Angular variation of induced anisotropy: A new method for characterization of atomic centers in magnetic crystals

Variation of externally induced anisotropy, under rotation, is discussed as a sensitive method for investigating the symmetry properties of atomic centers. The high sensitivity to symmetry is accounted for by the second-or der character, with respect to site anisotropy, of the induced effects. Atomic centers having a weakly bound charge (electron or hole) constitute an interesting category to which the method is applicable. In these cases, the weakly bound charge can be transferred between sites under interaction with an external field, thus providing one of the general mechanisms for second-order induced anisotropy: the redistribution of active centers. The angular variation of induced anisotropy (shortly AVIA) is demonstrated in doped garnets, with a redistribution of centers such as the ones described above effected either by polarized electromagnetic radiation or an external magnetic field. In particular, the theoretical angular dependence of induced linear dichroism is considered for the three cases of centers occupy ing octahedrel, tetrahedrel or dodecahedral sites. The three crystallographic sites are found to exhibit markedly different behaviour, thus demonstrating the resolving power of this sensitive tool for site identification. Some applications to the interpretation of experimental AVIA curves of doped garnets are discussed.