Spin-momentum locking breakdown of evanescent electromagnetic waves in a metal-gyromagnetic interface

Spin-momentum locking is widely regarded as an inherent property of evanescent waves, where the transverse spin angular momentum is intrinsically tied to wave's polarization. This principle is well-established in systems such as surface plasmon polaritons, surface elastic waves, and other evanescent modes. Here, we theoretically unveil an anomalous breakdown of spin-momentum locking in evanescent electromagnetic waves at a metal-gyromagnetic interface. We show that the hybrid polarization of the field induces two successive reversals of transverse spin near the interface ― directly violating the conventional locking between spin and momentum. As a result, identical chiral sources placed at different heights above the interface excite evanescent waves propagating in opposite directions, defying standard expectations. This discovery challenges the presumed universality of spin-momentum locking and opens new degrees of freedom for controlling wave propagation in photonic and plasmonic systems.