Chiral Locking of Magnon Flow and Electron Spin Accumulation in Their Near-Field Radiative Spin Transfer

Abstract: We report a non-contact mechanism for directional injection of magnons in magnetic films when driven by a spin accumulation $\pmb{\mu}_s$ of electrons of a nearby metallic layer, governed by the long-range dipolar coupling between magnons and electron spins, which spontaneously generates a magnon current ${\bf J}_m$ flowing in the film plane. Crucially, in such near-field radiative spin transfer, the magnon flow ${\bf J}_m$ is always perpendicular to the spin accumulation $\pmb{\mu}_s$, showing a universal chiral locking relation. The spin injection is efficient even when $\pmb{\mu}_s$ is parallel to the magnetization, a feature breaking the limitation of the spin transfer by contact exchange interaction. Our findings reveal the critical role of dipolar chirality in driving the magnon thermal current and paving the way for the functional design of magnonic devices based on near-field radiative spin transfer.