Isotropic Spin Transport in Epitaxial Lithium Aluminum Ferrite Thin Films

Ferromagnetic insulating thin films are key materials for investigating magnon transport owing to their intrinsically low magnetic damping. Such films are promising for the realization of magnonic waveguides, where isotropic in-plane magnon propagation is particularly desirable. Here, we report non-local spin transport measurements based on spin Hall magnon injection and detection in epitaxial (001) lithium aluminum ferrite, Li_0.5Al_0.7Fe_1.8O₄ (LAFO) films. By measuring the inverse spin Hall signal as a function of the distance of magnon propagation, we deduce a magnon diffusion length of 3.1±0.2 µm along the [100] crystallographic direction and of 2.9±0.1 µm along the [110] direction at 250 K. Moreover, we find that the magnon diffusion lengths along the [100] and [110] directions are nearly identical at 30 K as well, despite the presence of increasing fourfold in-plane magnetic anisotropy. This demonstrates that magnetic anisotropy does not necessarily give rise to anisotropic spin diffusion. We suggest that the observed isotropic spin diffusion can be explained by an isotropic exchange stiffness. These results highlight that isotropic magnon transport can persist in LAFO and possibly other magnetically anisotropic spinel structure ferrites, broadening the range of viable materials for magnonic waveguides.