Two-dimensional XY behavior observed in quasi-two-dimensional quantum Heisenberg antiferromagnets

The magnetic properties of a family of molecular-based quasi-two- dimensional 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= C4 H4 N2). 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 Néel 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 two-dimensional 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 three-dimensional transition appearing parasitically. The PF6 compound remains Heisenberg type at all temperatures, with its transition to the Néel state due to the interlayer interactions. Effects of field-induced anisotropy have been observed. © 2009 The American Physical Society