An S=1 antiferromagnetic polymeric chain, [Ni(HF2)(3-Clpy)4]BF4 (py = pyridine), has previously been identified to have intrachain, nearest-neighbor antiferromagnetic interaction strength J/kB=4.86 K and single-ion anisotropy (zero-field splitting) D/kB=4.3 K, so the ratio D/J=0.88 places this system close to the D/J≈1 gapless critical point between the topologically distinct Haldane and Large-D phases. The magnetization was studied over a range of temperatures, 50 mK ≤T≤1 K, and magnetic fields, B≤10 T, in an attempt to identify a critical field, Bc, associated with the closing of the Haldane gap, and the present work places an upper bound of Bc≤(35±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.
@article{arxiv.1409.5971,
title = {Unusual Magnetic Response of an $S = 1$ Antiferromagetic Linear-Chain Material},
author = {Jian-Sheng Xia and Andrzej Ozarowski and Peter M. Spurgeon and Adora G. Baldwin and Jamie L. Manson and Mark W. Meisel},
journal= {arXiv preprint arXiv:1409.5971},
year = {2014}
}