Influence Of Spin-transfer Torque Research Articles

Current-driven dynamics of antiferromagnetic skyrmions: from skyrmion Hall effects to hybrid inter-skyrmion scattering

Antiferromagnetic (AFM) skyrmions have emerged as a highly promising avenue in the realm of spintronics, particularly for the development of advanced racetrack memory devices. A distinguishing feature of AFM skyrmions is the cancellation of their net topological charge, leading to an anticipated absence of the skyrmion Hall effect (SkHE). Here, we unveil that the latter is finite under the influence of spin-transfer torque, depending on the direction of the injected current impinging on intrinsic AFM skyrmions emerging in Cr/Pd/Fe trilayer on Ir(111) surface. Hinging on first principles combined with atomistic spin dynamics simulations, we identify the origin of the SkHE, which is due to the ellipticity of the skyrmions, and we uncover that FM skyrmions in the underlying Fe layer act as effective traps for AFM skyrmions, confining them and affecting their velocity. These findings hold significant promise for spintronic applications, the design of multi-purpose skyrmion tracks while advancing our understanding of AFM–FM skyrmion interactions and hybrid soliton dynamics in heterostructures.

Annihilation mechanisms for interacting skyrmions in magnetic nanowire

Abstract Magnetic skyrmions are considered potential candidates for spintronics-based memory and logic devices. For achieving high-density and high-speed devices, it is essential to study their interactions. In this paper, the interaction, dynamics, and annihilation mechanisms of Néel skyrmions in nanowire confinement under the influence of spin-transfer torque (STT) and edge forces have been studied. Initially isolated, two Néel skyrmions are brought into proximity, leading to distinct interaction scenarios characterized by varying current densities. We explore the impact of these interactions on skyrmion trajectories, size evolution, and annihilation phenomena. Our findings reveal the interplay of skyrmion-skyrmion repulsive forces, edge effects, and the influence of STT, shedding light on the rich dynamics of these topological magnetic textures. Furthermore, we unveil the distinct annihilation mechanisms of the leading and trailing skyrmions under different forces, providing valuable insights into the fundamental physics of skyrmion behavior in confined geometries.

External Bias Field Control of Domain Wall to Skyrmion Conversion in a Typically Designed Magnetic Nanotrack

Understanding the evolution and tunable control of skyrmion dynamics is crucial for the effective development of next generation Racetrack Memory (RM) devices. In this study, we present the micromagnetic simulation demonstrating precise control of skyrmion motion in a magnetic nanotrack composed of two (three) narrow and a wider nanochannels under the influence of spin transfer torque (STT) and out of plane bias field. Our results reveal that out of plane magnetic field assists in efficient conversion of Domain wall (DW) pair to skyrmion and results in the achievement of uniform velocity for each skyrmion. Furthermore, a thorough analysis of individual and total energy terms during the DW pair to skyrmion conversion provides detailed insight into the role of bias field in overcoming the pinning potential at the junction of the narrow and wide nanochannel. The ability to precisely control the skyrmion motion stabilized from a DW pair under the influence of an external bias field is of significant importance for developing skyrmion based memory devices.

Spin transfer torque induced domain wall oscillations in ferromagnetic nanowire with a nanoscale Dzyaloshinskii–Moriya interaction region

The interfacial Dzyaloshinskii–Moriya Interaction (iDMI) in ferromagnetic (FM)/heavy metal (HM) structures has been under intense investigation due to the interesting observation of many novel chiral structures such as domain walls (DWs) and skyrmions. In all the studies so far, the iDMI is present in the whole of the FM layer. In this study, we report our investigations on a FM nanowire where iDMI is confined to a nanoscale region. We observe that the local modulation of magnetic properties results in oscillations of DWs under the influence of spin-transfer torque (STT). For such oscillations, the nanoscale iDMI acts as a restoring force. We have studied the dependence of oscillation frequency upon the current density, geometrical parameter (width of nanoscale iDMI region), strength of iDMI and magnetic parameters such as anisotropy constant (Ku), exchange constant (A) and saturation magnetization (Ms). The variation in the above-mentioned parameters results in the tunability of oscillation frequency within a few GHz. Moreover, the amplitude of oscillations is also tunable with the width of the nanoscale iDMI region. These observations make this study potentially useful for applications in neuromorphic computing.

Spin Torque Effect in Magnetic Low-Frequency Noise Due to Asymmetric Oscillation

In this letter, the influence of spin-transfer torque (STT) on magnetic read sensor noise in the test band of 1–200 MHz has been investigated experimentally using the macrospin model with STT term. The results reveal a dependence of the integrated noise power on the direction of bias current when an external field is applied. In a positive field direction, positive bias current produces stronger noise than negative current. In the presence of negative field direction, the relationship is reversed. The hypothesis of asymmetric oscillation noise modification by STT is confirmed by corresponding changes of a correlation between the signal in the test band, and amplitude fluctuations of thermally driven ferromagnetic resonance in the sensor free layer, both derived from high-bandwidth waveforms. The effect is a consequence of the spin torque impact on the effective damping constant, which adjusts the shape of the asymmetric oscillation spectrum in a test band.

Influence of the thermal contact resistance in current-induced domain wall depinning

In this work we study the influence of the thermal contact resistance on the temperature of a typical nanostripe used in current induced magnetic domain wall movement or depinning. The thermal contact resistance arises from an imperfect heat transport across the interface between the metallic ferromagnetic nanostripe and the substrate. We show that this parameter, which is likely non-zero in any experimental device, increases the temperature in the nanostripe considerably. When the current is injected in the nanostripe in nanosecond long pulses, the larger temperature also implies a reduction of the effective current density delivered by the pulse generator. Both the thermal contact resistance and the dynamic response of the pulse generator are usually neglected in theoretical estimations of the influence of spin transfer torque on domain wall displacement and depinning. Here we show that only if the thermal contact resistance and the electric resistivity of the ferromagnetic nanostripe are optimized to the best values reported in the bibliography, the Joule heating may not be so crucial for current densities of the order of 108 A cm−2. Also, the use of physical constrictions (notch) to pin the magnetic domain wall may complicate the interpretation of the results as they always come together with relevant thermal gradients.

Off-centred immobile magnetic vortex under influence of spin-transfer torque

Formation of the ‘dip’ structure which foregoes switching of magnetic vortex polarity is studied numerically in magnetic nanodisc. A new method based on influence of the spin-transfer torque is used. The method allows one to obtain the dip structure for immobile vortex which significantly improves studying accuracy in comparison with the case of moving vortex. Free out-of-plane vortices as well as in-plane vortices pinned on hole defects are considered. It is shown that the process of the dip formation is different for free and pinned vortices and direction of the dip does not directly depend on the vortex polarity.

Minimum action paths for spin-torque assisted thermally induced magnetization reversal

We calculate the most probable reaction paths for thermally induced magnetization reversal of a nanomagnet under the influence of spin transfer torque. The presence of the spin transfer torque implies that the standard reaction rate theory of Kramers cannot be used since the dynamics no longer shows detailed balance and so the magnetization reversals are nonequilibrium transitions. Thin film nanomagnets with a biaxial anisotropy, a shape anisotropy that leads to in-plane magnetization with a preferred axis in the plane, are considered. The reaction pathways and rates are computed using geometrical Minimum Action Method. Our results indicate that the transition state has an out-of-plane magnetization component, in contrast to the case without an applied spin transfer torque.

Influence of spin-transfer torque on thermally activated ferromagnetic resonance excitations in magnetic tunnel junctions

Voltage noise measurements on magnetic tunnel junctions show that thermal fluctuations of the magnetization are either amplified or quenched by subcritical spin-transfer torque depending on the current direction. We present an analytical model that describes the dependence of thermally activated ferromagnetic resonance on bias current. The evolution of the peak amplitude and linewidth with the applied current is directly related to the longitudinal torque, whereas the shift of the resonance frequency is sensitive to the transverse torque. Both spin torque terms are independently extracted from the measured noise spectra. Our results support the general idea that it is more pertinent to describe spin torque in terms of voltage rather than current in magnetic tunnel junctions.

Spin-Torque Influence on the High-Frequency Magnetization Fluctuations in Magnetic Tunnel Junctions

Voltage noise measurements were performed in the 3-7 GHz frequency range on magnetic tunnel junctions biased with a dc current. Magnetic noise associated with ferromagnetic resonance excitations is either amplified or reduced depending on the direction of the bias current. This effect is interpreted as the influence of spin transfer torque on the magnetization fluctuations and described using Gilbert dynamics equation including spin transfer torque and effective field terms.

Understanding the current-induced magnetization dynamics in the presence of large magnetic fields

We have theoretically studied the magnetization behavior in the magnetic trilayer nanostructures under the influence of spin-transfer torque using the Landau–Lifshitz–Gilbert equation. The focus is the coherent transition of the magnetization from a static state to another static state induced by the spin current even in the presence of a large applied field. Based on the study, the experimental observations, which were thought to be due to spin-wave excitations previously, can be directly and quantitatively explained by the out-of-plane static states of the magnetization induced by the spin-current in the presence of large in-plane magnetic fields in the range of several Teslas.

Spin-current effect on ferromagnetic resonance in patterned magnetic thin film structures

We have theoretically investigated the ferromagnetic resonance in the magnetic thin film structures under the influence of spin-transfer torque using a modified Landau–Lifshitz–Gilbert equation in the linearization regime. The study shows that spin currents do not shift the resonance field but rather change both the resonance amplitude and the linewidth. Ferromagnetic resonance under this circumstance can be characterized by an effective damping constant. Depending upon its direction, the spin current can pump energy into or dissipate energy from the magnetic system. In addition, the quality factor of the resonance can be tuned by changing the current intensity. Ferromagnetic resonance excited by ac electrical currents is also theoretically demonstrated and discussed in this article.

In-plane magnetization dynamics driven by spin-polarized currents in magnetic nanostructures

We have theoretically studied the magnetization behavior in the magnetic trilayer nanostructures under the influence of spin-transfer torque using the Landau-Lifshitz-Gilbert equation. The focus is the large angle dynamic precession mode of the magnetization in the film plane that can be induced and sustained by the spin current under certain circumstance although the Gilbert damping is present. Based on the study, the recent experimental observations of microwave oscillation and its characteristics can be well explained and its origin, which is in controversy, can be understood.