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- Front Matter: Volume 9167
- Spin Laser
- Spin Coherence in Quantum Dots
- Optical Pumping I
- Spin Coherence II
- Spin Photonics
- Spin Coherence and Magnetic Polarons
- Optical Pumping II
- Multiferroics, Half Metals, and Oxides
- Skyrmions
- Helical Order and Organics
- Magnetooptics and Landé Factor
- Magnetic RAM I
- Magnetic RAM and Logic Devices
- Spin Caloric Transport I
- Spin Caloric Transport II
- Spin Transfer and Spin-Orbit Interaction
- Spin Transfer and Domain Walls
- Spin-Orbit Coupling
- Optical and Electrical Control
- Spin Pumping I
- Ultrafast Optical Control and Topological Insulators I
- Spin Pumping II
- Spin Pumping and Spin Noise
Front Matter: Volume 9167
Front Matter: Volume 9167
Show abstract
This PDF file contains the front matter associated with SPIE Proceedings Volume 9167, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.
Spin Laser
Polarization dynamics in spin-polarized vertical-cavity surface-emitting lasers
Show abstract
Spin-polarized lasers and especially spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) are at- tractive novel spintronic devices providing functionalities and characteristics superior to their conventional purely charge-based counterparts. This applies in particular to ultrafast dynamics, modulation capability and chirp control of directly modulated lasers. Here we demonstrate that ultrafast oscillations of the circular polarization degree can be generated in VCSELs by pulsed spin injection which have the potential to reach frequencies beyond 100 GHz. These oscillations are due to the coupling of the carrier-spin-photon system via the optical birefringence for the linearly polarized laser modes in the micro-cavity and are principally decoupled from conventional relaxation oscillations of the carrier-photon system. Utilizing these polarization oscillations is a very promising path to ultrafast directly modulated spin-VCSELs in the near future as long as an effective concept can be developed to modulate or switch these polarization oscillations. After briefly reviewing the state of research in the emerging field of spin-VCSELs, we present a novel concept for controlled switching of polarization oscillations by use of multiple optical spin injection pulses. Depending on the amplitude and phase conditions of the excitation pulses, constructive or destructive interference of polarization oscillations leads to an excitation, stabilization or switch-off of these oscillations. Furthermore even short single polarization bursts can be generated with pulse widths only limited by the resonance frequency of the polarization oscillation. Consequently, this concept is an important building block for using spin controlled polarization oscillations for future communication applications.
Spin Coherence in Quantum Dots
Applications of femtosecond pulse engineering in the control of excitons in quantum dots
Reuble Mathew,
Angela Gamouras,
Eric Dilcher,
et al.
Show abstract
Pulse shaping techniques are used to demonstrate quantum control of exciton qubits in InAs quantum dots. Linearly chirped laser pulses are used to demonstrate adiabatic rapid passage in a single quantum dot on a subpicosecond timescale. The observed dependence of the exciton inversion efficiency on the sign of the pulse chirp identifies phonons as the dominant source of dephasing, which can be suppressed for positive chirp at low temperatures. The use of optimal quantum control theory to engineer a single optical pulse to implement simultaneous π and 2π single qubit gates in two uncoupled quantum dots is demonstrated. This work will support the use of pulse shaping in solid-state quantum hardware.
Room-temperature initialization, dynamics, and measurement of coherent electron spins in strongly confined quantum dots
Show abstract
Semiconductor quantum dots provide a platform for studying and exploiting individual electron spins as they
interact with a complex solid state environment. Colloidal nanocrystal quantum dots are of particular interest
for potential applications, because they can achieve sufficient confinement to operate at room temperature with
relatively robust electron spin coherence. The strong confinement in these nanostructures leads to significant
effects caused by mixing of valence subbands and variation in particle size and shape. These effects influence
the processes of carrier spin initialization and detection. We have performed ensemble time-resolved Faraday
rotation experiments as well as single-dot photoluminescence excitation measurements to study how the strong
quantum confinement affects the spin physics in these systems. Single dot PLE measurements reveal mechanisms
of transition broadening that are relevant at room temperature, including thermal broadening and spectral
diffusion due to mobile charges in the surrounding environment. We find that the mixing of valence subbands
in the confined hole states largely determines the efficiency of optical spin pumping and Faraday-rotation-based
spin detection. By studying these effects, we take a step towards controlling and exploiting spin coherence in
this flexible room temperature platform.
Epitaxial growth of CdMnTe quantum dots directly on Si(111)
Marielle H. M. B. Lage,
Sukarno O. Ferreira,
Angelo S. Malachias
Show abstract
The presence of magnetic ions in a diluted magnetic semiconductor (DMS) leads to a variety of electronic, optical and
magneto-optical properties. For instance, the exchange interaction between the magnetic ions spin and the spin of
carriers leads to formation of bound magnetic polarons (BMPs). In a diluted magnetic quantum dot (DMQD), the
possibility of tuning the three dimensional confinement originates new magnetic effects not present in bulk or quantum
wells and makes MPs a very interesting system. Recently, formation of robust MPs has been observed in type-II
DMQDs, due to the spatial separation of electrons and holes. In this work, we report the growth and structural
characterization of CdMnTe/Si quantum dots. The samples were grown by molecular beam epitaxy directly on Si(111)
substrates, in contrast with the previously studied systems, where the DMS islands were grown on II-VI buffers layers.
The use of Silicon as substrates is advantageous for its compatibility with most processes of the microelectronic industry.
We have used atomic force microscopy, high-resolution transmission electron microscopy and high-resolution x-ray
diffraction to investigate the effect of growth time and temperature on the morphology and structural characteristics of
the quantum dots. Our results show that this system follows the Volmer-Weber growth mode and almost perfect epitaxial
islands can be grown despite a lattice mismatch around 19%. The introduction of a small concentration Mn ions
improves the structural quality of the islands, as observed by high resolution x-diffraction around the (111) Bragg
reflection.
Optical Pumping I
Determination of spin diffusion length in Germanium by optical and electrical spin injection
Show abstract
We report on the measurements of spin diffusion length and lifetime in Germanium with both magneto-electro-optical
and magneto-electrical techniques. Magneto-electro-optical measurements were made by optically inject in Fe/MgO/Ge
spin-photodiodes a spin polarized population around the Γ point of the Brillouin zone of Ge at different photon energies.
The spin diffusion length is obtained by fitting by a mathematical model the photon energy dependence of the spin
signal, due to switching of the light polarization from left to right, leading to a spin diffusion length of 0.9±0.2 μm at
room temperature. Non-local four-terminals and Hanle measurements performed on Fe/MgO/Ge lateral devices, at room
temperature, instead lead to 1.2±0.2 μm. The compatibility of these values among the different measurement methods
validates the use all of all of them to determine the spin diffusion length in semiconductors. While electrical methods are
well known in semiconductor spintronics, in this work we demonstrate that the optical pumping versus photon energy is
an alternative and reliable method for the determination of the spin diffusion length whereas the band structure of the
semiconductor allows for a non-negligible optical spin orientation.
Optical orientation of electron spins in GaAs L-valleys
Andrea Balocchi,
Philippe Barate,
Tiantian Zhang,
et al.
Show abstract
We report on optical orientation experiments in GaAs epilayers with excitation energies in the 3 eV region,
leading the photo-generation of spin-polarized electrons in the satellite L valley. From both continuous-wave and
time resolved measurements we show that a significant fraction of the electron spin memory can be conserved
when the electron is scattered from the L to the Γ valley following an energy relaxation of several hundreds
of meV. A typical L-valley electron spin relaxation time of 200 fs is deduced, in agreement with theoretical
calculations.
Spin-dependent transport as a consequence of Pauli blockade in a degenerate electron gas
Show abstract
Degeneracy of a photoelectron gas is shown to strongly affect spin polarized electron transport since the Pauli
principle dictates a concentration dependence of the spin stiffness and of the mobility. This causes a spin
dependence of the diffusion constant D. A spin-dependence of D as large as 50 % is measured using polarized
microluminescence imaging in p+ GaAs thin films, revealing a novel spin filter effect. The charge diffusion
constant also depends on spin via a second order effect.
Spin Coherence II
Spin-orbit interaction and spin coherence in narrow-gap semiconductor and semimetal wires
J. J. Heremans,
R. L. Kallaher,
M. Rudolph,
et al.
Show abstract
Spin-dependent quantum transport experiments on InSb and InAs heterostructures and Bi thin films are
discussed, focusing on mesoscopic geometries where spin-orbit interaction and quantum coherence determine the
properties. The narrow-bandgap semiconductors InSb and InAs, and the semimetal Bi have substantial spin-orbit
interaction. The experiments use antilocalization to study spin-orbit interaction and spin coherence lengths in nanolithographic
wires fabricated on the materials. In the three systems the spin coherence lengths increase with decreasing
wire widths if other parameters stay constant, of technological importance for spin-based devices. The experiments
also indicate that Bi has surface states with Rashba-like spin-orbit interaction. A quasi-one-dimensional model of antilocalization,
as fitted to the data, is explained and its consequences for quantum coherence in mesoscopic structures is
explored. A united understanding of the experiments is presented relying on the duality between the Aharonov-Bohm
and the Aharonov-Casher phases, the latter resulting from spin-orbit interaction. The duality strengthens the analogy
between phenomena under magnetic fields and under spin-orbit interaction.
Spin Photonics
Spin-resolved study of direct band-gap recombination in bulk Ge
Fabio Pezzoli,
Anna Giorgioni,
Giovanni Isella,
et al.
Show abstract
Recent investigations have demonstrated how temperature and doping can be employed as effective turning knobs to
fully control the angular momentum of the photons emitted at the direct gap recombination of carriers with optically
oriented spin in bulk Germanium. Here we emphasize how cooling of hot electrons via Coulomb collisions and
intervalley scattering affect spin distribution within the conduction band, and explore the role of an additional degree of
freedom, namely the excitation power density, in contributing to the electron spin relaxation.
Photonic spin Hall effect for precision metrology
Show abstract
The photonic spin Hall effect (SHE) is generally believed to be a result of an effective spin-orbit coupling, which
describes the mutual influence of the spin (polarization) and the trajectory of the light beam. The photonic SHE
holds great potential for precision metrology owing to the fact that the spin-dependent splitting in photonic SHE
are sensitive to the physical parameter variations of different systems. Remarkably, using the weak measurements,
this tiny spin-dependent shifts can be detected with the desirable accuracy so that the corresponding physical
parameters can be determined. Here, we will review some of our works on using photonic SHE for precision
metrology, such as measuring the thickness of nanometal film, identifying the graphene layers, detecting the
strength of axion coupling in topological insulators, and determining the magneto-optical constant of magnetic
film.
The charge-magnet paradoxes of classical electrodynamics
Show abstract
A number of charge-magnet paradoxes have been discussed in the literature, beginning with
Shockley’s famous 1967 paper, where he introduced the notion of hidden momentum in electromagnetic
systems. We discuss all these paradoxes in a single, general context, showing that the conservation laws
of linear and angular momenta can be satisfied without the need for hidden entities, provided that the
Einstein-Laub laws of force and torque are used in place of the standard Lorentz law. Einstein and Laub
published their paper in 1908, but the simplicity of the conventional Lorentz law overshadowed the subtle
features of their formulation which, at first sight, appears somewhat complicated. However, that slight
complication turns out to lead to subsequent advantages in light of Shockley’s discovery of hidden
momentum, which occurred more than a decade after Einstein had passed away. In this paper, we show
how the Einstein-Laub formalism handles the underlying problems associated with certain paradoxes of
classical electrodynamics involving a static distribution of electric charges and a magnet whose
magnetization slowly fades away in time. The Einstein-Laub laws of electromagnetic force and torque
treat these paradoxes with elegance and without contradicting the existing body of knowledge, which has
been confirmed by more than one and a half century of theoretical and experimental investigations.
Spin Coherence and Magnetic Polarons
Individual cobalt and manganese ions in semiconductor quantum dots and photonic structures
Wojciech Pacuski
Show abstract
This work reviews recent progress in technology of optical operating single magnetic ions. Photoluminescence
spectra of nonmagnetic QDs and QDs containing Mn or Co individual ions are shown. Emission of
quantum dots with single magnetic ions is compared to emission of bulk diluted magnetic semiconductors.
Micropillar cavity with a CdTe/ZnTe QD containing a single manganese ion is presented. Possibilities of
applying various microcavity systems for sophisticated studies of single magnetic dopants are discussed.
Conventional versus unconventional magnetic polarons: ZnMnTe/ZnSe and ZnTe/ZnMnSe quantum dots
B. Barman,
Y. Tsai,
T. Scrace,
et al.
Show abstract
We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in
two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photoexcited
holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding
non-magnetic ZnSe matrix. In sample B (ZnTe/ZnMnSe) on the other hand, the holes are confined in the non-magnetic
ZnTe QDs and the electrons move in the magnetic ZnMnSe matrix. The magnetic polaron formation energies, EMP , in
these samples were measured from the temporal red-shift of the excitonic emission peak. The magnetic polarons in the
two samples exhibit distinct characteristics. In sample A, the magnetic polaron is strongly bound with EMP=35 meV.
Furthermore, EMP has unconventionally weak dependence of on both temperature T and magnetic field Bappl . In
contrast, magnetic polarons in sample B show conventional characteristics with EMP decreasing with increasing
temperature and increasing external magnetic field. We attribute the difference in magnetic polaron properties between
the two types of QDs to the difference in the location of the Mn ions in the respective structures.
Optical Pumping II
Direct observation of non-thermal influence in the process of photo-induced ferromagnetic resonance in (Ga,Mn)As
Show abstract
Excitation dynamics of photo-induced ferromagnetic resonance (phi-FMR) in (Ga,Mn)As is investigated at both ps and
ns time regions with time-resolved magneto-optical (MO) and reflectivity measurements by using pump-probe technique
with a fs-laser source in the regime of weak excitation (0.34 - 3.4 μJ/cm2 per pulse). Magnetization precession is
observed as oscillating MO signals in ns timescale for the excitation energy hv extending from 1.41 eV to 1.65 eV.
Rapidly oscillating and spike-like signals appearing at the onset of phi-FMR within 1ps are analyzed with Landau-
Lifshitz-Bloch (LLB) equation and convoluted autocorrelation function, from which those signals are identified as the
optical effects due to autocorrelation between pump and probe, but not due to ultrafast demagnetization. Our results, in
particular, the occurrence of phi-FMR with the photon energy smaller than the GaAs bandgap energy (hv 1.51 eV at
10 K) without ultrafast demagnetization, suggests a new mechanism of controlling magnetic anisotropy with Mninduced
states in the band gap of (Ga,Mn)As.
Modeling optically pumped NMR and spin polarization in GaAs/AlGaAs quantum wells
D. Saha,
R. Wood,
J. T. Tokarski III,
et al.
Show abstract
Optically-pumped nuclear magnetic resonance (OPNMR) spectroscopy is an emerging technique to probe electronic
and nuclear spin properties in bulk and quantum well semiconductors. In OPNMR, one uses optical
pumping with light to create spin-polarized electrons in a semiconductor. The electron spin can be transferred
to the nuclear spin bath through the Fermi contact hyperfine interaction which can then be detected by conventional
NMR. The resulting NMR signal can be enhanced four to five orders of magnitude or more over the
thermal equilibrium signal. In previous work, we studied OPNMR in bulk GaAs where we investigated the
strength of the OPNMR signal as a function of the pump laser frequency. This allowed us to study the spin-split
valence band. Here we report on OPNMR studies in GaAs/AlGaAs quantum wells. We focus on theoretical
calculations for the average electron spin polarization at different photon energies for different values of external
magnetic field in both unstrained and strained quantum wells. Our calculations allow us to identify the Landau
level transitions which are responsible for the peaks in the photon energy dependence of the OPNMR signal
intensity. The calculations are based on the 8- band Pidgeon-Brown model generalized to include the effects
of the quantum confinement potential as well as pseudomorphic strain at the interfaces. Optical properties are
calculated within the golden rule approximation. Detailed comparison to experiment allows one to accurately
determine valence band spin splitting in the quantum wells including the effects of strain.
Experimental measurements of optically-pumped NMR (OPNMR) and spin polarization in bulk GaAs and GaAs/AlGaAs quantum wells
Dustin D. Wheeler,
Erika L. Sesti,
Dipta Saha,
et al.
Show abstract
Optically-pumped nuclear magnetic resonance (OPNMR) is a measurement scheme that utilizes optical pumping of
conduction electrons within a semiconductor to polarize systems of nuclear spins to which they are coupled. The
spectroscopic power of NMR techniques is brought to bear on these rare spin systems through enhancement of the
nuclear spin polarization, here in direct-gap semiconductors such as bulk semi-insulating GaAs and GaAs/AlGaAs
quantum wells. The nuclear spins act as reporters of the electron spins that are oriented during optical pumping with
circularly polarized laser light, at specific photon energies. The effects of penetration depth of the laser in the sample
can be understood when irradiating at energies less than the bandgap energy, as well as details of coupling to interband
transitions originating from Landau levels at photon energies greater than the bandgap energy. We show that OPNMR is
particularly sensitive to the sign of magnetization that results from light hole-to-conduction band transitions because the
sign of magnetization is reversed when the light hole states in the valence band are accessed.
Multiferroics, Half Metals, and Oxides
Magneto-optical investigation of Fe/CoO/Fe(001) trilayers
Show abstract
In this paper, we report on the characterization of the magnetic properties of layered Fe/CoO/Fe(001) magnetic structures
by means of Magneto-Optical Kerr Effect. Hysteresis loops were acquired on samples with variable CoO thickness, from
1 nm to 4 nm, and at different temperatures, from 30 K to room temperature. This characterization offers the opportunity
of exploiting the differences in the layer-dependent sensitivity of Kerr rotation and Kerr ellipticity in order to disentangle
the contribution of the different Fe layers in the hysteresis loops. Moreover, it allows us to give a detailed overview of
the magnetic behavior of the trilayers.
Static and dynamic magnetic property of MBE-grown Co2FeAl films
Shuang Qiao,
Shuaihua Nie,
Yan Huo,
et al.
Show abstract
In this work, the static and dynamic magnetic properties of Co2FeAl films grown by molecular beam epitaxy (MBE)
were studied by employing the magneto-optical Kerr rotation and ferromagnetic resonance (FMR) measurements. The
growth temperature dependent magnetocrystalline anisotropy of MBE-grown Co2FeAl films were first investigated by
employing the rotating magneto-optical Kerr effect. Then the magnetization dynamics and Gilbert damping property for
high quality Co2FeAl films were investigated in detail by combining both the FMR and time-resolved magneto-optical
Kerr rotation techniques. The apparent damping parameter was found to show strong dependence on the strength of the
applied magnetic field at low-field regime, but decrease drastically with increasing magnetic field and eventually
become a constant value of 0.004 at high-field regime. The inhomogeneity of magnetocrystalline anisotropy and
two-magnon scattering are suggested to be responsible for the observed abnormal damping properties observed
especially at low field regime. The intrinsic damping parameter of 0.004 is deduced for our highly-ordered Co2FeAl
film. Our results provide essential information for highly-ordered MBE-grown Co2FeA film and its possible application
in spintronic devices.
Ultra-low-energy straintronics using multiferroic composites
Show abstract
The primary impediment to continued improvement of traditional charge-based electronic devices in accordance
with Moore's law is the excessive energy dissipation that takes place in the devices during switching of bits. One
very promising solution is to utilize strain-mediated multiferroic composites, i.e., a magnetostrictive nanomagnet
strain-coupled to a piezoelectric layer, where the magnetization can be switched between its two stable states
in sub-nanosecond delay while expending a minuscule amount of energy of ~1 attojoule at room-temperature.
Apart from devising digital memory and logic, these multiferroic devices can be also utilized for analog signal
processing, e.g., voltage amplifier. First, we briefly review the recent advances on multiferroic straintronic devices
and then we show here that in a magnetostrictive nanomagnet, it is possible to achieve the so-called Landauer
limit (or the ultimate limit) of energy dissipation of amount kT ln(2) compensating the entropy loss, thereby
linking information and thermodynamics.
Skyrmions
Dynamics and topological mass of skyrmionic spin structures (presentation video)
Christoforos Moutafis,
Felix Büttner,
Andre Bisig,
et al.
Show abstract
Skyrmions are topologically protected particle-like configurations, with a topological
complexity described by their Skyrmion number. In magnetic systems, they have been
numerically predicted to exhibit rich dynamics, such as the gyrotropic and breathing
modes, dominated by their topology. Recent experimental advances brought their static
manipulation well under control. However, their dynamical behaviour is largely
unexplored experimentally. In this work, we provide with the first direct observation of
eigenmode skyrmion dynamics. In particular, we present dynamical imaging data with
high temporal and spatial resolution to demonstrate the GHz gyrotropic mode of a
single skyrmion bubble, as well as the breathing-like behaviour of a pair of skyrmionic
configurations. We use the observed dynamical behaviour to confirm the skyrmion
topology and show the existence of an unexpectedly large inertia that is key for
accurately describing skyrmion dynamics. Our results demonstrate new ways for
experimentally observing skyrmion dynamics and provide a framework for describing
their behaviour. Furthermore, the results outline a link between the dynamical behaviour
of skyrmions and their distinct topological properties, with possible ramifications for
skyrmionic spin structures research including technological applications.
Interaction between vortex walls and asymmetric notches in Permalloy nanowires (presentation video)
Luiz Sampaio Lima
Show abstract
We have investigated the injection and transmission of vortex domain walls in Permalloy (Py) nanowires through asymmetric triangular notches using Kerr microscopy and micromagnetic simulations. Through coercivity histogram measurements we observed that different vortex chiralities (counterclockwise and clockwise) have different probabilities to be observed after the notch, meaning that the interaction between the vortex wall and the notch is able to change the vortex chirality. Using the experimental results and micromagnetic simulations we were able to obtain the potential produced by the notch on the vortex wall.
Helical Order and Organics
Helical order in one dimensional semiconductors (presentation video)
Peter Stano
Show abstract
I will talk about the helical spin order which arises in the thermodynamical equilibrium in a one-dimensional (semi)conductor with spin impurities (e.g., nuclear spins, or spins of localized magnetic impurities). The helical order in localized spins is a consequence of the dimensionality, and thus very general, arising in metals, semiconductors, and even gapped phases, like superconductors.
I will show that the order follows from the resonant peak of the response (spin susceptibility) of a one-dimensional system. I will discuss recent low temperature transport experiments with semiconducting wires which suggest that such helical order was established in nuclear spins of atoms of the wire.
I will explain how such a helical order can be useful in the semi-super hybrid platform to stabilize Majorana fermions and to produce even more exotic many body excitations like fractionally charged fermions and parafermions.
Direct observation of a highly spin-polarized organic spinterface at room temperature
F. Djeghloul,
F. Ibrahim,
M. Cantoni,
et al.
Show abstract
Toward the design of large-scale electronic circuits that are entirely spintronics-driven, organic semiconductors
have been identified as a promising medium to transport information using the electron spin. This requires a
ferromagnetic metal-organic interface that is highly spin-polarized at and beyond room temperature, but this key building
block is still lacking. We show how the interface between Co and phthalocyanine molecules constitutes a promising
candidate. In fact, spin-polarized direct and inverse photoemission experiments reveal a high degree of spin polarization
at room temperature at this interface.
Magnetooptics and Landé Factor
Time resolved magneto-optical studies of InAsP ternary alloys
Show abstract
The recent rapid progress in the field of spintronics requires extensive studies of carrier and spin relaxation dynamics in
semiconductors. In this work, we employed time and spin resolved differential transmission measurements in order to
probe carrier and spin relaxation times in several InAsP ternary alloys. In addition, the dynamics of the excitonic
radiative transitions of InAs0.13P0.87 epitaxial layer were studied through the time-resolved photoluminescence
spectroscopy.
Magnetic RAM I
Status and outlook of STT-MRAM development
T. Min,
G. S. Kar,
J. Swerts,
et al.
Show abstract
MTJ stack is optimized for TMR at low RA region, high PMA and 400oC post annealing
capability. Atomic level smooth bottom electrode with 0.5A roughness was developed and positive effects on
annealing capability and PMA was demonstrated. The scaling challenge of STT-MRAM read operation down
to sub-10nm is discussed. Various contributing factors to the MTJ cell resistance variation were investigated
with focus on MRAM cell variation due to advanced lithography patterning techniques. With SADP or DSA,
the MRAM cell size can be scaled down to 18nm physical dimension with 4.2% σ/μ cell area variation, good
enough for sub-10nm technology node.
Magnetic RAM and Logic Devices
Time resolved transport studies of magnetization reversal in orthogonal spin transfer magnetic tunnel junction devices
Show abstract
In this work we report on time resolved magnetization reversal driven by spin transfer torque in an orthogonal
spin transfer (OST) magnetic tunnel junction device. We focus on the transitions from parallel (P) to antiparallel
(AP) states and the reverse transitions (AP to P) under the influence of 10 ns voltage pulses. The electrical
response is monitored with a fast real-time oscilloscope and thus timing information of the reversal process
is obtained. The P to AP transition switching time decreases with increasing current and shows only direct
switching behavior. The AP to P transition shows similar behavior, but has a broader distribution of switching
times at high currents. The rare AP to P switching events that occur at later times are due to the occurrence of a
pre-oscillation, which could be identified in time resolve voltage traces. A possible origin of these pre-oscillations
is seen in micromagnetic simulations, where they are associated with the breakdown of the uniform precession
of the magnetization, and lead to reversal of the magnetization at later times.
High-performance computing based on spin-diode logic
Show abstract
The cascading of logic gates is one of the primary challenges for spintronic computing, as there is a need to dynamically
create magnetic fields. Spin-diode logic provides this essential cascading, as the current through each spin-diode is
modulated by a magnetic field created by the current through other spin-diodes. This logic family can potentially be
applied to any device exhibiting strong positive or negative magnetoresistance, and allows for the creation of circuits
with exceptionally high performance. These novel circuit structures provide an opportunity for spintronics to replace
CMOS in general-purpose computing.
Spin Caloric Transport I
Anomalous spin and charge Seebeck effect in a quantum well with spin orbit interaction
Show abstract
We discuss the possible existence of an anomalously high low-temperature charge and spin thermopower in
a two dimensional electron system with Rashba and Dresselhaus spin-orbit coupling in the special case when
the two interactions have equal strengths. The fundamental premise of the theory is the establishment of an
weak itinerant antiferromagnetic order in the ground state, a spin alignment favored in the minimum-energy
many-body state when the Coulomb interaction is considered. The transport in this state is modeled by using
the solutions of a Boltzmann equation obtained within the relaxation time approximation. We show that when
scattering on magnetic impurities is introduced, the energy dependence of the relaxation time enhances the value
of the thermoelectric coefficient for both charge and spin currents. An estimate of the effect is provided for the
case of a standard InAs quantum well and its variation with the strength of the magnetic scattering is studied.
Spin Caloric Transport II
Spin-Hall effects: from the two-channel model to Dyakonov-Perel equations
Show abstract
The anisotropic properties of thermal transport in insulating or conducting ferromagnets are derived on the basis of the Onsager reciprocity relations applied to a magnetic system. It is shown that the angular dependence of the temperature gradient takes the same form as that of the anisotropic magnetoresistance, including anomalous and planar Hall contributions [1].
The experimental study [2] shows that the voltage measured between the extremities of the non-magnetic electrode in thermal contact to the Py or YIG ferromagnetic layers follows the predicted angular dependence. Furthermore, the sign and the amplitude of the magneto-voltaic signal measured is in agreement with the thermocouples calculated from the corresponding Seebeck coefficients, for the three different electrodes used in the study (Pt, Cu, Bi).
Spin Transfer and Spin-Orbit Interaction
Spin-orbit torques in ferromagnetic heterostructures: fundamentals and applications (presentation video)
Kevin Garello,
Can Onur Avci,
Mihai Miron,
et al.
Show abstract
Current-induced spin torques are of great interest to manipulate the orientation of nanomagnets without applying external magnetic fields. They find direct application in non-volatile data storage and logic devices. Recent demonstrations of perpendicular magnetization switching induced by in-plane current injection in ferromagnetic heterostructures have drawn attention to a class of spin torques based on orbital-to-spin momentum transfer (SOTs), which is alternative to pure spin transfer torque (STT) between non collinear magnetic layers and amenable to more diversified device functions. We will present advance made to build first perpendicular SOT-MRAM devices and to describe the symmetry and amplitudes of SOTs, revealing unexpected physics.
Spin Transfer and Domain Walls
Correlation between spin structure oscillations and domain wall velocities (presentation video)
Andre Bisig,
Martin Stärk,
Mohamad-Assaad Mawass,
et al.
Show abstract
Magnetic sensing and logic devices based on the motion of magnetic domain walls rely on the precise and deterministic control of the position and the velocity of individual magnetic domain walls. Varying domain wall velocities have been predicted to result from intrinsic effects such as oscillating domain wall spin structure transformations and extrinsic pinning due to imperfections. We use direct dynamic imaging of the nanoscale spin structure to directly check these predictions. We find a new regime of oscillating domain wall motion even below the Walker breakdown correlated with periodic spin structure changes and we show that the extrinsic pinning from defects in the nanowire only affects slow domain walls.
Spin-Orbit Coupling
Spin control and manipulation in (111) GaAs quantum wells
A. Hernández-Mínguez,
K. Biermann,
R. Hey,
et al.
Show abstract
The control of spin dephasing is an essential requirement for quantum information processing using electron spins in IIIV
semiconductors. GaAs quantum wells grown along the non-conventional [111] crystallographic direction are
particularly interesting for spintronics due to the long spin lifetimes, which can be electrically controlled. Here, we show
electron spin dynamics in (111) quantum wells by combining spatially-resolved with time-resolved photoluminescence
measurements. The latter allows us to experimentally demonstrate the field induced enhancement of the spin lifetime as
well as the transport of spin over several micrometers along the quantum well plane.
Spin injection and spin-orbit coupling in low-dimensional semiconductor nanostructures
Sebastian Heedt,
Isabel Wehrmann,
Thomas Gerster,
et al.
Show abstract
Due to their strong spin-orbit coupling III-V semiconductor nanowires are excellent candidates for electrical spin
manipulation. Therefore, a major goal is to tailor spin-orbit coupling in these devices. Direct electrical spin injection into
quasi one-dimensional nanowires is demonstrated. Furthermore, the weak antilocalization effect was investigated in InAs
nanowires. The quantum corrections to the conductivity are interpreted by developing a quasi-one-dimensional diffusive
model. It turns out that by means of doping and electric gating the spin-lifetimes can be tuned significantly. By creating
few-electron quantum dots inside these devices the impact of the confinement on the spin relaxation properties is
investigated.
Optical and Electrical Control
Electric-field modulation of spin-wave phase in yttrium iron garnet film (presentation video)
Show abstract
We study the electric control of spin-wave in yttrium iron garnet (YIG) both theoretically and experimentally. It is found that an applied electric field can be used to manipulate the phase of spin waves through spin-orbit interaction and first-order magnetoelectric effect in YIG film with no piezoelectric layer coupling to it.
Spin Pumping I
Spin pumping and inverse spin Hall effect in platinum and other 5d metals: the essential role of spin-memory loss and spin-current discontinuities at interfaces
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It was recently realized that the spin Hall effect (SHE) can be very useful in the area of spintronics, due to its ability to
generate pure spin current from charge current, without the use of any magnetic materials or magnetic field. The
maximum conversion factor is given by the spin Hall angle 𝜃SH, which can take rather important values (above 10% in
absolute value was reported for β-Ta and β-W). This phenomenon is usually observed in materials with large spin-orbit
coupling, either intrinsic (Pt, Ta, W, etc.) or induced by heavy impurities (Cu doped with Bi or Ir). To investigate this
property, several groups studied the reciprocal effect, the so-called inverse spin Hall effect (ISHE), converting a pure
“pumped” spin current into a charge current (measured by voltage detection in an “open circuit”). We focus here on the
5d Pt material. Values published nowadays for 𝜃SH in Pt are scattered over one order of magnitude, with a clear
correlation between the spin diffusion length ℓsf and the 𝜃SH, both quantities being related to the spin-orbit strength or its
inverse. We performed measurements of spin pumping in a cavity and measured the resulting ISHE voltage. We propose
a model including spin-current discontinuity or spin memory loss at the interfaces that may reconcile all the different
observations. In particular, we demonstrate consistent values of spin diffusion length (ℓsf = 3.4 ± 0.4 nm) and spin Hall
angle (𝜃SH = 0.056 ± 0.010) for Pt in different metallic multilayer systems.
Ultrafast Optical Control and Topological Insulators I
Attempting nanolocalization of all-optical switching through nano-holes in an Al-mask
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We investigate the light-induced magnetization reversal in samples of rare-earth transition metal alloys, where we aim to
spatially confine the switched region at the nanoscale, with the help of nano-holes in an Al-mask covering the sample.
First of all, an optimum multilayer structure is designed for the optimum absorption of the incident light. Next, using
finite difference time domain simulations we investigate light penetration through nano-holes of different diameter. We
find that the holes of 200 nm diameter combine an optimum transmittance with a localization better than λ/4. Further,
we have manufactured samples with the help of focused ion beam milling of Al-capped TbCoFe layers. Finally,
employing magnetization-sensitive X-ray holography techniques, we have investigated the magnetization reversal with
extremely high resolution. The results show severe processing effects on the switching characteristics of the magnetic
layers.
Ultrafast spin dynamics in metallic layers and strongly correlated oxides
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The spin dynamics triggered by an ultrashort optical excitation can lead to a variety of behaviors depending on the
specific spin and electronic structure of the material. In metallic films, electron-quasiparticles (phonons and magnons)
interactions takes place on sub-picosecond timescale and demagnetization is established within 100 fs. In half-metal
oxides, spin dynamics is much slower (100 ps) due to the inhibition of spin-flip processes. Furthermore, the dynamics of
magnetic anisotropies can be exploited to control the magnetization in ferromagnets. Optically-induced reversible
switching of the magnetization has been recently demonstrated in thin magnetic layers on the 100 picoseconds timescale.
Weak localization and weak anti-localization in topological insulators
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Weak localization and weak anti-localization are quantum interference effects in quantum transport in a disor- dered electron system. Weak anti-localization enhances the conductivity and weak localization suppresses the conductivity with decreasing temperature at very low temperatures. A magnetic field can destroy the quantum interference effect, giving rise to a cusp-like positive and negative magnetoconductivity as the signatures of weak localization and weak anti-localization, respectively. These effects have been widely observed in topological in- sulators. In this article, we review recent progresses in both theory and experiment of weak (anti-)localization in topological insulators, where the quasiparticles are described as Dirac fermions. We predicted a crossover from weak anti-localization to weak localization if the massless Dirac fermions (such as the surface states of topo- logical insulator) acquire a Dirac mass, which was confirmed experimentally. The bulk states in a topological insulator thin film can exhibit the weak localization effect, quite different from other system with strong spin- orbit interaction. We compare the localization behaviors of Dirac fermions with conventional electron systems in the presence of disorders of different symmetries. Finally, we show that both the interaction and quantum interference are required to account for the experimentally observed temperature and magnetic field dependence of the conductivity at low temperatures.
Weak antilocalisation in topological insulators with strong spin-orbit scattering (presentation video)
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Topological insulators (TI) have revolutionised our understanding of insulating behaviour. They are insulators in the bulk but conducting along their surfaces, thanks to surface states in which the spin and the charge are strongly coupled by means of the spin-orbit interaction. Much of the recent research on TI focuses on overcoming the transport bottleneck [1], namely the fact that surface state transport is overwhelmed by bulk transport stemming from unintentional doping. The key to overcoming this bottleneck is identifying unambiguous signatures of surface state transport. This talk will discuss one such signature, which is manifest in the coherent backscattering of electrons in TI. Because of the strong spin-orbit coupling in TI one expects to observe weak antilocalisation rather than weak localisation, meaning that coherent backscattering increases the electrical conductivity [2]. The features of this effect, however, are rather subtle, because in TI the impurities have strong spin-orbit coupling as well, greatly increasing the complexity of the problem [3]. I will show that spin-orbit coupled impurities introduce an additional time scale, which is expected to be shorter than the dephasing time, and the resulting conductivity has a logarithmic dependence on the carrier number density, a behaviour hitherto unknown in 2D electron systems. The result we predict is directly observable experimentally and would provide a smoking gun test of surface transport. Furthermore, I will also discuss the effect of electron-electron interactions on transport in this regime.
[1] D. Culcer, Physica E 44, 860 (2012).
[2] G. Tkachov and E. M. Hankiewicz, Phys. Rev. B 84, 035444 (2011).
[3] W. Liu, , P. Adroguer, X. Bi, E. M. Hankiewicz, and D. Culcer, to be published.
Spin Pumping II
Three-magnon splitting controlled by temperature
Saki Matsuura,
Takaharu Tashiro,
Kazuya Ando
Show abstract
A nonlinear magnon interaction has been investigated in an yttrium iron garnet/Pt film. In the bilayer film,
multiple microwave absorption signals due to the excitation of magnetostatic spin waves, including magnoetostatic
surface waves and backward volume magnetostatic spin waves, were observed at room temperature. The
microwave absorption spectrum was found to be changed drastically by decreasing temperature. This drastic
change in the microwave absorption spectrum is attributed to the appearance of the three-magnon splitting
process; the spin-wave dispersion calculated by taking into account the temperature variation of the saturation
magnetization indicates that the change in the microwave absorption spectrum appears when the lowest frequency
of the spin-wave dispersion becomes lower than a half of the microwave excitation frequency. At this
condition, the energy and momentum conservation laws can be fulfilled in the process where a pumped magnon
splits into two magnons. These results demonstrate that the nonlinear magnon interaction can be controlled not
only by excitation frequency but also by temperature.
Dual-frequency ferromagnetic resonance to measure spin current coupling in multilayers
Rohan Adur,
Chunhui Du,
Hailong Wang,
et al.
Show abstract
Spin pumping is a method for injecting a pure spin current into a non-magnetic metal (NM) by inducing precession
of a neighboring ferromagnet (FM) at its ferromagnetic resonance frequency. A popular method to detect spin
current uses the Inverse Spin Hall Effect (ISHE) to convert the spin current to a detectable charge current and
hence a voltage. In order to better understand the role of time independent and high frequency contributions to
spin pumping, we sought to detect we attempt to detect spin currents by using a second microwave frequency
to detect changes in linewidth of a second ferromagnet due to the spin-torque induced by the spin current from
the first ferromagnet. This dual resonance is achieved by pairing a custom broadband coplanar transmission line
with the high-Q resonant cavity of a commercial electron paramagnetic resonance spectrometer. This technique
is general enough that it should enable the investigation of spin currents in any FM-NM-FM system, for any
orientation of external field, and is not sensitive to voltage artifacts often found in ISHE measurements. We
find that the condition for simultaneous resonance generates a dc spin current that is too small to produce
a measurable change in linewidth of the second ferromagnet, confirming the dominance of ac spin currents in
linewidth enhancement measurements.
Spin Pumping and Spin Noise
Insight about spin Hall effects from spin pumping (presentation video)
Axel Hoffmann,
Wei Zhang,
Vincent Vlaminck,
et al.
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We present measurements of the spin Hall effect using macroscopic spin currents generated from spin pumping. Measurements as a function of layer thickness enable the determination of the spin diffusion length, which is an essential parameter for the proper quantification of the spin Hall conductivity and the related spin Hall angle. Furthermore we discuss the temperature dependence of these measurements and how possible proximity effects may be reflected in the data.
Spin noise spectroscopy in semiconductors: from a billion down to single spins
J. Hübner,
R. Dahbashi,
F. Berski,
et al.
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Spin noise spectroscopy in semiconductors has matured during the past nine years into a versatile and well developed
technique being capable to unveil the intrinsic and unaltered spin dynamics in a wide range of semiconductor systems.
Originating from atom and quantum optics as a potential true quantum non-demolition measurement technique, SNS is
capable of unearthing the intricate dynamics of free or localized electron and hole spins in semiconductors being
eventually coupled to the nuclear spin bath as well. In this contribution, we review shortly the major steps which inspired
the success of spin noise spectroscopy in semiconductors and present the most recent extensions into the low-invasive
detection regime of the spin dynamics for the two extreme limits of very high and extremely low rates of spin
decoherence, respectively. On the one hand, merging ultrafast laser spectroscopy with spin noise spectroscopy enables
the detection of spin noise with picosecond resolution, i.e., with THz bandwidths yielding access to otherwise concealed
microscopic electronic processes. On the other hand, we present very high sensitivity SNS being capable to measure the
extremely long spin coherence of single holes enclosed in individual quantum dots venturing a step forward towards true
optical quantum non-demolition experiments in semiconductors. In addition, higher-order spin noise statistics of, e.g.,
single charges can give information beyond the linear response regime governed by the fundamental fluctuationdissipation
theorem and thereby possibly shed some light on the nested coupling between electronic and nuclear spins.