Field-induced anisotropy in the quasi-two-dimensional weakly anisotropic antiferromagnet [CuCl(pyz) 2 ]BF 4

We measured NMR and magnetic susceptibility for the quasi-two-dimensional, weakly XY-like, spin-1/2 square-lattice Heisenberg antiferromagnet [CuCl(pyz) 2 ]BF 4 (pyz = pyrazine = N 2 C 4 H 4 ) near the critical temperature. The Neel 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 μ 0 H > 2T.