Unconventional energy-selective magneto-optical polarity reversal in thermal-mismatch-strained TmIG films

Hidden vertical inhomogeneity in ferrimagnetic oxide films can strongly reshape their energy-resolved magneto-optical response. Here, we show that thermal-expansion mismatch between thulium iron garnet, Tm3Fe5O12 (TmIG), and quartz produces different degrees of lattice distortion in nominally monolithic TmIG films. Cross-sectional transmission electron microscopy and Fe K-edge extended x-ray absorption fine structure (EXAFS) reveal that, compared with the 35 nm film, the 25 nm film exhibits enhanced interfacial lattice distortion and a broader Fe–O bond-length distribution. Energy-resolved magnetic circular dichroism (MCD) shows that the 25 nm film retains the principal TmIG spectral features but develops a pronounced polarity reversal and hump-like MCD–H loops in the 3–4 eV range. A constrained two-component decomposition reproduces the loop evolution and reveals the spectral superposition of vertically differentiated magneto-optical responses. EXAFS and x-ray absorption near-edge structure results support an interpretation based on distortion-induced redistribution of Fe(Td)- and Fe(Oh)-related contributions in the 25 nm film. These results establish thermal-mismatch strain as a route to photon-energy-selective magneto-optical polarity control in a nominally single-layer ferrimagnetic oxide.