Infrared Sensitive Ferrimagnetics

Abstract

The relaxation and magnetic anneal of the magnetic anisotropy which are observed in silicon-doped yttrium iron garnet are summarized. These arise from centers in the crystals at which the magnetization is bound to a crystallographic cube body diagonal. Redistribution of centers between the four body diagonals causes the thermal relaxation of the magnetic anisotropy. At low temperatures T≲100°K, this relaxation becomes very slow and if the specimen is exposed to infra-red radiation the absorption of photons redistributes the centers, overriding the thermal effect; the anisotropy can thereby be controlled by radiation. By assuming that the absorption probability for a photon by a center depends upon the angles between (i) M0 and the body diagonal (ii) plane of polarization of the radiation and the body diagonal, we are able to account for the effect upon the anisotropy observed when a sample is exposed to plane polarized radiation while M0 is held in a fixed orientation. Radiation controlled changes of several hundred gauss in the anisotropy field are possible at low temperatures for samples of composition Y3Fe5−αSiαO12 with α≳0.03. The initial permeability and coercive field also show a large sensitivity to radiation at 77°K, but the sensitivity is greatest for α≲0.03 and the observed reduction in μ is not reversible following further radiation exposure in the way that the changes in anisotropy are. The detailed mechanisms of the μ and anisotropy sensitivity may be different, though both show greatest response to radiation of wavelength 1 to 2 μ. The nature of the centers referred to and their relaxation is discussed, but we are not confident that the simplest explanation, in terms of Fe2+ ions and valence exchange on octahedral sites, is yet confirmed. Technical application of the photosensitivity for radiation detection and information storage is mentioned briefly.