The impact of radiofrequency (rf) currents on the direct current (dc) driven switching dynamics in current-perpendicular-to-plane nanoscale spin valves is demonstrated. The rf currents dramatically alter the dc driven free layer magnetization reversal dynamics as well as the dc switching level. This occurs when the frequency of the rf current is tuned to a frequency range around the dc driven magnetization precession frequencies. For these frequencies, interactions between the dc driven precession and the injected rf induce frequency locking and frequency pulling effects that lead to a measurable dependence of the critical switching current on the frequency of the injected rf. Based on macrospin simulations, including dc as well as rf spin torque currents, we explain the origin of the observed effects.
Voltage-driven spin transfer torque in a magnetic tunnel junction comprising magnetic insulating electrodes is studied theoretically. In contrast with the conventional magnetic tunnel junctions comprising transition metal ferromagnets, the spin transfer torque presents unconventional bias dependencies, related to the presence of spin-dependent Fowler-Nordheim tunneling processes. In particular, we find that (i) the out-of-plane torque generally dominates the in-plane torque, (ii) out-of-plane torque and in-plane torque are symmetric and asymmetric at low bias voltage, respectively, and (iii) b…
An efficient first principles method was developed to calculate spin transfer torques in layered system with noncollinear magnetization. The complete scattering wave function is determined by matching the wave function in the scattering region with the Bloch states in the leads. The spin transfer torques are obtained with aid of the scattering wave function. We applied our method to the ferromagnetic spin valve and found that the material (Co, Ni and Ni80Fe20) dependence of the spin transfer torques could be well understood by the Fermi surface. Ni has much longer spin injection penetration le…
We numerically compute current-induced spin-transfer torques for antiferromagnetic domain walls, based on a linear response theory in a tight-binding model. We find that, unlike for ferromagnetic domain wall motion, the contribution of adiabatic spin torque to antiferromagnetic domain wall motion is negligible, consistent with previous theories. As a result, the non-adiabatic spin-transfer torque is a main driving torque for antiferromagnetic domain wall motion. Moreover, the non-adiabatic spin-transfer torque for narrower antiferromagnetic domain walls increases more rapidly than that for fer…
The electrical manipulation of magnetization by current-induced spin torques has given access to realize a plethora of ultralow power and fast spintronic devices such as non-volatile magnetic memories, spin-torque nano-oscillators, and neuromorphic computing devices. Recent advances have led to the notion that relativistic spin-orbit coupling is an efficient source for current-induced torques, opening the field of spin-orbitronics. Despite the significant progress, however, the fundamental mechanism of magnetization manipulation, the requirement of spin currents in generating current-induced t…
Spin transport in magnetic tunnel junctions in the presence of spin diffusion is considered theoretically. Combining ballistic tunneling across the barrier and diffusive transport in the electrodes, we solve the spin dynamics equation in the metallic layers. We show that spin diffusion mixes the transverse spin current components and dramatically modifies the bias dependence of the effective spin transfer torque. This leads to a significant linear bias dependence of the out-of-plane torque, as well as a non-conventional thickness dependence of both spin torque components.
Spin valves incorporating perpendicularly magnetized materials are promising structures for memory elements and high-frequency generators. We report the angular dependence of the spin-transfer torque in spin valves with perpendicular equilibrium magnetization computed by first-principles circuit theory and compare results with experiments by W.H. Rippard c.s. [Phys. Rev. B 81, 014426 (2010)] on the CoFe|Cu|CoNi system. Furthermore, we predict a non-monotonous ("wavy") spin-transfer torque when the Cu spacer is replaced by a Ru layer.
We investigate an interfacial spin-transfer torque and $β$-term torque with alternating current (AC) parallel to a magnetic interface. We find that both torques are resonantly enhanced as the AC frequency approaches to the exchange splitting energy. We show that this resonance allows us to estimate directly the interfacial exchange interaction strength from the domain wall motion. We also find that the $β$-term includes an unconventional contribution which is proportional to the time derivative of the current and exists even in absence of any spin relaxation processes.
We calculate the spin-transfer torque in Fe/MgO/Fe tunnel junctions and compare the results to those for all-metallic junctions. We show that the spin-transfer torque is interfacial in the ferromagnetic layer to a greater degree than in all-metallic junctions. This result originates in the half metallic behavior of Fe for the $Δ_1$ states at the Brillouin zone center; in contrast to all-metallic structures, dephasing does not play an important role. We further show that it is possible to get a component of the torque that is out of the plane of the magnetizations and that is linear in the bias…
We present a Non-Equilibrium Green's Function based model for spin torque transfer (STT) devices which provides quantitative agreement with experimentally measured (1) differential resistances, (2) Magnetoresistance (MR), (3) In-plane torque and (4) out-of-plane torque over a range of bias voltages, using a single set of three adjustable parameters. We believe this is the first theoretical model that is able to cover this diverse range of experiments and a key aspect of our model is the inclusion of multiple transverse modes. We also provide a simple explanation for the asymmetric bias depende…
We propose an experimental scheme to determine the spin-transfer torque efficiency excited by the spin-orbit interaction in ferromagnetic bilayers from the measurement of the longitudinal magnetoresistace. Solving a diffusive spin-transport theory with appropriate boundary conditions gives an analytical formula of the longitudinal charge current density. The longitudinal charge current has a term that is proportional to the square of the spin-transfer torque efficiency and that also depends on the ratio of the film thickness to the spin diffusion length of the ferromagnet. Extracting this cont…
We study the excitation of spin waves in magnetic insulators by the current-induced spin-transfer torque. We predict preferential excitation of surface spin waves induced by an easy-axis surface anisotropy with critical current inversely proportional to the penetration depth and surface anisotropy. The surface modes strongly reduce the critical current and enhance the excitation power of the current-induced magnetization dynamics.
Electron transport in magnetic orders and the magnetic orders dynamics have a mutual dependence, which provides the key mechanisms in spin-dependent phenomena. Recently, antiferromagnetic orders are focused on as the magnetic order, where current-induced spin-transfer torques, a typical effect of electron transport on the magnetic order, have been debatable mainly because of the lack of an analytic derivation based on quantum field theory. Here, we construct the microscopic theory of spin-transfer torques on the slowly-varying staggered magnetization in antiferromagnets with weak canting. In o…
Spin pumping and spin-transfer torques are two reciprocal phenomena widely studied in ferromagnetic materials. However, pumping from antiferromagnets and its relation to current-induced torques have not been explored. By calculating how electrons scatter off a normal metal-antiferromagnetic interface, we derive pumped spin and staggered spin currents in terms of the staggered field, the magnetization, and their rates of change. For both compensated and uncompensated interfaces, spin pumping is of a similar magnitude as in ferromagnets with a direction controlled by the polarization of the driv…
In ferromagnet/heavy metal bilayers, an in-plane current gives rise to spin-orbit spin transfer torque which is usually decomposed into field-like and damping-like torques. For two-dimensional free-electron and tight-binding models with Rashba spin-orbit coupling, the field-like torque acquires nontrivial dependence on the magnetization direction when the Rashba spin-orbit coupling becomes comparable to the exchange interaction. This nontrivial angular dependence of the field-like torque is related to the Fermi surface distortion, determined by the ratio of the Rashba spin-orbit coupling to th…
Current induced domain wall (DW) motion in perpendicularly magnetized nanostripes in the presence of spin orbit torques is studied. We show using micromagnetic simulations that the direction of the current induced DW motion and the associated DW velocity depend on the relative values of the field like torque (FLT) and the Slonczewski like torques (SLT). The results are well explained by a collective coordinate model which is used to draw a phase diagram of the DW dynamics as a function of the FLT and the SLT. We show that a large increase in the DW velocity can be reached by a proper tuning of…
We demonstrate a simple network-analyzer technique to make quantitative measurements of the bias dependence of spin torque in a magnetic tunnel junction. We apply a microwave current to exert an oscillating spin torque near the ferromagnetic resonance frequency of the tunnel junction's free layer. This produces an oscillating resistance that, together with an applied direct current, generates a microwave signal that we measure with the network analyzer. An analysis of the resonant response yields the strength and direction of the spin torque at non-zero bias. We compare to measurements of the …
We predict that the magnetization direction of a ferromagnet can be reversed by the spin-transfer torque accompanying spin-polarized thermoelectric heat currents. We illustrate the concept by applying a finite-element theory of thermoelectric transport in disordered magnetoelectronic circuits and devices to metallic spin valves. When thermalization is not complete, a spin heat accumulation vector is found in the normal metal spacer, i.e., a directional imbalance in the temperature of majority and minority spins.
The spin transfer torque in all-metal dual spin valve, in which two antiparallelly aligned pinned ferromagnetic layers are on the two sides of a free ferromagnetic layer with two thin nonmagnetic spacers in between, is studied in the ballistic regime. It is argued that, similar to the results in the diffusion regime, the spin transfer torque is dramatically enhanced in comparison to that in a conventional spin valve although no spin accumulation exists at the magnetic-nonmagnetic interfaces. Within the Slonczewski's approach, an analytical expression of the torque on the free magnetic layer is…
Picosecond switching of the staggered antiferromagnetic order is shown to be realizable through spin-transfer torques from a short current pulse. The coupled dynamics of sublattice magnetization is mapped onto a classical pendulum subject to gravity and a driving pulse, where switching occurs if the pendulum acquires sufficient kinetic energy during the pulse to overcome the maximum of the effective gravity potential. The optimal switching scheme is explored through the dependence of switch angle and magnetic loss on the duration and strength of the current pulse. The physics discussed here pr…
Several experimental techniques have been introduced in recent years in attempts to measure spin transfer torque in magnetic tunnel junctions (MTJs). The dependence of spin torque on bias is important for understanding fundamental spin physics in magnetic devices and for applications. However, previous techniques have provided only indirect measures of the torque and their results to date for the bias dependence are qualitatively and quantitatively inconsistent. Here we demonstrate that spin torque in MTJs can be measured directly by using time-domain techniques to detect resonant magnetic pre…
We consider theoretically a current flowing perpendicular to interfaces of a spin-valve type ferromagnetic metallic junction. For the first time an effective approach is investigated to calculate a simultaneous action of the two current effects, namely, the nonequilibrium longitudinal spin injection and the transversal spin-transfer surface torque. Dispersion relation for fluctuations is derived and solved. Nonlinear problem is solved about steady state arising due to instability for a thick enough free layer.
The spin wave transportation through a transverse magnetic domain wall (DW) in a magnetic nanowire is studied. It is found that spin wave passes through a DW without reflection. A magnon, the quantum of the spin wave, carries opposite spins on the two sides of the DW. As a result, there is a spin angular momentum transfer from the propagating magnons to the DW. This magnonic spin-transfer torque can efficiently drive a DW to propagate in the opposite direction to that of the spin wave.
The dynamics of a magnetic domain wall in a semi circular nanowire loop is studied by an analytical model and micromagnetic simulations. We find a damped sinusoidal oscillation of the domain wall for small displacement angles around its equilibrium position under an external magnetic field in the absence of currents. By studying the effect of current induced nonadiabatic spin transfer torque on the magnetic domain wall resonance frequency and mass, a red shift is found in the resonance frequency and domain wall mass increases by increasing the ratio of nonadiabatic spin torque to adiabatic con…
The quantum field theoretical approach with the Kubo formula has successfully captured spin torques, such as spin-transfer torques and spin-orbit torques, for continuum systems. We examine the field theoretical approach to current-induced spin-transfer torques in a magnetic junction system. We first give a brief overview of the field theoretical approach to spin torques. Then, we consider a five-layers system consisting of three nonmagnetic metal layers separated by two ferromagnetic metal layers and apply an electric field perpendicular to the layers. We demonstrate that the Slonczewski-type …
Currents induce magnetization torques via spin-transfer when the spin angular momentum is conserved or via relativistic spin-orbit coupling. Beyond simple models, the relationship between material properties and spin-orbit torques is not known. Here, we present a novel phenomenology of current-induced torques that is valid for any strength of intrinsic spin-orbit coupling. In $\rm Pt|Co|AlO_x$, we demonstrate that the domain walls move in response to a novel relativistic dissipative torque that is dependent on the domain wall structure and that can be controlled via the Dzyaloshinskii-Moriya i…
Comments on the formula to extract current-induced torques from the harmonic Hall voltage measurements
The University of Tokyo（东京大学）；Research Institute of Electrical Communication (RIEC), Tohoku University（东北大学电气通信研究所）；Trans-Scale Quantum Science Institute (TSQS), The University of Tokyo（东京大学跨尺度量子科学研究所）
We explore the possibility of exciting spin waves in insulating antiferromagnetic films by injecting spin current at the surface. We analyze both magnetically compensated and uncompensated interfaces. We find that the spin current induced spin-transfer torque can excite spin waves in insulating antiferromagnetic materials and that the chirality of the excited spin wave is determined by the polarization of the injected spin current. Furthermore, the presence of magnetic surface anisotropy can greatly increase the accessibility of these excitations.
We report quantum and semi-classical calculations of spin current and spin-transfer torque in a free-electron Stoner model for systems where the magnetization varies continuously in one dimension.Analytic results are obtained for an infinite spin spiral and numerical results are obtained for realistic domain wall profiles. The adiabatic limit describes conduction electron spins that follow the sum of the exchange field and an effective, velocity-dependent field produced by the gradient of the magnetization in the wall. Non-adiabatic effects arise for short domain walls but their magnitude decr…
We use three-terminal magnetic tunnel junctions (MTJs) designed for field-free switching by spin-orbit torques (SOTs) to systematically study the impact of dual voltage pulses on the switching performances. We show that the concurrent action of an SOT pulse and an MTJ bias pulse allows for reducing the critical switching energy below the level typical of spin transfer torque while preserving the ability to switch the MTJ on the sub-ns time scale. By performing dc and real-time electrical measurements, we discriminate and quantify three effects arising from the MTJ bias: the voltage-controlled …
We report the theoretical investigation of noise spectrum of spin current and spin transfer torque for non-colinear spin polarized transport in a spin-valve device which consists of normal scattering region connected by two ferromagnetic electrodes. Our theory was developed using non-equilibrium Green's function method and general non-linear $S^σ-V$ and $S^τ-V$ relations were derived as a function of angle $θ$ between magnetization of two leads. We have applied our theory to a quantum dot system with a resonant level coupled with two ferromagnetic electrodes. It was found that for the MNM syst…
The dynamics of magnetization in the presence of spin-transfer torque was studied. We derived the equation for the motion of magnetization in the presence of a spin current by using the local equilibrium assumption in non-equilibrium thermodynamics. We show that, in the resultant equation, the ratio of the Gilbert damping constant, $α$, and the coefficient, $β$, of the current-induced torque, called non-adiabatic torque, depends on the relaxation time of the fluctuating field $τ_{c}$. The equality $α=β$ holds when $τ_c$ is very short compared to the time scale of magnetization dynamics. We app…
We have studied spin orbit torque in heavy metal (HM)/ferromagnetic metal (FM) bilayers using magneto-optical Kerr effect. A double modulation technique is developed to separate signals from spin orbit torque and Joule heating. At a current density of ~1x10$^{10}$ A/m$^2$, we observe optical signals that scale linearly and quadratically with the current density, both in similar magnitude. The spin orbit torque estimated using this technique is consistent with that evaluated using spin transport measurements. We find that changes in the refractive index of the film with temperature is the main …
We review a unified approach for computing: (i) spin-transfer torque in magnetic trilayers like spin-valves and magnetic tunnel junction, where injected charge current flows perpendicularly to interfaces; and (ii) spin-orbit torque in magnetic bilayers of the type ferromagnet/spin-orbit-coupled-material, where injected charge current flows parallel to the interface. Our approach requires to construct the torque operator for a given Hamiltonian of the device and the steady-state nonequilibrium density matrix, where the latter is expressed in terms of the nonequilibrium Green's functions and spl…
Current-induced spin-transfer torques (STT) and spin-orbit torques (SOT) enable the electrical switching of magnetic tunnel junctions (MTJs) in nonvolatile magnetic random access memories. In order to develop faster memory devices, an improvement of the timescales underlying the current driven magnetization dynamics is required. Here we report all-electrical time-resolved measurements of magnetization reversal driven by SOT in a three-terminal MTJ device. Single-shot measurements of the MTJ resistance during current injection reveal that SOT switching involves a stochastic two-step process con…
We study the effects of Rashba spin-orbit coupling on the spin torque induced by spin waves, which are the plane wave dynamics of magnetization. The spin torque is derived from linear response theory, and we calculate the dynamic spin torque by considering the impurity-ladder-sum vertex corrections. This dynamic spin torque is divided into three terms: a damping term, a $distortion$ term, and a correction term for the equation of motion. The $distorting$ torque describes a phenomenon unique to the Rashba spin-orbit coupling system, where the distorted motion of magnetization precession is subj…
Low crystal symmetry of magnetic van der Waals materials naturally promotes spin-orbital complexity unachievable in common magnetic materials used for spin-orbit torque switching. Here, using first-principles methods, we demonstrate that an interplay of spin and orbital degrees of freedom has a profound impact on spin-orbit torques in a prototype van der Waals ferromagnet: Fe$_3$GeTe$_2$ (FGT). While we show that bulk FGT hosts strong "hidden" current-induced torques harvested by each of its layers, we uncover that their origin alternates between the conventional spin flux torque and the so-ca…
We have recently reported that the spin transfer torque switching current density is very sensitive to not only the junction sizes but also the exchange stiffness constants of the free layer according to the micromagnetic simulations. The results are very complicate and far from the macro-spin model because of the non-coherent spin switching processes. The dependence of the switching current density on the junction sizes and the exchange stiffness constants becomes systematic when we employ the non-collinear polarizer layer. It is found that the non-collinear polarizer layer enhances the coher…
The reversal of perpendicular magnetization (PM) by electric control is crucial for high-density integration of low-power magnetic random-access memory (MRAM). Although the spin-transfer torque (STT) and spin-orbit torque (SOT) technologies have been used to switch the magnetization of a free layer with perpendicular magnetic anisotropy, the former has limited endurance because of the high current density directly through the junction, while the latter requires an external magnetic field or unconventional configuration to break the symmetry. Here we propose and realize the orbit-transfer torqu…
In systems with strong spin-orbit coupling, the relationship between spin-transfer torque and the divergence of the spin current is generalized to a relation between spin transfer torques, total angular momentum current, and mechanical torques. In ferromagnetic semiconductors, where the spin-orbit coupling is large, these considerations modify the behavior of the spin transfer torques. One example is a persistent spin transfer torque in a spin valve: the spin transfer torque does not decay away from the interface, but approaches a constant value. A second example is a mechanical torque at sing…
We study spin accumulation and spin transfer torque in a domain wall by solving the Boltzmann equation with a diffusion approximation. We obtained the analytical expressions of spin accumulation and spin transfer torque. Both the adiabatic and the non-adiabatic components of the spin transfer torque oscillate with the thickness of the domain wall. We show that the oscillation plays a dominant role in the non-adiabatic torque when the domain wall thickness is less than the spin-flip length, which is defined by the product of the Fermi velocity and the spin-flip scattering time.
Impact of spin transfer torque (STT) on the write error rate of a voltage-torque-based magnetoresistive random access memory is theoretically analyzed by using the macrospin model. During the voltage pulse the STT assists or suppresses the precessional motion of the magnetization depending on the initial magnetization direction. The characteristic value of the current density is derived by balancing the STT and the external-field torque, which is about 5$\times$ 10$^{11}$ A/m$^{2}$. The results show that the write error rate is insensitive to the STT below the current density of $10^{10}$ A/m$…
We predict the existence of a new metastable magnetization state in a single-domain nanomagnet with uniaxial shape anisotropy. It emerges when a spin-polarized current, delivering a spin-transfer-torque, is injected into the nanomagnet. It can trap the magnetization vector and prevent spin-transfer-torque from switching the magnetization from one stable state along the easy axis to the other. Above a certain threshold current, the metastable state no longer appears. This has important technological consequences for spin-transfer-torque based magnetic memory and logic systems.
The magnetization dynamics equation predicts that a domain wall that changes structure should undergo a displacement by itself - automotion - due to the relaxation of the linear momentum that is associated with the wall structure. We experimentally demonstrate this effect in soft nanostrips,transforming under spin transfer torque a metastable asymmetric transverse wall into a vortex wall. Displacements more than three times as large as under spin transfer torque only are measured for 1~ns pulses. The results are explained by analytical and numerical micromagnetics. Their relevance to domain wa…
Ferromagnetic spin-valves and tunneling junctions are crucial for spintronics applications and are one of the most fundamental spintronics devices. Motivated by the potential unique advantages of antiferromagnets for spintronics, we theoretically study here junctions built out of non-collinear antiferromagnets. We demonstrate a large and robust magnetoresistance and spin-transfer torque capable of ultrafast switching between parallel and anti-parallel states of the junction. In addition, we show that the non-collinear order results in a spin-transfer torque that is in several key aspects diffe…
Here we study the effect of an additional interfacial spin-transfer torque, as well as the well established spin-orbit torque and bulk spin-transfer torque, on skyrmion collections - group of skyrmions dense enough that they are not isolated from on another - in ultrathin heavy metal / ferromagnetic multilayers, by comparing modelling with experimental results. Using a skyrmion collection with a range of skyrmion diameters and landscape disorder, we study the dependence of the skyrmion Hall angle on diameter and velocity, as well as the velocity as a function of diameter. We show the experimen…
We present an implementation of the steady state Keldysh approach in a Green's function multiple scattering scheme to calculate the non-equilibrium spin density. This density is used to obtain the spin transfer torque in junctions showing the magnetoresistance effect. We use our implementation to study the spin transfer torque in metallic Co/Cu/Co junctions.
We report a theoretical study on the spin and electron transport in the nonlocal lateral spin valve with non-collinear magnetic configuration. The nonlocal magnetoresistance, defined as the voltage difference on the detection lead over the injected current, is derived analytically. The spin transfer torques on the detection lead are calculated. It is found that spin transfer torques are symmetrical for parallel and antiparallel magnetic configurations, which is different from that in conventional sandwiched spin valve.
We study the optically induced torques in thin film ferromagnetic layers under excitation by circularly polarized light. We study cases both with and without Rashba spin-orbit coupling using a 4-band model. In the absence of Rashba spin-orbit coupling, we derive an analytic expression for the optical torques, revealing the conditions under which the torque is mostly derived from optical spin transfer torque (i.e. when the torque is along the direction of optical angular momentum), versus when the torque is derived from the inverse Faraday effect (i.e. when the torque is perpendicular to the op…
It is shown that the current-induced torques between a ferromagnetic layer and an antiferromagnetic layer with a compensated interface vanish when the ferromagnet is aligned with an axis of spin-rotation symmetry of the antiferromagnet. For properly chosen geometries this implies that the current induced torque can stabilize the out-of-plane (or hard axis) orientation of the ferromagnetic layer. This current-induced torque relies on phase coherent transport, and we calculate the robustness of this torque to phase breaking scattering. From this it is shown that the torque is not linearly depend…
We theoretically study the spin current and spin-transfer torque generation in a conventional spin- valve hybrid structure of type ferromagnetic/normal metal/ferromagnetic (FM/NM/FM) made of the topological insulator (TI), in which a gate voltage is attached to the normal layer. We demonstrate the penetration of the spin-transfer torque into the right ferromagnetic layer and show that, unlike graphene spin-valve junction, the spin-transfer torque in TI is very sensitive to the chemical potential of the NM region. As an important result, by changing the chemical potential of the NM spacer and m…
We theoretically investigate current-induced magnetization dynamics in chiral-lattice helimagnets. Spin-orbit coupling in non-centrosymmetric crystals induces a reactive spin-transfer torque that has not been previously considered. We demonstrate how the torque is governed by the crystal symmetry and acts as an effective magnetic field along the current direction in cubic B20-type crystals. The effects of the new torque are computed for current-induced dynamics of spin-spirals and the Doppler shift of spin waves. In current-induced spin-spiral motion, the new torque tilts the spiral structure.…
Spin-transfer torques induced by temperature gradients in conducting ferromagnets are calculated microscopically for smooth magnetization textures. Temperature gradients are treated a la Luttinger by introducing a fictitious gravitational field that couples to the energy density. The thermal torque coefficients obtained by the Kubo formula contain divergences caused by equilibrium components that should be subtracted before applying the Einstein-Luttinger relation. Only by following this procedure a familiar Mott-like formula is obtained for the dissipative spin-transfer torque. The result ind…
Spin currents can exert spin-transfer torques on magnetic systems even in the limit of vanishingly small net magnetization, as is the case for antiferromagnets. Here, we experimentally show that a spin-transfer torque is operative in a material with weak, short-range magnetic order -- namely, a macroscopic ensemble of superparamagnetic-like Co nanomagnets. We employ element- and time-resolved X-ray ferromagnetic resonance (XFMR) spectroscopy to directly detect sub-ns dynamics of the Co nanomagnets, excited into precession with cone angle $\geq$0.003$^{\circ}$ by an oscillating spin current. XF…
We investigate the torque field and skyrmion movement at an interface between a ferromagnet hosting a skyrmion and a material with strong spin-orbit interaction. We analyze both semiconductor materials and topological insulators using a Hamiltonian model that includes a linear term. The spin torque inducing current is considered to flow in the single band limit therefore a quantum model of current is used. Skyrmion movement due spin transfer torque proves to be more difficult in presence of spin orbit interaction in the case where only interface in-plane currents are present. However, edge eff…
Spin accumulation and spin transfer torques induced by Spin Hall Effect in bi-layer structures comprising ferromagnetic and paramagnetic materials are theoretically investigated. The charge and spin diffusion equations taking into account spin-flip and spin Hall effect are formulated and solved analytically and numerically for in structures with simplified and complex geometry, respectively. It is demonstrated that spin torques could be efficiently produced by means of Spin Hall effect which may be further enhanced by modifying structure geometry.
The miniaturisation of magnetic read heads and random access memory elements makes them vulnerable to thermal fluctuations. We demonstrate how current-induced spin-transfer torques can be used to suppress the effects of thermal fluctuations. This enhances the fidelity of perpendicular magnetic spin valves. The simplest realization is a dc current to stabilize the free magnetic layers. The power can be significantly reduced without losing fidelity by simple control schemes, in which the stabilizing current-induced spin-transfer torque is controlled by the instantaneous resistance.
The dynamics of a macrospin model for a single magnetic domain is investigated in two cases: (i) under the action of a periodic magnetic field and (ii) under the external field plus a spin transfer torque from spin-polarized current or spin current. It has been proved that (i) without spin transfer torque, the trajectory of magnetization (hysteresis) is always a closed curve oscillating between two stable points and following the same path each time; (ii) under the action of a constant field (or with small periodic perturbation) and a constant spin transfer torque, the spin would always turn t…
We analyse the influence of current induced torques on the magnetization configuration of a ferromagnet in a circuit containing a compensated antiferromagnet. We argue that these torques are generically non-zero and support this conclusion with a microscopic NEGF calculation for a circuit containing antiferromagnetic NiMn and ferromagnetic Co layers. Because of symmetry dictated differences in the form of the current-induced torque, the phase diagram which expresses the dependence of ferromagnet configuration on current and external magnetic field differs qualitatively from its ferromagnet-onl…
