Superconductivity, magnetic order, and quadrupolar order in the filled skutterudite system Pr1−xNdxOs4Sb12

Abstract

Superconductivity, magnetic order, and the high field ordered phase have been investigated in the filled skutterudite system Pr${}_{1\ensuremath{-}x}$Nd${}_{x}$Os${}_{4}$Sb${}_{12}$ as a function of composition $x$ in magnetic fields up to 9 T and at temperatures between 50 mK and 10 K. Electrical resistivity measurements indicate that the high field ordered phase, which has been identified with antiferroquadruoplar order, persists to $x$ $~$ 0.5. The superconducting critical temperature ${T}_{c}$ of PrOs${}_{4}$Sb${}_{12}$ is depressed linearly with Nd concentration to $x$ $~$ 0.55, whereas the Curie temperature ${T}_{\mathrm{FM}}$ of NdOs${}_{4}$Sb${}_{12}$ is depressed linearly with Pr composition to ($1\ensuremath{-}x$) $~$ 0.45. In the superconducting region, the upper critical field ${H}_{c2}(x,0)$ is depressed quadratically with $x$ in the range 0 $<$ $x$ $\ensuremath{\lesssim}$ 0.3, exhibits a kink at $x$ $\ensuremath{\approx}$ 0.3, and then decreases linearly with $x$ in the range 0.3 $\ensuremath{\lesssim}$ $x$ $\ensuremath{\lesssim}$ 0.6. The behavior of ${H}_{c2}(x,0)$ appears to be a result of pair breaking caused by the applied magnetic field and the exchange field associated with the polarization of the Nd magnetic moments in the superconducting state. From magnetic susceptibility measurements, the correlations between the Nd moments in the superconducting state appear to change from ferromagnetic in the range 0.3 $\ensuremath{\lesssim}$ $x$ $\ensuremath{\lesssim}$ 0.6 to antiferromagnetic in the range 0 $<$ $x$ $\ensuremath{\lesssim}$ 0.3. Specific-heat measurements on a sample with $x$ $=$ 0.45 indicate that magnetic order may occur in the superconducting state, as is also inferred from the depression of ${H}_{c2}(x,0)$ with $x$.