Tuning spatial exchange and single-ion anisotropies in low-dimensional Ni(II) quantum magnets*

Spatial exchange anisotropy is key to determining $J$'/$J$ and $T_{N}$/$J$ critical ratios and unveiling $B$/$T$ phase diagrams in low-dimensional quantum magnets. Systematic design strategies have enabled us to synthesize a series of model $S=$1 Ni(II) systems whereby this anisotropy, in combination with the single-ion anisotropy, can be tuned by adjusting the nature of the coordinating ligands. For example, the coordination polymers [Ni$L_{x}$(pyz)$_{2}$]$Y$ (pyz $=$ pyrazine; $L=$ HF$_{2}$, $x=$ 1, $Y=$ PF$_{6}$, SbF$_{6}$; $L=$ Cl, Br, I, $x=$ 2, $Y=$ nil), possess 2D [Ni(pyz)$_{2}$]$^{2+}$ square lattices that are spaced apart by bridging or non-bridging $L$ anions such that 1.7 $\le T_{N}\le $ 12 K depending on the magnitude of $J$'. Chemical substitution of pyz for other organic ligands leads to quasi-1D [Ni(HF$_{2})$(3-Clpy)$_{4}$]BF$_{4}$ (Clpy $=$ chloropyridine) and the 2D Kagome lattice [Ni(H$_{3}$F$_{4})$(3-Fpy)$_{4}$]SbF$_{6}$ (Fpy $=$ fluoropyridine) which contain HF$_{2}^{-}$ or H$_{3}$F$_{4}^{-}$ bridges, respectively. Furthermore, the inherent flexibility of strong F\textbullet \textbullet \textbullet H\textbullet \textbullet \textbullet F and O-H\textbullet \textbullet \textbullet F bonds also renders them highly sensitive to external stimuli such as high pressure. Time permitting, these examples and others will be presented.