Fast vortex wall motion in wide permalloy strips from double switching of the vortex core

Abstract

We study vortex domain wall dynamics in wide permalloy strips driven by applied magnetic fields and spin-polarized electric currents. As recently reported [V. Estévez and L. Laurson, Phys. Rev. B, 93, 064403 (2016)], for sufficiently wide strips and above a threshold field, periodic dynamics of the vortex core are localized in the vicinity of one of the strip edges, and the velocity drop typically observed for narrow strips is replaced by a high-velocity plateau. Here, we analyze this behavior in more detail by means of micromagnetic simulations. We show that the high-velocity plateau originates from a repeated double switching of the magnetic vortex core, underlying the periodic vortex core dynamics in the vicinity of the strip edge, i.e., the "attraction-repulsion" effect. We also discuss the corresponding dynamics driven by spin-polarized currents, as well as the effect of including quenched random structural disorder to the system.

Fast micromagnetic simulations on GPU—recent advances made with $\mathsf{mumax}^3$

“…When β = α (figure 11(b)), the adiabatic and nonadiaba tic transverse torques balance each other, resulting only in a longitudinal vortex movement along the strip axis. In wider nanostrips, the mobility curves are similar, but in this case, the highvelocity regime physically originates in a doublevortex switching mechanism [99].…”

“…This is the case when differ ent domain wall components acquire different propagation velocities, and the domain wall increases size indefinitely. In other cases, starting from a certain threshold field, the vortex core dynamically switches polarity close to the nanostrip edge via a double switching mechanism, leading to a highvelocity plateau at large external fields [99], contrasting the velocity drop typically observed above the Walker breakdown for nar rower nanostrips. In these complex structures especially, the dynamics can no longer be reduced to simple models, and micromagnetic simulations are the only available tool to study the dynamics.…”

“…Figure 8 shows the phase diagram of the different domain wall types in nanostrips made out of this material. Depending on the width and thickness, the equilibrium domain wall is a transverse wall, a vortex domain wall [96], or in very wide nanostrips, even double or triple vortex domain walls [97][98][99]. Indeed, in wide strips, the shape anisotropy has less pronounced effects, favoring fluxclosure states containing multiple vortices.…”

“…As the GPU performance improves over time, ever larger simulations become feasible. In recent years, this allowed one to study domain wall motion for longer timescales [150], or in larger thin film geometries [97][98][99]152] than was previ ously attainable. Furthermore, also large threedimensional structures [153][154][155] are starting to become computationally tractable.…”

Exchange explosions of topological edge defects in a square micromagnet

“…The situation is comparable to a strip with a small but finite width, where a DW consists of two oppositely charged topological edge defects on each side of the strip [12]. In such strips, Walker breakdown occurs through intermittent exchange of Vs/AVs between the two edge defects, as the DW reaches a certain critical velocity [24,[27][28][29]. In the present case, there is no oppositely charged defect to "absorb" the 014450-3 (A)V in our square micromagnet model system, and the edge defect thus builds up exchange energy which leads to an exchange explosion.…”