Anisotropic magnon spin diffusion length in ultra-thin spinel ferrite thin films

Magnon-mediated spin transport in magnetically ordered insulators is of interest in the field of spintronics as it enables the transport of spin information with ultra-low-dissipation. Although the spin diffusion length, which is important to characterize spin transport, has been studied previously in yttrium iron garnet (YIG) films, its dependence on magnetic anisotropy remains largely unexplored due to the low in-plane magnetic anisotropy of YIG. Here we demonstrate anisotropic magnon spin transport in coherently-strained ultra-thin epitaxial films of magnesium aluminum ferrite (MgAl_0.5Fe_1.5O₄, MAFO) with low Gilbert damping and a thickness of 6 nm. Using nonlocal measurements with the spin polarization injected from Pt bars, we found a ~30 % enhancement of the spin diffusion length for propagation parallel to the easy axes as compared to the hard axes, for both electrically and thermally excited magnons. We correlate this anisotropy to the biaxial magnetic anisotropy in MAFO induced by tetragonal distortion.