Magneto-rotation coupling dominates surface acoustic wave driven ferromagnetic resonance in the longitudinal geometry

Abstract

We present a phonon-magnon extension for the mumax+ micromagnetic framework that implements three surface acoustic wave (SAW) coupling mechanisms: magnetoelastic strain coupling, magneto-rotation coupling arising from the antisymmetric displacement gradient, and spin-rotation (Barnett) coupling from the lattice angular velocity. Six benchmark simulations validate the implementation through SAW-driven domain-wall motion, magnetization switching, magneto-rotation and Barnett field validation, nonreciprocal SAW-magnon absorption from Rayleigh-wave chirality, and spatially resolved coupling in a standing SAW cavity. For the longitudinal geometry (m_0 parallel to k_SAW), we show that the magnetoelastic coupling produces zero transverse torque despite generating a 50 times larger effective field; the magneto-rotation channel provides the sole driving mechanism. The crossover angle below which MR dominates is theta_c approximately 1.1 degrees for YIG parameters. Treating the magneto-rotation coupling constant K_mr as a tunable parameter, we map out the cooperativity phase diagram and show that MR alone can achieve strong coupling (C = 257 for K_mr = 1 MJ/m^3) with an avoided-crossing splitting of 13.6 MHz.

Figures (11)

• Figure 1: SAW-driven domain-wall motion benchmark. (a) Domain-wall displacement versus time for representative SAW strain amplitudes. (b) Peak domain-wall velocity versus $\varepsilon_0^2$; below threshold ($\varepsilon_0 \lesssim 10^{-3}$) the DW remains pinned, above threshold the velocity increases monotonically.
• Figure 2: SAW-assisted switching benchmark. (a) Time-domain switching trajectories for representative SAW amplitudes. (b) Switching phase diagram in the strain--assist-field plane.
• Figure 3: Magneto-rotation field validation. Numerical values of $\bm{H}_\mathrm{mr}$ (dashed) are compared with Eq. \ref{['eq:hmr']} (solid) for (a) $\bm{m} \parallel \hat{z}$, (b) $\bm{m}$ tilted $45^\circ$, (c) $\bm{m} \parallel \hat{x}$, and (d) short wavelength, showing near-exact agreement.
• Figure 4: Direction-dependent SAW--magnon coupling via magneto-rotation in PMA CoFeB. (a) $m_y(t)$ reverses sign for $+k$ vs. $-k$ propagation, reflecting the sign flip of the rotation pseudovector $\Omega_y$. (b) Transverse amplitude $|\delta m_\perp|$ is identical for both directions; the $K_\mathrm{mr}=0$ control (gray) confirms that MEL alone produces no torque for PMA.
• Figure 5: Barnett (spin-rotation) field validation. Numerical values of $\bm{H}_\mathrm{Barnett}$ (dashed) are compared with Eq. \ref{['eq:hbarnett']} (solid) for (a) default parameters, (b) higher velocity, (c) reduced $\gamma$, and (d) shorter wavelength.