Influence of exchange bias on spin torque ferromagnetic resonance for quantification of spin–orbit torque efficiency

Antiferromagnet(AFM)/ferromagnet(FM)heterostructure is a popular system for studying the spin–orbit torque(SOT)of AFMs.However,the interfacial exchange bias field induces that the magnetization in FM layer is noncollinear to the external magnetic field,namely the magnetic moment drag effect,which further influences the characteristic of SOT efficiency.In this work,we study the SOT efficiencies of IrMn/NiFe bilayers with strong interfacial exchange bias by using spin-torque ferromagnetic resonance(ST-FMR)method.A full analysis on the AFM/FM systems with exchange bias is performed,and the angular dependence of magnetization on external magnetic field is determined through the minimum rule of free energy.The ST-FMR results can be well fitted by this model.We obtained the relative accurate SOT efficiencyξ_(DL)=0.058 for the IrMn film.This work provides a useful method to analyze the angular dependence of ST-FMR results and facilitates the accurate measurement of SOT efficiency for the AFM/FM heterostructures with strong exchange bias.

Interaction region of magnon-mediated spin torques and novel magnetic states

We determine the region in which the magnon-mediated spin torques exist.This region can be controlled by the spin waves.In terms of stability analysis of magnetization dynamics based on the spin-wave background,we obtain the instability conditions of spin waves.With these results,we find the relationship between unstable regions and the formation of Akhmediev breather,Kuznetsov-Ma breather and rogue waves.We establish the phase diagram of some novel magnetic excitaions.

Spin torque oscillator based on magnetic tunnel junction with MgO cap layer for radio-frequency-oriented neuromorphic computing

Recently,it has been proposed that spin torque oscillators(STOs)and spin torque diodes could be used as artificial neurons and synapses to directly process microwave signals,which could lower latency and power consumption greatly.However,one critical challenge is to make the microwave emission frequency of the STO stay constant with a varying input current.In this work,we study the microwave emission characteristics of STOs based on magnetic tunnel junction with MgO cap layer.By applying a small magnetic field,we realize the invariability of the microwave emission frequency of the STO,making it qualified to act as artificial neuron.Furthermore,we have simulated an artificial neural network using STO neuron to recognize the handwritten digits in the Mixed National Institute of Standards and Technology database,and obtained a high accuracy of 92.28%.Our work paves the way for the development of radio-frequency-oriented neuromorphic computing systems.

Giant spin torque efficiency in single-crystalline antiferromagnet MnAu films

Antiferromagnetic (AFM) materials have attracted wide attention in spin-orbit torque (SOT)-based spintronic due to its abundant spin-dependent properties and unique advantage of immunity against external field perturbations.To act as the charge-to-spin conversion source in energy-saving spintronic devices,it is of great importance for the AFM material to possess a large spin torque efficiency(ξDL).In this work,using the spin torque ferromagnetic resonance (ST-FMR) technique and a Mn2Au/Ni Fe(Py) bilayer system,we systemically study the ξDLof AFM Mn2Au films with different crystal structures.Compared with polycrystalline Mn2Au with effective ξDL<0.051,we show a much larger ξDLof~0.333 in single-crystal Mn2Au,which arises from the large spin Hall conductivity instead of electrical resistivity.Moreover,with a further contribution of interfacial effects,the effective ξDLof single-crystalline Mn2Au/Py system increases to 0.731,which is more than two times larger than the value of ~0.22 reported for the Mn2Au/CoFeB system.By utilizing the largeξDLof Mn2Au in a perpendicularly magnetized Mn Ga/Mn2Au system,energy-efficient deterministic magnetization switching with a current density at ~10^(6)A cm^(-2)is achieved.Our results reveal a significant potential of Mn2Au as an efficient SOT source and shed light on its application in future AFM material-based SOT integration technology.

Nonadiabatic spin-torque and damping effects on vortex core switching triggered by a single current pulse

Switching the orientation of a vortex core by spin-polarised pulse current introduces a promising concept for the reliable addressing of a single nanodisc element inside dense arrays. In this paper, micromagnetic simulations are employed to study the vortex core switching behaviour excited by a short in-plane Gaussian current pulse. We find that both the switching mechanism and the switching time are not sensitive to changes in the phenomenological parameters of spin-torque nonadiabaticity and Gilbert damping. The switching time, however, strongly depends on the current strength. In addition, we have theoretically predicted the parameter range of current pulses to achieve a single switching event.

Spin orbit torques in Pt-based heterostructures with van der Waals interface

Spin orbit torques(SOTs)in ferromagnet/heavy-metal heterostructures have provided great opportunities for efficient manipulation of spintronic devices.However,deterministically field-free switching of perpendicular magnetization with SOTs is forbidden because of the global two-fold rotational symmetry in conventional heavy-metal such as Pt.Here,we engineer the interface of Pt/Ni heterostructures by inserting a monolayer MoTe_(2)with low crystal symmetry.It is demonstrated that the spin orbit efficiency,as well as the out-of-plane magnetic anisotropy and the Gilbert damping of Ni are enhanced,due to the effect of orbital hybridization and the increased spin scatting at the interface induced by MoTe_(2).Particularly,an out-of-plane damping-like torque is observed when the current is applied perpendicular to the mirror plane of the MoTe_(2)crystal,which is attributed to the interfacial inversion symmetry breaking of the system.Our work provides an effective route for engineering the SOT in Pt-based heterostructures,and offers potential opportunities for van der Waals interfaces in spintronic devices.

Spin–orbit torque in perpendicularly magnetized[Pt/Ni]multilayers

The performance of spin–orbit torque(SOT)in heavy metal/ferromagnetic metal periodic multilayers has attracted widespread attention.In this paper,we have successfully fabricated a series of perpendicular magnetized[Pt(2-t)/Ni(t)]_4 multilayers,and studied the SOT in the multilayers by varying the thickness of Ni layer t.The current induced magnetization switching was achieved with a critical current density of 1×10^(7)A/cm^(2).The damping-like SOT efficiencyξ_(DL)was extracted from an extended harmonic Hall measurement.We demonstrated that theξ_(DL)can be effectively modulated by t_(Pt)/t_(Ni)ratio of Pt and Ni in the multilayers.The SOT investigation about the[Pt/Ni]N multilayers might provide new material candidates for practical perpendicular SOT-MRAM devices.

Spin transfer torque in the semiconductor/ferromagnetic structure in the presence of Rashba effect

Spin transfer torque in magnetic structure occurs when the transverse component of the spin current that flows from the nonmagnetic medium to ferromagnetic medium is absorbed by the interface. In this paper, considering the Rashba effect on the semiconductor region, we discuss the spin transfer torque in semiconductor/ferromagnetic structure and obtain the components of spin-current density for two models：（i） single electron and（ii） the distribution of electrons. We show that no matter whether the difference in Fermi surface between semiconductor and Fermi spheres for the up and down spins in ferromagnetic increases, the transmission probability decreases. The obtained results for the values used in this article illustrate that Rashba effect increases the difference in Fermi sphere between semiconductor and Fermi sphere for the up and down spins in ferromagnetic. The results also show that the Rashba effect, brings an additional contribution to the components of spin transfer torque, which does not exist in the absence of the Rashba interaction. Moreover, the Rashba term has also different effects on the transverse components of the spin torque transfer.

Spin logic devices based on negative differential resistance -enhanced anomalous Hall effect

Owing to rapid developments in spintronics,spin-based logic devices have emerged as promising tools for next-generation computing technologies.This paper provides a comprehensive review of recent advancements in spin logic devices,particularly focusing on fundamental device concepts rooted in nanomagnets,magnetoresistive random access memory,spin–orbit torques,electric-field modu-lation,and magnetic domain walls.The operation principles of these devices are comprehensively analyzed,and recent progress in spin logic devices based on negative differential resistance-enhanced anomalous Hall effect is summarized.These devices exhibit reconfigur-able logic capabilities and integrate nonvolatile data storage and computing functionalities.For current-driven spin logic devices,negative differential resistance elements are employed to nonlinearly enhance anomalous Hall effect signals from magnetic bits,enabling reconfig-urable Boolean logic operations.Besides,voltage-driven spin logic devices employ another type of negative differential resistance ele-ment to achieve logic functionalities with excellent cascading ability.By cascading several elementary logic gates,the logic circuit of a full adder can be obtained,and the potential of voltage-driven spin logic devices for implementing complex logic functions can be veri-fied.This review contributes to the understanding of the evolving landscape of spin logic devices and underscores the promising pro-spects they offer for the future of emerging computing schemes.

Macroscopic spin and charge transport theory

According to the general principle of non-equilibrium thermodynamics, we propose a set of macroscopic transport equations for the spin transport and the charge transport, In particular, the spin torque is introduced as a generalized ＇current density＇ to describe the phenomena associated with the spin non-conservation in a unified framework. The Einstein relations and the Onsager relations between different transport phenomena are established. Specifically, the spin transport properties of the isotropic non-magnetic and the isotropic magnetic two-dimensional electron gases are fully described by using this theory, in which only the macroscopic-spin-related transport phenomena allowed by the symmetry of the system are taken into account.

Symmetry ensemble theory of the spin wave emitting effect driven by current in nanoscale magnetic multilayers

This paper proposes a symmetry ensemble model for the magnetic dynamics caused by spin transfer torque in nanoscale pseudo-spin-valves, in which individual spin moments in the free layer are considered as subsystems to form a spinor ensemble. The magnetization dynamics equation of the ensemble was developed. By analytically investigating the equation, many magnetization dynamics properties excited by polarized current reported in experiments, such as double spin wave modes and the abrupt frequency jump, can be successfully explained. It is pointed out that an external field is not necessary for spin wave emitting （SWE） and a novel perpendicular configuration structure can provide much higher SWE efficiency in zero magnetic field.

Spin transport in a Zigzag normal/ferromagnetic graphene junction

We study magnetic proximity effect induced low-energy spin transport in the normal/ferromagnetic junction of a semi-infinite zigzag graphene nanoribbon. Due to the absence of a spin flip in a single interface, the spin transfer in this model can be described by the ＂two-spin channel＂ model. We identify each spin channel as either a perfect conducting or a non-conducting channel. This feature leads to spin filter in symmetric zigzag graphene nanoribbon and spin precession in antisymmetric zigzag graphene nanoribbon, and helps to directly determine the exchange-splitting intensity directly, even without an external auxiliary bias.

Control of spins in a nano-sized magnet using electric-current

With the development of spintronics,spin-transfer torque control of magnetic properties receives considerable attention.In this paper the Landau-Lifshitz-Gilbert equation including the torque term is used to investigate the magnetic moment dynamics in the free layer of the ferromagnet/non-magnetic/ferromagnet（FM1/N/FM2） structures.It is found that the reverse critical time τ_c decreases with the current increasing.The critical time τ_c as a function of current for the perpendicular and parallel easy magnetic axes are the same.The critical time τ_c increases with the damping factor α increasing.In the case of large current the influence of the damping factor α is smaller,but in the case of little torque the critical time τ_c increases greatly with the damping increasing.The direction of the magnetization in the fixed layer influences the critical time,when the angle between the magnetization and the z direction changes from 0.1π to 0.4π,the critical time τ_c decreases from 26.7 to 15.6.

Recent progress on excitation and manipulation of spin-waves in spin Hall nano-oscillators

Spin Hall nano oscillator(SHNO),a new type spintronic nano-device,can electrically excite and control spin waves in both nanoscale magnetic metals and insulators with low damping by the spin current due to spin Hall effect and interfacial Rashba effect.Several spin-wave modes have been excited successfully and investigated substantially in SHNOs based on dozens of different ferromagnetic/nonmagnetic(FM/NM)bilayer systems(e.g.,FM=Py,[Co/Ni],Fe,CoFeB,Y3Fe5O12;NM=Pt,Ta,W).Here,we will review recent progress about spin-wave excitation and experimental parameters dependent dynamics in SHNOs.The nanogap SHNOs with in-plane magnetization exhibit a nonlinear self-localized bullet soliton localized at the center of the gap between the electrodes and a secondary high-frequency mode which coexists with the primary bullet mode at higher currents.While in the nanogap SHNOs with out of plane magnetization,besides both nonlinear bullet soliton and propagating spin-wave mode are achieved and controlled by varying the external magnetic field and current,the magnetic bubble skyrmion mode also can be excited at a low in-plane magnetic field.These spin-wave modes show thermal-induced mode hopping behavior at high temperature due to the coupling between the modes mediated by thermal magnon mediated scattering.Moreover,thanks to the perpendicular magnetic anisotropy induced effective field,the single coherent mode also can be achieved without applying an external magnetic field.The strong nonlinear effect of spin waves makes SHNOs easy to achieve synchronization with external microwave signals or mutual synchronization between multiple oscillators which improve the coherence and power of oscillation modes significantly.Spin waves in SHNOs with an external free magnetic layer have a wide range of applications from as a nanoscale signal source of low power consumption magnonic devices to spin-based neuromorphic computing systems in the field of artificial intelligence.

Experiments and SPICE simulations of double MgO-based perpendicular magnetic tunnel junction

We investigate properties of perpendicular anisotropy magnetic tunnel junctions(pMTJs) with a stack structure MgO/CoFeB/Ta/CoFeB/MgO as the free layer(or recording layer),and obtain the necessary device parameters from the tunneling magnetoresistance(TMR) vs.field loops and current-driven magnetization switching experiments.Based on the experimental results and device parameters,we further estimate current-driven switching performance of pMTJ including switching time and power,and their dependence on perpendicular magnetic anisotropy and damping constant of the free layer by SPICE-based circuit simulations.Our results show that the pMTJ cells exhibit a less than 1 ns switching time and write energies <1.4 pJ;meanwhile the lower perpendicular magnetic anisotropy(PMA) and damping constant can further reduce the switching time at the studied range of damping constant α <0.1.Additionally,our results demonstrate that the pMTJs with the thermal stability factor■73 can be easily transformed into spin-torque nano-oscillators from magnetic memory as microwave sources or detectors for telecommunication devices.

Electric and thermo spin transfer torques in Fe/Vacuum/Fe tunnel junction

We present first-principle calculations of electric and thermo spin transfer torques （STT） in Fe/Vacuum（Vac）/Fe magnetic tunnel junctions （MTJs）. Our quantitative studies demonstrate rich bias dependence of STT and tunnel inagneto resistance （TMR） behaviors with respect to the interface roughness. Thermoelectric effects in Fe/Vac/Fe MTJs is remarkable. We observe larger ZT of 6.2 in 8 ML clean Vacuum barrier, where responsible for. Thermo-STT in Fe/Vac/Fe with similar barrier thickness. the heavily restraitmd thermal conductance should be MTJs show same order as that in Fe/MgO/Fe MTJs with similar barrier thickness.

RF magnetron sputtering induced the perpendicular magnetic anisotropy modification in Pt/Co based multilayers

We demonstrate that radio frequency(RF)magnetron sputtering technique can modify the perpendicular magnetic anisotropy(PMA)of Pt/Co/normal metal(NM)thin films.Influence of ion irradiation during RF magnetron sputtering should not be neglected and it can weaken PMA of the deposited magnetic films.The magnitude of this influence can be controlled by tuning RF magnetron sputtering deposition conditions and the upper NM layer thickness.According to the stopping and range of ions in matter(SRIM)simulation results,defects such as displacement atoms and vacancies in the deposited film will increase after the RF magnetron sputtering,which can account for the weakness of PMA.The amplitude changes of the Hall resistance and the threshold current intensity of spin orbit torque(SOT)induced magnetization switching also can be modified.Our study could be useful for controlling magnetic properties of PMA films and designing new type of SOT-based spintronic devices.

Perpendicular magnetic tunnel junction and its application in magnetic random access memory

Recent progresses in magnetic tunnel junctions with perpendicular magnetic anisotropy （PMA） are reviewed and summarized. At first, the concept and source of perpendicular magnetic anisotropy （PMA） are introduced. Next, a historical overview of PMA materials as magnetic electrodes, such as the RE-TM alloys TbFeCo and GdFeCo, novel tetragonal manganese alloys Mn-Ga, L10-ordered （Co, Fe）/Pt alloy, multilayer film [Co, Fe, CoFe/Pt, Pd, Ni, AU]N, and ultra-thin magnetic metal/oxidized barrier is offered. The other part of the article focuses on the optimization and fabrication of CoFeB/MgO/CoFeB p-MTJs, which is thought to have high potential to meet the main demands for non-volatile magnetic random access memory.

Thermal spin transfer torque in Fe｜Ag｜YIG multilayers

We investigated the thermal spin transfer effect in FM｜NM｜YIG multilayers using the first princi- ples scattering theory. At room temperature, the spin Seebeck torque TZSE -1.0 μJ/（K-m^2） in an Ag＋｜Fe｜Ag｜YIG multilayer, which is around 40% larger than that estimated from mixing conductance. The quantum effects such as interlayer exchange coupling between FM and YIG could be responsible for the enhancements. Based on the LLG equation, reverse the magnetic configurations, circularly, in a we predict that a temperature bias of -10 K can multilayer at room temperature.

Ballistic current induced effective force on magnetic domain wall

The collective dynamics of magnetic domain wall under electric current is studied in the form of spin transfer torque(STT). The out-of-plane STT induced effective force is obtained based on the Landau-Lifshitz-Gilbert(LLG) equation including microscopic STT terms. The relation between microscopic calculations and collective description of the domain wall motion is established. With our numerical calculations based on tight binding free electron model, we find that the non adiabatic out-of-plane torque components have considerable non-local properties. It turns out that the calculated effective forces decay significantly with increasing domain wall widths.