Photomagnetism in Ca2+Fe4+-doped yttrium iron garnet

Abstract

The temperature and time dependence of the initial susceptibility of the optical magnetization curve has been measured on epitaxial films of nominally pure yttrium iron garnet (YIG) and Ca: YIG, during illumination with white light after cooling in the dark. Depending on sample composition, the maximum temperature for the occurrence of the light-induced effects on the susceptibility is 100 K<T<150 K. The change to the low-susceptibility state by light excitation is a three-step process. The first process is characterized by time constants of the order of minutes, during the second process the susceptibility does not change, and finally, at 93 K after 4 h of illumination by 15 mW/cm2 light intensity the susceptibility of Ca0.11:YIG is observed to decrease to a saturation value in less than an hour. After turning off the light the susceptibility remains constant for about an hour, then increases with a similar time dependence. No light-induced effects were detected in YIG over the same range of temperature, time, or light intensity. The results are interpreted in terms of light excitation of holes from the tetrahedral Fe4+ sites nearest to the Ca2+ impurity first to orientationally inequivalent sites on the same coordination sphere, then to nonmagnetic centers with intermediate energies, and finally to the next-nearest tetrahedral Fe3+ sites of higher anisotropy energy. The energy difference between the two states is measured to be in the 3–10-meV range.