Magneto-Transport and Nanomagnet Dynamics in MgO-Based Magnetic Tunnel Junctions(MTJs)

We investigate bias and different barrier thicknesses effects on quantities related to spin and charge currents in MgO-based magnetic tunnel junctions. Using the non-Equilibrium Green's function formalism, we demonstrate that the in-plane and out-of-plane components of the spin-transfer torque have asymmetric and symmetric behaviors respectively. Magneto-resistance also decreases with increasing barrier thickness. The Landau–Lifshits–Gilbert equation describes the dynamics of the magnetization made by spin transfer torque. Increasing in spin current above its critical value or smaller the magnet reduces the switching time which is major result for making of new memory devices.     

Damping-like effects in Heisenberg spin chain caused by the site-dependent bilinear interaction

We investigate a continuous Heisenberg spin chain equation which models the local magnetization in ferromagnet with time-and site-dependent inhomogeneous bilinear interaction and timedependent spin-transfer torque.By establishing the gauge equivalence between the spin chain equation and an integrable generalized nonlinear Schr?dinger equation,we present explicitly a novel nonautonomous magnetic soliton solution for the spin chain equation.The results display how the dynamics of the magnetic soliton can be controlled by the bilinear interaction and spin-polarized current.Especially,we find that the site-dependent bilinear interaction may break some conserved quantity,and give rise to damping-like effect in the spin evolution.     

Domain-wall dynamics and spin-wave excitations with spin-transfer torques

A generalization of spin-transfer torques in ferromagnetic structures is proposed. For a spatially nonuniform magnetization, the spin torque has a form nearly identical to that in magnetic multilayers. We show that the domain-wall motion driven by the current has many unique features that do not exist in the conventional domain-wall motion driven by a magnetic field. We also demonstrate that the spin torque can generate bulk and surface spin excitations that have been seen in point-contact experiments.     

Dynamics of magnetization in ferromagnet with spin-transfer torque

We review our recent works on dynamics of magnetization in ferromagnet with spin-transfer torque. Driven by constant spin-polarized current, the spin-transfer torque counteracts both the precession driven by the effective field and the Gilbert damping term different from the common understanding. When the spin current exceeds the critical value, the conjunctive action of Gilbert damping and spin-transfer torque leads naturally the novel screw-pitch effect characterized by the temporal oscillation of domain wall velocity and width. Driven by space- and time-dependent spin-polarized current and magnetic field, we expatiate the formation of domain wall velocity in ferromagnetic nanowire. We discuss the properties of dynamic magnetic soliton in uniaxial anisotropic ferromagnetic nanowire driven by spin-transfer torque, and analyze the modulation instability and dark soliton on the spin wave background, which shows the characteristic breather behavior of the soliton as it propagates along the ferromagnetic nanowire. With stronger breather character, we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave mainly arises from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized current can control the exchange rate of magnons between the envelope soliton and the background, and the critical current condition is obtained analytically. At last, we have theoretically investigated the current-excited and frequency-adjusted ferromagnetic resonance in magnetic trilayers. A particular case of the perpendicular analyzer reveals that the ferromagnetic resonance curves, including the resonant location and the resonant linewidth, can be adjusted by changing the pinned magnetization direction and the direct current. Under the control of the current and external magnetic field, several magnetic states, such as quasi-parallel and quasi-antiparallel stable states, out-of-plane precession, and bistable states can be realized. Th     

Width dependence of the nonadiabatic spin-transfer torque in narrow domain walls

We analytically determine the spatially varying spin-transfer torque within a domain wall. In the case of ballistic spin and diffusive charge transport, the spin-transfer torque as well as the local degree of nonadiabaticity oscillate within a domain wall. In narrow domain walls, the degree of nonadiabaticity ceases to be a constant material parameter but depends on the domain-wall width including a possible sign change, which is crucial for experiments and the technological utilization in spin-transfer-torque-based storage devices.     

Contribution of electron–magnon scattering to the spin-dependent Seebeck effect in a ferromagnet

We solve the Boltzmann’s transport equations for conduction electrons in a ferromagnet considering electron–magnon scattering. Such scattering gives rise to a spin dependent Seebeck coefficient, which in turn implies that the spin current can be generated by applying a temperature gradient. We estimate the temperature gradient required for switching a nano-magnet by using spin-transfer torque.     

垂直磁各向异性自旋阀结构中的铁磁共振

本文在理论上研究了垂直磁各向异性自旋阀结构中磁场激发和调节的铁磁共振. 通过线性展开包含自旋转移矩项的Landau-Lifshitz-Gilbert方程,获得了磁场激发和调节的铁磁共振谱. 给出了共振线宽、共振频率和共振磁场随直流电流密度大小和方向以及直流磁场的变化关系. 通过调节直流电流密度的大小和方向,系统的有效阻尼可以达到最小. 关键词： 自旋阀 自旋转移矩 垂直磁各向异性 铁磁共振     

Current-driven destabilization of both collinear configurations in asymmetric spin valves

Spin-transfer torque in spin valves usually destabilizes one of the collinear configurations (either parallel or antiparallel) and stabilizes the second one. Apart from this, balance of the spin-transfer and damping torques can lead to steady precessional modes. In this Letter we show that in some asymmetric nanopillars, spin current can destabilize both parallel and antiparallel configurations. As a result, stationary precessional modes can occur at zero magnetic field. The corresponding phase diagram as well as frequencies of the precessional modes have been calculated in the framework of macrospin model. The relevant spin-transfer torque has been calculated in terms of the macroscopic model based on spin diffusion equations.     

Resonance measurement of nonlocal spin torque in a three-terminal magnetic device

A pure spin current generated within a nonlocal spin valve can exert a spin-transfer torque on a nanomagnet. This nonlocal torque enables new design schemes for magnetic memory devices that do not require the application of large voltages across tunnel barriers that can suffer electrical breakdown. Here we report a quantitative measurement of this nonlocal spin torque using spin-torque-driven ferromagnetic resonance. Our measurement agrees well with the prediction of an effective circuit model for spin transport. Based on this model, we suggest strategies for optimizing the strength of nonlocal torque.     

Ab initio studies of the spin-transfer torque in magnetic tunnel junctions

We calculate the spin-transfer torque in Fe/MgO/Fe tunnel junctions and compare the results with those for all-metallic junctions. The spin-transfer torque is interfacial due to the half-metallic nature of the Fe Delta1 states. For samples with typical interfacial roughness, the in-plane torque varies linearly with bias and the out-of-plane torque varies quadratically, both in quantitative agreement with experiment. For ideal samples, we predict that the out-of-plane component of the torque varies linearly with bias and oscillates as a function of the ferromagnetic layer thickness.     

Amplification of spin waves by thermal spin-transfer torque

We observe amplification of spin-wave packets propagating along a film of single-crystal yttrium iron garnet subject to a transverse temperature gradient. The spin waves are excited and detected with standard techniques used in magnetostatic microwave delay lines in the 1-2 GHz frequency range. The amplification is attributed to the action of a thermal spin-transfer torque acting on the magnetization that opposes the relaxation and which is created by spin currents generated through the spin-Seebeck effect. The experimental data are interpreted with a spin-wave model that gives an amplification gain in very good agreement with the data.     

Current-induced modification of spin wave mode interference

We have studied the effect of adiabatic spin-transfer torque on mode interference of spin waves. The mode interference generates amplitude-localized spots at special positions which do not move with time. When applying current, the wavevector of spin wave is modified, resulting in current-dependent displacement of amplitude-localized spots. This current-dependent change in the mode interference may allow to probe current-induced spin wave Doppler shift in space-domain. In favorable situations, it can be used to estimate the intrinsic properties of magnetic materials such as spin polarization.     

Thermal spin-transfer torque in magnetoelectronic devices

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.     

Changing exchange bias in spin valves with an electric current

We show that a high-density electric current, injected from a point contact into an exchange-biased spin valve, systematically changes the exchange bias. The bias can either increase or decrease depending upon the current direction. This observation is not readily explained by the well-known spin-transfer torque effect in ferromagnetic metal circuits, but could be evidence for the recently predicted current-induced torques in antiferromagnetic metals.     

Magnetization noise in magnetoelectronic nanostructures

By scattering theory we show that spin current noise in normal electric conductors in contact with nanoscale ferromagnets increases the magnetization noise by means of a fluctuating spin-transfer torque. Johnson-Nyquist noise in the spin current is related to the increased Gilbert damping due to spin pumping, in accordance with the fluctuation-dissipation theorem. Spin current shot noise in the presence of an applied bias is the dominant contribution to the magnetization noise at low temperatures.     

All-magnonic spin-transfer torque and domain wall propagation

The spin-wave transportation through a transverse magnetic domain wall (DW) in a magnetic nanowire is studied. It is found that the 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.     

自旋转移矩效应激发的非线性磁化动力学

本文基于宏观磁矩(macrospin)的Landau-Lifshitz-Gilbert方程,模拟研究了磁性自旋阀结构中由垂直膜面流向的自旋极化电流所激发的磁化转动动力学特性.直流自旋极化电流借助自旋转移矩效应可驱动磁矩翻转或作周期性振荡,交流电可以激发出具有混沌行为的磁矩振荡.展示了磁矩振荡行为随电流强度变化而发生倍周期分岔、直至混沌振荡的行为规律. 关键词： 自旋转移矩效应 微磁模拟 磁性自旋阀 混沌     

Joule heating and spin-transfer torque investigated on the atomic scale using a spin-polarized scanning tunneling microscope

The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.     

Spin injection-driven switching in a magnetic junction

We investigate a perpendicular electric current passing through a “ferromagnetic nanojunction”, that is through some layered nanosized structure of spin-valve type, containing two ferromagnetic metallic layers. Spacer may be used between the metallic layers to prevent the rotation of the moving spin phases. Such an arrangement is typical for spin valves: one of the metallic layers has strongly pinned magnetic lattice and the other one has free magnetic lattice and free mobile spins. Further the conditions are derived to provide a very high nonequilibrium spin injection level. It appears that the so-called spin resistances of the constitutive layers should be in definite relations to each other. These relations lead to the situation where the spin injection becomes dominant and significantly suppresses the “ordinary” spin-transfer torque. As a result, the threshold current becomes lowered down to 2-3 and even more orders of magnitude.     

Magnetic rogue wave in a perpendicular anisotropic ferromagnetic nanowire with spin-transfer torque

We present the current controlled motion of a dynamic soliton embedded in spin wave background in ferromagnetic nanowire. With the stronger breather character we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave mainly arises from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized current can control the exchange rate of magnons between the envelope soliton and the background, and the critical current condition is obtained analytically. Even more interesting is that the spin-transfer torque plays the completely opposite role for the cases below and above the critical value.