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 the 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.