A single crystal high-temperature pyrochlore antiferromagnet

TL;DR

This study characterizes NaCaCo$_2$F$_7$, a highly frustrated pyrochlore antiferromagnet with large single crystals, revealing its magnetic properties, low entropy at freezing, and potential as a model for geometric frustration.

Contribution

First detailed magnetic characterization of NaCaCo$_2$F$_7$, demonstrating its high frustration and suitability for studying pyrochlore magnetic phenomena.

Findings

High frustration index (~56) indicating strong magnetic frustration.

Spin freezing occurs at 2.4 K despite large Curie-Weiss temperature.

Magnetic entropy remains significant at 0.6 K, suggesting persistent magnetic fluctuations.

Abstract

We report the magnetic characterization of the frustrated transition metal pyrochlore NaCaCo$_2$F$_7$. This material has high spin Co$^{2+}$ in CoF$_6$ octahedra in a pyrochlore lattice, and disordered non-magnetic Na and Ca on the large-atom sites in the structure. Large crystals grown by the floating zone method were studied. The magnetic susceptibility is isotropic, the Co moment is larger than the spin-only value, and in spite of the large Curie Weiss theta (-140 K), freezing of the spin system, as characterized by peaks in the ac and dc susceptibility and specific heat, does not occur until around 2.4 K. This yields a frustration index of f = $-\theta_{CW}$/$T_f$ $\approx$ 56, an indication that the system is highly frustrated. The observed entropy loss at the freezing transition is low, indicating that magnetic entropy remains present in the system at 0.6 K. The compound may be the realization of a frustrated pyrochlore antiferromagnet with weak bond disorder. The high magnetic interaction strength, strong frustration, and the availability of large single crystals makes NaCaCo$_2$F$_7$ an interesting alternative to rare earth oxide pyrochlores for the study of geometric magnetic frustration in pyrochlore lattices.