  Superconductivity, magnetic order, and quadrupolar order have been
investigated in the filled skutterudite system
Pr$_{1-x}$Nd$_{x}$Os$_4$Sb$_{12}$ 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 $x$
$\sim$ 0.5. The superconducting critical temperature $T_c$ of PrOs$_4$Sb$_{12}$
is depressed linearly with Nd concentration to $x$ $\sim$ 0.55, whereas the
Curie temperature $T_{FM}$ of NdOs$_4$Sb$_{12}$ is depressed linearly with Pr
composition to ($1-x$) $\sim$ 0.45. In the superconducting region, the upper
critical field $H_{c2}(x,0)$ is depressed quadratically with $x$ in the range 0
$<$ $x$ $\lesssim$ 0.3, exhibits a kink at $x$ $\approx$ 0.3, and then
decreases linearly with $x$ in the range 0.3 $\lesssim$ $x$ $\lesssim$ 0.6. The
behavior of $H_{c2}(x,0)$ 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
$\lesssim$ $x$ $\lesssim$ 0.6 to antiferromagnetic in the range 0 $<$ $x$
$\lesssim$ 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 $H_{c2}(x,0)$ with $x$.
