Dynamic helimagnetism via charged spin current

Abstract

We present a theoretical and numerical study of classical, localized spins coupled to conduction electrons that carry both charge and spin currents, deriving the effective equations of motion from the s-d exchange Hamiltonian. Applied to a periodically arranged one-dimensional spin chain, these equations yield two current-driven helical phases: a static helix stabilized by a pure spin current alone, and a coherently precessing helix that appears once a small charge-current component is added. For a given helical wavenumber, the precession angular frequency $ω$ and the canting ratio $Δ$ toward the hard axis follow analytically from the equations of motion, while the wavenumber $q_{\rm helix}$ of the precessing helix is selected by a heuristically constructed effective Lagrangian. Direct numerical integration of the same equations of motion reproduces all three observables wherever it yields a coherent helix. The helix chirality is set by a long-range, current-generated Dzyaloshinskii-Moriya (DM)-type coupling rather than an intrinsic DM interaction, so materials without bulk or interfacial DM interactions could also host current-controlled helical order. Because the helical pitch, precession frequency, and canting ratio are all continuously tunable by the current amplitudes, these helices could serve as probes of spin currents.