Theory of the Magnetic Properties of the Ilmenites MTIO3

摘要

Crystal-field, superexchange, and molecular-field theories have been used to analyze available experimental data for the ilmenites MnTiO3, FeTiO3, CoTiO3, and NiTiO3. The magnetic order and spin directions are shown to be compatible with negative trigonal fields in all compounds. Crystallographic considerations indicate that FeTiO3 has its negative trigonal field enhanced by spin-orbit coupling and by a large magnetic anisotropy orienting spins along the C sub h axis, even in the paramagnetic region. This invalidates the interpretation of the paramagnetic susceptibility of FeTiO3 by an isotropic model. MnTiO3 exhibits three anomalies: a reduced atomic moment (4.55 micron B) in the magnetically ordered state, a broad maximum in X sub m versus T above T sub N, and a discrepancy in the molecular-field exchange parameters obtained from high-temperature susceptibilities and low-temperature resonance with only dipole-dipole magnetic anisotropy. A similar but smaller discrepancy in exchange parameters was also found for NiTiO3. These difficulties disappear if exciton transfer between superexchange-coupled cations is introduced. This is a correlated superexchange involving the simultaneous transfer of electrons on neighboring cations to one another. (Author)