Muon spin rotation and relaxation in Pr${}_{1-x}$Nd${}_x$Os${}_4$Sb${}_{12}$: Magnetic and superconducting ground states

Abstract

Muon spin rotation and relaxation ($\mu$SR) experiments have been carried out to characterize magnetic and superconducting ground states in the Pr${}_{1-x}$Nd${}_x$Os${}_4$Sb${}_{12}$ alloy series. In the ferromagnetic end compound NdOs${}_4$Sb${}_{12}$ the spontaneous local field at positive-muon (${\mu }^{+}$) sites below the ordering temperature $T_C$ is greater than expected from dipolar coupling to ferromagnetically aligned Nd${}^{3+}$ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For 0.45 $\le x\le$ 0.75, ${\mu }^{+}$ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at ${\mu }^{+}$ sites below $T_C$ are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd${}^{3+}$ moments, and speculate that the Nd${}^{3+}$-${\mu }^{+}$ interaction is suppressed and disordered by Pr doping. In an $x$ = 0.25 sample, which is superconducting below $T_c$ = 1.3 K, there is no sign of "spin freezing" (static Nd${}^{3+}$ magnetism), ordered or disordered, down to 25 mK. Dynamic ${\mu }^{+}$ spin relaxation is strong, indicating significant Nd-moment fluctuations. The ${\mu }^{+}$ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below $T_c$, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For 0.25 $\le x\le$ 0.75, the $\mu$SR data provide evidence against phase separation; superconductivity and Nd${}^{3+}$ magnetism coexist on the atomic scale.

arXiv comment14 pages, 17 figures. To be published in Physical Review B

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