Magnetic Polarization of the Americium $J=0$ Ground State in ${\mathrm{AmFe}}_2$

Trivalent americium has a nonmagnetic ($J=0$) ground state arising from the cancellation of the orbital and spin moments. However, magnetism can be induced by a large molecular field if ${\mathrm{Am}}^{3+}$ is embedded in a ferromagnetic matrix. Using the technique of x-ray magnetic circular dichroism, we show that this is the case in ${\mathrm{AmFe}}_2$. Since $⟨J_z⟩=0$, the spin component is exactly twice as large as the orbital one, the total Am moment is opposite to that of Fe, and the magnetic dipole operator $⟨T_z⟩$ can be determined directly; we discuss the progression of the latter across the actinide series.

Figure 1

Observed (circles), calculated (full red line), and difference (lower trace) x-ray diffraction pattern recorded at room temperature for the ${\mathrm{AmFe}}_2$ sample used in this study. Vertical ticks indicate the position of Bragg peaks. The broad peaks at low angles are due to the sample holder. Inset: cubic $C\text{−}15$ structure of the Laves phase ${\mathrm{AmFe}}_2$ (space group $Fd\overline{3}m$, room temperature lattice constant $a_0=7.300\AA$). Am atoms are represented by dark large spheres, and Fe atoms by smaller spheres.

Figure 2

Magnetization curve measured for ${\mathrm{AmFe}}_2$ at 10 K. The inset shows the temperature dependence of the magnetization $M$ (open circles) fitted by a $J=5/2$ Brillouin function (solid line), providing an estimate of the Curie temperature $T_C\sim 700\mathrm{K}$.

Figure 3

The x-ray absorption (XANES) and x-ray magnetic circular dichroism (XMCD) spectra as a function of photon energy through the Am $M_5$ and $M_4$ edges in ${\mathrm{AmFe}}_2$. The experiment was conducted at room temperature in an applied field of 3 T. The spectra have been corrected for self-absorption effects and incomplete circular polarization of the incident beam. The inset shows the XANES signal from the Np $M_4$ edge. ${}^{237}\mathrm{Np}$ is a decay product of ${}^{241}\mathrm{Am}$ and the gamma spectra from the sample showed this to be present at the $\sim 1\%$ level (Np in ${\mathrm{NpFe}}_2$ is strongly magnetic—see Ref. [14]).

Figure 4

XMCD spectroscopic shapes for the An $M_5$ and $M_4$ edges of ${\mathrm{AnFe}}_2$ compounds (Np through Am; ${\mathrm{NpFe}}_2$ and ${\mathrm{PuFe}}_2$ from Ref. [14], ${\mathrm{AmFe}}_2$—this work) with the energy of the $M_5$ edge taken as zero and the amplitude of the $M_4$ edge normalized to unity. Note that although for any one element the two signals are correctly represented, there is no scaling between the signals for different elements. For example, the signal for ${\mathrm{AmFe}}_2$ is much smaller in absolute terms than that found for the large Np moment in ${\mathrm{NpFe}}_2$. The narrow linewidth of the $M_4$ XMCD signal for Am in ${\mathrm{AmFe}}_2$ (about 50% of that found for the actinides in ${\mathrm{NpFe}}_2$ and ${\mathrm{PuFe}}_2$) is consistent with the assumption of localized $5f$ states, and found also in PuSb [29].

Figure 5

Ratio ($r$) between the expectation values of the magnetic dipole operator $3⟨T_z⟩$ and the spin operator $⟨S_z⟩$ as a function of the $5f$ shell occupation number $n_f$. Experimental data for different compounds are represented by symbols identifying their crystallographic structure (circles for $C\text{−}15$ cubic Laves phases, squares for the NaCl-type structure, and triangles for hexagonal lattice groups). Calculated values are shown for $LS$ (dashed line), $jj$ (dotted line), and IC (solid line). All the calculated values refer to the free-ion ground state, except for the $f^6$ configuration, where $J$ mixing with the first excited state is taken into account; otherwise, $r$ cannot be determined since $⟨T_z⟩=⟨S_z⟩=0$.