Frustrated three-dimensional antiferromagnet Li$_{2}$CuW$_{2}$O$_{8}$: $^7$Li NMR and the effect of non-magnetic dilution
We report a $^7$Li nuclear magnetic resonance (NMR) study of a frustrated three-dimensional \mbox{spin-$\frac12$} antiferromagnet Li$_{2}$CuW$_{2}$O$_{8}$ and also explore the effect of non-magnetic dilution. The magnetic long-range ordering in the parent compound at $T_{\rm N}\simeq$ 3.9~K was detected from the drastic line broadening and a peak in the spin-lattice relaxation rate ($1/T_1$). The NMR spectrum above $T_{\rm N}$ broadens systematically, and its full width at half maximum (FWHM) tracks the static spin susceptibility. From the analysis of FWHM vs. static susceptibility, the coupling between the Li nuclei and Cu$^{2+}$ ions was found to be purely dipolar in nature. The magnitude of the maximum exchange coupling constant is $J_{\rm max}/k_{\rm B}\simeq 13$\,K. NMR spectra below $T_{\rm N}$ broaden abruptly and transform into a double-horn pattern reflecting the commensurate nature of the spin structure in the ordered state. Below $T_{\rm N}$, 1/$T_1$ follows a $T^5$ behavior. The frustrated nature of the compound is confirmed by persistent magnetic correlations at high temperatures well above $T_{\rm N}$. The dilution of the spin lattice with non-magnetic Zn atoms has dramatic influence on $T_N$ that decreases exponentially similar to quasi-one-dimensional antiferromagnets, even though Li$_2$CuW$_2$O$_8$ has only a weak one-dimensional anisotropy. Heat capacity of doped samples follows power law ($C_{\rm p} \propto T^{\alpha}$) below $T_{\rm N}$, and the exponent ($\alpha$) decreases from 3 in the parent compound to 1 in the 25\% doped sample.