Magnonics has emerged as a promising route toward next-generation information technologies beyond the scaling limits of Moore's law. Compared with the extensively studied yttrium iron garnet (YIG), thulium iron garnet (TmIG) has attracted growing interest for magnonics due to its ability to sustain robust perpendicular magnetic anisotropy (PMA). This work moves beyond the conventional strain-dominated picture of PMA by establishing a unified framework for magnetic anisotropy control in TmIG films, where epitaxial strain, oxygen vacancies, and cation stoichiometry are integrated as coupled factors. High-quality epitaxial TmIG films of varying thicknesses were deposited via magnetron sputtering on YSGG ($\eta =0.82\%$) and GGG ($\eta =0.49\%$) substrates. The results show that TmIG/YSGG films exhibit stable PMA, with the PMA strength reaching a maximum at 30 nm, whereas those grown on GGG exhibit dominant in-plane anisotropy because of insufficient tensile strain. Through a combination of transmission electron microscopy, magnetic characterization, and theoretical modeling, we further reveal that oxygen deficiency together with near-ideal cation stoichiometry enhances the effective magnetostriction coefficient $|{\lambda }_{111}|$ in TmIG/YSGG films, thereby strengthening the magnetoelastic contribution to PMA. These findings establish TmIG as a tunable PMA platform through coupled strain and defect chemistry, providing insights for engineering garnet-based magnonic materials.

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• Composition
• Defects
• Magnetic anisotropy
• Magnetism
• Stress