Tuning skyrmion helicity for racetrack memory and quantum computing applications

Three distinct topological degrees of freedom are used to define all topological spin textures based on out-of-plane and in-plane spin configurations: the topological charge, representing the number of times the magnetization vector m wraps around the unit sphere; the vorticity, which quantifies the angular integration of the magnetic moment along the circumferential direction of a domain wall; and the helicity, defining the swirling direction of in-plane magnetization.

Electrical manipulation of these three degrees of freedom has garnered significant attention due to their potential applications in future spintronic devices. Among these, the helicity of a magnetic skyrmion—a critical topological property—is typically determined by the Dzyaloshinskii-Moriya interaction (DMI). However, controlling skyrmion helicity remains a formidable challenge.

A team of scientists led by Professor Yan Zhou from The Chinese University of Hong Kong, Shenzhen, and Professor Senfu Zhang from Lanzhou University successfully demonstrated a controllable helicity switching of skyrmions using spin-orbit torque, enhanced by thermal effects.

The work is published in the journal Nature Communications.

In their work, electric current pulses were applied to a magnetic multilayer stripe composed of [Pt/Co]₃/Ru/[Co/Pt]₃. The researchers observed skyrmion motion opposite to the current direction.

Upon continuous pulsing, an unexpected reversal in the particles’ motion direction was noted. Experimental and micromagnetic simulation analyses revealed that skyrmions in the upper and lower ferromagnetic layers of the multilayer system exhibit different helicities, forming a hybrid synthetic ferromagnetic skyrmion.

The team discovered that as Joule heating accumulates during the application of current, the spin-orbit torque disrupts the equilibrium among various energy interactions, including DMI, Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, and dipolar interaction. This disruption triggers a helicity flip in the skyrmions, resulting in the observed sudden reversal in their motion.

This work marks another significant advancement following the team’s earlier experimental observation of interconversion between different spin topological structures and the successful synthesis of artificial antiferromagnetic skyrmions.

These findings further validate the feasibility of early concepts based on spin topological structures, such as racetrack memory, logic circuits, microwave devices, and topological neuromorphic computing, which have become hot topics in the field of topological spintronics.

Professor Zhou said, “This study introduces innovative methods for controlling helicity, pushing the boundaries of spintronic device applications such as data storage and quantum computing based on skyrmion helicity.”