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

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Superconductivity, magnetic order, and quadrupolar order have been investigated in the filled skutterudite system Pr₁₋ₓNdₓOs₄Sb₁₂ as a function of composition x in magnetic fields up to 9 tesla and at temperatures between 50 mK and 10 K. Electrical resistivity measurements indicate that the high field ordered phase (HFOP), which has been identified with antiferroquadruoplar order, persists to x 0.5. The superconducting critical temperature Tc of PrOs₄Sb₁₂ is depressed linearly with Nd concentration to x 0.55, whereas the Curie temperature TFM of NdOs₄Sb₁₂ is depressed linearly with Pr composition to (1-x) 0.45. In the superconducting region, the upper critical field Hc₂(x,0) is depressed quadratically with x in the range 0 < x 0.3, exhibits a kink at x ≈ 0.3, and then decreases linearly with x in the range 0.3 x 0.6. The behavior of Hc₂(x,0) appears to be due to pair breaking caused by the applied magnetic field and the exhange 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 x 0.6 to antiferromagnetic in the range 0 < x 0.3. Specific heat measurements on a sample with x = 0.45 indicate that magnetic order occurs in the superconducting state, as is also inferred from the depression of Hc₂(x,0) with x.