Exploring low-temperature spin dynamics in the doped spinel ferrite Li_0.5AlFe_1.5O₄ (LAFO)

Epitaxial ferrimagnetic insulators are promising platforms for low-dissipation spin transport and coherent magnonics, but their cryogenic performance is often limited by poorly understood spin relaxation. We investigate low-temperature spin dynamics in epitaxial thin films of lithium aluminum ferrite (LAFO) using broadband ferromagnetic resonance (FMR) spectroscopy from 68 K to 0.44 K. LAFO's tunable spinel structure—with magnetocrystalline anisotropy controlled via Al substitution—provides a platform to study intrinsic magnetic properties and defect-driven damping. We measure FMR with the magnetic film aligned along both the [100] and [110] axes. We observe changes in anisotropy and linewidth broadening as a function of temperature, indicating the role of antisite defects as fluctuating spin centers. The emergence of non-monotonic linewidth evolution at low temperatures suggest coupling between magnons and two-level systems (TLS), revealing a previously unexplored damping mechanism in disordered ferrites. These results position LAFO as a model system for studying spin coherence in complex oxides and highlight the importance of defect engineering in low-loss magnetic materials. A companion talk will discuss theoretical modeling of TLS-induced damping.