The magnetic properties of a new family of molecular-based quasi-two dimension S=1/2 Heisenberg antiferromagnets are reported. Three compounds, (Cu(pz)2(ClO4)2, Cu(pz)2(BF4)2, and [Cu(pz)2(NO3)](PF6)) contain similar planes of Cu2+ ions linked into magnetically square lattices by bridging pyrazine molecules (pz = C4H4N2). The anions provide charge balance as well as isolation between the layers. Single crystal measurements of susceptibility and magnetization, as well as muon spin relaxation studies, reveal low ratios of N\'{e}el temperatures to exchange strengths (4.25/17.5=0.243, 3.80/15.3=0.248, and 3.05/10.8=0.282, respectively) while the ratio of the anisotropy fields HA (kOe) to the saturation field HSAT (kOe) are small (2.6/490=5.3×10−3, 2.4/430=5.5×10−3, and 0.07/300=2.3×10−4, respectively), demonstrating close approximations to a 2D Heisenberg model. The susceptibilities of ClO4 and BF4 show evidence of an exchange anisotropy crossover (Heisenberg to XY) at low temperatures; their ordering transitions are primarily driven by the XY behavior with the ultimate 3D transition appearing parasitically. The PF6 compound remains Heisenberg-like at all temperatures, with its transition to the N\'{e}el state due to the interlayer interactions. Effects of field-induced anisotropy have been observed.
@article{arxiv.0903.2470,
title = {2D XY Behavior observed in quasi-2D quantum Heisenberg antiferromagnets},
author = {F. Xiao and F. M. Woodward and C. P. Landee and M. M. Turnbull and C. Mielke and N. Harrison and T. Lancaster and S. J. Blundell and P. J. Baker and P. Babkevich and F. L. Pratt},
journal= {arXiv preprint arXiv:0903.2470},
year = {2015}
}