Skyrmion Dynamics and Transverse Mobility: Skyrmion Hall Angle Reversal on 2D Periodic Substrates with dc and Biharmonic ac Drives

Abstract

We numerically examine the dynamics of a skyrmion interacting with a two-dimensional periodic substrate under dc and biharmonic ac drives. We show that the Magnus force of the skyrmion produces circular orbits that can resonate with the ac drive and the periodicity of the substrate to create quantized motion both parallel and perpendicular to the dc drive. The skyrmion Hall angle exhibits a series of increasing and/or decreasing steps along with strongly fluctuating regimes. In the phase locked regimes, the skyrmion Hall angle is constant and the skyrmion motion consists of periodic orbits encircling an integer number of obstacles per every or every other ac drive cycle. We also observe phases in which the skyrmion moves at 90 • with respect to the driving direction even in the presence of damping, a phenomenon called absolute transverse mobility that can exhibit reentrance as a function of dc drive. When the biharmonic ac drives have different amplitudes, in the two directions we find regimes in which the skyrmion Hall angle shows a sign reversal from positive to negative, as well as a reentrant pinning effect in which the skyrmion is mobile at low drives but becomes pinned at higher drives. These behaviors arise due to the combination of the Magnus force with the periodic motion of the skyrmions, which produce Shapiro steps, directional locking, and ratchet effects.

Late stages in the ordering of magnetic skyrmion lattices

“…Intriguing new phenomena can emerge under these circumstances. For example, in driven systems a constant angle, known as the skyrmion Hall angle, appears between the direction of the drive and the direction of displacement 7,[13][14][15][16][17][18][19] . Other studies have focused on the appearance of avalanches when the driven skyrmions interact with random pins 20 , the pattern formation of geometrically confined skyrmions 21 , the existence of nonequilibrium phase transitions between different dynamic phases 22,23 , shear banding of driven skyrmions in inhomogeneous pinning arrays 24 , laning transitions in presence of damping 25 , as well as the formation of skyrmion crystals 26,27 and the flow of skyrmion lattices 28 .…”