Steering Magnetic Skyrmions with Currents: A Nonequilibrium Green's Functions Approach

Magnetic skyrmions, topologically protected vortex-like configurations in spin textures, are of wide conceptual and practical appeal, notably in relation to the making of so-called race-track memory devices. Skyrmions can be created, steered, and destroyed with magnetic fields and/or (spin) currents. Here the authors focus on the latter mechanism, analyzed via a microscopic treatment of the skyrmion–current interaction. The system considered is an isolated skyrmion in a square-lattice cluster, interacting with electron spins in a current-carrying quantum wire. For the theoretical description, a quantum formulation of spin-dependent currents via nonequilibrium Green's functions (NEGF) within the generalized Kadanoff–Baym ansatz (GKBA) is employed. This is combined with a treatment of skyrmions based on classical localized spins, with the skyrmion motion described via Ehrenfest dynamics. With the mixed quantum–classical scheme, the authors assess how time-dependent currents can affect the skyrmion dynamics, and how this in turn depends on electron–electron and spin–orbit interactions in the wire. This study shows the usefulness of a quantum–classical treatment of skyrmion steering via currents, as a way for example to validate/extract an effective, classical-only, description of skyrmion dynamics from a microscopic quantum modeling of the skyrmion–current interaction.