Unusual Magnetic-Pressure Response of an $S=1$ Antiferromagnetic Linear-Chain near the $D/J \approx 1$ Critical Point.*

An $S = 1$ chain, [Ni(HF$_2$)(3-Clpy)$_4$]BF$_4$ (py = pyridine), has been identified to have nearest-neighbor antiferromagnetic interaction $J/k_B = 4.86$~K and single-ion anisotropy $D/k_B = 4.3$~K, while avoiding long-range order to 25~mK.\footnote{J.L.~Manson \emph{et al.}, Inorg.~Chem. 51 (2012) 7520.} With $D/J = 0.88$, this system is close to the $D/J \approx 1$ gapless quantum critical point between the Haldane and Large-$D$ phases. The magnetization was studied at 50~mK $\leq T \leq~1$~K and with $B \leq 10$~T.\footnote{J.-S.~Xia \emph{et al.}, arxiv.1409.5971 (2014).} Using a magnetometer equipped with a pressure cell, the low-field (0.1~T), high temperature ($T \geq 2$~K) magnetic susceptibility was studied to 1.47~GPa. These data suggest the response at ambient pressure$^2$ changes between 0.24~GPa and 0.35~GPa. These studies are being extended by $^1$H NMR experiments capable of varying the pressure and of spanning from 300~K to below 100~mK.