准二维量子海森堡反铁磁体中的二维XY行为
2D XY Behavior observed in quasi-2D quantum Heisenberg antiferromagnets
摘要：
报道了一类新的准二维分子基$S=1/2$海森堡反铁磁体的磁性。三种化合物（$Cu(pz)_2(ClO_4)_2$、$Cu(pz)_2(BF_4)_2$和$[Cu(pz)_2(NO_3)](PF_6)$）含有通过桥连吡嗪分子（pz = $C_4H_4N_2$）连接成磁性正方晶格的Cu $^{2+}$离子的相似平面。阴离子提供电荷平衡以及层之间的隔离。磁化率和磁化强度的单晶测量以及μ子自旋弛豫研究揭示了Néel温度与交换强度的低比率（分别为$4.25 / 17.5 = 0.243$、$3.80/15.3=0.248$和$3.05/10.8=0.282$），而各向异性场$H_A$（kOe）与饱和场$H_\mathrm{SAT}$（kOe）的比率较小（分别为$2.6/490 = 5.3\times10^{-3}$、$2.4/430=5.5\times10^{-3}$和$0.07/300=2.3\times10^{-4}$），证明了与2D海森堡模型的密切近似。ClO $_4$和BF $_4$的结晶性显示了在低温下交换各向异性交叉（海森堡到$XY$）的证据;它们的有序转变主要由$XY$行为驱动，最终的3D转变寄生地出现。PF $_6$化合物在所有温度下都保持海森堡样，由于层间相互作用而转变为Néel态。观察到了场致各向异性效应。
英文摘要：
The magnetic properties of a new family of molecular-based quasi-two dimension $S=1/2$ Heisenberg antiferromagnets are reported. Three compounds, ($Cu(pz)_2(ClO_4)_2$, $Cu(pz)_2(BF_4)_2$, and $[Cu(pz)_2(NO_3)](PF_6)$) contain similar planes of Cu$^{2+}$ ions linked into magnetically square lattices by bridging pyrazine molecules (pz = $C_4H_4N_2$). 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 $H_\mathrm{SAT}$ (kOe) are small ($2.6/490 = 5.3\times10^{-3}$, $2.4/430=5.5\times10^{-3}$, and $0.07/300=2.3\times10^{-4}$, respectively), demonstrating close approximations to a 2D Heisenberg model. The susceptibilities of ClO$_4$ and 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 3D transition appearing parasitically. The PF$_6$ compound remains Heisenberg-like at all temperatures, with its transition to the Néel state due to the interlayer interactions. Effects of field-induced anisotropy have been observed.
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