Theory of Magnetic Properties of Actinide Compounds. I

UP, UAs, NpC, and crystals of other actinide compounds have metallic conductivity and are magnetically ordered. They exhibit discontinuous changes in the magnetic moment per actinide ion as a function of temperature. These are accompanied by large changes of the specific heat, magnetic susceptibility, and electrical conductivity, as well as in some cases by changes in the magnetic symmetry. The transition at the Néel temperature may also be of first order. A theory is developed that explains these phenomena quantitatively. Abandoning the concept of a specific configuration (valency) of the actinide ion, we show that the transition $5f^n\to 5f^{n-1\mathrm{}}{\mathrm{}(6d-7s)\mathrm{}}^1$ occurs for a certain fraction of the ions at different temperatures. The transition is induced by the combined effect of a small energy gap (or overlap) between the $5f$ localized and $6d-7s$ band states, the shielded and therefore short-range Coulomb interaction between band and localized electrons, and the variation of the exchange interaction between ions with the occupation of the band. The free energy is miminized with respect to the occupation numbers of the band states and of the magnetic sublevels of the ions in the two different configurations. This yields the temperature dependence of the band occupation and sublattice magnetization. The other observables are calculated from these quantities and fit the experimental data well.