Two-Level Fluctuator Induced Magnon Damping in Aluminum-Doped Lithium Ferrite

Low magnetic damping is essential for coherent magnonic and spintronic applications, where long-lived spin excitations enable efficient information processing. Aluminum-doped lithium ferrite Li_0.5(Al_xFe_2.5-x)O₄ (LAFO) has recently emerged as a promising magnetic insulator alternative to yttrium iron garnet due to its ultra-low magnetic damping, tunable magnetic anisotropy, and compatibility with thin-film growth.¹ While various sources of magnon loss can broaden the ferromagnetic resonance (FMR) linewidth and reduce the spin diffusion length, conventional damping mechanisms cannot account for their anomalous temperature dependence. This anomalous temperature dependence suggests the presence of microscopic two-level systems (TLS) that couple to magnonic modes and induce magnon damping.² In this work, we identify magnetic impurities as two-level fluctuators that couple to the magnon modes and examine their role in contributing to the FMR linewidth and in limiting the spin diffusion length in LAFO. Our results highlight the critical role of TLS in governing low-temperature magnon dissipation, providing microscopic insight into engineering low-loss materials for magnonic and spintronic applications.

1X. Y. Zheng et al. Nat Commun 14, 4918 (2023)

2J. H. Van Vleck et al. Rev. Lett. 11, 303 (1963)