NUCLEAR MAGNETIC-RELAXATION IN DISORDERED SOLIDS - A MONTE-CARLO STUDY OF METAL HYDROGEN SYSTEMS

The dipolar nuclear magnetic relaxation rate associated with the hoppi ng diffusion of atoms in a disordered solid is calculated by Monte Car lo methods. The model is intended to simulate the diffusion of hydroge n atoms trapped at interstitial positions in the matrix of metal atoms in amorphous alloys. The principal features of the model system are t hat the atoms hop on a spatially disordered array of traps and the tra pping energy varies from trap to trap so that the diffusion of the hyd rogen is characterized by a distribution of jump rates. The effective jump rate from a trap is assumed to have an Arrhenius dependence on te mperature. Calculated at constant temperature, the characteristic peak in the relaxation rate, which occurs in ordered solids when the produ ct of the average jump rate and the Larmor frequency is approximately unity, is found to be broadened and shifted in frequency, particularly when the occupancy of the traps is high. The long-range diffusion con stant is also calculated and used to evaluate the effect of atom-vacan cy correlations. It is found that the shifts in the relaxation peak ca nnot be accounted for solely by these correlation effects and it is su ggested that multiple hopping of the more rapidly diffusing spins is a contributory factor. The shifts have a profound effect on the tempera ture dependence of the relaxation rate when the distribution of jump r ates is also dependent on temperature. The adjustments to the peak in the relaxation caused by the distribution are small when the temperatu re dependence is taken into account, showing that experiments involvin g only the temperature variation of the relaxation are unlikely to be a sensitive method for detecting the presence of a jump-rate distribut ion. This aspect of the results of the computer model is illustrated b y comparison with experimental data.