Anisotropy of the hyperfine field on Fe-57 in pure and substituted yttriumiron garnet

Using NMR, anisotropy of the hyperfine field on Fe-57 nuclei was measured i n Y3Fe5O12 with nonmagnetic, trivalent substitutions on d (Ga3+) and a (y(3 +)-yttrium antisite defect) sites which are occupied by ferric ions in a pu re, stoichiometric system. The measurements were per formed by the spin-ech o method at liquid helium temperature. Thorough analysis of the hyperfine f ield anisotropy on the Fe-57 nuclei in an ideal environment and in an envir onment where one of Fe3+ nearest cation neighbours is replaced by the nonma gnetic impurity is given. When the Fe3+ on which the NMR is measured is on the a site and the impurity is Ga3+(d), the results may he interpreted in t erms of the superposition model. On the other hand, the results for Fe3+ on the d site and Y3+(a) are in clear disagreement with the predictions of th is model.