Anisotropy of the hyperfine field on 57Fe in pure and substituted yttrium iron garnet

Using NMR, anisotropy of the hyperfine field on Fe-57 nuclei was measured in Y3Fe5O12 with nonmagnetic, trivalent substitutions on d (Ga3+) and a (y(3+)-yttrium antisite defect) sites which are occupied by ferric ions in a pure, stoichiometric system. The measurements were per formed by the spin-echo method at liquid helium temperature. Thorough analysis of the hyperfine field anisotropy on the Fe-57 nuclei in an ideal environment and in an environment where one of Fe3+ nearest cation neighbours is replaced by the nonmagnetic 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 terms 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 this model.