Determining single-ion and spatial-exchange anisotropies in a $S=$1 quantum antiferromagnet*

The magnetic ground-state of the Q1D $S=$ 1 antiferromagnetic (AFM) chain is sensitive to the single-ion anisotropy ($D)$ and the relative strength of intra- ($J)$ and interchain ($J$') exchange interactions. The ratios $D$/$J$ and $J'$/$J$ dictate the material's placement on the phase diagram for which three competing phases are known to theoretically exist: Haldane, \textit{XY} and quantum paramagnetic. We have identified [Ni(HF$_{\mathrm{2}})$(pyz)$_{\mathrm{2}}$]SbF$_{\mathrm{6}}$ as a candidate in which to explore proximity to these phases. Combining neutron scattering (elastic and inelastic) and high-field magnetization we obtained the ground state Hamiltonian and phase diagram for a powdered sample. Long-range \textit{XY}AFM order ($D$ \textgreater 0) occurs below $T_{\mathrm{N}}=$ 12.2 K and independent simulations of inelastic neutron scattering and $M(H)$ data show excellent consistency for the parameters; $D=$ 13.3 K, $J=$ 10.4 K (Ni-FHF-Ni) and $J'=$ 1.4 K (Ni-pyz-Ni).