Electron, muon, and nuclear spin dynamics in SmRh/sub 4/B/sub 4/ and ErRh/sub 4/B/sub 4/

Zero-field positive muon spin relaxation (ZF-..mu../sup +/SR) studies of SmRh/sub 4/B/sub 4/ and ErRh/sub 4/B/sub 4/ have revealed several features of magnetic moment dynamics in these magnetic superconductors. The shape of the ..mu..SR signal is not simple over most of the temperature range studied in either SMRh/sub 4/B/sub 4/ or ErRh/sub 4/B/sub 4/ because of the complicated nature of both materials. In SmRh/sub 4/B/sub 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 /sup 11/B nuclear moments. The muon spin-relaxation rate in SmRh/sub 4/B/sub 4/ increases smoothly as the temperature is lowered through the superconducting transition toward the antiferromagnetic ordering at 0.87 K. In ErRh/sub 4/B/sub 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 themore » 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 SmRh/sub 4/B/sub 4/ and above 50 K (i.e., where the signal is fully resolved) in ErRh/sub 4/B/sub 4/.« less