Title: Probing magnetism in the vortex phase of PuCoGa$_5$ by X-ray magnetic circular dichroism,
Abstract: We have measured 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 $^{242}$PuCoGa$_{5}$, exhibiting a critical
temperature $T_{c} = 18.7~{\rm K}$. The experiment probes the vortex phase
below $T_{c}$ and shows that an external magnetic field induces a Pu 5$f$
magnetic moment at 2 K equal to the temperature-independent moment measured in
the normal phase up to 300 K by a SQUID device. This observation is in
agreement with theoretical models claiming that the Pu atoms in PuCoGa$_{5}$
have a nonmagnetic singlet ground state resulting from the hybridization of the
conduction electrons with the intermediate-valence 5$f$ 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 5$f$ moment.