Noise fluctuations and drive dependence of the skyrmion Hall effect in disordered systems

Abstract

Using a particle-based simulation model, we show that quenched disorder creates a drive-dependent skyrmion Hall effect as measured by the change in the ratio =R V V of the skyrmion velocity perpendicular (V ⊥ ) and parallel (V ) to an external drive. R is zero at depinning and increases linearly with increasing drive, in agreement with recent experimental observations. At sufficiently high drives where the skyrmions enter a free flow regime, R saturates to the disorder-free limit. This behavior is robust for a wide range of disorder strengths and intrinsic Hall angle values, and occurs whenever plastic flow is present. For systems with small intrinsic Hall angles, we find that the Hall angle increases linearly with external drive, as also observed in experiment. In the weak pinning regime where the skyrmion lattice depins elastically, R is nonlinear and the net direction of the skyrmion lattice motion can rotate as a function of external drive. We show that the changes in the skyrmion Hall effect correlate with changes in the power spectrum of the skyrmion velocity noise fluctuations. The plastic flow regime is associated with f 1 noise, while in the regime in which R has saturated, the noise is white with a weak narrow band signal, and the noise power drops by several orders of magnitude. At low drives, the velocity noise in the perpendicular and parallel directions is of the same order of magnitude, while at intermediate drives the perpendicular noise fluctuations are much larger.

Manipulation of Magnetization by Spin–Orbit Torque

“…A similar phenomenon was also observed in Pt/CoFeB/MgO, where the velocity linearly varies with the current, in agreement with magnetic simulations. However, some features of the motion are not consistent with theoretical expectations, such as the dependence of the velocity and the size of skyrmion, and the relation between skyrmion Hall angle and the velocity . Therefore, the underlying mechanism remains controversial …”

“…However, some features of the motion are not consistent with theoretical expectations, such as the dependence of the velocity and the size of skyrmion, [22,86,87] and the relation between skyrmion Hall angle and the velocity. [86,89] Therefore, the underlying mechanism remains controversial. [86] The skyrmion Hall effect hinders the practical application of skyrmion in racetrack memory.…”

Skyrmion dynamics and topological sorting on periodic obstacle arrays

“…This model is appropriate for lower skyrmion densities where there is no overlap between the skyrmions, as well as for systems in which the skyrmions are relatively stiff. It has been used previously to capture a wide variety of dynamical skyrmion behaviors, including the drive dependent skyrmion Hall effect which has also been observed in experiment [62,68]. We use an in-house simulation code based on standard molecular dynamics methods [75].…”

“…The skyrmion Hall angle is known to have a strong drive dependence in the presence of pinning. At low drives, the skyrmion Hall angle is small or zero, but θ sk increases with increasing drive before saturating at a value close to the pin-free limiting value for high drives [61][62][63][64][65][66][67][68][69][70]. This drive dependence can arise due to a side jump or swirling motion of the skyrmions that occurs when they interact with pinning sites, where faster moving skyrmions undergo a smaller side jump [71,72].…”

Skyrmions in magnetic multilayers: chirality, electrical detection and current-induced motion

“…14,22 Note that similar results were also obtained for a particle-based model with pinning sites. 23 We can even find a quantitative match with experiments, but for sets of parameters (magnitude and dispersion of the grain-to-grain magnetic properties) that we cannot verify yet. Below a current threshold, we can observe the same phenomena than by MFM: for example, in the case of DMI variation ( Fig.7(a)), skyrmions can be trapped in a grain of large DMI, will move around low DMI grains, sometimes will get stuck at some grain boundaries and then freed by another skyrmion.…”