Signatures of Auto-oscillation in a Ferrimagnetic Insulator with a Vertical Easy-plane Anisotropy

Driving large-angle magnetic auto-oscillations has attracted interest for applications in high-frequency signal generation. Demonstrations of current-driven auto-oscillation have been largely limited to ferromagnets, but in antiferromagnetically coupled multi-sublattice systems, auto-oscillation of individual sublattices can be excited by a different mechanism that relies on current-induced canting of the moments. Auto-oscillation is expected to occur in the terahertz regime in oxide ferrimagnets with strong superexchange coupling but has yet to be observed experimentally. Here we demonstrate the signature of auto-oscillation in a ferrimagnetic europium iron garnet thin film capped with Pt. The carefully engineered vertical easy plane of the garnet satisfies the geometrical requirements for excitation, and the uncompensated magnetic moment allows for convenient detection. The critical current density in the Pt to excite auto-oscillation in the garnet is 3×10¹⁰Am^-2, decreasing as the anisotropy energy barrier decreases according to theoretical predictions. Excitation of exchange-driven auto-oscillation in ferrimagnets away from angular momentum compensation opens up possibilities for a wide range of dynamical applications.