Anomalous magnetic moment suppression in the superconducting and ferromagnetic coextistence region in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$

A previous study [1] of the effect of the ferromagnetism (FM) on unconventional superconductivity (SC) in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ found that SC and FM are both suppressed toward a critical concentration $x_{cr,1} \sim 0.6$, and the $x$ dependence of the upper critical field $H_{c2}$ has a curvature break at $x_{cr,2} \sim 0.3$. The specific heat measurements indicate that FM extends into the SC region. In order to probe the FM in the SC-FM coexistence region, $\mu$-SR measurements are performed on the samples near $x_{cr,1}$ ($x =$ 0.55, 0.5, and 0.45). A small quasistatic field $\sim 40$\,Gauss was found in the field cooled state of these samples ($H = 10$\,Oe) and this field is corresponding to a frozen Nd moment of $\sim 0.1$\,$\mu_B$, which is much smaller than the CEF ground state moment of the Nd$^{3+}$ ion ($\sim 1.36 \mu_B$). The origin of the moment reduction in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ is unclear currently. The Kondo effect, which is usually involved in such a reduction, has never been observed in Nd-based materials. [1] Ho, et. al., 2009 APS March Meeting, A41.00005 (2009); manuscript in preparation (2009).