Exchange narrowing of NMR line shapes in randomly diluted magnetic systems.

Abstract

An analysis of $^{19}\mathrm{F}$ NMR linewidths in the randomly diluted magnetic system ${\mathrm{KMn}}_{\mathrm{x}}$${\mathrm{Mg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{F}}_{3}$ is presented. It is shown that good agreement with measured linewidths can be obtained if in the usual asymptotic spin-diffusion assumption for the spin autocorrelation function (0)${\mathrm{〉}}_{\mathrm{av}}$ \ensuremath{\alpha}${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}\mathrm{d}(\mathrm{x})/2}$, d(x) is taken to be independent of x above the percolation concentration. Experimental results in the system ${\mathrm{KNi}}_{\mathrm{x}}$${\mathrm{Mg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{F}}_{3}$ are also presented. These data exhibit striking differences with the behavior of isostructural ${\mathrm{KMn}}_{\mathrm{x}}$${\mathrm{Mg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{F}}_{3}$ whose origin is discussed.