Thermoreflectance studies of yttrium-iron garnet crystals

The thermoreflectance of YIG single crystals was measured in the visible energy range. Spectra of this compound support a model that incorporates both localised and one-electron band states. All structures of the thermoreflectance spectra exhibit temperature dependence. Those at low energies, which occur at 16390, 17220, 19321, 20350, 21450, 22410, 23609 and 24271 cm-1 and exhibit a small temperature shift and broadening, were assigned to crystal-field-split localised interionic transitions between the 3d states of Fe3+. The other structures, occurring at 25775, 27367, 30453, 32500, 35248, 39514 and 44202 cm-1 with a considerable temperature shift (between 2*10-3 and 8*10-4 eV K-1) and broadening were ascribed to pair transitions between the 3d states of iron and/or to charge-transfer transitions involving delocalised valence-band oxygen states and iron d-like bands.