Auto-oscillations and directional magnon emission induced by spin current injection into large magnetic volumes

The ability to manipulate magnons using electronic currents holds transformative potential for high-frequency signal processing architectures based on insulating magnetic materials. A critical challenge, however, lies in achieving efficient magnon emission and amplification through damping compensation, which typically requires ultra-thin films. In this study, we break this limitation by demonstrating a three-order-of-magnitude increase in magnon population, consistent with the onset of auto-oscillations upon reaching damping compensation, by injecting a spin current from a μm-wide Pt wire into a continuous 150 nm-thick yttrium iron garnet film. Using nonlocal magnon transport and Brillouin light scattering, we reveal that damping compensation occurs due to magnon self-localization beneath the Pt injector, which precludes radiation from the excited region. As a result, the nonlocal magnon conductance becomes mode-dependent and is significantly amplified by multi-magnon scattering at high magnon populations. Finally, we demonstrate that interfacial spin injection breaks yttrium iron garnet's inversion symmetry, leading to unidirectional magnon emission. Our results pave the way for the development of advanced magnonic devices, including directional magnon emitters, and offer a new approach to achieving damping compensation in thick magnetic films.