The NMR of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in Bi:YIG and La-YIG (where YIG is yttrium iron garnet) polycrystalline samples was measured between liquid-helium and room temperatures. In spectra of both tetrahedral and octahedral ${\mathrm{Fe}}^{3+}$ ions, satellite lines appear, corresponding to ${\mathrm{Fe}}^{3+}$ ions with ${\mathrm{Bi}}^{3+}$ or ${\mathrm{La}}^{3+}$ ions in their vicinity. The resonance frequency $f_{\mathrm{res}}$ of the satellites decreases with increasing temperature more slowly than $f_{\mathrm{res}}$ of the main lines. The different temperature dependences reflect the enhancement of the ${\mathrm{Fe}}^{3+}$-${\mathrm{Fe}}^{3+}$ superexchange interaction in the vicinity of the diamagnetic substitution. In the simplest model, the presence of ${\mathrm{Bi}}^{3+}$ or ${\mathrm{La}}^{3+}$ leads to two modified exchange integrals. Corresponding changes obtained by the molecular-field analysis are 4.5 and 1.6% for Bi:YIG, while they are 1.3 and 0.5% in the La-doped system. The smaller effect of the ${\mathrm{La}}^{3+}$ ion indicates that local deformation around the substitution plays only a minor role in the enhancement of the superexchange.

• Received 29 December 1987

DOI:https://doi.org/10.1103/PhysRevB.37.9712