Field-induced anisotropy in the quasi-two-dimensional weakly anisotropic antiferromagnet [CuCl(pyz)2]BF4

We measured NMR and magnetic susceptibility for the quasi-two-dimensional, weakly XY -like, spin-1/2 square-lattice Heisenberg antiferromagnet [CuCl(pyz)₂]BF₄ (pyz = pyrazine = N₂C₄H₄) near the critical temperature. The Néel temperature T_N and the order-parameter critical exponent β were obtained from the NMR line broadening as a function of temperature. As the applied field strength (H || c) was increased, T_N increased and β decreased. This behavior indicates that the field effectively enhanced XY anisotropy. The susceptibility as a function of temperature did not show a clear feature for T_N, but showed field-dependent minima below T_N for both H || c and H || ab, where minimum features disappeared for μ₀H > 2 T.