On the 169Tm NMR Line Shape and Local-Field Distribution in Dilute LiY0.98Tm0.02F4: Monte Carlo Analysis

A microscopic Monte Carlo analysis of the 169Tm NMR line shape in dilute LiY0.98Tm0.02F4 is presented. The random occupation of rare-earth lattice sites by Tm3+ ions is treated explicitly, and the distribution of dipolar local fields at the resonant 169Tm nuclei is calculated for 5 × 106 independent configurations. It is shown that, in the 2% diluted system, the nuclear magnetic resonance spectrum is strongly non-Gaussian. Instead of a single smooth line, the calculated profile consists of a dominant central resonance produced by relatively isolated thulium ions and weak satellite components arising from rare close Tm–Tm configurations. This discreteness strongly affects the interpretation of the anomalously large linewidth reported for H0 || [001]. In particular, the global second moment is found to be highly sensitive to the far wings of the distribution and therefore does not characterize the intrinsic width of the main spin ensemble. The full width at half maximum of the central peak provides a more physically transparent measure of the linewidth in the dilute limit. The calculated angular dependence reproduces the strong anisotropy associated with the van Vleck response of Tm3+ ions and supports a microscopic reinterpretation of the observed broadening in terms of conventional dipolar local fields in a spatially disordered rare-earth sublattice.