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 transport of spin information with ultra-low-dissipation. Long-distance spin transport has been demonstrated previously in low-damping iron garnets with the film thickness typically above 100 nm. For this large thickness, strain relaxation in the film impedes the study of strain control of magnons. 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 thickness of 6 nm. Using nonlocal measurements with the spin polarization injected from Pt bars, we found a ~50 % enhancement of the magnon spin diffusion length for propagation in the [110] direction compared to [100], for both electrically and thermally excited magnons. We correlate this anisotropy to the biaxial magnetic anisotropy in MAFO induced by tetragonal distortion. Our finding suggests that epitaxial strain can be used to further engineer magnon spin transport.