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-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.

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.

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.

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.

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.

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.

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-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.

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.

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.

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.

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.

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, E_MP , 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 E_MP=35 meV. Furthermore, E_MP has unconventionally weak dependence of on both temperature T and magnetic field B_appl . In contrast, magnetic polarons in sample B show conventional characteristics with E_MP 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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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 InAs_0.13P_0.87 epitaxial layer were studied through the time-resolved photoluminescence spectroscopy.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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 𝜃_SH, 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 𝜃_SH in Pt are scattered over one order of magnitude, with a clear correlation between the spin diffusion length ℓ_sf and the 𝜃_SH, 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 (ℓ_sf = 3.4 ± 0.4 nm) and spin Hall angle (𝜃_SH = 0.056 ± 0.010) for Pt in different metallic multilayer systems.

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.

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 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.

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.

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.

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.

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 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.