Muon-hydrogen correlation in amorphous alloys

Abstract

Muon spin relaxation has been measured in the amorphous transition-metal alloy Zr₂Pd containing a high concentration of interstitial hydrogen. The nuclear spin dipole moments of the metal atoms in this alloy are relatively small with the result that the local magnetic field at the muon arises almost entirely from the nuclear moments of the hydrogen atoms. Even at the lowest temperature of measurement, ∼ 10 K, the relaxation profiles are consistent with fluctuating local fields. The correlation time is typically a few microseconds and varies only slowly with temperature up to about 100 K. On the other hand, above 250 K, where the diffusion hopping rate of the hydrogen atoms is about 10⁷-10⁸ s⁻¹ as measured by nuclear relaxation methods, the correlation times vary more rapidly with temperature and may be described in terms of activation energies which are similar to the activation energies of the hydrogen diffusion. In addition the correlation time of the local field at the muon was found to be greater than the average interval between hops of the hydrogen atoms, a puzzling result which has been found in earlier experiments on similar alloys. The present data confirm the generality of this feature. However, it has not been possible to offer a fully convincing explanation of the mechanism which may lead to it.