Magnetic behavior of single-crystal Co(SCN)_2(CH_3OH)_2: Three-dimensional Ising metamagnetism and field-induced spin reorientations

摘要

The magnetic behavior of single-crystal Co(SCN)_2(CH_3OH)_2 has been studied by dc susceptibility and magnetization measurements. Pronounced anisotropy in the susceptibility is present at all temperatures. Anti-ferromagnetic maxima appear in three orthogonal crystal susceptibilities at 4.45±0.02 K. One of the axes, the monoclinic a~* direction of much the largest susceptibility, has the appearance of an easy axis and exhibits a transition at 4.15±0.02 K. There are no indications of lower-dimensional behavior, probably because the spacing between SCN~-coordinated layers of cobalt ions characterizing the structure is not large. A fit to the a~* susceptibility data using a high-temperature series expansion (HTSE) for the simple cubic three-dimensional Ising model yields g=5.83 and J/k=0.49_0 K; the positive sign indicates a dominant ferromagnetic interaction and large maximum susceptibility value, despite overall antiferromagnetism. A low-temperature series expansion fit for the same model, at temperatures below that of the susceptibility maximum, yields g=5.79 and J/k=-0.63_5 K. Along the same a~* axis magnetization versus field isotherms reveal an abrupt transition at 3.73 kG for 1.84 K, the magnetization then gradually approaching near saturation as a 15.9-kG field limit is approached. Along the two orthogonal b and c crystal axes much less abrupt transitions are observed in the several kG range, one transition along b and two along c. None have the appearance of transitions to a polarized paramagnetic state; they are believed to be spin reorientation in nature. Alternative scenarios for the transition along a~* are considered, spin-flop and metamagnetic. A mean-field analysis based on the former yields an exchange field H_E=19.6-kG and an anisotropy field H_A=0.35 kG. But this interpretation entails inconsistencies regarding the appearance of M vs H and conflicting estimates of the exchange interaction. Analysis based on the metamagnetic hypothesis yields for the interlayer antiferromagnetic exchange J_2/k =-0.36_5 K. In conjunction with the HTSE result, an intralayer ferromagnetic exchange J_1/k=1.10 K is inferred. The ordering temperature and Weiss parameter predicted from these exchange parameters, again on a mean-field basis, are reasonably consistent with observation. In either interpretation a multisublattice model with hidden canting suggests itself in order to rationalize the spin reorientation transitions along the b and c axes.