Electron, muon, and nuclear spin dynamics in SmRh4B4 and ErRh4B4.

Abstract

Zero-field positive muon spin relaxation (ZF-${\ensuremath{\mu}}^{+}$SR) studies of ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$ and ${\mathrm{ErRh}}_{4}$${\mathrm{B}}_{4}$ have revealed several features of magnetic moment dynamics in these magnetic superconductors. The shape of the \ensuremath{\mu}SR signal is not simple over most of the temperature range studied in either ${\mathrm{SMRh}}_{4}$${\mathrm{B}}_{4}$ or ${\mathrm{ErRh}}_{4}$${\mathrm{B}}_{4}$ because of the complicated nature of both materials. In ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$ above 200 K we observe complete decoupling of the muon spins from the stable, localized samarium ionic moments due to the extremely fast fluctuation of those moments; below 200 K there is a nearly simultaneous onset of coupling of the samarium electronic moments to both the muon spins and the $^{11}\mathrm{B}$ nuclear moments. The muon spin-relaxation rate in ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$ increases smoothly as the temperature is lowered through the superconducting transition toward the antiferromagnetic ordering at 0.87 K. In ${\mathrm{ErRh}}_{4}$${\mathrm{B}}_{4}$ the muon spin-relaxation function is a sum of two exponentials down to 50 K, below which the relaxation becomes too fast to be detected by our instruments, leaving only a signal of much-reduced amplitude at lower temperatures. We attribute the two exponentials to two distinct types of muon site in the crystal structure: a (high rms field) channel structure around the rare earths, and a (low rms field) site at the center of the RhB cluster. The deduced fluctuation rates of the rare-earth dipole fields at the muons are generally consistent with a model of paramagnetic rare-earth fluctuation mechanisms (Ruderman-Kittel-Kasuya-Yosida, Korringa, and spin-lattice interactions) above 4 K in ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$ and above 50 K (i.e., where the signal is fully resolved) in ${\mathrm{ErRh}}_{4}$${\mathrm{B}}_{4}$. Below 4 K in ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$, there is excess relaxation that seems to be associated with the magnetic ordering and that cannot be explained by the model which fits above 4 K.