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In the optically pumped systems, the spin polarization of the atoms is generally derived by employing additional modulations, in which way an extra perturbation will be introduced and thus affect the normal operation of the sensors. By investigating the absorption principle of the circularly polarized pump laser, here, we demonstrate the feasibility of extracting the electron-spin polarization from the transmitted intensity of the pump beam. This scheme can detect real-time polarization without disturbing the normal operation of the sensor, and the experimental result agrees well with the theory. This research is of value to the research and development of optically pumped systems, especially the magnetometer, and the comagnetometer operated in the spin-exchange relaxation free regime.

In the last decades, the research in spintronics has mainly concentrated on ferromagnetic materials, which respond to the different kinds of spintransfer torques with gigahertz-frequency dynamics [1]–[3]. A jump to higher-frequency devices, working in the range of terahertz, is promised by the use of antiferromagnets (AFM) [4]–[6]. AFM materials will, probably, be in the research spotlight in the near future. In order to study their behavior, however, a new theoretical approach is needed, which, for instance, has to account for the strong internal exchange field in AFMs, for the absence of macroscopic magnetization, and possibility to operate without an external bias field. When it comes to modeling, AFMs are generally studied by considering the magnetizations M 1 and M 2 of the two sublattices. Dynamics of M 1 and M 2 is ruled by two coupled Landau-Lifshitz-Gilbert equations [6]. In this work, we model micromagnetically an antiferromagnetic spin-Hall oscillator (ASHO). This device consists of an AFM layer coupled to a layer of a heavy metal, as sketched in Fig. 1, where a coordinate system $x-y-z$is also shown. The AFM layer is square-shaped, with dimensions 40×40 nm 2, whereas the dimension along $z$, namely its thickness $d$, varies from 1 to 5 nm. The heavy metal is designed with 4 terminals that can be used to apply a charge current, and/or to read the device resistance [7]. According to the well-known spin-Hall effect, when a current is applied (in our case in the plane $x- y$), it creates a spin polarization in the heavy metal. The consequent spin current, flowing along the $z$direction, creates a spin accumulation at the interface with the AFM, and, therefore, a torque on the AFM. The direction of the spin-Hall polarization p is perpendicular to the plane of the charge and spin currents, namely, if the charge current is applied along the axis $x$and the spin current is along the axis $z$, the spin polarization is along the axis $y[{2 ,7}]$. The spin-Hall-driven torque, therefore, acts on the magnetizations M 1 and M 2 of the two AFM sublattices in the same direction, thus allowing dynamics with a non-zero net magnetization. We investigate this dynamics, solving the equations of motion for the two sublattices by means of a custom-developed micromagnetic code. The following parameters are used in our simulations: saturation magnetization $M_{S}=350 \mathrm {x}10 ^{3}\mathrm {A} /\mathrm {m}$, spinHall angle $\theta _{SH}=0.10$. The exchange constant $A$is a variable parameter in our simulations, and assumes the values of 0.5, 1.0 and $1.5\mathrm {x} 10 ^{11}\mathrm {J} /\text{m}$. Similarly, the Gilbert damping constant $\alpha $is set equal to 0.01, 0.05 and 0.1. The AFM is assumed to have the in-plane anisotropy along the $x$direction, the corresponding constant is $K_{u}= 10 ^{5}\mathrm {J}/ \mathrm {m}^{3}$, and simulations start from the equilibrium AFM configuration of M 1 and M 2 along the easy axis, shown in Fig. 1. Last, but not least, the current is applied differently at the terminals, in order to modify the direction of the spin-Hall polarization with respect to the easy axis $x$. We consider three cases: (i) current at the terminals A-A' along $x$direction (p along $y$), (ii) current at the terminals B-B' along $- y$direction (p along $x$axis), and (iii) current at both A-A' and B-B' with equal intensity (p is directed 45° w.r.t. to x and y axes). By means of this configuration, it is, therefore, possible to manage the direction of the spin-Hall polarization with respect to the AFM easy axis, simply by tuning the two currents at the two couples of terminals. The above described systematic study has provided important data on the behavior of an AFM under the action of the spinHall effect. Dynamics of magnetization is excited above a certain threshold current. However, the same dynamics is turned off at the lower values of the driving current, highlighting a clear hysteretic behavior of the excitation. The values of the threshold currents and the width of the hysteresis region depend on different parameters. For instance, the threshold current increases with the increase of the AFM thickness, or the damping, or the exchange constant. When the polarization is moved from the axis $y$to the AFM easy axis, the threshold current slightly increases, and to get the excitation of dynamics we need to include a small thermal field. Above the threshold, the magnetizations of the two AFM sublattices show a precession around the spin-polarization direction, as shown in the inset of Fig. 2. The net magnetization is, mainly, given by the sum of the components of M 1 and M 2 along the spin polarization p, whereas the other two components give almost zero contribution when the sum of M 1 and M 2 is evaluated. The frequency of the net magnetization dynamics shows a blue-shift with the increase of the applied current (see the example in Fig. 2). The values of the generated frequency are from hundreds of GHz up to several THz, as expected. The hysteretic excitation, the blue-shift of the frequency, the order of threshold current, and of output frequencies confirm the predictions of the previous analytical studies [6], and confirm the validity of our numerical approach.

The generation of spin currents in a ferromagnet/paramagnet junction from acoustically driving magnetization precession, acoustic spin pumping (ASP), in different magnetoelectric structures draws a lot of attention because it offers great opportunities for low-power microwave spintronics. Recently, we have demonstrated for the first time the generation of pure spin current (which does not include charge current) in the high overtone bulk acoustic wave resonator (HBAR) based on YIG-Pt hybrid under resonant excitation of the magnetic dynamics in YIG [N.I. Polzikova, et al., AIP Adv. 6 (2016), 56306.]. For the practical application of this method it is necessary to understand the various factors affecting its efficiency.

A novel antenna based on spin waves in magnetic materials is presented. By biasing magnetic field on the magnetic materials, the spin waves are generated and used to create electromagnetic radiation, thus forming the proposed antenna. Different with the conventional antennas, the proposed one excites the radiation by spin waves in magnetic materials instead of charge currents in conductors. Due to the intrinsic low-loss feature of the spin waves, the proposed antenna shows higher radiation efficiency compared with conventional ones where charge currents loss happens. To explain the proposed idea, the corresponding theory is deduced and presented, and also the schematic view of the proposed antenna is demonstrated.

We present an analytical study of effects that circularly polarized light irradiation has on spin-valley currents in a dual ferromagnetic-gated silicene. Two identical electric fields are applied to both ferromagnetic (FM) gates. Two exchange field configurations, parallel (P) and anti-parallel (AP), are applied along with chemical potential to the FM gates in this investigation. The results show that application of circularly polarized light has an impact on polarized spin and valley current characteristics, particularly at the off-resonant frequency region. It also enhances the amplitude of tunneling magnetoresistance (TMR) significantly. In addition, we found that exchange field configuration has an effect on both spin polarization and valley polarization. Our study suggests the potential of this scheme in applications, such as spin-valleytronic photo-sensing devices under polarized-photo irradiation.

Spin plays a fundamental role in the electronic structure of matter, from single atoms and molecules through to condensed matter. However, spin dynamics and the spin response of electrons to ultra-short, strong laser pulses has remained largely unexplored until today. In 2011 and 2013, pioneering theoretical work [1, 2] extended the fundamental PPT theory [3] to treat ionization of p+ and p” electrons by circular fields. This work predicted the sensitivity of ionization probabilities in strong field ionization using circular fields to the magnetic quantum number, showing that counter rotating electrons can tunnel ionize easier. It also predicted the possibility to obtain spin polarized electrons from strong, circularly polarized fields with a high degree of efficiency. The preference of ionization was later confirmed experimentally by [4], however, the experiment in [4] did not provide a confirmation of the spin polarization predictions.

The pseudospin dynamics of exciton-polaritons in a wedged 2D cavity with ultrahigh quality factor has been studied theoretically. The effect of the external magnetic field applied normally to the cavity plane has been taken into account as well. The energy gradient caused by the cavity width variation can be described as the “artificial gravitational” force acting on an exciton-polariton. It has been demonstrated that one is possible to switch the polariton polarization in the ballistic regime by choosing the specific combination of the initial polariton wave vector, the energy gradient and the magnetic field magnitude.

Nuclear Magnetic Resonance Gyro (NMRG) utilizes the atomic spin to sense system angular rate. It has the advantages of small volume and high precision. The influence of the pump laser frequency fluctuation on the bias drift of NMRG has been studied in this paper. The output of NMRG in open loop is derived, and the process of 129Xe polarization via Spin-Exchange Optical Pumping (SEOP) is analyzed. The pump laser frequency fluctuation leads to the instability of the alkali atom spin polarization. As the consequence, it makes the nuclear magnetic moment instability, which results to the bias drift of NMRG. By analysis, the pump laser frequency needs to be stabilized at the alkali atoms resonance transition frequency to get high and stable alkali atomic spin polarization and low bias drift of NMRG. The pump laser frequency can be stabilized by wavelength modulation frequency stabilization method. In experiment, the bias drift of NMRG is improved from 889.68°/h (1σ) to 98.74°/h (1σ) after the pump laser frequency is controlled. We conclude that the major factor that leads to the bias drift of NMRG under the existing conditions is the pump laser frequency fluctuation.

The study of spin waves (SW) excitation in magnetic devices is one of the most important topics in modern magnetism due to the applications of the information carrier and the signal processing.

It was recently shown that the presence of thermal gradients in magnetic nano-devices influences the magnetization state and dynamics in several ways, as for example by the generation of spin currents, spin waves, spin transfer torque, etc. [1–3].

The Bottom Spin Valve (BSV) geometry has been the standard HDD Reader design for well over 10 years [1].

Terahertz (THz) magnetic field directly interacts with spin systems through Zeeman torque, enabling selective excitation of magnetic order dynamics. Here we demonstrate coherent THz-control of macroscopic magnetization in ErFeO3 using metamaterial-enhanced magnetic nearfields. We fluctuated initial process of laser-induced spin reorientation transition to modify the coherent path of phase transition, resulting in macroscopically magnetized domain. We found that the direction of created magnetization is determined either by the instantaneous phase of spin precession or of applied THz magnetic field. Our results extend the potential applicability of THz magnetic fields to ultrafast magnetic recording in future memory devices.

The control of the spin Hall Effect with the polarization degree is an emerging strategy for including new functionality in spintronic devices. The dependence of spin hall conductance on spin polarization is reported. It is shown that skew scattering term depends on electrons temperature and spin polarization. We use the spherical harmonics method to calculate the dependence of the spin Hall angle on energy shift in the spin-polarized paramagnetic semiconductor. Transport coefficients obtained from the density matrix kinetic equation and spherical harmonics expansion. The skew scattering coefficient is computed numerically. It is shown that spin Hall coefficients nonlinearly depend on the degree of spin polarization. The results indicate that the spin Hall angle can nonlinearly depend on the spin polarization of the electron gas. This feature opens new way to control spin transport in paramagnetic materials like semiconductors or heavy metals.

We provide an overview of recent theoretical and experimental studies, which revisited the basic dynamical properties of light: momentum and angular momentum. Recently, we described qualitatively new types of the spin and momentum in structured optical fields. These are: (i) the transverse spin, which is orthogonal to the wave vector and is independent of the helicity, and (ii) the anomalous transverse momentum, which depends on the helicity of light. Both of these quantities were described and measured experimentally in various optical systems, and they are currently attracting rapidly growing attention. In particular, the transverse spin in evanescent waves has found promising applications for robust spin-controlled unidirectional coupling to surface and waveguide optical modes. In turn, the transverse momentum provides a weak spin-dependent optical force, which is orthogonal to both the propagation direction and the intensity gradient in a wave field.

The coupling between the spin and momentum degrees of freedom due to Spin Orbit Interactions (SOI) allows the control of one degree of freedom to manipulate the other. For example, the momentum of the charge carriers can be modified by introducing geometric curvature into the waveguides. Aspects of the influence of curvature on spin accumulation have been studied by various authors [1]. In this work, we show via two distinct techniques that curvature in SOI systems leads to the generation of spin polarization perpendicular to the plane of motion.

We have made transient photoluminescence (PL) study on electron-spin dynamics in InGaAs-based coupled nanostructures of quantum dots (QDs) with quantum wells (QWs). Self-assembled InGaAs QDs were grown integrated with an InGaAs QW through a GaAs tunneling barrier or embedded in a GaAs QW. Time-resolved circularly polarized PL in the QDs was measured as a function of temperature after optical spin excitation selectively in the QW, reflecting electron-spin polarization injected from the QW into QDs. We show the spin injection dynamics induced by spin tunneling and subsequent energy relaxation from the QW into QDs in the former coupled QDs. Spin relaxation at excited states in the QDs after the dynamical spin injection is shown as a function of temperature. These coupled QD samples exhibit thermally persistent spin polarization up to 200 K, originating from ultrafast and thus efficient spin injection as well as longer spin-relaxation times compared to radiative decay times in the QDs after the injection.

In this paper, we investigate the propagation characteristics of spin electromagnetic wave (SEW) in ferromagnetic film. We calculate the amplitude and phase distribution of spin wave along propagating direction for MSSW mode and BVMSW mode. Two kinds of antenna probes are considered. And we simulate the amplitude and phase distribution of magnetic field around the antenna probe.

Since its first introduction over twenty years ago, weak measurements have gradually became an useful metrology tool in the laboratory. Due to its desirable amplification effect, a small physical variable can be detected with weak measurements. Spin splitting of light, which stems from the interaction between the spin and orbital angular momentum in optical reflection or refraction at the interface of mediums, is always a weak effect. However, the spin splitting of light can be detected easily with weak measurements. Remarkably, the spin splitting of light is sensitive to the physical parameter variations of system. Using the spin splitting as a probe, the physical parameter can be accurately estimated with weak measurements. Here, we review some of our works on precision metrology with weak measurements using the spin splitting of light as a probe, such as estimation of the magneto-optical constant of a magnetic film, determination of the focused beam waist, and measurement of the polarization state of light.

We investigated the dependence of spatial spin distribution on bias voltage in a GaAs quantum well with Kerr rotation microscopy. The spin precession frequency of drifting spins depends on in-plane electric fields, which indicates that effective magnetic fields induced by spin-orbit interactions are modulated by in-plane electric fields. By comparing an experimental result and a theoretical simulation based on the Monte Carlo approach, we confirmed that the in-plane electric field dependence of effective magnetic fields can be attributed to the k-cubic term of the Dresselhaus spin-orbit interaction. We found that spin coherent length can be elongated by balancing a Rashba SOI and all the terms of a Dresselhaus spin-orbit interaction. This technique will provide a new way of both suppressing spin relaxation and controlling the spin precession frequency when using the cubic Dresselhaus SOI.

Spin angular momentum is a fundamental property of the propagating optical waves. In this work, we reveal a possibility of the continuous transition from the transverse spin angular momentum as for the conventional surface plasmon-polariton to the longitudinal spin angular momentum as for the left and right circularly polarized bulk electromagnetic modes.

We investigate electromagnetic fields spinning around an axis perpendicular to the local propagation direction of light. Such transverse components of the spin density occur naturally when light is highly confined, for example, in waveguides or tightly focused beams. On this account, we discuss different measurement schemes for the detection of transversely spinning fields and demonstrate an exemplary utilization of the transverse spin density.

In this work, the inverse spin Hall effect was separated from the spin rectification effect by two methods. One is originated from that the inverse spin Hall effect which is an odd function of the spin injection direction while the spin rectification effect is independent on it. The second method is based on the relation between the direction of static magnetic field and the symmetric and anti-symmetric components of voltage.

We compare some experimental techniques for detection of the ferromagnetic resonance and spin waves resonance in thin ferromagnetic films and patterned magnetoresistance devices. For a 20 nm NiFe film the microwave frequency dispersion relation and damping parameter were obtained by inductive microwave magnetometer and scalar network analysis. Finally, on a 2×90 µm2 magnetoresitive stripe FMR and standing spin wave spectra were measured by means of spin diode effect.

Spin-dependent resonant tunneling in ferromagnet-oxide-semiconductor structures is currently of great interest due to the promising potential of semiconductors and silicon in particular for spin-driven applications. Trap-assisted tunneling explains the larger than predicted signal in three-terminal spin-injection experiments. However, for realistic comparison to experiments at elevated temperatures the master equation describing the occupation and spin evolution at an electron trap coupled to the contacts must be augmented to include spin relaxation and dephasing. A short spin relaxation time suppresses the “spin blockade”, thus reducing the magnetoresistance modulation. However, intensive dephasing does not strongly affect the magnetoresistance. The substantial magnetoresistance modulation is present at an arbitrary trap position relative to the contacts. Finally, an unusual non-monotonic dependence of the magnetoresistance half-width as a function of the perpendicular magnetic field with dephasing increased is observed.

Emerging electronic devices utilizing spin based effects and phenomena, such as spin transfer torque magnetic random access memory (STT-MRAM), have gathered a great spectrum of effort, ranging from basic scientific research to technology development [1][2][3][4]. In this talk, we present a novel all spin logic circuits, referred to as mLogic, based on a four terminal device, referred to as mCell, that utilizes Spin Hall Effect (SHE) along with Dzyaloshinskii-Moriya-Interaction (DMI) at metallic interfaces for its operation [5].

It is shown according to analytical and numerical calculations that in a lossless and in-plane saturated ferromagnetic film the dynamic permeability calculated in the magnetostatic limit is negative for frequencies corresponding to those of the so called backward volume spin waves. These spin excitations have frequencies in the lowest part of the spectrum lying between the ferromagnetic resonance and the Larmor resonance. The negative sign of the dynamic permeability (real part) is connected to the peculiar dynamical properties of the backward spin waves. At the Larmor resonance frequency and just above it the electromagnetic radiation can be coupled to backward spin waves allowing the ferromagnetic film to behave as a magnetic metamaterial with negative permeability.

Impulsive excitation of terahertz magnetic field induces spin precession motions and the spin dynamics are probed. The results of temperature dependence in ErCrO 3 and enhancement of spin precession amplitudes using metamaterial in ErFeO 3 is reported.

Optical pumping is known as a most efficient tool to achieve a high value of nuclear spin polarization. In negatively charged QDs containing, in average, a single resident electron, the angular momentum of a photon is transferred to the electron spin thanks to non-zero spin-orbit coupling in III-V semiconductor. Due to a strong localization, the electron spin is rapidly relaxes via hyperfine interaction with nuclei. In spite of this fast relaxation, the long electron-nuclear correlation time allows one to monitor the nuclear spin system state via electron-spin polarization dynamics. Combining the optical pumping and optical detection with some advances of NMR, we come up with optically detected NMR (ODNMR), which very recently has become a very promising tool for studying nuclear spin dynamics in single QDs and QD ensembles. Here, a brief review of recent studies in the QD ensembles will be presented.

In this paper, fundamental limits of injection of spin-polarized electrons in a non-magnetic material via optical pumping is investigated. Unlike electrical spin injection which can be used for injecting spins into either metallic or semiconducting non-magnets, optical spin pumping is restricted to non-magnets that have a bandgap (e.g. GaAs). Physical models of energy dissipation for both optical and electrical spin-injection schemes are derived. A comparison of their energy dissipation provides useful insights into their relative merits and limits, while also providing guidelines for selecting a specific fabric and topology for spin-based nanoarchitectures.

Magnetic Bipolar Transistor is a promising novel device that provides electrical as well as magnetic control in order to exert desired output. In this paper relative performance of Magnetic Bipolar Transistor and its equivalent representation by Ebers-Moll model has been theoretically studied in terms of their common emitter current gain. We simulated the device to extract its behavior under varying applied field. A range of magnetic field is suggested within which the model follows the actual device characteristics. An analytical relation between common emitter current gain of Magnetic Bipolar Transistor and applied magnetic field is established. Finally a modification in the existing EbersMoll model is proposed to ensure better compatibility with actual characteristics.

We studied the spin motive force induced in a 1D ferromagnetic ring with presence of Rashba spin orbit coupling[1]. The domain wall (DW) is assumed to precess periodically between Bloch-like and Neel-like DW (Fig. 1) at frequency ω. In strong exchange coupling limit (adiabatic approximation), we showed that the induced spin motive force between any two points separated by an angle Δφ on the ring is simply given by V=V 0 ω sin(Δφ/2)cos(ωt+ Δφ/2), where is the precession frequency, and V 0 =2mRα/e being dependent on material and geometric parameters, i.e. Rashba coupling α and ring radius R. In strong Rashba system, the SMF is significantly large comparing with the conventional SMF due to time-varying chiral magnetization, in the absence of Rashba SOC [2]. Since the induced spin motive force is opposite for spin up and spin down, it would be possible to generate a pure spin current circulating around the ring, which can be extracted via attached leads. Due to the sin(Δφ/2) factor, the maximum spin current is obtained by placing the leads diametrically opposite to one another. Therefore, the set-up can achieve a spin current battery function.

The investigation of propagating spin waves is a key topic of contemporary magnetism research [1]. For the excitation of spin waves with short wavelengths, it was typically necessary to either use transducers with sizes on the order of the desired wavelengths (striplines or point-contacts) or to generate those spin waves parametrically by a double-frequency spatially uniform microwave signal [2]. Only recently, a novel mechanism for the local excitation of spin waves, which overcomes the wavelength limit given by the minimum patterning size has been discovered. This method utilizes the translation of natural topological defects, namely the gyration of spin vortex cores [3]. A spin vortex is characterized by a planar, flux-closing magnetization curl, which tilts out of the plane in the central nanoscopic core region [cf. Fig. 1(a)]. Both, the in-plane rotation sense of the curl (circulation) and the orientation of the perpendicular core (polarity), are independently either positive or negative. The initial study [3] was carried out on a vortex pair system with opposite circulations and equal polarities, in which the two vortices were stacked via a nonmagnetic inter-layer [cf. Fig. 1(b) and 1(c)]. In such a system, spin waves can be generated by lateral magnetic field excitation at the vortex cores. Scanning transmission x-ray microscopy (STXM) was used to directly image these spin waves propagating to the rim of the sample in a spiraling manner [cf. Fig. 1(d)]. Thereby, the resulting spin wave length was found to be directly tunable by the excitation frequency. Moreover, the resulting spin waves were analytically calculated to exhibit a gapless, linear, and non-reciprocal dispersion relation with much shorter wave lengths compared to spin waves of the same frequency in corresponding single layer films.

Magnetic tunnel junction (MTJ) are at the center of the magnetic random access memories (MRAM) technology. Nowadays, the research and development efforts are focusing on perpendicularly magnetized layers in the MTJ, taking advantage of the anisotropy provided by spin-orbit coupling at the interfaces with heavy metals such as Pt or Ta [1]. Recently, interfaces between ferromagnetic and heavy metals are also studied in a more fundamental context, because an antisymmetric interaction might appear at the interface, namely an interfacial Dzyaloshinskii-Moriya interaction (DMI) [2]. We will present a micromagnetic study of the reversal of the free layer magnetization direction by spin transfer torque under different amplitudes of the DMI. In particular, we will evaluate the energy barrier E b or equivalently the thermal stability Δ=E b /(300k B ) and the minimal current density necessary for switching J c0 as a function of the DMI amplitude.

Magnetic skyrmions are topologically protected spin textures typical of chiral magnets characterized by a special kind of exchange interaction known as Dzyaloshinkii-Moriya interaction (DMI). In the last years growing interest has been addressed to the study of these topological defects both from a fundamental point of view and for its several technological applications like for instance their usage as information carriers [1]. Great attention has been also given to skyrmion nucleation, stability and motion in confined magnetic systems under the influence of a spin-current [2,3]. Recently, the effect of confinement has been studied in the presence of spin-polarized current [2], but there are not yet studies focusing on the skyrmion motion driven by SHE in constrained geometries.

Magnetic skyrmions are viewed as promising candidates as information carriers in future spintronic devices, especially when it is very recently reported that magnetic skyrmions in perpendicular [Co/ Pt] N multilayers can be well stable at room temperature [1]. For example, magnetic racetrack memory based on magnetic skyrmions has recently been proposed [2] and has attracted great interest. It has been demonstrated that spin waves (SWs) can drive DW motion [3,4]. One advantage of the driving DW motion by SWs is that SWs can create a flow of magnetic momentum without any electron ow which will cause Joule heating.

Magnetic vortex is characterized by a magnetization curling in-plane around a vortex core which points out of the plane.[1] Recently another topologic defect, skyrmion having nanometer scale size, attracted great interest since it can be moved by low current density and thus opening new routes for massive information storage devices.[2] However, skyrmions exist only within a relatively narrow range of magnetic fields and temperatures and thus are difficult to stabilize and manipulate at RT. Therefore, one purpose of our study is to focus on ultra-small vortices in a ferromagnetic system with strong easy-plane magnetic anisotropy (so-called XY model[3,4]). Depending on the easy-plane anisotropy, the size of the vortex core can be less than 10nm while typically in soft ferromagnetic material such as NiFe, it is more than 20nm. On the other hand, current induced magnetic vortex dynamics has been studied both from theoretical and experimental points of view,[5,6] especially taking into account the spin-transfer torque (STT) generated by a spin polarized current. However, effects of another novel phenomenal, i.e., current induced spin-orbit torque (SOT) by spin Hall effect (SHE) on magnetic vortices starts to be of great interest. A comparison between the dynamics under STT and SOT was carried out in this study.

The existing spintronic devices typically use spin-polarized electron currents in metals to transfer spin angular momentum. Thus, in metallic systems the spin transfer requires also a transfer of charge, which limits the applicability of these systems due to the ohmic losses. Recently, a concept of a “pure spin current” has been introduced in which the transfer of the spin angular momentum can be achieved without any charge current [1]. Thus, the effective transfer of pure spin currents in metallic and insulating magnetic systems is a “hot topic” in modern magnetism.

In this digest, we discuss a robust mechanism for electron spin relaxation in a broad range of materials, which have a substantial part of conduction electrons in different valleys that are not connected by time reversal (see Fig. 1 for examples). While the intervalley phonon scattering is important at relatively pure material and high temperature [1,2], we show that the intervalley scattering driven by donor impurities again is more significant than their intravalley counterpart. These leading-order spin flips are dictated by the crystal and band structure symmetry. Further, we find out the microscopic mechanism to be the spin-orbit interaction with the central cell region of the donors rather than due to the ions' long-range electric field, and associate the interaction strength with the fine spectrum of the donor states.

Following the tutorial sessions on materials and physics of topological insulators, we will discuss important unique properties of TI and potential applications. In particular, we will outline the properties that are related to the spin-momentum lock for spin injection applications. I will then discuss the use of the magnetic order in topological insulators (TIs) to break the time-reversal-symmetry (TRS) to drive the non-trivial topological surface into a new massive Dirac-fermions state. An overview of the utilization of both the strong spin-orbit coupling (SOC) and the ferromagnetic (FM) orders in such TRS-breaking systems will be introduced. I will then summarize the progress on magnetic TIs and related TRS-breaking physics in the past few years. In Bi 2 Te 3 /Sb 2 Te 3 -based TI materials, FM moments can be developed through two major mechanisms: the Ruderman-Kit-tel-Kasuya-Yosida (RKKY) coupling and the van Vleck mechanism. In the former case, we find the neighboring magnetic ions can also be coupled through the mediation of conduction carriers, indicating that the magnetic strength can be modulated by electric-field; on the other hand, unlike conventional dilute magnetic semiconductors (DMS), due to the unique band inversion in magnetic TIs, valence electrons themselves can generate large spin susceptibility, and the magnetic ions can thus be directly coupled through these local valence electrons without the assistance of the itinerant electrons. With gate-controlled magneto-transport measurements, the hole-mediated RKKY coupling and the carrier-independent van Vleck magnetism may be separated by controlling, for example, the Cr doping level.

The spin-transfer torque (STT) phenomenon has recently been utilized for magnetic nano spin-transfer torque oscillators (STO) in patterned or nano-contact spin valves and magnetic tunnel junctions (MTJ). For applications, synchronization of multiple oscillators to achieve a higher microwave power and lower bandwidth is required. In order to realize such synchronization, a mutual feedback system among the nano STOs is required. Spin wave mediated interaction of two nano-contact STOs was the first demonstration of STOs synchronization.[1] Despite many efforts, maximum four STOs have been experimentally synchronized so far.[2] A new type of oscillator based on the spin-Hall effect (SHE) has been introduced, in which the oscillation mode is a self-localized spin wave bullet,[3] and its injection locking has been demonstrated as well.[4] The spin Hall oscillators (SHO) can be also realized in nano-magnetic elements.[5]

Spin waves propagating in artificial magnetic periodic structures could be used for low-loss microwave signal processing. Arrays of dipolarly coupled magnetic nanodots having sufficient shape or/ and crystallographic anisotropy constitute such magnonic periodic structures that can operate without an external bias magnetic field and are dynamically reconfigurable [1]. Our previous work has shown that edges of finite arrays of magnetic dots can support propagation of edge spin-wave modes and those modes have many interesting physical properties, including strong localization and non-reciprocity [2]. It is also known that spin-wave modes can be localized not only at the physical edges of the arrays, but also at the internal boundaries inside the array defined, for example, by the regions of a non-uniform internal bias magnetic field or by the borders between the different ground states of the array — internal domain walls.

Current-induced spin-orbit torque (SOT) is a very recently discovered phenomenon which provides efficient ways to control and manipulate the magnetization. It allows to use the spin-orbit interaction in the bulk or at the interface of a heavy metal (HM) to inject angular momentum in an adjacent ferromagnet (FM) [1-3]. Even though its origin either from Rashba or spin Hall effects is still under debate, switching the magnetization of nanostructures by spin-orbit torques is very promising for applications both in terms of speed and energetic efficiency. In this work, we study the influence of one of the components of the torques, the field-like torque, for example in Ta/CoFeB/MgO trilayers [4]. This study focuses on the macrospin-based magnetization switching of small devices with a uniform magnetization (50×50 nm2) which are of significant interest for applications.

Nuclear spins in semiconductors are an ideal system for implementing quantum bits (qubits) for quantum computation because of an extremely long coherence time. Dynamic nuclear polarization (DNP) through hyperfine interaction with electron spins and control of nuclear spin states by nuclear magnetic resonance (NMR) in semiconductors provide basic technologies for implementing nuclear-spin based qubits. A half-metallic ferromagnet could be considered an excellent candidate for a spin source for efficient DNP because it provides complete spin polarization at the Fermi level. In previous studies, we demonstrated high tunneling magnetoresistance ratios of up to 1995% at 4.2 K and up to 354% at 290 K in magnetic tunnel junctions (MTJs) having Mn-rich Co 2 MnSi (CMS) electrodes [1], and 2610% at 4.2 K and 429% at 290 K in Mn-rich Co 2 (Mn, Fe)Si MTJs [2]. Furthermore, we have achieved efficient spin injection from Mn-rich CMS into GaAs via an ultra-thin CoFe insertion layer, resulting in electron spin polarization of up to 52% in GaAs at 4.2 K [3]. This value is more than one order of magnitude higher than those obtained for a Fe electrode or a CoFe electrode [4].

A skyrmion is a topological particle-like excitation in classical continuum field theory [1-3]. Skyr-mion can be driven by spin-polarized current [4, 5]. However, to move the skyrmion along the central line of a nanotrack by in-plane spin-polarized current requires severe matching between the damping coefficient and the non-adiabatic coefficient, i.e. α is close to β, limiting possible material systems for skyrmion applications [5-7]. Another possibility of controlling a skyrmion is to use spin wave. Spin wave produces less heat than electric current, which therefore is promising for practical applications. We investigate the skyrmion dynamics driven by spin wave in constricted geometries with the Dzyaloshinskii-Moriya interaction (DMI) such as nanotracks, L-corners, T- and Y-junctions, which are the basic ingredients of circuits based on skyrmions.

The origin of electrical spin signals observed in ferromagnet/insulator/semiconductor (FM/I/SC) junctions has been the subject of heated debates in recent years. Experimental data obtained by many research groups using the three-terminal configuration are orders of magnitude larger than predicted by the theory of spin injection, accumulation and diffusion [1]. Nonetheless, the spin signals, which are obtained from Hanle measurements, show all the characteristic features of precession of an induced non-equilibrium spin population [2], indicating that intriguing and yet unknown physics is at play.

The heat management bottleneck drives the search for alternative approaches for energy-efficient information technologies. Temporal and spatial magnetic spin textures are of great interest, since their dynamics can occur with minimal dissipation. Typical examples include spin waves (magnons), which are the fundamental quasi-particle excitations of magnetically ordered systems, and skyrmions, which are spatial spin textures that that are stabilized by their distinct topology, and can behave as independent quasi-particles. In bulk helimagnetic systems it has been demonstrated at low temperatures that skyrmions can be efficiently manipulated via electric currents with ultra-low threshold current densities.

In recent years, spin-orbit torques induced by charge current in heavy metal/magnetic structures have attracted wide attention among researchers. Since the experimental demonstration of spin-orbit torques due to the spin Hall effect (SHE) being able to manipulate the magnetization direction in an in-plane MTJ structure[1], it has been heavily explored for applications in memory and logic. It provides a potential low power alternative to other techniques such as spin transfer torque or electric field control for magnetization reversal. Recently, it has been shown that the spin Hall effect can provide a clocking mechanism for logic applications where the spin Hall effect is used to change the magnetization of a perpendicularly magnetized device along the hard axis (in the plane of the film)[2]. In [2], a series of three nanomagets are spaced closely together so they interact through dipole interactions. Each nanomagnet serves as a logic bit. An additional nearby magnet is used as the input. However, due to the dipole interactions, only logic states of 101 or 010 can be obtained. For logic applications, it would be beneficial to be able to individually control the states of the individual bits to obtain a full range of logic functions.

Micromagnetic studies of current driven domain-wall motion nowadays usually employ a local spin-transfer torque model by Zhang and Li [1]. In this model the right-hand side of the Landau-Lifshitz-Gilbert equation is extended by two torque terms referred to as adiabatic and nonadiabatic spin torque. The prefactor of the nonadiabatic term β describes the degree of nonadiabacity. Both the adiabatic and the nonadiabatic term depend on the local magnetization and its spatial derivatives only. Hence the model does not describe any nonlocal effects caused by spin diffusion. However, recent investigations show, that these effects have a non-negligible influence on current driven domain-wall dynamics [2]. Sharp magnetization transitions as in domain walls cause spin accumulation. Due to diffusion the accumulated spins interact with the magnetization in a nonlocal fashion.