Single-ion properties of the Seff = ½ XY antiferromagnetic pyrochlores NaA'Co2F7 (A'=Ca2+, Sr2+).

The antiferromagnetic pyrochlore material NaCaCo2F7 is a thermal spin liquid over a broad temperature range (≈140 K down to TF=2.4 K), in which magnetic correlations between Co2+ dipole moments explore a continuous manifold of antiferromagnetic XY states [K. A. Ross et al., Phys. Rev. B 93, 014433 (2016)]. The thermal spin liquid is interrupted by spin freezing at a temperature that is ~2% of the mean-field interaction strength, leading to short-range static XY clusters with distinctive relaxation dynamics. Here we report the low-energy inelastic neutron scattering response from the related compound NaSrCo2F7, confirming that it hosts the same static and dynamic spin correlations as NaCaCo2F7. We then present the single-ion levels of Co2+ in these materials as measured by inelastic neutron scattering. An intermediate spin-orbit coupling model applied to an ensemble of trigonally distorted octahedral crystal fields accounts for the observed transitions. The single-ion ground state of Co2+ is a Kramers doublet with a strongly XY-like g tensor (gxy/gz~3). The local disorder inherent from the mixed pyrochlore A sites (Na+/Ca2+ and Na+/Sr2+) is evident in these measurements as exaggerated broadening of some of the levels. A simple model that reproduces the salient features of the single-ion spectrum produces approximately 8.4% and 4.1% variation in the z and xy components of the g tensor, respectively. This study confirms that an Seff=½ model with XY antiferromagnetic exchange and weak exchange disorder serves as a basic starting point in understanding the low-temperature magnetic behavior of these strongly frustrated magnets. [ABSTRACT FROM AUTHOR]