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

TL;DR

This study combines NMR experiments and Monte-Carlo simulations to investigate spin freezing in the disordered pyrochlore magnet NaCaCo$_2$F$_7$, revealing bond-disorder-induced freezing and the importance of quantum effects.

Contribution

It provides new insights into spin freezing mechanisms in disordered pyrochlore magnets through combined experimental and computational approaches.

Findings

NMR spectra broaden below 3.6 K indicating spin freezing.

Peak in spin-lattice relaxation rate at 2.9 K suggests freezing temperature.

Monte-Carlo simulations support bond-disorder-induced freezing, highlighting quantum effects.

Abstract

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$_7$.