Spin Orbit Torque Dynamics of Magnetic Skyrmions in GdCo Ferrimagnetic Thin-Films

Abstract

Magnetic skyrmions are a peculiar kind of chiral magnetic textures stabilized by chiral interactions like the Dzyaloshinskii-Moriya interaction (DMI) [1]. Their topological properties offer interesting robustness qualities but, as a counterpart, add a complexity to their dynamics. Since their first observation in a bulk chiral magnet in 2009 [2], rapid experimental advances demonstrated their stability at room temperature without any magnetic field in ferromagnet/heavy metal bilayers and multi-stacks [3]. The experimental demonstrations of smaller and smaller skyrmions at room temperature (down to few 10s of nm) suggest that they are the perfect candidates for information storage and logic devices. Before building such devices, we first need to master skyrmions' static and dynamical properties, namely the nucleation and stabilization of small skyrmions that can move at high speed inside tracks.One significant issue with skyrmion dynamics is the topological deflection. Also known as Skyrmion Hall effect, it is caused by the topological charge of skyrmion and consists in a deflecting force proportional and perpendicular to the skyrmion velocity. Deflection can lead to the expulsion of skyrmions from the tracks that contain them, and thus imposes a limit to the skyrmion velocity. Experiments and calculations found that the deflection angle varies with skyrmion velocity and diameter probably due to its interaction with pinning disorder [4].It has been suggested that antiferromagnetically-coupled systems, such as antiferromagnets, synthetic antiferromagnets (SAFs) or ferrimagnets, may be used to overcome these issues. The low net magnetization of these systems reduces the size of the skyrmions, and their low angular momentum density should suppress the troublesome topological deflection.Ferrimagnetic Rare-Earth/Transition Metal alloys possess two anti-parallel sublattices whose relative magnetic moments can be changed by temperature or alloy composition. Ferrimagnets share properties from ferromagnets and antiferromagnets (at the magnetic compensation) with which they present many similarities. They are perfect systems to study the advantages of antiferromagnetically-coupled materials, while preserving the effects that have already been used to stabilize and drive skyrmions in ferromagnets.Recent results on SAF showed the promising advantages of using multi-lattices coupled antiferromagnetically [5], and in 2018 a huge breakthrough was achieved in Rare-Earth/Transition Metal ferrimagnets where the smallest skyrmions were observed (10-30nm) [6] as well as a reduced topological deflection [7].Using co-evaporation in Ultra-High-Vacuum [8], we grew ferrimagnetic thin film of Ta(1nm)/Pt(5nm)/Gd0.3Co0.7(5nm)/Ta(5nm) with an amorphous structure that decreases pinning of skyrmions. By optimizing the material stack and the process conditions, we were able to strengthen the interfacial DMI while keeping a balanced interfacial anisotropy to obtain out of plane magnetized samples (around room temperature) that can host skyrmions.We then observed the skyrmions by Magneto-Kerr Microscopy with sizes smaller than the technique’s resolution limit (around 500 nm) (Fig. 1). After saturating the sample, we were able to nucleate skyrmions in the tracks and to tune their overall density by applying magnetic fields. Skyrmions were also nucleated on the edges of the track by nanosecond-long current pulses. Thanks to the tunability of the magnetic properties of the ferrimagnet, we studied the sample in a range of magnetic field and temperature (0–40 mT, and 295–320 K). We observed different regimes: saturated magnetic textures, skyrmions with different densities, and stripes textures. Skyrmions were observed between ~295-310 K. The field necessary for nucleating skyrmions increased with temperature (8 mT at 295 K, 15 mT at 305 K), probably due to a decreasing Ms (as the compensation temperature of this sample is above the tested range).We also observed their dynamics under the application of nanosecond current pulses. The current going through the adjacent Pt layer generates a spin orbit torque (SOT) due to the spin Hall effect, which drives the skyrmions. We observed speeds higher than 150 m/s (for ~10 GA/m2) in a denser skyrmion regime and up to 40 m/s in the most diluted regime (Fig. 2), limited for higher current density by edge nucleations. The speed seems to follow a linear dependence with current. In the conditions of higher density, the velocity was higher and the depinning current lower, probably due to a change of the skyrmion diameter. We also studied the evolution of the deflection angle of the skyrmions, which increased from 40° at low current to finally saturate around 60°.This work shows that ferrimagnetic skyrmions can be nucleated, stabilized and propagated by SOT in amorphous rare earth/transition metal thin films. In these results, we stabilized skyrmions far away from the magnetic compensation of the sample (also far away from the angular compensation). We have since prepared new samples with accessible magnetic and angular compensation in order to study the dynamical properties of our sub-micron skyrmions in these very peculiar regimes. **