Unusual Magnetic Response of an $S = 1$ Antiferromagetic Linear-Chain Material

An $S=1$ antiferromagnetic polymeric chain, [Ni(HF$_2$)(3-Clpy)$_4$]BF$_4$ (py = pyridine), has previously been identified to have intrachain, nearest-neighbor antiferromagnetic interaction strength $J/k_{mathrm{B}} = 4.86$ K and single-ion anisotropy (zero-field splitting) $D/k_{mathrm{B}} = 4.3$ K, so the ratio $D/J = 0.88$ places this system close to the $D/J approx 1$ gapless critical point between the topologically distinct Haldane and Large-$D$ phases. The magnetization was studied over a range of temperatures, 50 mK $leq T leq 1$ K, and magnetic fields, $B leq 10$ T, in an attempt to identify a critical field, $B_{mathrm{c}}$, associated with the closing of the Haldane gap, and the present work places an upper bound of $B_{mathrm{c}} leq (35 pm 10)$ mT. At higher fields, the observed magnetic response is qualitatively similar to the excess signal observed by other workers at 0.5 K and below 3 T. The high-field (up to 14.5 T), multi-frequency (nomially 200 GHz to 425 GHz) ESR spectra at 3 K reveal several broad features considered to be associated with the linear-chain sample.