Probing Magnetism in the Vortex Phase of ${\mathrm{PuCoGa}}_5$ by X-Ray Magnetic Circular Dichroism

We measure x-ray magnetic circular dichroism (XMCD) spectra at the Pu $M_{4,5}$ absorption edges from a newly prepared high-quality single crystal of the heavy-fermion superconductor ${}^{242}\mathrm{Pu}{\mathrm{CoGa}}_5$, exhibiting a critical temperature $T_c=18.7\mathrm{K}$. The experiment probes the vortex phase below $T_c$ and shows that an external magnetic field induces a Pu $5f$ magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a superconducting quantum interference device. This observation is in agreement with theoretical models claiming that the Pu atoms in ${\mathrm{PuCoGa}}_5$ have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence $5f$ electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the $5f$ magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local $5f$ moment.

Figure 1

Magnetic susceptibility in the normal state of ${\mathrm{PuCoGa}}_5$ measured with an applied field of 1 mT on warming the sample after zero-field cooling (filled black dots). The blue open circles represent values deduced from polarized neutron diffraction measurements [6]. Inset: temperature dependence of the magnetic susceptibility measured under zero-field cooling (filled black dots) and field cooling conditions (open red dots) in an applied field of 1 mT, providing $T_c=18.7\mathrm{K}$.

Figure 2

The x-ray absorption near-edge structure (XANES, solid black lines) and x-ray magnetic circular dichroism (XMCD) spectra as a function of photon energy through the Pu $M_5$ (red line) and $M_4$ (green line) edges in ${\mathrm{PuCoGa}}_5$.

Figure 3

Temperature dependence of the XMCD signal measured at the $M_4$ absorption edge of Pu for ${\mathrm{PuCoGa}}_5$, with a 17-T magnetic field applied along $c$. The red circles correspond to the integral of the XMCD spectra over the whole $M_4$-edge energy range, whereas the black triangles are the XMCD value measured at the $M_4$ peak energy (3968 eV). The line is a guide to the eye. Inset: XMCD spectra at the $M_4$-edge of Pu measured for ${\mathrm{PuCoGa}}_5$ (present work) and ${\mathrm{PuFe}}_2$ [24]. The spectra are normalized to their respective maxima. Note that the signal measured on ${\mathrm{PuFe}}_2$ was about 30 times larger.