Zero-Field Nucleation and Fast Motion of Skyrmions Induced by Nanosecond Current Pulses in a Ferrimagnetic Thin Film

Abstract

Skyrmion racetrack memories are highly attractive for next-generation data storage technologies. Skyrmions are noncollinear spin textures stabilized by chiral interactions. To achieve a fast-operating memory device, it is critical to move skyrmions at high speeds. The skyrmion dynamics induced by spin–orbit torques (SOTs) in the commonly studied ferromagnetic films is hindered by strong pinning effects and a large skyrmion Hall effect causing deflection of the skyrmion toward the racetrack edge, which can lead to information loss. Here, we investigate the current-induced nucleation and motion of skyrmions in ferrimagnetic Pt/CoGd/(W or Ta) thin films. We first reveal field-free skyrmion nucleation mediated by Joule heating. We then achieve fast skyrmion motion driven by SOTs with velocities as high as 610 m s–1 and a small skyrmion Hall angle |θSkHE| ≲ 3°. Our results show that ferrimagnets are better candidates for fast skyrmion-based memory devices with low risk of information loss.

Magnetic skyrmions: Basic properties and potential applications

“…[ 136,164 ] Experimentally, a suppressed SkHE has been achieved in ferrimagnets and SAF structures (Figure 9C–E). [ 26,27,29 ] A reduced SkHE (~20°) is observed in a ferrimagnetic multilayer which was not totally compensated compared to their ferromagnetic counterparts. [ 26 ] Very recently, fast skyrmion motion accompanied by very small SkHE (~3°) was achieved in ferrimagnetic Pt/CoGd/(W or Ta) films.…”

“…[126] STXM can show ferromagnetic skyrmions in different sublattices and facilitate the study of current-driven behavior for skyrmions in ferrimagnetic systems (Figure 6E). [26,29] Scanning nitrogen-vacancy (NV) magnetometry is another suitable approach to studying skyrmion structures. [49,51,[143][144][145] This method provides a noninvasive detection of magnetic stray fields produced by skyrmions by measuring the induced Zeeman splitting effect in a single NV center.…”

“…Many experiments have demonstrated that spin torque can not only be used to transport skyrmions but also to facilitate the creation of skyrmions. [29,[41][42][43][44][45][46][47][48][49][50][51][52] Therefore, the FM/HM structures capable of generating SOT by injecting currents in HM layers are potential candidates for skyrmion applications due to their convenience of both creating and manipulating skyrmions. Current flows with inhomogeneous density due to geometric confinement in tracks and specially engineered injector structures can be used to create skyrmions in multilayer thin films.…”

“…It was experimentally shown that this issue could probably be addressed by using AFSKs in ferrimagnets and SAFs. [26,27,29] Instead of suppressing it, Göbel et al showed that the SkHE can even be favorable by driving skyrmion ratchet propagation in mirror-asymmetric racetracks via alternating currents. [186] The geometric constrictions along one side of the racetrack will also trap skyrmions after the currents are turned off, thus enhancing the position stability.…”

Systematic Control of Ferrimagnetic Skyrmions via Composition Modulation in Pt/Fe1–xTbx/Ta Multilayers

“…However, ferromagnetic skyrmions show an undesired skyrmion Hall effect, leading to an additional transverse motion toward the edges of the nanotrack, which is believed to be detrimental for applications such as skyrmion racetrack memory. , It is suggested that the skyrmion Hall effect can be suppressed in ferrimagnet-based multilayers. − Specifically, ferrimagnetic skyrmions with reduced net magnetization exhibit a nearly vanishing skyrmion Hall angle and fast current-driven dynamics. , Along this intriguing direction, important progress has been made. − …”