Effect of Impurities on the Optical Properties of Yttrium Iron Garnet

The optical absorption spectrum of single-crystal YIG is dominated at the short wavelengths by electronic transitions due to iron ions. For the long wavelengths, it is dominated by lattice vibrational transitions. Between about 1 μ and 6 μ there is a region of great transparency which can be modified in several ways. The valence of the iron ions can be altered by heat-treatment of crystals in oxidizing or reducing atmospheres, or by doping with divalent or tetravalent ions. The altered valence leads to increased optical absorption, as well as increased electrical conductivity. Foreign ions such as cobalt or the rare earths may have characteristic absorption lines in the region of transparency, and when yttrium is completely replaced by a rare earth, the short-wavelength edge absorption may be shifted appreciably due to lattice expansion or contraction. Another anomaly in the transparent region of YIG is found in hydrothermally-grown crystals, where OH-group stretching vibrations occur.