Single-ion properties of the S eff = 1/2 XY antiferromagnetic pyrochlores, NaA The antiferromagnetic pyrochlore material NaCaCo2F7 is a thermal spin liquid over a broad temperature range (≈ 140 K down to TF = 2.4K), in which magnetic correlations between Co 2+ dipole moments explore a continuous manifold of antiferromagnetic XY states.1 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 Co 2+ 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 Co 2+ 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 + /Ca 2+ and Na + /Sr 2+ ) is evident in these measurements as exaggerated broadening of some of the levels. A simple model that reproduces the salient features of the singleion 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 S eff = 1/2 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.

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We present observations of highly frustrated quasi two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the Seff= 1/2 compound γ-BaCo2(PO4)2 (γ-BCPO). Specific heat shows a broad peak comprised of two weak kink features at TN1 ∼ 6 K and TN2 ∼ 3.5 K, the relative weights of which can be modified by sample annealing. Neutron powder diffraction measurements reveal short range quasi-2D order that is established below TN1 and TN2, at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length ξc = 60 ± 2 Å (TN1) and in quasi-2D helical domains with ξ h = 350 ± 11 Å (TN2). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on γ-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ J1-J2-J3 model with ferromagnetic nearest-neighbor exchange J1 are favored, both near regions of high classical degeneracy. High energy coherent excitations (∼ 10 meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above TN . These data show that γ-BCPO is a rare highly frustrated, quasi-2D Seff= 1/2 honeycomb lattice material which resists long range magnetic order and spin freezing. arXiv:1712.06208v2 [cond-mat.str-el]

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