Magnetic Susceptibility of Multiorbital Systems

TL;DR

This paper develops a mean-field theory to analyze how orbital degeneracy influences magnetic susceptibility in multiorbital systems, explaining deviations from Curie-Weiss behavior observed in certain f- and d-electron materials.

Contribution

It introduces a model accounting for independent spin and orbital moments, leading to a sum of two Curie-Weiss relations, and applies this to explain experimental differences in actinide oxides.

Findings

Magnetic susceptibility can be expressed as a sum of two Curie-Weiss laws.

Deviations from Curie-Weiss law occur due to orbital degeneracy effects.

The theory explains the different temperature behaviors of UO₂ and NpO₂.

Abstract

Effects of orbital degeneracy on magnetic susceptibility in paramagnetic phases are investigated within a mean-field theory. Under certain crystalline electric fields, the magnetic moment consists of two independent moments, e.g., spin and orbital moments. In such a case, the magnetic susceptibility is given by the sum of two different Curie-Weiss relations, leading to deviation from the Curie-Weiss law. Such behavior may be observed in d- and f-electron systems with t_{2g} and Gamma_8 ground states, respectively. As a potential application of our theory, we attempt to explain the difference in the temperature dependence of magnetic susceptibilities of UO_2 and NpO_2.