Current-driven skyrmion motion in granular films

Abstract

Current-driven skyrmion motion in random granular films is investigated with interesting findings. For a given current, there exists a critical disorder strength below which its transverse motion could either be boosted below a critical damping or be hindered above the critical damping, resulting in current and disorder dependences of skyrmion Hall angle. The boosting comes mainly from the random force that is opposite to the driving force (current). The critical damping depends on the current density and disorder strength. However, the longitudinal motion of a skyrmion is always hindered by the disorder. Above the critical disorder strength, skyrmions are pinned. The disorder-induced random force on a skyrmion can be classified as static and kinetic ones, similar to the friction force in the Newtonian mechanics. In the pinning phase, the static (pinning) random force is transverse to the current density. The kinetic random force is opposite to the skyrmion velocity when skyrmions are in motion. Furthermore, we provide strong evidences that the Thiele equation can perfectly describe skyrmion dynamics in granular films. These findings provide insight to skyrmion motion and should be important for skyrmiontronics.

Thermal motion of skyrmion arrays in granular films

“…4(c) and 4(d), the presence of grain boundaries also enhances the diffusivity at longer lag times, on the order of 1 ns. This suggests that the gyrotropic motion induced by the interaction of skyrmions with grain boundaries leads to an increased diffusivity at this intermediate timescale, consistent with findings of an enhanced mobility for current driven motion in the presence of grains [17]. Considering the effect of a single boundary on a skyrmion, the skyrmion will tend to move along this boundary due to its gyrotropic motion until it meets a vertex.…”

“…However, in order to create such devices the realities of experimental systems need to be taken into account. Particularly, experimental systems contain pinning sites, which have been shown to play an important role in skyrmion dynamics in various simulations of single skyrmion systems under temperature [4,13,14] and activation of skyrmions by electric currents [15][16][17][18][19]. Furthermore, by assuming skyrmions are rigid particles, molecular dynamics simulations have also been used to study the properties of skyrmion lattices pinned by point defects [20] and the corresponding depinning transition under a driving force [21,22].…”

Controlled motion of skyrmions in a magnetic antidot lattice

“…Unfortunately, skyrmions driven by the electric current migrate towards the edges of a magnetic strip, due to the skyrmion Hall effect, which leads to an unstable transverse position and annihilation. Furthermore, the skyrmion motion is randomized by the thermal diffusion and the presence of pining centers in real samples [21,22]. Therefore, key challenges regarding skyrmion motion are related to stabilization, confinement, and control of the movement of skyrmions at room temperature [23,24].…”

Skyrmion crystal formation and magnetic field

“…Skyrmion crystals (SkXs) have attracted enormous attention in the past decade not only because they established the notion of skyrmion [1][2][3][4][5][6] but also because they are good platform for studying various physics [7][8][9][10][11] such as the emergent electromagnetic fields and topological Hall effect [11][12][13]. Three types of circular skyrmions in chiral magnets [11,14,15] have been identified,namely, Bloch skyrmions, hedgehog skyrmions, and anti-skyrmions.…”