Trapping and manipulating skyrmions in two-dimensional films by surface acoustic waves

Abstract

Skyrmions, topologically stable spin structures with particle-like properties, are promising for spintronics applications such as skyrmion racetrack memory. Though reliable control of skyrmion motion is essential for the operation of spintronics devices, the straight motion of skyrmions along the driving force is in general difficult due to an inevitable transverse force originating from their topology. Here, we propose a method of precise manipulation of skyrmions based on surface acoustic waves (SAWs) propagating in two dimensions. Using two standing SAWs, saddle-shape local potentials like quadrupole ion traps are created to trap skyrmions robustly. Furthermore, by tuning the frequencies of the SAWs, we show that trapped skyrmions not only move in straight lines but also move precisely in any direction in a two-dimensional thin film. These results could be helpful for the future design of spintronics devices based on skyrmions.

Transport of skyrmions by surface acoustic waves

“…18 The dynamic strain induced by surface acoustic waves (SAWs) has also been suggested as an attractive approach to control thin film magnetization. [19][20][21][22][23][24][25][26][27][28][29][30][31] In particular, Yokouchi et al experimentally observed the creation of skyrmions by SAWs in a Pt/Co/Ir thin film owing to the inhomogeneous effective torque arising from both SAWs and thermal fluctuations via magnetoelastic coupling. 32 Nepal et al theoretically studied the dynamical pinning of skyrmion bubbles at the anti-nodes of standing SAWs in a FePt nano-wire, revealing the strain gradient induced by SAWs is the driving force for skyrmion motion.…”

“…of the skyrmion, D is a dissipative tensor in the form of D 0 0 D for an isolated skyrmion with D ¼ Ð @m @x Á @m @y dxdy, and FðRÞ is the effective force per unit thickness due to the strain induced by SAWs. The effective force FðRÞ generated by the strain e ii (i ¼ x; y) of the SAWs at the given time t can be expressed as 29,33 FðR;…”

“…where C ii is a shape factor due to strain and their values depend upon the shape and size of the bubble, 29,33 and r is the center of the skyrmion at the give time t. The skyrmion velocity is proportional to the strain gradient. The SAW-induced force acting upon the skyrmion is only in the direction of the applied SAWs, whereas the force exerted by the SAWs in the direction perpendicular to their application is zero, as there is no strain gradient present in that direction.…”