ROTATING-FRAME NUCLEAR MAGNETIC-RELAXATION OF SPINS DIFFUSING ON A DISORDERED LATTICE - A MONTE-CARLO MODEL

The rotating-frame nuclear magnetic relaxation rate of spins diffusing on a disordered lattice has been calculated by Monte Carlo methods. T he disorder includes not only variation in the distances between neigh bouring spin sites but also variation in the hopping rate associated w ith each site. The presence of the disorder, particularly the hopping rate disorder, causes changes in the time-dependent spin correlation f unctions which translate into asymmetry in the characteristic peak in the temperature dependence of the dipolar relaxation rate. The results may be used to deduce the average hopping rate from the relaxation bu t the effect is not sufficiently marked to enable the distribution of the hopping rates to be evaluated. The distribution, which is a measur e of the degree of disorder, is the more interesting feature and it ha s been possible to show from the calculation that measurements of the relaxation rate as a function of the strength of the radiofrequency sp in-locking magnetic field can lead to an evaluation of its width. Some experimental data on an amorphous metal-hydrogen alloy are reported w hich demonstrate the feasibility of this novel approach to rotating-fr ame relaxation in disordered materials.