This PDF file contains the front matter associated with SPIE Proceedings Volume 8100, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The original motivation of great interest to topological insulators was the hope to observe the quantum spin Hall effect. Therefore if a material is in the topological insulator state they frequently call it the quantum spin Hall state. However, despite impressive experimental results confirming the existence of the quantum spin Hall state, the quantum spin Hall effect has not yet been detected. After a short overview of what was originally suggested as the quantum spin Hall effect (quantum spin conductance determined by the topological Chern number) the paper analyzes the crucial role of the boundary condition on the observation of the effect and finally discusses whether and how the quantum spin Hall effect could be observed.

We present photoluminescence (PL) and spin polarized photo-emission (SPPE) measurements performed on bulk Ge, and Ge/Si(100) epilayers. A set of bi-axially strained Ge epilayers featuring different strain levels has been deposited by low energy plasma enhanced CVD (LEPECVD) and characterized by high resolution X-ray diffraction (HR-XRD). SPPE data indicate that compressive strain effectively lifts the heavy holes - light holes degeneracy raising the polarization of injected electrons above the P=50% limit of bulk material. Circularly polarized light PL measurements performed on p-type bulk crystals confirm the suitability of Ge for spintronics application showing a robust electron spin inizialization at the direct gap of Ge and giving a lower bound value for the spin relaxation time of ≈ 230 fs.

The photocurrent obtained under polarized optical excitation and the polarized electroluminescence recorded under forward electric bias have been measured in the same hybrid Semiconductor/Ferromagnetic metal structures (Spin-Light Emitting Diode). Systematic investigations have been performed on devices with different ferromagnetic spin injectors, consisting e.g. of MgO tunnel barriers with a CoFeB ferromagnetic layer. Though a very efficient electrical spin injection is demonstrated, very weak polarization of the photocurrent is evidenced: the photocurrent polarization measured under continuous resonant circularly polarized excitation of the quantum well excitons is below 3%. This demonstrates that the investigated devices do not act as efficient spin filters for the electrons flowing from the semiconductor part to the ferromagnetic part of these structures though these systems are very efficient spin aligners for electrical spin injection. We interpret the weak measured photocurrent polarization of the as a consequence of the Zeeman splitting of the quantum well excitons which yields different absorption coefficients for the polarized excitation laser with different helicities. This leads to different intensities of photocurrent collected for the two different circularly polarized excitations. This interpretation is confirmed by an experiment exhibiting the same results for photocurrent measured on a device with a non ferromagnetic electrical contact.

Electron spin transmission across ferromagnetic metal/semiconductor interfaces with different ferromagnetic contacts, i.e., Fe and FeGa, is studied using optical spin orientation method. The bias dependence of spin dependent photocurrent, which is the difference between the photocurrents excited with left- and right- handed circularly polarized lights, is found to show a dip-like feature at -0.058 and 0.021 V for Fe and FeGa contacts, respectively. The origin of the bias dependence of the spin dependent photocurrent is discussed on the basis of the Breit-Wigner type resonant tunneling process via interface resonant states, comparing the results for the both contacts. The results also indicate that the control of interface states is crucial to achieve efficient spin filtering effect at the ferromagnet/semiconductor interfaces.

The intimate relation between the angular momentum and the magnetization - expressed through the gyromagnetic relation - is well known and is easy to evidenced at the macroscopic scale with magnetomechanical measurements. On the other hand, the conservation of the angular momentum find also a simple illustration in the behavior of a spinning top. Accordingly, the dynamics of a single domain ferromagnet should follow the same laws as a symmetrical spinning top. Paradoxically, this is not true since the equations that govern the dynamics of the magnetization do not contain inertial terms. We investigate under what conditions the inertial terms that are initially present in the conservation laws disappear, in order to lead to the well-known expressions of the Landau-Lifshitz-Gilbert equation

Spin wave excitations in dipolarly coupled nanodisks from permalloy are investigated. We address, both, onedimensional (1D) chains and two-dimensional (2D) arrays consisting of nanodisks of different diameter. An out-of-plane magnetic field allows us to initialize the so-called vortex state in each of them. Our micromagnetic simulations show that in such 1D and 2D periodic devices the low-frequency excitations of the individual disks couple and form allowed frequency bands for spin-wave propagation. The devices operate as magnonic wave guides and magnonic crystals. The diameter is found to allow one to control both the center frequency and bandwidth of the allowed miniband in the few GHz frequency regime. We discuss a hybrid nanodisk device which might allow one to control and slow down the spin waves, i.e., the transmitted GHz signal.

Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studied in photoluminescence experiments at 4K. First we report strong mixing of heavy hole-light hole states. Using the neutral and charged exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k•p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. Second we report optical orientation experiments. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence. Optical injection of spin polarized electrons into a GaAs dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build-up time of the nuclear polarization in a single GaAs dot in the order of one second.

We describe a spin filter at the single-electron level that produces pure spin currents with no net bias. Our device is based on the ground-state energetics of a single-electron transistor comprising a superconducting island connected to normal leads via tunnel barriers with different resistances that break spatial symmetry. The current has opposite spin polarization when the current is reversed, which leads to a dc spin current when applying an alternating charge current with zero mean, as expected in a spin ratchet. We demonstrate spin transport and quantify the spin ratchet efficiency by means of ferromagnetic leads with known spin polarization.

There are a variety of methods available for confining and manipulating single spins in solid state systems. While heterostructures can be engineered to the requirements of the problem, their variability is a disadvantage compared to identical impurities. I will discuss theoretical calculations of electronic states in both quantum dot heterostructures and bound to impurities. These include calculations of the spin state itself, the effective coupling to a magnetic field, the response to an electric field, and include both electrons and holes.

I will discuss recent advances in understanding the nuclear spin dynamics in low dimensional systems. The focus will be on the hyperfine interaction between nuclear and electronic spins and the role this interaction plays on the dynamical nuclear polarization of semiconductor nanostructures. I will address topics such as nuclear spin relaxation time, nuclear spin polarization, and nuclear spin diffusion. As an example I will consider the case of semiconductor quantum wells and discuss theoretical results in connection with experimental data.

We have investigated spin-density excitations of holes in one-sided p-modulation-doped GaAs-AlGaAs single quantum wells by means of resonant inelastic light scattering. The experiments yield a direct measure of the Rashba spin splitting of holes in quantum wells with an asymmetric potential profile. In the low-energy range of the inelastic light scattering spectra, we observe in all samples well-defined excitations with energies between about 1 meV and 4 meV, which can be attributed to spin-density excitations of the two-dimensional hole systems due to polarization selection rules. We interpret the excitations as spin-density excitations, where holes are excited between the Rashba spin-split ground states, performing a spinflip process. Comparison to k · p bandstructure calculations shows good agreement of the measured and calculated wave-vector-dependent spin splittings. Details of the spectra show a distinct dependence on the directions of light polarizations with respect to crystallographic axes. In particular, we have detected a doublet structure of the hole spin excitations, which might be attributed to the anisotropic spin-split hole dispersion within the quantum-well plane.

We perform low-temperature spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy measurements in magnetic fields to gain new insights into nanomagnetism. We use the magnetic field to change and control magnetizations of a sample and a magnetic tip, and measure the magnetic hysteresis loops of individual Co nano-islands on Cu(111). We also exploit the high spatial resolution of SP-STM in magnetic fields to measure maps of the differential conductance within a single Co nano-island. In connection with ab initio calculations, we find that the spin polarization is not homogeneous but spatially modulated within the nano-island. We ascribe the spatial variation of the spin polarization to spin-dependent electron confinement within the Co nano-island.

We report a reproducible top down fabrication procedure for a single domain wall magnetoresistance H-shaped device. A bi-layer e-beam lift-off process is used and the e-beam exposure dose sensing technique and proximity effect correction are discussed, together with a method to reduce the alignment tolerance to below 20 nanometer. The domain wall width is constrained down to 37nm and room temperature domain wall magnetoresistance ratio of 0.3% was detected. The dependence of switching magnetic field to domain width will be discussed, as well as the maximum domain width which can retain its magnetisation aligned along the long axis at zero field which is found to be 210nm in our experiment.

Enhanced spin injection efficiency and extended spin lifetimes are achieved in graphene spin valves. Spin injection efficiency is enhanced via tunneling spin injection into graphene through an MgO barrier. A large nonlocal magetoresistance of 130 Ω is observed for a single layer graphene (SLG) spin valve at room temperature (RT) with spin injection efficiency of ~ 26-30%. Extended spin lifetimes are observed using tunneling contact to suppress the contact induced spin relaxation. In SLG, spin lifetime as long as 771 ps is observed at RT. In bilayer graphene (BLG), we observe the spin lifetime of 6.2 ns at 20 K, which is the longest value reported in any graphene spin valve. Furthermore, contrasting spin relaxation behaviors are observed in SLG and BLG, which suggests that Elliot-Yafet spin relaxation dominates in SLG at low temperatures, while Dyakonov-Perel spin relaxation dominates in BLG at low temperatures.6

Surfaces are often different materials, and typically have a different electronic structure from the bulk and since the dawn of surface science, surface-localized electronic states, surface states, have been extensively studied and investigated with growing accuracy. Of particular importance to spintronics are magnetic surface states. Interfaces will play a very important role in many spintronics devices, yet the interface properties are often ignored, poorly understood or badly characterized. For many nominally half metal materials, materials that in some ground state calculations exhibit 100% spin polarization, the magnetic surface states may significantly reduce the effective spin polarization. We review the magnetic surface states of several well known and often highly touted high spin polarized materials such as NiMnSb, Fe₃O₄, CoS₂ and CrO₂. Finally, we summarize surface state measurements of magnetoelectric antiferromagnets Cr₂O₃, which has electrically controllable net surface spins, a major complication to the study of CrO₂ by photoemission.

This paper presents a novel design concept for spintronic nanoelectronics that emphasizes a seamless integration of spin-based memory and logic circuits. The building blocks are magneto-logic gates based on a hybrid graphene/ferromagnet material system. We use network search engines as a technology demonstration vehicle and present a spin-based circuit design with smaller area, faster speed, and lower energy consumption than the state-of-the-art CMOS counterparts. This design can also be applied in applications such as data compression, coding and image recognition. In the proposed scheme, over 100 spin-based logic operations are carried out before any need for a spin-charge conversion. Consequently, supporting CMOS electronics requires little power consumption. The spintronic-CMOS integrated system can be implemented on a single 3-D chip. These nonvolatile logic circuits hold potential for a paradigm shift in computing applications.

Graphene can be grown directly on MgO(111) by industrially practical and scalable methods: free radical-assisted chemical vapor deposition (CVD), and physical vapor deposition (PVD). Single layer and double layer films can be produced by PVD, with a ~ 2 monolayer (ML) thick film as the apparent limiting thickness. C(1s) x-ray photoemission spectra (XPS) indicate that in both layers, carbon atoms are in two different oxidation states. Low energy electron diffraction (LEED) data are consistent with this, showing unequal graphene A site/B site intensities for both single and double layer graphene, yielding C_3V symmetry. This lifts the A site/B site chemical equivalence in the graphene lattice, and therefore also the HOMO/LUMO degeneracy at the Dirac point. Consistent with this, a band gap of ~ 0.5 -1 eV has been observed for the two layer film. The XPS, LEED and band gap findings indicate that the graphene/MgO interface is commensurate, and that the MgO surface layer is reconstructed, resulting in carbon→MgO charge transfer. In addition, graphene growth by PVD is self-limiting at 2 monolayers thickness. These findings have implications for graphene growth on other (111) oxide surfaces. The ability to grow MgO(111) films on Si(100)-reported in the literature- points to a direct path to the development of graphene-based field effect transistors (FETs) and spin-FETs on MgO(111)/Si(100).

Fe films grown on Ag(001) as well as MgO films on Fe(001) have been studied by spin-polarized electron reflection experiments. The three central observations are: 1) Oscillations with monolayer periodicity of the electron-spin motion angles ε and Φ are observed as a function of the Fe thickness. They are attributed to the oscillatory behavior of the surface-lattice strain that is relaxed at island edges of the incompletely filled top Fe layer. 2) For strongly relaxed thick Fe films a giant spin precession angle of 180o, which is accompanied by a pronounced minimum in the reflected electron intensity, is observed for an electron energy of 7.3 eV. 3) For the interface system MgO/Fe(001) a very strong sensitivity of the spin motion angles on the MgO coverage is observed for certain energy ranges. Ab-initio band structure and spin-dependent electron reflection calculations reveal that lattice relaxations during growth of Fe on Ag(001) as well as MgO on Fe(001) are responsible for the strong changes of the electron-spin motion angles.

We propose a procedure, based on momentum power series expansion Hamiltonian of nth general order, to obtain a coherent expression of the probability current operator that is valid also when Spin-Orbit Interaction (SOI) terms are included. We prove that we recover the standard definition when the free electron-like term is included in the Hamiltonian, but when taking into account higher order Spin-Orbit Interaction (SOI) terms, a more general definition of the probability current operator is mandatory, due to its different symmetrization, compared to the Hermitian velocity operator expression.

We find a dramatic enhancement of electron propagation along a narrow range of real-space angles from an isotropic source in a two-dimensional quantum well made from a zincblende semiconductor. This "electron beam" formation is caused by the interplay between spin-orbit interaction originating from a perpendicular electric field to the quantum well and the intrinsic spin-orbit field of the zincblende crystal lattice in a quantum well, in situations where the two fields are different in strength but of the same order of magnitude. Beam formation is associated with caustics and can be described semi-classically using a stationary phase analysis.

Spin dynamics in zincblende two-dimensional electron systems is usually dominated by the Dyakonov-Perel spin dephasing mechanism resulting from the underlying spin-orbit fields. An exceptional situation is realized in symmetrically grown and doped GaAs/AlGaAs quantum wells grown along the [110] direction, where the Rashba contribution is negligible and the effective Dresselhaus spin-orbit field is perpendicular to the sample plane. In such a system the spin dephasing times for in- and out-of-plane crystallographic directions are expected to be strongly different and the out-of-plane spin dephasing time is significantly enhanced as compared with conventional systems. We observe the spin relaxation anisotropy by resonant spin amplification measurements in a 30 nm wide double-sided symmetrically δ-doped single quantum well with a very high mobility of about 3•10⁶ cm²/Vs at 1.5K. A comparison of the measured resonant spin amplification traces with the developed theory taking into account the spin dephasing anisotropy yields the dephasing times whose anisotropy and magnitudes are in-line with the theoretical expectations.

RF vortex spin-transfer oscillators based on low RA magnetic tunnel junctions were investigated. A very high power of excitations has been obtained characterized by a power spectral density containing a very sharp peak at the fundamental frequency and a series of harmonics. The observed behaviour is attributed to the combined effect of Oersted-Ampère field generated by the large applied dc-current and of the spin transfer torque. We furthermore show the synchronization of a vortex oscillation by applying a RF bias microwave which frequency is twice the oscillator fundamental frequency.

Spin-torque oscillators are a promising application for the spin-transfer torque effect. The major challenge lies in pushing their microwave output power to useful levels, e.g. by operating an array of spin-torque oscillators in a synchronized, phase-locked mode. Our experiments on metallic, GMR-type nanopillars focus on the influence of external high-frequency signals on the current-driven vortex dynamics in single-vortex and double-vortex spintorque oscillators. For both cases we observe injection locking behavior according to the nonlinear oscillator theory, which is a prerequisite for the synchronization of spin-torque oscillators via microwaves in common electrical electrodes.

Several geometries have recently been proposed for spin-transfer oscillators, with the purpose of optimizing the output power and the frequency dependence on the applied current, as well as minimizing the external magnetic field required to stabilize magnetization dynamics. The two structures most compatible with applications involve hybrid multilayers, including magnetic films magnetized in the plane, as well as perpendicular to the plane of the layers - alternatively acting as polarizing layer for the current and as excited layer. Here, we present a quantitative numerical comparison between the two geometries. We find that multilayers with perpendicularly magnetized polarizer and easy-plane anisotropy have considerably better frequency tunability versus current and require lower threshold currents. Nevertheless, steady state dynamics can only be excited from specific, field-dependent initial states, which complicates the design of potential applications. Structures with an in-plane polarizer and perpendicular anisotropy free layer are more reliable, as they are insensitive to the initial state. In exchange, devices based on this geometry require a small (but finite) external magnetic field for operation.

We report the electrical transport properties of a hybrid organic/inorganic diode device consisting of a layer of an organic ferrimagnetic semiconductor V[TCNE]_x (x~2, TCNE: tetracyanoethylene; T_C ~ 400 K, EG ~ 0.5 eV, σ~ 10^-2 S/cm) and a GaAs/AlGaAs p-i-n diode. Comparison with a control excluding the V[TCNE]_x~2 reveals that the addition of the V[TCNE]_x~2 layer shifts the turn-on voltage and ideality of the diode in accordance with bulk V[TCNE]_x~2 properties. This result has implications for the use of inorganic systems as probes of spin physics in organic and molecular systems.

The development of spin MOSFETs has been stymied by low values of magnetoresistance (MR) measured in experiments. Simulation studies have largely focused on diffusion driven or ballistic transport in the semiconductor and a simplistic treatment of spin injecting contacts. Here we demonstrate a novel framework to simulate spin injection and spin transport in semiconductors in the drift-diffusion regime and a tunneling based model for spin injecting contacts thereby enabling simulation and optimization of experimental devices.

Zinc oxide is a II-VI semiconductor with a wide direct band gap of about 3.37 eV at room temperature. Among the various growth techniques of 'Mn' doped ZnO thin films, pulsed laser deposition (PLD) offers the advantages such as deposition at relatively high oxygen pressure, high deposition rate and growth of highly oriented crystalline films at low substrate temperature. The substitution of 'Mn' in ZnO host lattice affects the lattice dynamics. Raman scattering provides a great deal of information in the optical modes of vibrations at the center of Brillouin zone. The parameters of Raman mode such as frequency, line width and lifetime provide the basic information of lattice dynamics. The PLD grown films were analysed using x-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis-NIR spectroscopy and Raman spectroscopy. Good optical quality of the films was confirmed from the transmittance of the film greater than 80% in the visible region. The presence of non-polar E₂h^igh and E₂^low Raman modes in thin films indicates that 'Mn' doping didn't change the wurtzite structure of ZnO. The intensity of E₂^high mode and the peak position shifted towards the lower frequency with increase of 'Mn' concentration. Apart from the normal modes of ZnO the Zn_1-xMnxO ceramic targets shows two additional modes at 332 cm^-1 (I₂) and 524 cm^-1 (I₄). The modes I₂ and I₄ are assigned as multi-phonon scattering considering the two phonon process in the disordered lattice due to Mn doping.