Direct Observation of Tunable Magnons in Epitaxial Lithium Aluminum Ferrite Thin Films.

Ferrimagnetic insulators offer a promising route toward low-loss magnonic devices. Here, we report the first direct observation of magnons in high-quality epitaxial lithium aluminum ferrite (LAFO) thin films using inelastic Brillouin light scattering. We observe remarkably sharp magnon resonances with instrument-limited line widths whose frequencies can be tuned by film thickness and applied magnetic field (<300 mT) across a broad 8-28 GHz range. The film thickness dependence confirms that they are discrete standing-wave magnons confined in the direction perpendicular to the film. The field-dependence of the magnon frequencies allows us to extract the exchange constant and magnetic anisotropy. Electronic structure calculations reveal the microscopic origin of the ferrimagnetic order. The insights into magnons in LAFO establish the low-damping spinel ferrite, with its controllable dynamic magnetic properties, as a versatile and Si-compatible platform for future magnonic technologies.