NaCaN i2 F7: A frustrated high-temperature pyrochlore antiferromagnet with S=1 N i2+

NaCaNi2F7 is an insulating, frustrated A2B2F7 pyrochlore with magnetic S=1Ni2+ on the pyrochlore B site. Nonmagnetic 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 300K, with an effective moment of 3.7μB/Ni. Despite displaying a large Curie-Weiss theta (-129K), spin-ordering-related features are not seen in the susceptibility or specific heat until a spin glass transition at 3.6K. This yields an empirical frustration index of f=-θCW/Tf≈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 NaCaNi2F7 is likely a realization of a frustrated spin 1 pyrochlore antiferromagnet with weak bond disorder.