$^7$Li NMR Study of Magnetic Defects in Heavy Fermion LiV$_2$O$_4$

A small concentration ($n \approx 0.7$ mol\%) of magnetic defects within the normal spinel structure of heavy fermion ${\rm LiV_2O_4}$ was recently found to have a strong effect on the $^7 $Li NMR at low temperatures $T <$ 4.2 K\@.(1) The $^7$Li nuclear magnetization relaxation versus time after saturation, $M(t)$, changed from a pure exponential in a pure sample with $n$ = 0.01 mol\% to a stretched exponential for $n$ = 0.7 mol\%. Here we present a systematic study of the variations of the $^7$Li NMR versus $n$ for additional $n$ values from 0.05 to 0.8 mol\%. Non-exponential $M(t)$ recovery was consistently obtained for samples with $n \geq 0.2$ mol\% and the nuclear spin lattice relaxation rate versus temperature evolved monotonically with increasing $n$, consistent with the interpretation in Ref.\ (1). In addition, we obtained relaxation data for much shorter times than studied in Ref.\ (1), which indicate a previously unknown initial $M(t) \propto \sqrt{t}$ dependence for all samples with $n \geq 0.5$ mol\%, for times $t <$ 20 ms. At present, there exists no microscopic theory for the influence of magnetic defects on our $^7$Li NMR results. Phenomenological models that may help to understand the data will be discussed.\newline (1). D. C. Johnston et al., Phys.\ Rev.\ Lett.\ \textbf{95}, 176408 (2005).