Neutron-Diffraction Study of Antiferromagnetism in UO2

Antiferromagnetism in U${\mathrm{O}}_2$ has been investigated in a detailed single-crystal neutron-diffraction study. Work on this material was prompted by a general interest in unpaired $5f$ and $6d$ electron distributions for different valence states of uranium in simple compounds. The form factor determined in the present case indicates a $5f^2$ electronic configuration with an effective moment slightly less than $1.8{\mu }_B$ for the ${\mathrm{U}}^{4+}$ ion. The magnetic ordering of the first kind, reported earlier by Henshaw and Brockhouse, is confirmed, but their proposed [111] orientation for the magnetization axis is not. The axis is found to be within the alternating ferromagnetic sheets. In the course of the diffraction study it was found that the paramagnetic-to-antiferromagnetic transition is an unusually sharp one, especially for such a simple magnetic system. Accordingly, the temperature dependence of magnetic neutron intensities was investigated through the Néel point. From the sharpness of the transition (50% of the magnetization is established within 0.03° below the Néel temperature), and the absence of a critical scattering peak, it is concluded that the transition is of the first order.