise of topology as a new ingredient to expand the scope of complex oxides for spintronics.

Magnetic damping impacts essential dynamics for spintronic device applications, but its fundamental mechanisms in various materials – including simple ferromagnetic metals – have yet to be understood. Here, we experimentally correlate damping with structural and transport properties of epitaxial thin films of Fe. At room temperature, the effective Gilbert damping parameter is independent of whether these films are coherently strained or partially relaxed. However, at low temperature, we find that coherently strained Fe films with higher crystalline quality and conductivity exhibit higher damping. The enhancement of low-temperature damping is greater than that from classical eddy current loss. Our observation of such conductivity-like damping, possibly governed by the intraband scattering mechanism [1], provides fundamental insight into the role of crystallinity in damping in ferromagnetic metals. [1] K. Gilmore, Y. U. Idzerda, M. D. Stiles, Phys. Rev. Lett. 99, 027204 (2007); M. A. W. Schoen et al. Nat. Phys. 12, 839 (2016).

Tailoring Gilbert damping of metallic ferromagnetic thin films is one of the central interests in spintronics applications. Here we report a giant Gilbert damping anisotropy in epitaxial Co50Fe50 (CoFe) thin film with a maximum-minimum damping ratio of 400 %, determined by broadband spin-torque as well as inductive ferromagnetic resonance (FMR). Our CoFe films are deposited via molecular beam epitaxy at room temperature. The films are then fabricated into micron-scale devices. The first sample series, with CoFe(10 nm) and CoFe(10 nm)/Pt(6 nm), are prepared for spin-torque FMR. The second sample series, with CoFe(20 nm), are prepared for vector network analyzer FMR measurements. In addition to the fourfold magnetocrystalline anisotropy, we also find a large fourfold Gilbert damping anisotropy, along with small and consistent inhomogeneous linewidth broadening. In order to exclude the two-magnon scattering influence on linewidth, we have also conducted spin-torque FMR on a CoFe/Pt sample up to 32 GHz and we don’t find any linewidth slope softening in the frequency range. We conclude that the origin of this damping anisotropy is the variation of the spin-orbit coupling (SOC) for different magnetization orientations in the cubic lattice. The large SOC anisotropy may come from the atomic short-range order in disordered Co-Fe alloy, which preserve global cubic symmetry but can have large effects on SOC. The SOC anisotropy is further corroborated from the large crystalline the anisotropic magnetoresistance in CoFe.

Magnonics research, i.e. the manipulation of spin waves for information processing, is a topic of intense research interest in the past years. FMR, BLS and MOKE measurements lead to tremendous success and advancement of the field. However, these methods are limited in their spatial resolution. X-ray microscopy opens up a way to push to spatial resolutions below 100 nm. Here, we discuss the methodology of STXM for pump-probe data acquisition with single photon counting and arbitrary excitation patterns. Furthermore, we showcase these capabilities using two magnonic crystals as examples: an antidot lattice and a Fibonacci quasicrystal.

Recent progress toward the fabrication of Majorana-based qubits has sparked the need for systematic approaches to optimize experimentally relevant parameters for the realization of robust Majorana bound states. In this talk, I will introduce an efficient numerical method for the real-space optimization of tunable parameters, such as electrostatic potential profiles and magnetic field textures, in Majorana wires. Combining ideas from quantum control and quantum transport, our algorithm operates on a largely unexplored parameter space and opens new routes for Majorana bound states with enhanced robustness. I will conclude by commenting on the applicability of the algorithm towards the realization of Majorana modes in low g-factor materials.

In this work, a novel fabrication technique is presented, which allows to construct topological insulator - superconductor (TI-SC) hybrid devices of high quality under UHV conditions. A stencil mask is applied to the substrate before growth of (Bi,Sb)2Te3 TI thin films by means of molecular beam epitaxy. The shadow mask is used for stencil lithography of superconductive electrodes on top of the topological thin films. Finally, the ready devices are covered with a capping layer to protect the surfaces of TI and S from oxidation at ambient conditions. Besides the full device fabrication under UHV conditions, it is important to choose the right material combination. While superconductive aluminum (Al) electrodes on top of (Bi,Sb)2Te3 thin films yielded a low interface quality, measurements on (Bi,Sb)2Te3 films with niobium (Nb) electrodes, however, provided a high interface transparency. In radio frequency experiments a missing first Shapiro step was detected in our Nb - (Bi,Sb)2Te3 - Nb junctions, indicating signatures of Majorana bound states. To exploit our stencil technique to more complex device layouts for proposed topological quantum computational applications, the mask was attached to pre-patterned Si substrates. This allowed us to combine selective area growth of topological nanostructures and stencil lithography of superconductive electrodes with nm precision, paving the way for fabrication of elaborated Majorana devices.

Transition metal dichalcogenides (TMDs) and 2-dim materials beyond graphene have shown excellent potential for future electronics. Controlling the heat flow across a hetero-structure will be crucial to developing high-speed electronic devices based on 2-dim materials. We have recently shown that the thermal expansion coefficient (TEC) dramatically increases in 2-dim materials when the thickness of the material shrinks from bulk to a few monolayers. Therefore, the TEC mismatch of 2-dim materials becomes an additional concern in designing electronic nano-devices. More specifically, we need to develop methods that enable us to control and tailor the TEC of TMDs through alloying or defect engineering. In this contribution, I will employ transition metal alloying in TMDs to tune the TEC of monolayer Mo1-xWxS2 and study the interplay between thermal expansion and local defects using a combination of the scanning transmission electron microscope (STEM), electron energy loss spectroscopy (EELS) and first-principles DFT calculations. More specifically, we will measure the thermal expansion coefficient based on the plasmon energy shift as a function of temperature and combine this with first-principles modeling of the low-loss EELS signals. Using DFT calculations in the random phase approximation (RPA) we model the the plasmon peak shift as a function of lattice expansion. Combining the experimental and modeling data, we can now predict the TEC for WSe2. Using this approach, we have determined the TEC of monolayer MoS2 and WS2 and found a significant mismatch between the two materials. To explore the influence of alloy engineering on the TEC, free-standing Mo0.7W0.3S2 2-dim materials are prepared. Finally, I will compare the TEC of alloyed Mo0.7W0.3S2 monolayer with that of MoS2/WS2 lateral heterointerfaces and explore the effects of strain or point defects on the local TEC using a combination of STEM imaging, EEL spectroscopy and DFT modeling.

We show that in bilayers of transition metal dichalcogenides (TMDCs) both intra-layer and inter-layer couplings give important contributions to the Berry curvature in the K and K valleys of the Brillouin zone. Because of the inter-layer contribution the Berry curvature is stacking dependent and the commonly available 3R type and 2H type bilayers have different and highly tunable Berry curvature properties. The Berry curvature leads to valley Hall and spin Hall effects and we study them in 2H stacked bilayer MoS₂. Interestingly, the Hall conductivities may change sign as a function of the external electric field in this system which is reminiscent of the properties of lattice Chern insulators.

The goal of our research is to establish the feasibility of the information transduction between electronic, spintronic, and photonic states in 2D hybrid systems. Our initial work demonstrated, for the first time, the efficient optical manipulation of spin polarization in a MoS2/graphene heterostructure. We observed that the magnitude and direction of spin polarization can be controlled by photon helicity and photon energy. In addition, we achieved the full functionality at room temperature, which is invaluable for the technological prospects of 2D materials. Currently, we are studying the underlying mechanisms of photon-to-spin and photon-to-charge conversions at 2D heterostructure interface. Experimentally, we implement dual-gated electrostatic manipulation of optical spin and charge transfer across monolayer MoS2/monolayer graphene interface, which will give us insights into the band alignment and tunability between the two materials. Theoretically, our DFT and analytic theory co-workers model the diffusion of optically excited spin and charge at the interface, as well as investigate the role of spin orbit coupling on the spin transfer process. Our initial findings reveal an intriguing bias enhancement of photoconductivity at MoS2/graphene interface, which behaves oppositely across graphene charge neutrality point. Doping dependence and photon intensity dependence indicate strongly towards the dominance of graphene photothermoelectric effect for below MoS2 gap excitation, while an interplay of MoS2 photoelectric effect and graphene photothermoelectric effect mediates above gap excitation. Our results establish the scientific foundation of photon-to-spin and photon-to-charge transduction in 2D hybrid systems, and thereby enable significant advance for future nanoscale ultra-low-power information processors.

Graphene exhibits excellent properties for spin transport including the long spin diffusion lengths at room temperature (up to 30 microns). This results from a small intrinsic spin-orbit coupling, leading to relatively slow spin relaxation rates, in combination with relatively high mobilities. The low spin-orbit coupling, however, also has some downsides, mainly that the electrical control of spin is difficult to achieve. In this talk, I will discuss one of the emerging approaches to address this limitation, namely the use of spin proximity effect in heterostructures. One type of effect is the proximity exchange coupling, where spins in graphene are exchange coupled to the magnetization of an adjacent ferromagnetic insulator. This can produce effective magnetic fields to induce precessional dynamics and magnetically-controlled spin relaxation [1]. Another type of effect is the proximity spin-orbit coupling, where spins in graphene experience substantially increased spin-orbit coupling due to interactions with an adjacent heavy metal layer or transition metal dichalcognide (TMDC) 2D semiconductor. The role of electrostatic gating as a proposed method for tuning the spin proximity effect will be discussed, as well as the surprising observation of intrinsic gate-tunable spin lifetime anisotropy in Bernal stacked bilayer graphene [2]. This latter result is one of the first instances of an intrinsic band structure spin-orbit effect observed in graphene spin valves. Relevant references from presenter: [1] Singh et al., Phys. Rev. Lett. 118, 187201 (2017). [2] Xu et al., Phys. Rev. Lett. 121, 127703 (2018).

Distinct from all the existing methods for determining the spin Hall angle with bilayers, we have developed a new approach based on the mesoscopic H-shaped structure, which generates and detects pure spin current full electrically in a single metal. Using this approach we have unequivocally addressed the long-standing controversy of the magnitude of the spin Hall angle of gold. By exploiting the long spin diffusion length of Cu and the large spin-orbit coupling of Bi, we have realized a very large spin Hall effect in 10 nm Cu films delta doped with 0.15 nm of Bi. This new artificial material can generate a large spin-orbit torque to switch an adjacent Fe layer. We have thus demonstrated an effective fabrication method for artificial materials with a simultaneously large spin Hall angle and long spin diffusion length.

Electric gating can strongly modulate a wide variety of physical properties in semiconductors and insulators, such as significant changes of conductivity in silicon, appearance of super- conductivity in insulator [1], the paramagnet–ferromagnet transition in (In,Mn)As [2], and so on. The key to such modulation is charge accumulation in solids. Thus, it has been believed that such modulation is out of reach for conventional metals where the number of carriers is too large. However, success in tuning the Curie temperature of ultrathin cobalt gave hope of finally achieving such a degree of control even in metallic materials [3]. In this presentation, reversible modulation of up to two orders of magnitude of the inverse spin Hall effect in ultrathin platinum by ionic gating is discussed [4]. References: [1] K. Ueno et al., Nature Mater. 7, 855 (2008). [2] H. Ohno et al., Nature 408, 944 (2000). [3] D. Chiba et al., Nature Mater. 10, 853. (2011). [4] S. Dushenko, M. Shiraishi et al., Nature Commun. 9, 3118 (2018).

Recently, spin-charge conversion induced magnetoresistance (MR) in a ferromagnet (FM) or its junction with a heavy metal (HM) has attracted great attention due to its potential applications in spintronics as well as being a powerful technique for studying spin-charge interaction and transport in magnetic thin films and heterostructures. In this talk, I will discuss our recent finding of an MR induced by the anomalous Hall effect (AHE) in a ferromagnet [1]. When a charge current flows in a ferromagnet, both transverse charge and spin accumulations will occur at boundaries of the sample at steady state. The spin accumulation in turn induces a spin current either in an adjacent material or in the ferromagnet itself due to backflow of spin from the boundary. We demonstrate that the spin accumulation, subsequent spin backflow, and spin-charge conversion can give rise to a different type of spin current related magnetoresistance, dubbed as the anomalous Hall magnetoresistance (AHMR), which has the same angular dependence as the recently discovered spin Hall magnetoresistance (SMR). The AHMR is observed in four types of samples: co-sputtered (Fe1-xMnx)0.6Pt0.4, Fe1-xMnx/Pt multilayer, Fe1-xMnx with x = 0.17 – 0.65 and Fe, and analyzed using the drift-diffusion model. The AHMR provides an alternative route to study charge-spin conversion in ferromagnets and to exploit it for potential spintronic applications. In addition to AHMR, I will also discuss spin-torque induced by AHE in both single layer and multilayer structures and its applications in magnetization switching.

Graphene provides an ideal table-top test-ground for relativistic physics due to the Dirac-type linear dispersion of  electrons of carbon atoms. Haldane and Kane and Mele discovered that, taking into account the hopping between next-nearest-neighbor carbon atoms, one can achieve topological electronic states known as quantum anomalous Hall effect and quantum spin Hall effect [1].　Recently we have found that introducing superstructures respecting C6v crystalline symmetry into graphene will also generate topological states [2-6], without resort to staggered flux and/or spin-orbit coupling. As an example of our approach, we reveal that triangular and honeycomb nano-hole arrays punctured in graphene yield topologically distinct states, and that in the patchwork of them topological interface states appear which are protected by an energy gap up to 0.5 eV [7]. The unidirectional propagating interface states are dominated by a pseudospin, an emergent degree of freedom intimately related to the orbital angular momentum of the Bloch wavefunctions, which can be exploited for orbitronics functionality. Using nano holes with unbalanced numbers of carbon atoms in the two sublattices of honeycomb lattice, one may obtain spinful topological states, which hopefully can be developed for spintronics applications. References: [1] H.-M. Weng, R. Yu, X. Hu, X. Dai and Z. Fang, Adv. Phys. 64, 227 (2015). [2] L.-H. Wu and X. Hu, Phys. Rev. Lett. 114, 223901 (2015). [3] Y.-T. Yang, X. Hu, Z.-H. Hang et al., Phys. Rev. Lett. 120, 217401 (2018). [4] Y. Li, H. Chen, X. Hu et al., Nat. Commun. 9, 4598 (2018). [5] L.-H. Wu and X. Hu, Sci. Rep. 6, 24347 (2016). [6] T. Kariyado and X. Hu, Sci. Rep. 7, 16515 (2017). [7] T. Kariyado, Y.-C. Jiang, H.-X. Yang and X. Hu, Phys. Rev. B 98, 195416 (2018).

We show that even for magnetic heterostructures with light transition metals such as Mo, deterministic switching can be realized if the Mo layer is deposited in an oblique way. Our results suggest that materials growth method can strongly affect the resulting SOT switching dynamics.

Ultrafast laser-induced demagnetization was discovered two decades ago, however, the underlying fundamental processes of the fast magnetization dissipation continues to be intensively debated. I shall review recent achievements in the field of ultrafast magnetism with a focus on the various proposed mechanisms and on technological relevant applications, such as THz spintronics. Ultrafast demagnetization of a ferromagnetic (FM) layer on a nonmagnetic (NM) metallic layer can cause injection of a large, ultrafast spin-polarized current into the NM layer, which can lead to a strong THz emission pulse. We quantify the amount of spin injection and find that the spin injection proceeds within femtoseconds. Studying a FM/NM/FM trilayer with orthonormal directions of the magnetization in the FM layers, we investigate how the emitted spin current pulse can excite THz spin waves in the 2nd FM layer. Investigating further ultrafast demagnetization and all-optical switching of high-anisotropy materials such as FePt we find that element-dependent spin dynamics plays a role in the picosecond demagnetization, and that ultrafast spin transport can enhance the demagnetization of nano-sized materials. This work was performed together with P. Maldonado, M. Berritta, R. Mondal, K. Carva, L. Salemi, U. Ritzmann, J. Hurst and P. Balaz.

Acoustic waves propagating in fluids like gas and liquid can carry angular momentum, similar to the angular momentum of light. Acoustic vortex beams with rotating helical phase front possess orbital angular momentum (OAM), whose radiation torque was applied for the control of particle rotations. The orthogonality between acoustic OAM with different topological charges was exploited to increase information capacity and communication speed of a single pulse. Meanwhile, the circular energy flux obtained in several phononic crystals forms another type of acoustic OAM named pseudospin was implemented for the realization of one-way topological edge states. In acoustic helical waveguides, the geometric phase induced by the curvature of the propagation paths was also observed. The properties of the acoustic OAM are similar to their optical counterparts. Besides OAM, optical waves can also carry spin angular momentum (SAM) induced by the rotation of optical polarization characterized by local electric vector like circular polarized light. The optical spin is an intrinsic quantity that induces many intriguing wave physics including optical spin Hall effect and spin-orbit coupling of light. On the other hand, the existence of acoustic spin is an important question that is still under debating. Because of the longitudinal nature of acoustic waves propagating in fluids like airborne sound, the acoustic waves are usually characterized by scalar pressure fields and are considered to be spinless. While most of acoustic phenomena can be characterized by the scalar pressure field, the local particle velocity vector field is a crucial quantity for the understanding of the complete acoustic physics. In this work, we analytically derive a density quantity that characterize the acoustic spin if exist. The acoustic spin can exist in waves whose local particle velocity vector is rotating about itself. This acoustic spin is experimentally observed in the interference of two beams propagating perpendicular to each other. The spin induced torque is measured by a design acoustic dipole particle that interacts with the local particle velocity field. The acoustic spin also exist in evanescent waves propagating along a periodic groove waveguide, where acoustic spin-momentum locking is observed experimentally.

Charge to spin current conversion due to the spin Hall effect (SHE) was intensely studied in the last decade. For metallic wires even direct magneto-optic detection of the current-induced spin accumulation has been reported [1]. Recently, Yang et al. demonstrated the direct visualization of current-induced spin accumulation at the wire edges using a novel light helicity-dependent scanning photovoltage measurement technique in metallic wires at room temperature [2]. Unfortunately, a resonable physical explanation of the observed signals was not presented. In our experiments we reproduce the results by Yang et al. qualitatively for Pt wires with a spatial resolution of 300 nm, using a very similar setup as discussed in [2]. We compare the spatial dependence of the photovoltages in Pt, Ta and Cu wire structures under DC bias. In Pt we find signals at the wire edges, which have a similar magnitude and the same symmetry as in [2] upon switching the current direction. However, we also observe very similar helicity dependent photo voltage signals also for Cu and Ta wires under DC bias. Therefore, we conclude that most of the signals we observe at the wire edges are not caused by the SHE induced spin accumulation. We perform a careful analysis of the possible origins of these signals. [1] C. Stamm et al. Phys. Rev. Lett. (2017) [2] L. Yang et al., Nat. Commun. 9, 2492 (2018)

Here, we study intrinsic Hall effects in 4d,5d transition metal compounds and f-electron compounds based on the realistic multiorbital tight-binding models. We calculate various transport coefficients using Nakano-Kubo linear response theory. Effect of strong spin-orbit interaction (SOI) is taken into account correctly. First, we analyze the intrinsic spin Hall conductivity (SHC) in transition metals and f-electron compounds, and found that the SHC is negative when the number of d, f electrons is less than half-filling (e.g., Ta, Nb, Ce), whereas it is positive in case of more than half-filling (e.g., Pt, Pd, Yb). In f-electron systems, various multipole degrees of freedom (such as quadrupole, octupole, hexadecapole) are active due to strong SOI. We find that such multipole degrees of freedom exhibit interesting Hall effects, by analyzing the multiorbital periodic Anderson model. For example, we predict that huge qudrupole Hall effect appears in some Ce-based compounds. In addition, we discuss the generation of spin current driven by surface induced superconductivity.

We have measured a spin Hall conductivity in Ni60Cu40 that is comparable to the value we reported for Pt [1], using broadband FMR measurements of ferromagnet/nonmagnet bilayers that do not require patterning of the bilayer. In a series of samples with the layer structure Ta(3)/Py(3.5)/NixCu1-x(d)/Ta(3) (thickness in nm), we varied Ni fraction x for d = 10 nm and varied thickness d for x = 0.6, a composition with a critical temperature for magnetic ordering of 140 K. FMR measurements at room temperature showed a substantial SHE for all compositions we deposited, 0.3 ≤ x ≤ 0.75. For the thickness series at x = 0.6, we use the approach detailed in [1] to extract a spin diffusion length of (8.3 ± 0.4) nm and a spin Hall ratio of 1.05 ± 0.22. [1] Berger, A. J., et al. "Determination of the spin Hall effect and the spin diffusion length of Pt from self-consistent fitting of damping enhancement and inverse spin-orbit torque measurements" Phys. Rev. B 98, 024402 (2018).

The aim of semiconductor spintronics is to exploit the spin degree of freedom of electrons to add new functionalities to electronic devices and boost their performances. The development of assets with the ability of efficiently injecting, transferring and detecting spins is a first step towards this goal. In this sense, a well established spin injection/detection scheme relies on an heavy metal grown on the top of a Ge substrate. The semiconductor is exploited to photogenerate spin-polarized carriers making use of the optical orientation technique. These carriers are then transferred to the heavy-metal layer where spin detection occurs by means of the inverse spin-Hall effect. A key point to get quantitative information from the investigation of such a platform is the knowledge of the total spin transferred from the semiconductor to the heavy-metal layer. Here, we address this problem by employing both an analytical and a numerical spin drift-diffusion model.

Electroluminescence with nearly pure circular polarization (CP) at room temperature (RT) together with electrical helicity control [1, 2] has been demonstrated by a lateral-type spin-polarized light emitting diodes (LT-spin-LEDs) consisting of AlGaAs/GaAs double-heterostructures and the crystalline AlOx (x-AlOx) tunneling barrier [3]. In this LT-spin-LED, relatively high current density of J > 100 A/cm2 was required to achieve the circular polarization of P ~ 0.95. Operation with J > 100 A/cm2, however, often resulted in irreversible breakdown and short-lived spin-LED. In order to suppress this breakdown, we have studied fabrication of LT-spin-LED devices incorporating x-AlOx/AlAs hybrid tunneling barriers. With AlAs layers that are inserted between the x-AlOx layer and a top n-GaAs layer of LT-spin-LED, we aim at reinforcing electrical robustness of x-AlOx layers that are formed by oxidation of Al epilayers at RT. Nearly pure CP emissions (~ 0.92) are obtained from LT-spin-LED devices with hybrid tunneling barriers at J ~ 10 A/cm2. To our surprise, current density for pure CP emission decreases to about one-tenth and the yield of device fabrication is significantly improved (~ 5 % -> ~ 67 %). It is supposed that growth of AlAs layers prior to the formation of x-AlOx layers gives rise to improvement of crystalline quality of x-AlOx layers in terms of suppression of defects in the oxide layer and/or those across x-AlOx/AlAs/n-GaAs interfaces. [1] N. Nishizawa et al., PNAS 114, 1783 (2017). [2] N. Nishizawa et al., APEX 11, 053003 (2018). [3] N. Nishizawa et al., JAP 114, 033507 (2013).

High-speed vertical-cavity surface-emitting lasers (VCSELs) are essential for future data communications with high speed, low cost, and low power consumptions. For exceeding intrinsic modulation bandwidth of VCSELs limited by the relaxation oscillation frequency, manipulation of electron spin polarizations in VCSELs has been attracting attention in recent years. In this study, we theoretically and experimentally investigate modulation characteristics of 1.55-m VCSELs under the spin polarization modulation to obtaining tailored modulation characteristics suitable for high-speed data communications. Spin-flip rate equation analyses reveal that a short spin relaxation time is suitable for a flat modulation response under the spin polarization modulation, and a 100-Gb/s operation is expected in InAlGaAs quantum well (QW) VCSELs with a spin relaxation time of 10 ps, linear birefringence of 100 GHz, and dichroism of 50 GHz. A wide 3-dB bandwidth of 23 GHz determined by a frequency split between two orthogonal polarization modes (~19.4 GHz) is experimentally confirmed under optical spin polarization modulations by using a commercially-available InAlGaAs QW VCSEL whose relaxation oscillation frequency is ~3 GHz. These results support the idea that the ultrahigh- speed optical signal generation is available at the telecom wavelength of 1.55 m by applying the spin polarization modulation to VCSELs. Additionally, a modulation format conversion technique for output lights from the polarization modulation to phase modulation by using a polarizer is suggested and confirmed by the spin-flip rate equation analyses.

Recent technological and scientific interest in spin-polarized vertical-cavity surface emitting lasers (spin-VCSELs) leads to development of advanced laser modeling tools [1]. The models describes arbitrary multilayer structure of spin-VCSELs including general anisotropy of optical properties of individual layers, and also anisotropy of gain. This important generalization allows not only for precise description of static properties of the resonant cavity, but can also be used to precisely calculate effective parameters used in dynamical models. [2] Applications of spin-lasers for high modulation and switching up to GHz/THz frequencies, advanced output beam shape manipulation, and use of active quantum dot structures leads to necessity to model spin-VCSELs with active and passive laterally periodic structures such as gratings, photonic crystals, and diffractive structures. In this paper, we will show the generalization of existing matrix-based models to describe spin-lasers with lateral periodicity. The generalized rigorous coupled-wave analysis (RCWA) will be discussed in details and it will be compared with grid-based techniques such as finite element methods (FEM) and finite-difference time-domain analysis (FDTD). We will also discuss effects of incoherent propagation and random phase during light propagation in the structure. The method will be applied to structures of practical interest consisting of anisotropic grating used for ultrafast laser modulation and photonic structure. [1] T. Fördös, et. al., J. Opt. 16 (2014) 065008, Phys. Rev. A 96, (2017) 043828. [2] M. Drong, et al. Proc. SPIE 10926, (2019) 1092614.

Magnetoplasmonic gratings offer a suitable platform to analyze fundamental aspects of the interaction of plasmons with magnetization. In particular, broken time-reversal symmetry induces frequency shifts in plasmon resonances that result in large magneto-optic responses at the plasmon frequency. Here we study the excitation of plasmons with light at oblique incidence, focusing on the effect of plasmon excitation on the diffracted modes of the gratings. This way, we were able to select one of the two counterpropagating plasmons allowed for each diffraction order. Interestingly, in these conditions we found large magneto-optic amplitudes driven by plasmons near grating diffraction modes. Finite-difference timedomain simulations confirm these experimental results and corroborate that the sign of the magneto-optic enhancement is exclusively dependent on the surface plasmon wavevector and does not depends on the angle of incidence.

With increasing demand for low-power embedded electronics for Internet of Things (IoT) applications, nanomagnetic devices have emerged as strong potential candidates to complement present day electronics. A variety of novel switching effects such as Spin Transfer Torque (STT), Spin Orbit Torque (SOT) and Giant Spin Hall Effect (GSHE) offer complementary ways to manipulate nano-sized magnets. However, the low intrinsic energy cost of switching spins is often compromised by the energy consumed in the overhead circuitry in creating the necessary switching fields. In fact, the two major challenges are size scalability – limited by the superparamagnetic limit, and the energy-delay-reliability cost, limited by spin stagnation in STT devices and defect pinning in SOT driven domain wall devices. We discuss two possible alternatives that attempt to bypass these limits. Skyrmions stabilized in thin magnetic films can bypass the super-paramagnetic limit and carry information bits with added protection accorded by size and topological barrier. We present candidate ferrimagnetic materials with low saturation magnetization and damping needed to enable ultrafast room temperature soliton like skyrmion propagation. A different solution at a systems level is to design interconnected low-barrier soft magnets with controlled stochastic dynamics that can be used to realize analog or digital hardware neurons. We show how such analog neurons can directly implement Reservoir Computing for temporal inferencing, tracking and real time pattern recognition.

Future computing requires new science and technology for processing, memory, and interconnections. Nanoscale devices built out of new materials are the most promising option to overcome this challenge. Nanophotonic, nanoelectronic, nanomagnetic, and nanoelectromechanical technologies, such as spintronics, plasmonics, and nanoelectromechanical systems (NEMS), are some of the technologies currently in the nascent stages of development that hold strong promise to enable computing structures technology to provide building blocks for energy-efficient computing and data systems of the future. Investigating the underlying nanoscience, overcoming the engineering hurdles to convert these technologies in order to build integrated devices, and designing new computing architectures harnessing such nanotechnologies are the primary scientific goals of this paper. We seek a comprehensive approach, based on innovative nanoscience and technology studies, to conduct a clean-slate redesign of computing architecture and system design, simulation, and prototyping. The impact of such studies can be immense. 1. Energy-efficient and scalable computing: The nanophotonics, nanoelectronics, nanomagnetics and nanoelectromechanical systems show strong promises as building blocks with extremely low-power consumption, and will help create energy-efficient, scalable, and miniature computing systems. 2. Balanced and optimized performance: While Amdahl’s law suggests that a system with balanced computation, memory and communications perform the best under most circumstances, today’s computing systems are typically misbalanced by more than two orders of magnitude. Massively parallel interconnections by nanophotonics and nanoelectronics bring the computing systems much needed communication bandwidth for more balanced and optimized operation for energy efficiency. 3. Fundamental changes in computing architecture: The new architecture with massively parallel interconnects enables a ‘flat’ computing architecture where locality and hierarchy considerations are no longer necessary. The emerging nanoscale science and technology offer a clean-slate approach that fundamentally changes the computing architectural design through parallel optical nanowaveguide and electrical nanowire interconnects. 4. Programmability: The new ‘flat’ computing platform greatly facilitates parallel programming and invites scientists to solve grand challenge problems.

We demonstrate that a magnetic field-free two-pulse scheme previously proposed to switch an in-plane magnetized free layer is also suitable for switching a perpendicularly magnetized layer. In the case of a symmetric square free layer, deterministic switching is achieved by running the second pulse over a part of the free layer. Applying the same approach to a rectangular free layer results in switching times as short as 0.25 ns. The optimal overlap of the second heavy metal wire with the free layer is found to be between 30-60%. It is shown that the switching scheme yields a large window for the time delay/overlap between the two pulses, still maintaining the switching times as short as 0.25 ns. Consequently, the scheme is extremely robust against pulse duration fluctuations and pulse synchronization failures.

In this work, a concise method is described for providing multi-axis pinning and used to fabricate two-axis GMR sensors on one wafer. Firstly, a thin film stack with structure of 2Ta/3Ru/7IrMn80/2CoFe10/0.85Ru/2.1CoFe10/1.9Cu/1.2CoFe10/2.5NiFe19/4Ta (layer thickness in nm) is deposited on SiO2 substrate. Then the fabrication process is performed on as-deposited film by three steps of lithography based on a special layout including a full-Wheatstone-bridge sensor and a half-bridge sensor. The patterned devices are annealed in vacuum under a magnetic field of 0.9T along X-axis for 1h, then a field of -80mT for a few minutes before cooling down to room temperature. Through this design, the half-bridge sensor is sensitive to the field in X-axis while the full-bridge one responses only to the field in Y-axis with good performance. The fabrication method is cost-efficient and highly interesting for industrial mass-production. Then a perpendicular TMR sensor is demonstrated using MTJs wih in-plane magnetized free layer and a perpendicularly magnetized reference layer. we studied the free layer CoFeB thickness dependence of tunneling magnetoresistance (TMR) ratio and resistance-area product (RA) in this kind of MTJ. An optimal thickness of free layer to achieve linear response is found.

The development of high-sensitivity magnetic field sensors at low frequencies and ambient temperatures is of great importance for many practical applications, where different aspects of the sensor performance need to be considered. In this paper, it is presented that by tuning magnetic nanostructures of the free layers in magnetic tunnel junctions, widedynamic-range or ultra-high-sensitivity tunneling magnetoresistive sensors can be obtained. Tunneling magnetoresistive sensors with a linear response from -75 mT to +75 mT are demonstrated. Also, it is demonstrated that an optimized ultra-high-sensitivity magnetic sensor with a sensitivity of 57,790 %/mT can be achieved. This sensitivity is currently the highest among all magnetoresistive sensors that have been reported. The estimated noise of our magnetic sensor is 2.3 pT/Hz^1/2 at 1 Hz and 190 fT/Hz^1/2 at 100 Hz respectively. This tunneling magnetoresistive sensor dissipates only 25 μW of power when it operates under an applied voltage of 1 V at room temperature.

Advances in machine intelligence have sparked interest in hardware accelerators to implement these algorithms, yet embedded electronics have stringent power, area budgets, and speed requirements that may limit non- volatile memory (NVM) integration. In this context, the development of fast nanomagnetic neural networks using minimal training data is attractive. Here, we extend an inference-only proposal using the intrinsic physics of domain-wall MTJ (DW-MTJ) neurons for online learning to implement fully unsupervised pattern recognition operation, using winner-take-all networks that contain either random or plastic synapses (weights). Meanwhile, a read-out layer trains in a supervised fashion. We find our proposed design can approach state-of-the-art success on the task relative to competing memristive neural network proposals, while eliminating much of the area and energy overhead that would typically be required to build the neuronal layers with CMOS devices.

The end of Moore’s Law and the rise of “smart” consumer electronics has wide opened the gate for creative hardware design for the next few decades. While linear algebra accelerators and emulated hardware on FPGA has made some advances in this direction, a fundamentally different approach is required for reaching the efficiency and performance that will be necessary to embed cognitive computing in-situ in these next generation devices. To address this problem, in this work, we present a collection of spintronic hardware building blocks, fabricable with present day technology, that can be used to build biologically inspired neuromorphic hardware. These hardware units provide neuromorphic behavior derived from their physics and manifested in their electrical characteristics, therefore opening the pathway for compact, low power and VLSI grade scalability using these units. The collection contains two types of stochastic neuron (SN) devices: Analog (ASN) and Binary (BSN) as well as multi-level programmable synaptic connections that can be used for implementing compact dendrites. We discuss the area and power savings brought on by these building blocks and compared with an example design using FPGAs. This functionally complete but minimal set of neuromorphic building blocks can be used to implement a variety of neuromorphic architectures, as demonstrated in this work. We end the discussion with design ideas for neuromorphic architectures, which do not merely implement fast linear algebra but go beyond to elevate compact, physics-based field programmable neuromorphic arrays as first class citizens in every designers toolkit.

The challenge of developing an efficient artificial neuron is impeded by the use of external CMOS circuits to perform leaking and lateral inhibition. The proposed leaky integrate-and-fire neuron based on the three terminal magnetic tunnel junction (3T-MTJ) performs integration by pushing its domain wall (DW) with spin-transfer or spin-orbit torque. The leaking capability is achieved by pushing the neurons’ DWs in the direction opposite of integration using a stray field from a hard ferromagnet or a non-uniform energy landscape resulting from shape or anisotropy variation. Firing is performed by the MTJ stack. Finally, analog lateral inhibition is achieved by dipolar field repulsive coupling from each neuron. An integrating neuron thus pushes slower neighboring neurons’ DWs in the direction opposite of integration. Applying this lateral inhibition to a ten-neuron output layer within a neuromorphic crossbar structure enables the identification of handwritten digits with 94% accuracy.

Modern cognitive computing workloads require computing systems tailored to the applications, where the underlying hardware fabrics should naturally match the characteristics of learning algorithms and compute kernels. With emerging memory technologies (e.g., resistive RAM (RRAM), magnetic RAM (MRAM)), we design neuro-inspired computing systems that exploit technology characteristics such as rich device physics, circuit architecture, and integration capabilities with CMOS and beyond-CMOS technologies. Our methodology is built upon a combination of experimental characterization, cross-stack modeling, and system integration, illustrated by case studies for neural networks and highdimensional (HD) computing. Finally, we discuss the prospects of heterogeneous learning machines that emphasize the integration of compute kernels and learning algorithms, as well as the integration of emerging nanotechnologies.

Giant magnetoresistive sensors (GMR) and tunnel magnetoresistive sensors (TMR) can be efficiently used to detect weak magnetic signals corresponding to the field created by the precession of nuclear spins. In this paper, we present two experiments with their advantages and limitations. The first one is Nuclear Magnetic Resonance (NMR) spectroscopy at micron scale using magnetoresistive sensors and the second is very low field Magnetic Resonance Imaging (MRI) using mixed sensors which is a combination of magnetoresistive sensors and superconducting loops.

In this work we are going to present the ability to tune the linear range of magnetic sensors based on magnetic tunnel junctions (MTJs) from 20 Oe to 400 Oe. Controlling the linear range we control the sensitivity, the lower the linear range is the higher is the sensitivity of the sensor but in a smaller field range. To control the linear range it is fundamental to tune all the thicknesses of MTJ stack and to control all its anisotropies. The MTJ stack under study is composed of: buffer / 20 PtMn / 2 CoFe30 / 0.7 Ru / 2.6 CoFe40B20 / MgO / 2 CoFe40B20 / 0.21 Ta / tNiFe / tCoFe / tRu / tIrMn / Top Contact. The free layer is softly pinned to an anti-ferromagnet (AFM) that allows to have the pinned layer and the free layer anisotropies independent to each other. To have a good linearity we need to have the pinned and the free layers to be perpendicular. By changing any of the free layer and the top AFM thicknesses we are able to change in a control way the linear range of the sensors. [1] We achieve a linear response with the magnetic field performing two annealing in the double pinned MTJs, one to define the direction of the bottom AFM and a second one perpendicular to the first to rotate the top AFM direction. The signal is not completely centred in zero field because the free layer crystalline anisotropy is along the first annealing, perpendicular to the top AFM. In order to improve this we develop a three-step annealing process instead of only two annealing that controls the three anisotropies present in the sample (bottom and top AFM and crystalline anisotropy). [2] [1] R. Ferreira, E. Paz, P. P. Freitas, J. Wang, and S. Xue, IEEE Trans. Magn., vol. 48, no. 11, pp. 3719–3722, Nov. 2012. [2] E. Paz, R. Ferreira, and P. P. Freitas, IEEE Trans. Magn., vol. 52, no. 7, p. 4002104, 2016.

We present investigation on sensing characteristics dependence on bias voltage in CoFeB/MgO/CoFeB based tunneling magnetoresistance sensors designed to have perpendicular magnetic anisotropy in a sensing layer. The voltage controlled magnetic anisotropy effect was used to modify the perpendicular magnetic anisotropy. The modulation of the anisotropy is achieved by changing bias voltage polarity and strength. The change in the magnetic anisotropy influences spin dependent tunneling current during magnetic field sweeping. As a result of the current modification sensitivity, sensing range, nonlinearity and hysteresis of the sensors changed with the bias voltage strength and polarity. The sensitivity, sensing range, nonlinearity and hysteresis show linear dependence on the bias voltage. Under bias reversal between +1V and -1 V, the sensing properties changed more than a factor of 2. The linear dependence of the sensing characteristics and significant changed upon bias polarity reversal indicate potential of their control and modification by bias voltage polarity and strength. The use of voltage controlled perpendicular magnetic anisotropy in TMR sensors design creates new possibilities in development and applications of the TMR sensors with bias voltage controlled sensing properties.

Electronic devices are continually evolving to offer improved performance, smaller sizes, lower weight, and reduced costs, often requiring state of the art manufacturing and materials to do so. An emerging class of materials and fabrication techniques, inspired by self-assembling biological systems shows promise as an alternative to the more traditional methods that are currently used in the microelectronics industry. Mimicking unique features of natural systems, namely flexibility and shapeability, the geometry of initially planar microelectronic structures can be tailored. Heavily relying on cylindrical geometry, fabrication of RF components as well as magnetic sensors is challenging, when conventional fabrication techniques are applied. Involving novel self-assembly strategies realization of these spatially non-trivial devices in a compact form and with a reduced number of fabrication steps become feasible. This spatial self-assembly process, triggered by an external stimulus, offers a possibility of an improved performance while reducing overall manufacturing complexity of devices and components by harnessing the relative ease in which it can produce mesoscopic 3D geometries such as a “Swiss-roll” architecture. These benefits can lead to system integration of electronic and magnetoelectronic components including circuits, capacitors, coils, sensors and antennas with reduced costs fabricated from a single wafer.

I will introduce a new platform based on spin transitions in the fractional quantum Hall effect regime where parafermions - higher order non-abelian excitations - can be realized. Local (gate) control of spin transition allows formation of isolated domain walls, which consist of counter-propagating edge states of opposite polarization with fractional charge excitations. When superconductivity is induced into such a domain wall from superconducting contacts via proximity effect, parafermions are expected to be formed at the domain wall boundaries. In a multi-gate device a re-configurable network of domain walls can be formed allowing creation, braiding, manipulation and fusion of parafermions. In respect to the quantum computing application parafermons are more computationally intense than Majoranas and are a building block for Fibonacci fermions, even high order non-Abelian particles that can perform universal gate operations within the topologically protected subspace.

The fundamental antagonism between superconductivity and ferromagnetism arises from the nature of electron pairing in these materials - parallel for ferromagnetism and antiparallel (singlet) for superconductivity. However, recent theoretical and experimental evidences [1] suggest a unique form of odd-frequency equal-spin (triplet) superconductivity that arises at carefully engineered interfaces between ferromagnets and superconductors. These equal-spin Cooper pairs are immune to the pair breaking exchange field in a ferromagnet and can propagate over length scales which are significantly longer than the singlet pair coherence lengths. These dissipationless triplet currents carry a net spin which raises the intriguing possibility of ultra-low-dissipation superconducting spin-electronics (superspintronics). In this talk, following a brief introduction [2,3], I will specifically focus on our recent experiments demonstrating the control of triplet Cooper pairs using a single homogeneous ferromagnet coupled to a singlet superconductor Nb through a spin-orbit coupled interface. Our results demonstrate the crucial role played by spin-orbit coupling in modulating the triplets in a superconductor/ferromagnet system. I will also briefly focus on our recent theoretical prediction of magnetization reorientation triggered purely by a superconducting transition in ferromagnet/superconductor heterostructures with spin-orbit coupling. 1. J. Linder and J. W. A. Robinson, Nature Physics 11, 307–315 (2015) 2. N. Banerjee et al., Nature Communications 5:4771 (2014) 3. N. Banerjee et al., Nature Communications 5:3048 (2014) 4. N. Banerjee et al., Phys. Rev. B 97, 184521 (2018) 5. Lina G Johnsen et al., arXiv:1901.00501v2 (2019)

Magnetoresistive sensors based on giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) play a major role towards the miniaturization in the industrial society. Typically, spin-valve-type magnetoresistive sensors are embedded in a Wheatstone bridge configuration with rectangular, meander-like or elliptically shaped thin film elements. Such elements usually switch via multi-domain, C- or S-shaped magnetization states and, therefore, often exhibit an open non-linear hysteresis curve. Linearity and hysteretic effects are key features in the improvement of such sensors. We will present a different approach by using circularly shaped elements exhibiting a different magnetization state of a magnetic vortex [1]. This is one of the fundamental magnetization ground states occurring in disk-shaped thin film elements and is characterized by minimization of the demagnetizing energy at the expense of exchange energy. Experimental data were generated on electrically contacted GMR and TMR disks which were fabricated by optical lithography. The following advantages will be discussed and compared to standard elliptical sensor elements. (a) The vortex state shows essentially no hysteresis in the minor loop. (b) Since the vortex nucleation happens prior to the zero field, the M(H=0)=0 crossing is independent of history. (c) The critical fields can be easily controlled by the element geometry. (d) The noise is low. All characteristic experimental values have been determined in dependence of free layer thickness, disk diameter and temperature. These findings are discussed in the frame of the semi-analytical rigid-vortex-model [2] and micromagnetic simulations. The financial support by the Austrian Federal Ministry of Science, Research and Economy and the Christian Doppler Research Association in Austria is gratefully acknowledged. [1] D. Suess, A. Bachleitner-Hofmann, A. Satz, H. Weitensfelder, C. Vogler, F. Bruckner, C. Abert, K. Prügl, J. Zimmer, C. Huber, S. Luber, W. Raberg, T. Schrefl, H. Brückl, „Topologically Protected Vortex Structures to Realize Low-Noise Magnetic Sensors with High Linear Range”, Nature Electronics 1, 362 (2018) [2] K. Y. Guslienko et al., “Magnetization reversal due to vortex nucleation, displacement, and annihilation in submicron ferromagnetic dot arrays”, Phys. Rev. B 65 (2001)

Spintronics is widely explored as a replacement of CMOS for next-generation computing systems. In particular, magnetic skyrmions efficiently carry information due to their topological stability, non-volatility, low-current motion, and small size. Here we propose a skyrmion logic system that includes a direct cascading mechanism, enabling the implementation of large-scale skyrmion computing systems. This system leverages the rich physics of magnetic skyrmions, including the spin-Hall effect, skyrmion-Hall effect, skyrmion-skyrmion repulsion, repulsion between skyrmions and the track boundaries, and electrical current-control of notch depinning. A force generated by the spin-Hall effect moves skyrmions within ferromagnetic tracks due to repulsion from the track boundaries, and the interplay between the skyrmion-Hall effect and skyrmion-skyrmion repulsion enable logical operations at track junctions. Direct cascading is achieved by using the output skyrmions of one logic gate as inputs to other logic gates. To ensure correct functionality and enable large-scale systems, a global clock provides periodic current pulses in order to the synchronize skyrmion motion past notches in the track. In this conservative logic system, skyrmions are never destroyed during logical operations, thus eliminating the need to generate skyrmions, reducing energy consumption, and simplifying experimental demonstration.

In the context of miniaturization, energy conservation, smart devices and IOT, magnetic field sensors based on magnetic tunnel junctions (MTJ) constitute an attractive choice, with small size, very high intrinsic sensitivity and low power consumption. The specific sensor response curve is determined by the magnetization configuration and hysteresis loop behavior of the soft (sensing) layer of the MTJ. A possible implementation is the vortex-based sensor, in which the junction sensing layer magnetization is in a vortex configuration at zero field, consisting of a small central core with out-of-plane magnetization and an in-plane magnetization rotating around the core with a clockwise or counterclockwise direction. Depending on the sensor element geometry, the vortex can be the natural stable spin configuration of the sensor, with the lowest energy at remanent state. This is the case for circular dots of soft ferromagnetic materials with sufficient thickness. Important characteristics of sensors include the sensitivity, the linearity, the measurement range and the hysteresis. Circular vortex-state sensors typically exhibit much lower sensitivity compared to uniform-magnetization sensors but are naturally highly linear and exhibit low hysteresis in a limited field range. Here we use micro-magnetic simulations to study the effect of sensor shapes, volume defects, and perimeter defects on the magnetic behavior of a NiFe sensing layer, including hysteresis and vortex stability. In the case of circular dots, we show that the application of a large field results in a hysteresis near zero field, which originates from the combined role of volume defects inside the free layer (modeled by vacancies in the free layer) and perimeter defects (modeled by deviations from circular shape). We explore multiple geometries, sizes and thicknesses, and correlate sensor shape asymmetries, sensitivity axis direction, and hysteresis. The detailed vortex behavior following cycles of vortex expulsion and nucleation is also examined for selected asymmetric dot shapes. The role of different kinds of volume and perimeter defects is explored in determining overall sensor properties. A transverse field bias (transverse to sensing direction) is also shown to result in an increase of hysteresis. Finally, we study the effect of the dot aspect ratio on the sensor sensitivity and linearity and show the impact of the direction of the applied field on the extracted parameters.

Nanowires with strong Rashba spin-orbit coupling, such as indium arsenide or indium antimonide nanowires, have been investigated in recent years as potential host systems for topological superconductivity and Majorana bound states. In our work, we have studied theoretically the ramifications of the interplay between electron-electron interactions and Rashba spin-orbit coupling in such nanowires. One of the main experimental probes of Rashba spin-orbit coupling in nanowires is a partially gapped electron spectrum due to the application of a magnetic field perpendicular to the Rashba axis. We have shown that in a quasi-one-dimensional wire, the interplay between interactions and Rashba spin-orbit coupling can open a very similar partial gap even in the absence of magnetic field. These two types of gap, caused by magnetic fields or interactions respectively, can be distinguished in experiments by probing response functions. We argue that analogous effects occur in the edge states of two-dimensional topological insulators. This interaction-induced gap changes the topological properties of the wire profoundly: when coupled to a superconductor via the proximity effect such a nanowire can host exotic topological bound states known as Z4-parafermions. The latter are generalizations of Majorana bound states and have great potential in topological quantum computation. In nanowires, they can be experimentally identified by their signatures in spin-dependent response functions and a characteristic topological Josephson effect. Our results thus provide an experimentally feasible avenue to engineer parafermionic bound states.

Hybrid superconductor/semiconductor devices constitute a powerful platform to investigate the emergence of new topological state of matter. Among all possible semiconductor materials, InAs represents a promising choice, owing to its high quality, large g-factor and spin–orbit component. Here, we report on InAs-based devices both in one–dimensional and two–dimensional configurations. In the former, low-temperature measurements on a suspended nanowire are presented, inspecting the intrinsic spin–orbit contribution of the system. In the latter, Josephson Junctions between two Nb contacts comprising an InAs quantum well are investigated. Supercurrent flow is reported, with Nb critical temperature up to Tc ∼ 8 K. Multiple Andreev reflection signals are observed in the dissipative regime. In both systems, we show that the presence of external gates represents a useful knob, allowing for wide tunability and control of device properties, such as spin–orbit coherence length or supercurrent amplitude.

We study the electronic structure of full-shell superconductor-semiconductor nanowires, which have recently been proposed for creating Majorana zero modes, using an eight-band k p model within a fully self-consistent Schrödinger-Poisson scheme. We find that the spin-orbit coupling induced by the intrinsic radial electric field is generically weak for sub-bands with their minimum near the Fermi energy. Furthermore, we show that the chemical potential windows consistent with the emergence of a topological phase are small and sparse and can only be reached by fine tuning the diameter of the wire. These findings suggest that the parameter space consistent with the realization of a topological phase in full-shell InAs/Al nanowires is, at best, very narrow.

Spin-charge conversion in a nonmagnetic material generally follows the spin Hall symmetry, i.e. the charge current, spin current and spin polarization are all orthogonal to each other. Similar spin-charge conversion has been studied in a ferromagnetic material, but only in the anomalous Hall configuration such that the spin polarization is parallel with the magnetization. There are very few studies on the configuration where the spin polarization is transverse to the magnetization, possibly due to the consideration of transverse spin dephasing. Here I will show that despite dephasing, transverse spin-orbit effects in ferromagnetic metals are significant and exhibit unique symmetries. From a purely symmetry-based argument, a charge current through a ferromagnet can generate a spin current with conventional spin Hall symmetry. If the spin polarization is transverse to magnetization, spin can precess, leading to a spin current with a different spin polarization. We refer to these two effects as the transverse spin Hall effects with conventional symmetry (TSHE-C) and spin rotation symmetry (TSHE-SR), respectively. Both TSHE-C and TSHE-SR can generate transversely polarized spin current flowing within and out of the ferromagnet. The transversely polarized spin current flowing within the ferromagnet leads to anomalous spin-orbit torques at the surface of the ferromagnet. The transversely polarized spin current flowing out of the ferromagnet may possess a unconventional symmetry, allowing magnetization control of a second ferromagnet. The fact that ferromagnet itself can generate transversely polarized spin current may challenge the current understanding of spin-orbit effects including spin-orbit torque and spin pumping-spin galvanic effect.

Generation and detection of pure spin currents circulating in magnetic insulator materials have proven its worth by enabling transport of spin information across large distances, much further than in metals, thanks to the abscence of decay mechanism through the delocalized electrons. Spin currents here propagate over localized magnetic moments via spin-waves (SW), or their quanta the magnons, with characteristic frequencies ranging from GHz to THz and associated wavelengths from μm to nm. Most of the work so far has focus on Yttrium Iron garnets (YIG), which is a ferrimagnetic insulator with the lowest known amount of magnetic damping. In this talk, I will present recent result of spin waves transport in high quality ultra-thin films exploiting spin-orbit tools to interconvert the spin signal into an electrical signal. By injecting a high current density in Pt electrodes deposited few microns apart on top of a YIG film, a pure magnon current propagating in the YIG can be induced/detected via the direct/inverse spin Hall effect. In this work, I will mainly focus on investigating the behavior at large energies. We have found that the magnon tranresistance in this system can cross several regimes that involve strong change in the magnon distribution. Throughout various techniques such as Brillouin light scattering spectroscopy, spin Seebeck and spin Hall magnetoresistance measurements, we provide a complete analysis of the different phenomena surrounding the spin transport in thin YIG films and we will show that our experimental findings do not support yet the emergence of new collective behaviors, such as Bose-Einstein condensation at room temperature.

Spinorbitronics in III-V semiconductors, e.g. involving the GaMnAs ferromagnetic semiconductors, uses the properties of spin-orbit coupling (SOC) to generate currents of angular momentum [1-2]. Thoses are now essential to control the magnetization state of a magnet [3], or moving a domain wall [4] via the generalized spin-Hall effect of III-V possibly involving Rashba and Dresselhaus terms [4]. The interplay between particle spin and orbital motion is also at the basis of new families of effects played e. g. by the Anomalous Tunnel Hall effect described by the appearance of a lateral charge current transverse to a tunneling spin-current [5–7] ; or the spin-galvanic effects [8]. The ensemble of those complex phenomena requires a clear description of the spin-currents anatomy with advance calculation tools. In this work, as an extension to previous contributions [5], we study unconventional quantum effects resulting in a giant transport asymmetry of carriers and spin-to-charge conversion in semiconductor interfaces, tunnel barriers or quantum wells. Those are composed of ferromagnets and strong spin-orbit materials, e. g. III-V compounds with magnetizations of opposite direction (AP) or in the geometry of spin-injection devices. The symmetry of the structure allows a difference of transmission upon respective positive or negative incidence vs. the reflection plane defined by the magnetization and the surface normal. We will restrict ourself to the effect of bulk Dresselhaus terms by using the simplest form of the quantum boundary conditions. We will first detail the robustness of our advanced 30-band and 40 band tunneling codes free of spurious states effects and involving the higher electronic bands involving the relevant spin-orbit contributions. We will demonstrate that refined boundary conditions involving surface potentials, like Rashba terms, arising from the symmetry breaking at interfaces may lead to equivalent effects by their own. In a second part, we emphasize on the perturbation calculation techniques needed to understand this phenomena and to the case of the core SOI in the valence band (VB). References: [1] Tomasz Dietl and Hideo Ohno, Rev. Mod. Phys. 86, 187 (2014). [2] T. Jungwirth et al., Rev. Mod. Phys. 86, 855 (2014). [3] M. Elsen et al., Phys. Rev. B 73, 035303 [4] L. Thevenard et al., Phys. Rev. B 95, 054422 (2017). [5] T. Huong Dang, H. Jaffrès, T. L. Hoai Nguyen, and H.-J. Drouhin, Phys. Rev. B 92, 060403(R) (2015). [6] A. Matos-Abiague and J. Fabian, Phys. Rev. Lett. 115, 056602 (2015). [7] M. Jamet, A. Barski, T. Devillers, V. Poydenot et al., Nat. Mat. 5, 653-659 (2006). [10] S. D. Ganichev et al., Spinpolarization by current, ""Handbook of spin-transport & magnetism"", (Chapman and Hall), 2016.

The growth in number of transistors on a semiconductor chip has followed the Moore's law over the years and simultaneously, the size of transistor has reached equivalent to atomic size. Hence, a novel approach is needed for replacement in CMOS technology. The existing technologies are unable to meet the complex computing challenges due to their inability of inherent parallel processing and adaptability. Hence, there is a need of unconventional methodologies such as quantum computing and neuromorphic computing; and technologies like spintronics to support the realization of these methodologies. Skyrmions are the primary components of skyrmionics, the next generation of spintronics. These are the promising candidates for neuromorphic computing due to their significant features such as topological stability, low driving current density and nanoscale size. In this paper, spintronics based neuromorphic computing has been reviewed and the performance is evaluated in comparison to conventional techniques.

Magnetic susceptibility differences between tissues can be utilized as a new type of contrast in MRI. In this paper we indicated paramagnetic properties of Deoxyhemoglobin and Aquomethemoglobin using the Density Functional Theory (DFT). Deoxyhemoglobin and Aquomethemoglobin are the heme derivatives in the human body. We calculated geometries, binding energies, electronic and magnetic properties of them using GGA+U (U=3.5 eV). We obtained a magnetic moment about 4 μB and 4.72 μB for deoxyhemoglobin and aquomethemoglobin. Our results show a good agreement with the experimental reports.