Fast current-induced domain wall and skyrmion motion in a nearly compensated amorphous ferrimagnetic alloy

Abstract

Magnetic racetrack memories offer the possibility of increasing the density and speed of data storage. Yet, achieving fast current-induced motion of domain walls (DWs) and skyrmions at room temperature remains a challenge. We study nearly compensated ferrimagnetic CoGd alloys inserted between two heavy metals for which we have already demonstrated the optimization of spin-orbit torques (SOTs)1 and interfacial Dzyaloshinskii-Moriya interactions2. Ferrimagnets are attractive materials for their low stray field and fast spin dynamics which has been theoretically predicted to enable formation of ultrasmall3 and fast skyrmions. However, so far, the reported skyrmion velocities4 are much lower than of DWs3.Here, we investigate the nucleation and dynamics of chiral magnetic textures induced by current pulses in Pt/CoGd(5 nm)/(W or Ta) films. First, we demonstrate fast motion of DWs driven by SOTs at room temperature. We observe a maximum DW velocity of about 460 m s-1 (Fig. 1). Hence, we confirm that our ferrimagnetic materials have a very low DW pinning and density of defects, which is ideal for fast current-induced motion of DWs and skyrmions.Most literature studies focus on materials with a skyrmionic ground state. Here, our CoGd films have a sharp magnetic hysteresis loop and skyrmions are metastable. In Fig. 2a, we show that we can achieve field free nucleation of skyrmion bubbles with a single 5-ns current pulse and with a density of 2.7 x1012 A m-2. In addition, sequential magneto-optical Kerr (MOKE) imaging (Fig. 2b-f) after applying a single 5-ns current pulse with a density of 1.5 x1012 A m-2 reveals fast skyrmion motion with a velocity of about 400 m s-1. A small out-of-plane magnetic field of -5.7 mT is applied to stabilize the skyrmion bubble. In this study, we demonstrate that our low pinning ferrimagnetic films enable one of the fastest skyrmion motion thus far reported5. Our results could pave the way for ultrafast racetrack memory operations.This research was supported by DARPA award No. D18AP00009.  Average DW velocity driven by SOTs in Pt/CoGd/(W or Ta).  Field free current-induced nucleation (a) and motion (b-f) of skyrmion bubbles by a single 5-ns current pulse.