Polarization-Dependent Photoinduced Effects in Silicon-Doped Yttrium Iron Garnet

Polarization-dependent effects of light on the magnetic properties of silicon-doped yttrium iron garnet are described theoretically. The effects are identified with a process wherein photons selectively detach electrons from orientationally inequivalent ${\mathrm{Fe}}^{2+}$ centers. A crystal-field theory (including cubic-, trigonal-, and nontrigonal-distortion crystal fields) and spin-orbit coupling and exchange, is used to compute the wave functions for the different centers. The wave functions of the inequivalent centers present quite different cross sections to incoming photons of a given polarization. With some simplifying assumptions, we compute the anisotropy of the photodetachment cross sections. It is found that prolonged irradiation along the [110] direction, for example, can cause a 5 to 10% imbalance in the distribution of electrons among centers. The uniaxial anisotropy and dichroism associated with this imbalance are calculated and compared with our measurements. Our experiments are arranged so as to characterize the polarization-dependent photoinduced uniaxial anisotropy and distinguish it from other anisotropies. For example, we find that a uniaxial anisotropy energy ${\mathcal{E}}_u=-{\gamma }_1{\gamma }_2\times (4.\mathrm{}1\pm 0.5)\times {10}^3$ erg ${\mathrm{cm}}^{-3\mathrm{}}$ is induced by polarization-dependent processes at 4.2°K in a sample in which 1% of the "$d$"-site Fe ions are replaced by Si. The experimental results can be explained by our theory in two ways. Either all ${\mathrm{Fe}}^{2+}$ centers contribute weakly to the effects or most centers are effectively inert and a few strong centers are responsible. These two alternatives are discussed in the light of previous observations of magnetic aftereffect and magnetic-resonance phenomena.