Atomic spin, molecular orbitals, and anomalous antiferromagnetism in insulating V2O3

Abstract

A theory of the orbital ordering and the anomalous antiferromagnetism in V2O3 is developed on the basis of a realistic description of the V3+ atomic states. The effective electronic degrees of freedom in the insulating phase are found to be successively reduced to a set of molecular orbitals of V pairs along the trigonal axis. We derive the molecular interactions for the lowest orbital doublet and analyze their possible ordered phases in analogy with the Kugel-Khomskii model for the cubic perovskites. It is shown that the complex spin structure of V2O3 is stabilized uniquely in a reasonable parameter region, and that it is associated with an unusual ferro-type orbital order involving the intramolecular correlation of V atomic orbitals. This characteristic orbital state is shown to be consistent with the monoclinic lattice distortion, the anisotropy of spin exchange couplings, and the spin orientation in the antiferromagnetic phase of V2O3. Based on those analyses, improved molecular orbital states are proposed and recent experiments on neutron and x-ray scattering are discussed.