Ultrathin bismuth-yttrium iron garnet films with tunable magnetic anisotropy

We report on the epitaxial growth of nm-thick films of bismuth-substituted yttrium iron garnet (BiYIG) by high-temperature off-axis radio-frequency magnetron sputtering. We demonstrate accurate control of the magnetic properties by tuning of the sputtering parameters and epitaxial strain on various (111)-oriented garnet substrates. BiYIG films with up to \ensuremath{-}0.80% lattice mismatch with the substrate remain fully strained up to 60 nm thick, maintaining a high crystalline quality. Transmission electron microscopy and energy-dispersive x-ray spectroscopy confirm coherent epitaxial growth, the absence of defects, and limited interdiffusion at the BiYIG/substrate interface. Varying the tensile or compressive strain between \ensuremath{-}0.80% and $+0.56%$ in BiYIG allows for accurate compensation of the total magnetic anisotropy through magnetoelastic coupling. The effective magnetic anisotropy of sputtered BiYIG films can be further tuned via the off-axis deposition angle and the oxygen flow during growth, which determine the cation stoichiometry. Under optimized growth conditions, a ferromagnetic resonance (FMR) linewidth of 1 mT at 10 GHz is reliably obtained even for thicknesses as low as 10 nm. We also report small FMR linewidths in ultrathin (2--5 nm) BiYIG films grown on diamagnetic substrate yttrium scandium gallium garnet. These findings highlight the promise of low-damping, strain-engineered nm-thick BiYIG films for implementing advanced functionalities in spin-orbitronic and magnonic devices. Specifically, the magnetic-anisotropy compensation and low damping enable large cone-angle magnetization dynamics immune to magnon-magnon nonlinear scattering.