Simulation study of NMR line shapes in a classical nuclear-spin system with two isotopes.

Abstract

We have used Monte Carlo simulations, combined with microscopic Heisenberg equations of motion, to calculate accurately the dynamics of a classical spin assembly. The system consists of two kinds of nuclear spins which are coupled by exchange interactions, either ferromagnetic or antiferromagnetic. The fluctuation-dissipation theorem is employed to calculate the NMR lineshapes from the autocorrelation matrix for magnetization. We investigate the polarization and field dependence of the line shapes in the paramagnetic phase. Our simulations reproduce the observed exchange-induced merging of isotopic NMR lines of thallium at low polarizations, as well as the suppression-enhancement effect found experimentally for highly polarized copper and silver isotopes. The results are employed to evaluate the accuracy of a previously used mean-field model in the description of the NMR response. The observed deviations from the model highlight the usefulness of accurate numerical simulations. The line shapes are analyzed by separating the total NMR absorption into contributions from like and unlike spins. The decomposition shows how the nature of exchange narrowing changes with polarization. The effect of isotopic site randomness on the NMR line shapes is also investigated and found to be important at high polarizations.