Surface acoustic waves can drive ferromagnetic resonance — the collective precession of magnetic moments in a ferromagnet. The standard explanation invokes magnetoelastic coupling: the acoustic wave strains the lattice, the strain modifies the magnetic anisotropy through magnetostriction, and the modified anisotropy exerts a torque on the magnetization. This magnetoelastic mechanism produces an effective field 50 times larger than any competing effect.

The effective field doesn’t matter. In the longitudinal geometry — where the acoustic wave propagates along the magnetization direction — the magnetoelastic coupling produces zero transverse torque despite its enormous effective field. The field is parallel to the magnetization; parallel fields don’t precess. The 50x advantage is irrelevant because the force pushes in a direction that can’t do work.

The sole driving mechanism is magneto-rotation coupling. The acoustic wave’s shear component rotates the lattice locally, and this rotation drags the magnetization through angular momentum transfer. The magneto-rotation effective field is 50 times smaller than the magnetoelastic one, but it points in the transverse direction — perpendicular to the magnetization, where it can drive precession.

A crossover angle of approximately 1.1 degrees separates the regimes. Below this angle (close to longitudinal geometry), magneto-rotation dominates entirely. Above it, magnetoelastic coupling contributes increasingly. For yttrium iron garnet at typical parameters, treating the magneto-rotation coupling as tunable achieves a cooperativity of 257 — deep in the strong-coupling regime, with avoided-crossing splitting of 13.6 MHz.

The strongest force was pushing in the wrong direction. The weakest force, aimed correctly, did all the work.