Theory of the First-Order Magnetic Phase Change in UO2

Abstract

A simple model accounts semiquantitatively for the first-order magnetic phase change observed recently in U${\mathrm{O}}_{2}$ by Frazer, Shirane, Cox, and Olsen. The model assumes that the electronic structure of the paramagnetic ${\mathrm{U}}^{4+}$ ion consists of a nonmagnetic singlet ground state and a low-lying magnetic triplet, and that only bilinear isotropic exchange interactions are present. In a molecular-field theory the triplet is split by an internal field proportional to the magnetization. If the molecular field is sufficiently strong, one of the components of the triplet will lie, in the magnetic state, below the singlet, and a self-consistent magnetic solution is obtained at $T=0$. Increasing the temperature causes the magnetization to be reduced, and the low-lying component of the triplet is raised in energy. It is shown that a "catastrophe" may occur at some critical temperature so that the magnetization is reduced discontinuously to zero. It is also found that, depending on the ratio of the singlet-triplet energy difference to the molecular-field splitting of the triplet, one obtains either no magnetic ordering, a first-order phase change, or a second-order transition.