Magnetic susceptibility and spin fluctuations in the nonsuperconducting phase of PuCoGa$_{5}$

Within the Hubbard model with allowance for Hund's and spin–orbit interactions, the concepts of thermal spin fluctuations in a strongly correlated system of $f$ electrons in PuCoGa$_5$ are developed. The effect of fluctuations of the spin magnetic moments of different orbitals on the spin–orbit splitting of the $f$ electron spectrum is considered. The mean square magnetic moment obtained in this case makes it possible to describe the observed temperature dependence of the magnetic susceptibility $(\chi(T))$ of PuCoGa$_5$ near and above the electron pairing temperature. Estimates of the radius of spin correlations correspond in order of magnitude to the sizes of Cooper pairs in superconductors with the $d$ symmetry of the order parameter. In the high-temperature region, the magnetic susceptibility obeys the Curie–Weiss law. With decreasing temperature, when approaching the pairing temperature, the ferromagnetic instability is suppressed and the temperature maximum of $\chi(T)$ appears.