NaCaNi2F7: A frustrated high-temperature pyrochlore antiferromagnet withS=1Ni2+

Abstract

$\mathrm{NaCaN}{\mathrm{i}}_{2}{\mathrm{F}}_{7}$ is an insulating, frustrated ${A}_{2}{B}_{2}{\mathrm{F}}_{7}$ pyrochlore with magnetic $S=1\phantom{\rule{0.16em}{0ex}}\mathrm{N}{\mathrm{i}}^{2+}$ 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 $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, with an effective moment of $3.7\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Ni}$. Despite displaying a large Curie-Weiss theta $(\ensuremath{-}129\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, spin-ordering-related features are not seen in the susceptibility or specific heat until a spin glass transition at $3.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This yields an empirical frustration index of $f=\ensuremath{-}{\ensuremath{\theta}}_{\mathrm{CW}}/{\mathrm{T}}_{\mathrm{f}}\ensuremath{\approx}36$. 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 $\mathrm{NaCaN}{\mathrm{i}}_{2}{\mathrm{F}}_{7}$ is likely a realization of a frustrated spin 1 pyrochlore antiferromagnet with weak bond disorder.