NaCaNi2F7: A frustrated high temperature pyrochlore antiferromagnet with S=1 Ni2+

TL;DR

NaCaNi2F7 is a frustrated S=1 pyrochlore antiferromagnet exhibiting spin glass behavior at low temperatures, with strong magnetic frustration indicated by a high frustration index and disorder effects.

Contribution

This study provides the first detailed characterization of NaCaNi2F7 as a frustrated S=1 pyrochlore antiferromagnet with spin glass behavior and disorder effects.

Findings

Large frustration index (~36) indicating strong magnetic frustration.

Observation of spin glass transition at 3.6 K.

Evidence of weak bond disorder influencing magnetic properties.

Abstract

NaCaNi$_2$F$_7$ is an insulating, frustrated A$_2$B$_2$F$_7$ pyrochlore with magnetic S = 1 Ni$^{2+}$ on the pyrochlore B site. Non-magnetic Na and Ca are disordered on the A-site. Magnetic susceptibility measurements made on an oriented single crystal, grown in a floating zone furnace, show isotropic behavior at temperatures between 5 and 300 K, with an effective moment of 3.7 $\mu_B$/Ni. Despite displaying a large Curie-Weiss theta (-129 K), spin-ordering-related features are not seen in the susceptibility or specific heat until a spin glass transition at 3.6 K. This yields an empirical frustration index of f = -${\theta}_{CW}$/T$_f$ $\approx$ 36. The spin glass behavior is substantiated by a shift of the freezing temperature with frequency in the AC susceptibility, bifurcation in the DC susceptibility, and by a broad maximum in the magnetic specific heat. The observations as made on large single crystals suggest that NaCaNi$_2$F$_7$ is likely a realization of a frustrated spin 1 pyrochlore antiferromagnet with weak bond disorder.