Transferred Hyperfine Interactions and Spin Deviations in Magnetic Salts

Abstract

Magnetic resonance measurements on transition metal salts show that there is an appreciable hyperfine interaction between the electronic spin of one metal ion and the nuclei of neighboring metal ions. Recently reported measurements of this effect are described, particularly ENDOR studies of the 27Al hyperfine interaction in the structures Fe3+–O2−–Al3+ and Cr3+–O2−–Al3+. Mechanisms which have been proposed for explaining these metal-to-metal hyperfine interactions are also discussed with particular reference to the structures just mentioned and to the work of Huang et al. (1967) on KMnF3 and MnO. The most important process is thought to be the transfer of unpaired spin from d orbitals on the magnetic ion to s orbitals on the neighboring metal ion, usually via covalent and overlap effects with an intervening ligand. One result of such transfers, as emphasised by previous authors, is that the hyperfine interaction for a magnetic ion in a dilute salt is expected to be different from that in a concentrated salt because of the presence of magnetic neighbors. This effect is at least partly responsible for the lack of agreement between theoretical predictions and experimental estimates of zero-point spin deviations in antiferromagnets. Some examples are discussed, including KMnF3, MnO, LaFeO3, and LaCrO3.