Probing magnetism in the vortex phase of PuCoGa$_5$ by X-ray magnetic circular dichroism

TL;DR

This study uses X-ray magnetic circular dichroism to investigate the magnetic properties of PuCoGa$_5$ in its vortex phase, revealing field-induced moments and temperature-dependent changes in the 5f electronic state.

Contribution

First direct XMCD measurements of PuCoGa$_5$ in the vortex phase, showing field-induced moments and temperature-dependent orbital and spin components of 5f electrons.

Findings

Pu 5f magnetic moment is induced by magnetic field at low temperature.

Orbital component of 5f moment increases significantly between 30 K and 2 K.

Evidence suggests a breakdown of Kondo screening at low temperatures.

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.