Optical properties of epitaxial iron garnet thin films

Optical absorption, derivative absorption, reflectivity, and derivative reflectivity data are presented for several iron garnet thin-film compositions grown by liquid-phase epitaxy. In the case of yttrium iron garnet (YIG), the real and imaginary parts of the dielectric response function $\epsilon ={\epsilon }_1+i{\epsilon }_2$ are compared with previously published results for bulk crystals and shown to be substantially larger. Based on absorption and reflectivity data for iron garnet films in which the tetrahedral and octahedral iron sublattices have been selectively diluted with gallium or scandium as well as measured absorption data for bulk crystals of yttrium gallium garnet dilutely doped with ${\mathrm{Fe}}^{3+}$, we propose an Adler-Feinleib split-density-of-states diagram for YIG. The main results include a series of weak crystal-field transitions between 1.4 and 3.4 eV, a pair of much stronger transitions at 2.9 and 3.2 eV which we tentatively associate with excitons of the configuration $2p^{5}3d^6$, strong $2p\to 3d$ charge transfer bands above 3.4 eV where the final states are probably itinerant-one-electron band states, and very strong semiconductor like $2p\to 4s$ optical transitions above about 8 eV which make the major contributions to the low-frequency refractive index. Some of the above assignments are defended by comparing optical properties of YIG, NiO, and SrTi${\mathrm{O}}_3$.