Frustrated three-dimensional antiferromagnetLi2CuW2O8:Li7NMR and the effect of nonmagnetic dilution

Abstract

We report a $^{7}\mathrm{Li}$ nuclear magnetic resonance (NMR) study of a frustrated three-dimensional spin-$\frac{1}{2}$ antiferromagnet ${\text{Li}}_{2}{\text{CuW}}_{2}{\text{O}}_{8}$ and also explore the effect of nonmagnetic dilution. The magnetic long-range ordering in the parent compound at ${T}_{\mathrm{N}}\ensuremath{\simeq}3.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ was detected from the drastic line broadening and a peak in the spin-lattice relaxation rate ($1/{T}_{1}$). The NMR spectrum above ${T}_{\mathrm{N}}$ 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 ${\text{Cu}}^{2+}$ ions was found to be purely dipolar in nature. The magnitude of the maximum exchange coupling constant is ${J}_{\mathrm{max}}/{k}_{\mathrm{B}}\ensuremath{\simeq}13\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. NMR spectra below ${T}_{\mathrm{N}}$ broaden abruptly and transform into a double-horn pattern reflecting the commensurate nature of the spin structure in the ordered state. Below ${T}_{\mathrm{N}}$, $1/{T}_{1}$ follows a ${T}^{5}$ behavior. The frustrated nature of the compound is confirmed by persistent magnetic correlations at high temperatures well above ${T}_{\mathrm{N}}$. The dilution of the spin lattice with nonmagnetic Zn atoms has dramatic influence on ${T}_{N}$ that decreases exponentially similar to quasi-one-dimensional antiferromagnets, even though ${\text{Li}}_{2}{\text{CuW}}_{2}{\text{O}}_{8}$ has only a weak one-dimensional anisotropy. Heat capacity of doped samples follows power law (${C}_{\mathrm{p}}\ensuremath{\propto}{T}^{\ensuremath{\alpha}}$) below ${T}_{\mathrm{N}}$, and the exponent ($\ensuremath{\alpha}$) decreases from 3 in the parent compound to 1 in the 25% doped sample.