Magnetic behavior of curium dioxide with a nonmagnetic ground state

In order to understand magnetic behavior observed in CmO${}_2$ with a nonmagnetic ground state, we numerically evaluate magnetic susceptibility on the basis of a seven-orbital Anderson model with spin-orbit coupling. Naively we do not expect magnetic behavior in CmO${}_2$, since Cm is considered to be a tetravalent ion with six $5f$ electrons and the ground state is characterized by $J=0$, where $J$ is total angular momentum. However, there exists a magnetic excited state and the excitation energy is smaller than the value of the Landé interval rule due to the effect of crystalline electric-field potential. Then, we open a way to explain magnetic behavior in CmO${}_2$.

Figure 1

Excitation energy $\Delta E$ vs spin-orbit coupling $\lambda$ for $n=6$. Thick solid curves denote the calculated results with different values of the CEF potential $W$. Broken lines indicate the two limiting situations as $\Delta E=\lambda /6$ for small $\lambda$ and $\Delta E=7\lambda /2+\eta$ for large $\lambda$, where $\eta$ is the energy shift depending on the CEF potential.

Figure 2

Curie constant $C$ vs temperature $T$ for various values of $\lambda$.

Figure 3

Inverse susceptibility $1/\chi$ vs temperature $T$ for various values of $\lambda$. The CEF potential is set as $W=-105$ meV.