Unusual Magnetic Response of anS= 1 Antiferromagetic Linear-Chain Material

Abstract

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.PACS numbers: 75.50. Ee, 75.10.Kt, 74.40.Kb After Haldane reported the fundamental differences between one-dimensional Heisenberg antiferromagnets with integer and half-integer spins [1,2], theoretical [3][4][5] and experimental [6] activities focused on clarifying the magnetic phases and properties of a broad range of linear-chain magnets described by the Hamiltonian

Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet

“…Though the neutronscattering data suggest that there is no preferred direction for the moments at low temperatures, this feature could represent a precursor to very low temperature XY symmetry breaking due to rhombic anisotropy or distortion of the exchange energy 11 . Another possible explanation is the presence of antiferromagnetic domains 17 at low magnetic fields and temperatures.…”

“…More recently, interest in predicting and controlling the magnetic ground state of S = 1 quantum magnets has been fueled by the realization of a myriad of magnetic phases in a series of metal-organic coordination compounds. This includes the observation of field-induced Bose-Einstein condensation in NiCl 2 -4SC(NH 2 ) 2 [6][7][8], as well as the development of a Haldane phase in both [Ni(C 2 H 8 N 2 ) 2 NO 2 ]ClO 4 [9] and [Ni(HF 2 )(3-Clpy) 4 ]BF 4 (Clpy = C 5 H 4 NCl = chloropyridine) [10][11][12]. Groundstate diversity is attributable to the interplay between the single-ion anisotropy (D) and Heisenberg spin-exchange interactions in these materials, which are determined (in part) by the lattice geometry 12 .…”

“…Unlike classical systems, ideal S = 1 chains (J = 0) are vulnerable to strong quantum fluctuations 11 , which can have a profound influence on the magnetic ground state and act to suppress long-range order. Quantum Monte-Carlo (QMC) simulations predict that for easyplane anisotropy, a Haldane ground state gives way to quantum paramagnetism (QP) as the D/J ratio increases.…”

Inelastic neutron scattering study of the anisotropic S=1 spin chain [Ni(HF2)(3−Clpy

“…The INS ∆ E = 0.054 (0.05 ± 0.01) meV implies E/J < 0.07 for NBCT and then E/J ≈ 0.05 (E/D ≈ 0.03) assuming that ∆ E ∝ E/J near E/J = 0.07. Indeed, the absence of magnetic Bragg scattering at temperatures significantly less than the exchange energy scale is in opposition to a magnetically ordered ground state for NBCT, while the low temperature specific heat [14] and magnetization data [15] may indicate the nearby presence of strong quantum fluctuations. Finally, NBCT may be plotted on the (D/J, E/J) phase diagram [10], Fig.…”

“…No E value has been reported. The proximity of NBCT to a phase boundary was also inferred from isothermal magnetization studies that used randomly arranged microcrystals cooled down to 50 mK, and an upper limit for a possible critical magnetic field (which could indicate existence of the gap) was given as H C 35 ± 10 mT [15].…”