Photoinduced Magnetic Effects in Magnetic Rare Earth Oxides

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In single crystals of yttrium iron garnet irradiated with laser (the most effective wavelength is 700 nm) or white light at 77K,double peaks of DA around 140K and 210K below room temperature were found to be induced with correlative onset of the irreversible decrease of permeability insensitive to demagnetization at low temperature. These crystals were grown by a modified floating zone method. These is very pure, with neither Si nor Ca incorporated except for intentional dopants (Ga 0.0625 mol or Ca 0.001 mol). In all samples a considerable oxygen vacancy is incorporated of the order of 0.1 mol %. These phenomena are observable below 250K,while the original state may be restored by warming the sample to room temperature. Reversible and irreversible components of the temperature dependence of complex permeabilities mu' and mu" shows a thermal hysteresis. In several samples, an additional DA peak independent of irradiation is observed around room temperature. These results are interpreted within the context of configuration coordinate (CC) diagrams with large lattice relaxations. ; the basic concept of our explanation of both processes is as follows. Cooled in the dark, oxygen vacancies or deep donors capture the double electrons and so called F-centers are formed. Pinninng centers for domain walls is created with negligibly small capture rates, when the trapped electrons are photoexcited from the deep levels to the shallow levels or conduction band. An origin of DA will be considered with the main emphasis on a charge dependent mobility of oxygen vacancies.