Two-dimensionalXYbehavior observed in quasi-two-dimensional quantum Heisenberg antiferromagnets

Abstract

The magnetic properties of a family of molecular-based quasi-two-dimensional $S=1/2$ Heisenberg antiferromagnets are reported. Three compounds, [$\text{Cu}{\left(\text{pz}\right)}_2{\left({\text{ClO}}_4\right)}_2$, $\text{Cu}{\left(\text{pz}\right)}_2{\left({\text{BF}}_4\right)}_2$, and $\left[\text{Cu}{\left(\text{pz}\right)}_2\left({\text{NO}}_3\right)\right]\left({\text{PF}}_6\right)$] contain similar planes of ${\text{Cu}}^{2+}$ ions linked into magnetically square lattices by bridging pyrazine molecules $\left(\text{pz}={\text{C}}_4{\text{H}}_4{\text{N}}_2\right)$. 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 $H_A$ (kOe) to the saturation field $H_{\text{SAT}}$ (kOe) are small ($2.6/490=5.3\times {10}^{-3}$, $2.4/430=5.5\times {10}^{-3}$, and $0.07/300=2.3\times {10}^{-4}$, respectively), demonstrating close approximations to a two-dimensional Heisenberg model. The susceptibilities of ${\text{ClO}}_4$ and ${\text{BF}}_4$ 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 ${\text{PF}}_6$ 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.