Effect of Nd Substitution on PrOs$_4$Sb$_{12}$ Investigated by $\mu$SR Experiments

The pseudo ternary system Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been used as a model system to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC) and quantum critical behavior of PrOs$_4$Sb$_{12}$ [1]. SC in this system disappears near a critical concentration $x_{cr,1}\sim 0.58$ and FM appears above $x_{cr,2}\sim 0.33$ [1,2]. The new $\mu$SR measurements have been performed on samples with $x =$ 0.25, 0.75, and 1. For $x = 1$ and 0.75, the estimated frozen moments agree with the Nd$^{3+}$ CEF ground state moment. For $x = 0.25$, neither time reversal symmetry breaking nor evidence of freezing of Nd$^{3+}$ spins was observed in zero-field $\mu$SR measurements, the behavior of which is very different than what is observed for $x = 0.45-0.55$ [2]. In the SC state, an unexpected linear $T$ dependence of the Gaussian relaxation rate was also found in the transverse field $\mu$SR data for $x = 0.25$, which is different than the plateau in PrOs$_4$Sb$_{12}$ below 1.3K [3]. [1] Ho, et al., PRB 83, 024511 (2011).[2] Ho, et al., 2010 APS March Meeting, A38.00005 (2010). [3] MacLaughlin et al., PRL 105, 019701 (2010).