Spin freezing in the disordered pyrochlore magnet NaCaCo\(_2\)F\(_7\): NMR studies and Monte-Carlo simulations

We present results of \(^{23}\)Na and \(^{19}\)F nuclear magnetic resonance (NMR) measurements on NaCaCo\(_2\)F\(_7\), a frustrated pyrochlore magnet with a Curie-Weiss temperature, \(T_{cw}\) ~-140 K, and intrinsic bond disorder. Below 3.6 K both the \(^{23}\)Na and \(^{19}\)F spectra broaden substantially in comparison to higher temperatures accompanied by a considerable reduction (80 \%) of the NMR signal intensity: This proves a broad quasi-static field distribution. The \(^{19}\)F spin-lattice relaxation rate \(^{19}(1/T_1\)) exhibits a peak at 2.9 K already starting to develop below 10 K. We attribute the spin freezing to the presence of bond disorder. This is corroborated by large-scale Monte-Carlo simulations of a classical bond-disordered XY model on the pyrochlore lattice. The low freezing temperature, together with the very short magnetic correlation length not captured by the simulations, suggesting that quantum effects play a decisive role in NaCaCo\(_2\)F\(_7\).