NMR Study of the Low Temperature Phase of Fe₃O₄. I. Experiments

Abstract

The hyperfine field spectra of magnetite have been investigated at 4.2 K with and without magnetic cooling. From the spectra, it has been confirmed that three types of resonance lines exist and they are ascribed to Fe 3+ ( A ) ions, Fe 2+ ( B ) ions and Fe 3+ ( B ) ions. Fe 3+ ( B ) ions occupy five inequivalent positions of B sites. We have found that, with the application of the magnetic field, each Fe 3+ ( B ) line splits into two or four lines, even if the specimen has the fixed uniform a -, b - and c -axes. Angular dependence of the resonance frequencies of Fe 3+ ( B ) ions obtained by rotating external magnetic field was also observed. We have found that the splittings and the anisotropy of the spectra can be explained quantitatively by the symmetry of the site, which is approximately uniaxial along one of the <111> directions. For Fe 3+ ( B ) ions, the main part of the anisotropy can be explained by the magnetic dipole field of the surrounding Fe ions.