Effect of disorder on transverse domain wall dynamics in magnetic nanostrips

Abstract

We study the effect of disorder on the dynamics of a transverse domain wall in ferromagnetic nanostrips, driven either by magnetic fields or spin-polarized currents, by performing a large ensemble of graphics processing unit-accelerated micromagnetic simulations. Disorder is modeled by including small, randomly distributed nonmagnetic voids in the system. Studying the domain wall velocity as a function of the applied field and current density reveals fundamental differences in the domain wall dynamics induced by these two modes of driving: For the field-driven case, we identify two different domain wall pinning mechanisms, operating below and above the Walker breakdown, respectively, whereas for the current-driven case pinning is absent above the Walker breakdown. Increasing the disorder strength induces a larger Walker breakdown field and current, and leads to decreased and increased domain wall velocities at the breakdown field and current, respectively. Furthermore, for adiabatic spin-transfer torque, the intrinsic pinning mechanism is found to be suppressed by disorder. We explain these findings within the one-dimensional model in terms of an effective damping parameter α * increasing with the disorder strength. Domain wall (DW) dynamics in nanoscale ferromagnetic wires and strips driven by magnetic fields or spin-polarized currents is a subject of major technological importance for the operation of potential future nanoscale magnetic memory 1,2 and logic 3 devices. In these devices information is typically stored as magnetic domains along a nanostrip or wire and is processed by DW motion. For the reliable operation of such devices it is of fundamental importance to understand and control the effect of imperfections or disorder on the DW dynamics, necessarily present in any realistic samples, e.g., in the form of thickness fluctuations and grain structure of the sample, or various impurities and defects in the material. At the same time, such systems constitute a low-dimensional limit of the general problem of driven elastic manifolds in a random potential. 4 While the crucial importance of disorder for the dynamics of higher-dimensional DWs is well established, resulting in phenomena such as the Barkhausen effect, 5 a majority of studies of DW motion in systems with nanostrip or wire geometry neglect disorder effects. This applies to both theoretical studies and interpretations of experimental results. Some exceptions include studies demonstrating enhanced DW propagation due to the roughness of the edges of the strip. 6,7 Recently also the effect of spatially varying saturation magnetization M s on the dynamics of vortex walls was studied, resulting in an effective damping increasing with the disorder strength.8 Similar spatially distributed disorder has also been studied in a simplified, line-based model of a transverse DW.9,10 Experimental studies of DW dynamics in wires have revealed its stochastic nature in the case of short current pulses, 11 and has been attributed to the presence of disorder in...

Current-driven domain wall motion enhanced by the microwave field

“…23 For realistic nanostrips, the unavoidable edge/surface roughness, defects and/or random anisotropy lead to a disordered field and pin the DW motion. 12,13,37 Following, we will see that this extrinsic pinning can be greatly reduced for the microwave-assisted DW motion. As indicated in Ref.…”

“…If it is the nonadiabatic STT that drives the DW motion, the critical current density is mainly determined by the disordered extrinsic pinning caused by the unavoidable edge/surface roughness or defects in realistic nanostrips. [11][12][13] Although much effort has been done, the excessively high critical current density remains an issue to be resolved before the DW-based spintronic devices can be put into application. 9,[14][15][16][17][18][19] So far, in most theoretical studies of the current-induced DW motion, a DW is treated as a rigid particle.…”

Domain walls within domain walls in wide ferromagnetic strips

“…Possible origins for the symmetry breaking leading to incoherent precession of m DW in different parts of the DW could be edge effects, and/or quenched disorder, interacting with the DW [10,[14][15][16][17][18][19][20]; these may include dislocations, precipitates, grain boundaries, thickness fluctuations of the strip, etc. Here we explore the dynamics of extended DWs in wide CoPtCr PMA strips, with a Bloch wall equilibrium structure, using large-scale micromagnetic simulations with and without quenched disorder.…”

Influence of material defects on current-driven vortex domain wall mobility

“…Random distributions of these regions with densities ranging from σ = 500 to 1500 μm −2 were included. This differs from earlier approaches to simulate distributed disorder [32,33]. In Ref.…”

“…In Ref. [32] disorder was introduced in the material by introducing void cells. Reference [33], on the other hand, implemented disorder by introducing slight variations in the saturation magnetization.…”