Topological transformation of synthetic ferromagnetic skyrmions: thermal assisted switching of helicity by spin-orbit torque

Abstract

Abstract Electrical manipulation of topological spin textures, such as magnetic skyrmions, and their transitions between different topological states have attracted significant attention due to their potential applications in future spintronic devices. The helicity of a magnetic skyrmion, a crucial topological degree of freedom, is usually determined by the Dzyaloshinskii-Moriya interaction (DMI). Although there are methods to facilitate helicity flipping by choosing materials that lack DMI, in these materials, helicity reversal tends to occur in a random manner, which makes it unsuitable for practical applications. As of now, controlling the helicity of a skyrmion remains a challenging task. In this work, we successfully demonstrate a controllable switching of the helicity of skyrmion using spin-orbit torque, aided by thermal effects. When electric current pulses are applied to a magnetic multilayer stripe consisting of [Pt/Co]3/Ru/[Co/Pt]3, we observe that skyrmions move in the direction opposite to the current. Upon continuously applying pulses, we observe an unexpected reversal in the motion direction of the particles. Our investigation, which includes both experimental and micromagnetic simulation analyses, reveales that skyrmions in the upper and lower ferromagnetic layers of our multilayers exhibit distinct helicities, resulting in the formation of a hybrid synthetic ferromagnetic (SF) skyrmion. We discover that as Joule heating builds up during the current application process, the spin-orbit torque disrupts the balance between various energy factors, including DMI, Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, dipolar interaction, and others. This disruption leads to a helicity flip in the skyrmions, causing a sudden reversal in their motion. Our findings pave the way for new methods to control skyrmion helicity, offering enhanced versatility for future spintronic devices, such as advanced data storage systems and quantum computation technologies, that rely on skyrmion helicity.