Efficient Generation of Second-Harmonic Propagating Spin Waves in a Thin, Out-of-Plane-Magnetized Ferromagnetic Film

Spin waves are attractive information carriers owing to their gigahertz-to-terahertz frequencies, nanometric wavelengths, and negligible Joule heating. Yet the efficient excitation of short-wavelength, high-frequency spin waves and the exploitation of nonlinear effects remain challenging. We propose a hybrid ferromagnetic nanostructure composed of a small, in-plane-magnetized rim (a magnonic nanocavity) exchange-coupled to an out-of-plane-magnetized region. Micromagnetic simulations show that a spatially uniform out-of-plane microwave field excites the rim's fundamental mode; its second harmonic is then coherently and efficiently launched into the second region of the structure, yielding propagating spin waves. The process can be realized in strip or disk geometries, providing excitation of plane-wave or radial spin waves, respectively. The conversion efficiency grows nonlinearly with the pump amplitude and can be further improved when the frequency of a higher-order standing wave in the nanocavity matches the second-harmonic frequency. The emission frequency is tunable via the bias magnetic field or the width of the nanocavity, suggesting a compact route toward on-chip, short-wavelength, high-frequency spin-wave sources for artificial neural networks.