arXiv subjects

Katya Mikhailova

Publications and source records attributed to Katya Mikhailova.

5 recordsLinked to original sources

arXiv2026

Divergence between long- and short-wavelength magnon damping in spinel ferrites

The realization of practical, high-speed magnonic devices requires engineering magnetic materials with low dissipation over wide frequency ranges. While optical and microwave probes are used to infer the damping of low energy/long wavelength modes, the degree to which these $q\sim0$ properties translate into higher-energy, finite-momentum modes remains an important open question. Here, we utilize a combination of ferromagnetic resonance (FMR) and resonant inelastic x-ray scattering on spinel ferrites Li$_{0.5}$Al$_x$Fe$_{2.5-x}$O$_4$ to probe magnons in both the short- and long-wavelength limits. We observe that aluminum substitution both markedly reduces the magnon bandwidth and drastically shortens the high-$q$ magnon lifetimes, in sharp contrast to the ultralow magnon damping inferred from FMR. These findings demonstrate a disparity between how non-magnetic substituents impact magnon damping in the long- and short-wavelength limits, providing a new perspective for assessing candidate materials for magnonic devices.

cond-mat.mtrl-sci↗

arXiv2026

Microscopic Understanding of Thermal-magnon Transport in a Low-damping Ferrimagnetic Thin Films

Thermally generated magnons enable heat-driven spin transport in magnetic insulators, yet the relative importance of multiple microscopic mechanisms governing their propagation remains incompletely understood. Here, we investigate thermal magnon transport in low-damping Li$_{0.5}$Al$_{1.0}$Fe$_{1.5}$O$_4$/Pt nanodevices using a nonlocal spin Seebeck geometry that separates magnon transport from local thermoelectric effects. Thermal imaging establishes a detector region outside the thermal healing length, enabling intrinsic nonlocal measurements. We find that thermal magnon transport is strongly suppressed by magnetic fields far above saturation, and further that thermal magnon transport decreases with increasing temperature despite an increasing magnon population. Brillouin light scattering reveals the key microscopic mechanism driving this effect: increasing field reduces the group velocity of backward volume magnons, directly reducing the magnon spin diffusion length. Micromagnetic simulations reproduce this behavior only when a temperature-dependent exchange stiffness is included. These results identify magnon group velocity and exchange stiffness as key parameters governing thermal magnon transport in ferrimagnetic thin films.

cond-mat.other↗

arXiv2026

Understanding Damping Mechanisms via Spin Diffusion Length in Low-damping Li$_{0.5}$Al$_{1.0}$Fe$_{1.5}$O$_4$ Spinel Ferrite Thin Films

The mechanisms underlying magnon damping are of fundamental and technological interest in low-damping materials. We find low-damping ferrimagnetic insulator Li$_{0.5}$Al$_{1.0}$Fe$_{1.5}$O$_4$ (LAFO) thin films to be a promising model system for probing these mechanisms because of its distinct temperature dependent spin diffusion length (SDL) trends for electrically and thermally generated magnons. With increasing temperature, the electrical SDL shows minimal change, while the thermal SDL decreases. We attribute these trends to distinct magnon populations and scattering mechanisms: thermally generated high $k$ magnons are limited by magnon-phonon scattering, whereas electrically generated low $k$ magnons are limited by relaxational scattering from magnetic impurities.

cond-mat.mtrl-sci↗

arXiv2026

Nearly Isotropic Magnon Transport in Epitaxial Lithium Aluminum Ferrite Thin Films

Low-loss magnetic insulating thin films are promising for information transport via magnons, where isotropic in-plane magnon propagation is desirable. We report nonlocal measurements of electrically and thermally generated magnons in epitaxial (001) lithium aluminum ferrite Li$_{0.5}$Al$_{0.7}$Fe$_{1.8}$O$_4$ thin films with pronounced fourfold in-plane magnetic anisotropy. By measuring the inverse spin Hall signal as a function of the magnon diffusion distance, we deduce magnon diffusion lengths that are nearly identical along the [100] and [110] directions at 250~K. This isotropy is consistent with a nearly isotropic exchange stiffness. These results highlight spinel ferrites as viable platforms for isotropic magnon transport.

cond-mat.mtrl-sci↗

arXiv2026

Low-temperature spin dynamics in LAFO thin films: from cubic anisotropy to TLS-limited coherence

We investigate the low-temperature spin dynamics of epitaxial lithium aluminum ferrite (LAFO) thin films using broadband ferromagnetic resonance (FMR) spectroscopy from 0.44 K to 68 K. The results reveal a crossover from conventional cubic anisotropy-dominated behavior at higher temperatures to pronounced linewidth broadening and higher-order anisotropy contributions at cryogenic temperatures. With the magnetic field oriented along the [100] crystallographic direction, the resonance is well-captured by four-fold in-plane and out-of-plane uniaxial anisotropies. In contrast, measurements with the field along the [110] direction reveal the presence of an unusually large sixth-order cubic anisotropy term that is symmetry-suppressed for [100] but becomes apparent under this field orientation at ultralow temperatures, indicating a substantial modification of the anisotropy landscape. Independent linewidth analysis shows a pronounced peak near 8 K and a subtle monotonic enhancement with decreasing temperatures below 2 K, features consistent with dissipation mediated by a bath of two-level systems (TLS) arising from antisite defects and localized Fe$^{3+}$ moments. Comparison with TLS-based models demonstrates that both exchange-coupled impurities and nearly free paramagnetic centers contribute to the observed damping. Our results establish LAFO as a model ferrite system where disorder-induced TLS limit spin coherence at ultralow temperatures, providing new insights into anisotropy engineering, magnetic relaxation, and the design of ferrimagnetic insulators for coherent magnonics. These findings offer a framework for future optimization of growth conditions.

cond-mat.mtrl-sci↗