Site occupancy and valence state of optically active cobalt ions in yttrium iron garnet

An extensive study is reported of the magneto-optical properties of several film and bulk samples of Co-doped yttrium iron garnet, in the wavelength range 0.65–2.4 μm. The (paramagnetic) nature of all the transitions occurring in this range has been established, and the optical parameters quantitatively determined. The attribution of various transitions to either trivalent or divalent Co has been given, and the (tetrahedral-) site occupancy of the ions involved directly proved by the angular variation of induced anisotropy technique, in several instances. Reducing annealings in hydrogen have been found to reversibly change the valence state of large amounts (up to 90%) of ${\mathrm{Co}}^{3+}$ into ${\mathrm{Co}}^{2+}$, with consequent strong variations of the associated magneto-optical properties. On the other hand, the annealing and quenching experiments aiming to increase the tetrahedral occupancy of Co ions proved ineffective.