Muon spin relaxation in hydrogen-loaded amorphous Ni-Ti alloys

Abstract

Muon spin relaxation in the amorphous alloy Ni33Ti65 containing interstitial hydrogen has been measured in longitudinal geometry at zero applied magnetic field, at temperatures in the range 18-340 K. The relaxation profiles at all temperatures were consistent with time-dependent magnetic fields at the muon site and were analysed by means of a dynamic form of the Kubo-Toyabe relaxation theory. The correlation time of the local field at the muon below 80 K was found to be about 6 mu s and to vary only slowly with temperature. Above approximately 250 K, where the diffusion hopping rate of the hydrogen atoms is about 107-108 s-1, as measured by nuclear magnetic relaxation methods, the activation energy of the correlation time is similar to the activation energy of the hydrogen diffusion. The relaxation of the muon spin at these temperatures is ascribed to the highly correlated motion of the muon and the hydrogen atoms. Some Monte Carlo calculations of the fluctuation rates of the local fields under such conditions are reported. According to the outcome of these calculations the experimental data are consistent with the muon having an intrinsically lower diffusion rate than the hydrogen atoms.