Magnetic Behavior Of Single-Crystal Co(Scn)(2)(Ch3oh)(2): Three-Dimensional Ising Metamagnetism And Field-Induced Spin Reorientations

The magnetic behavior of single-crystal Co(SCN)(2)(CH3OH)(2) has been studied by dc susceptibility and magnetization measurements. Pronounced anisotropy in the susceptibility is present at all temperatures. Antiferromagnetic 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.