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

Abstract

An S = 1 antiferromagnetic polymeric chain, [Ni(HF2)(3-Clpy)4]BF4 (py = pyridine), also referred to as NBCT, has previously been identified to have intrachain, nearest-neighbor antiferromagnetic interaction strength J/k B = 4.86 K and single-ion anisotropy (zero-field splitting) D/k B = 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, B c, associated with the closing of a gap, and the present work places an upper bound of B c ≤ (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 (nominally 200 GHz to 425 GHz) ESR spectra at 3 K reveal several features associated with the sample.