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

TL;DR

This work demonstrates a non-contact, near-field mechanism for injecting magnons into a magnetic film via long-range dipolar coupling to a nearby metal’s spin accumulation ${\pmb \mu}_s$, generating a magnon current ${\bf J}_m$ that is universally perpendicular to ${\pmb \mu}_s$. By deriving the interlayer dipolar interaction and solving the kinetic equation, the authors reveal a chiral locking between ${\pmb \mu}_s$ and ${\bf J}_m$, with the injection efficiency and wave-vector selectivity governed by the dipolar scattering ${\bf g}(\mathbf{q})$. The findings indicate robust, non-contact spin transfer and a pathway to chiral magnon currents and non-reciprocal transport, offering new design rules for magnonic devices and potential magnetization-switching mechanisms without exchange transparency. The results are relevant for devices leveraging spin Hall-driven spin accumulations and extend the understanding of dipolar chirality in spin-transport phenomena.

Abstract

We report a non-contact mechanism for directional injection of magnons in magnetic films when driven by a spin accumulation $\pmbμ_s$ 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 $\pmbμ_s$, showing a universal chiral locking relation. The spin injection is efficient even when $\pmbμ_s$ 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.

Figures (6)

• Figure 1: Configuration for the chiral injection of magnons with $n_{\bf q}\ne n_{-{\bf q}}$ that generates the magnon current ${\bf J}_m$ flowing in the ferromagnet plane by nearby spin accumulations ${\pmb \mu}_s$ in metals. The red arrows indicate the spin polarization ${\pmb \mu}_s$ of electrons, while the blue arrows represent the magnon wave vector ${\bf q}$. An in-plane static magnetic field $H_0\hat{\bf z}$ is applied.
• Figure 2: Dipolar-field distribution and its circular polarization $\hat{\bf S}_T$ for the spin waves in the Damon-Eshbach [(a) and (b)] and bulk-volume [(c) and (d)] configurations.
• Figure 3: Wave-vector dependence of the interlayer dipolar coupling (unit: $s^{-1}$) between magnons in CoFeB thin film of thickness $d=100$ nm and electron spins in the metallic film of thickness $s=100$ nm. (a)-(d) plots $|g_x({\bf q})|$, $|g_y({\bf q})|$, $|g_z({\bf q})|$, and $|{\bf g}({\bf q})|$, respectively.
• Figure 4: Distribution of injected magnons $\delta n_{\bf q}$ in the Brillouin zone when biased by different spin-accumulation directions $\phi=\pi$ [(a)], $3\pi/4$ [(b)], $\pi/2$ [(c)], and $0$ [(d)].
• Figure 5: Driven magnon flow ${\bf J}_m$ as a function of the in-plane spin-accumulation directions $\phi$ with respect to the saturation magnetization direction. ${\bf J}_{m,\parallel}$ and ${\bf J}_{m,\perp}$ are the parallel and perpendicular components of ${\bf J}_m$ with respect to the spin-accumulation direction $\hat{\bf n}$.