Yttrium iron garnet (YIG, ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$) thin films are promising material systems for quantum magnonics due to their ultralow magnetic damping and the capability of building on-chip spin wave devices to be coupled with superconducting microwave circuits. However, the default substrate for epitaxial YIG thin-film growth, ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (GGG), suffers from strong microwave absorption at cryogenic temperature due to the magnetic moment induced by the rare-earth element Gd. In this work, we study strained epitaxial YIG thin films grown on ${\mathrm{Y}}_3{\mathrm{Sc}}_2{\mathrm{Ga}}_3{\mathrm{O}}_{12}$ (YSGG), a rare-earth-free substrate compatible with cryogenic applications. Using cross-sectional scanning transmission electron microscopy along with strain mapping, we show the existence of spot-shape domain defects in the epitaxial YIG films and their connection with strain and stoichiometry, leading to drastically different magnetic properties such as perpendicular magnetic anisotropy, magnetic hysteresis loop, and magnetic damping. Applying high-resolution orientation mapping, we identify localized lattice rotations as the structural origins of these domains. We hypothesize that these domains are formed by a self-composed off-stoichiometry during the growth of the YIG thin films, as a mechanism to accommodate the relatively large lattice mismatch between YIG and YSGG. By elucidating the structure-property relationship in YIG/YSGG thin films, this work highlights a strain accommodation mechanism in complex oxides and provides guidance for future works on the applications of strained YIG thin films in quantum information.

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• Magnetic anisotropy
• Magnetic thin films
• Electron diffraction
• Scanning transmission electron microscopy