Emergence of Strong Exchange Interaction in the Actinide Series: The Driving Force for Magnetic Stabilization of Curium

Using electron energy-loss spectroscopy, many-electron atomic spectral calculations, and density functional theory, we show that angular-momentum coupling in the $5f$ states plays a decisive role in the formation of the magnetic moment in Cm metal. The $5f$ states of Cm in intermediate coupling are strongly shifted towards the $LS$ coupling limit due to exchange interaction, unlike most actinide elements where the effective spin-orbit interaction prevails. Hund’s rule coupling is the key to producing the large spin polarization that dictates the newly found crystal structure of Cm under pressure.

Figure 1

(a) The $N_{4,5}$ EELS spectra of Am and Cm metal acquired in a TEM. (b) A plot of $⟨w^{110}⟩/(14-n)-\Delta$ as a function of the number of $5f$ electrons ($n_f$). The three theoretical angular momentum coupling schemes are shown: $LS$, $jj$, and intermediate. Data from the experimentally measured branching ratios of each metal are indicated by points.

Figure 2

The atomic (a) spin magnetic moment $m_s=-2⟨S_z⟩=-2\sum _i{s}_{z,i}$ and (b) orbital magnetic moment $m_l=-⟨L_z⟩=-\sum _i{l}_{z,i}$ (in ${\mu }_B$) for the actinide elements against the number of $5f$ electrons ($n_f$). The total magnetic moment is $m_s+m_l$ (not shown). The three theoretical angular momentum coupling schemes in the many-electron configurations are shown in each plot: $LS$, $jj$, and intermediate coupling. (c) The electron occupation numbers $n_{5/2}$ and $n_{7/2}$ in intermedicate coupling as a function of the $n_f$. The $n_{5/2}$ and $n_{7/2}$ occupation numbers from the spin-orbit analysis of the EELS spectra are indicated by points.

Figure 3

Total energies for the five polymorphic phases of Cm (I-V) as a function of atomic volume for AF and NM calculations. NM fcc and dhcp are nearly degenerate with less than 0.001 Ry difference throughout the entire volume range. Dhcp is lower than fcc at volumes above $28{\AA }^3$. The vertical black lines indicate the experimentally measured phase transition volumes [12].