We measured NMR and magnetic susceptibility for the quasi-two-dimensional, weakly $XY$-like, spin-$1/2$ square-lattice Heisenberg antiferromagnet $\left[\mathrm{CuCl}{\left(\mathrm{pyz}\right)}_2\right]{\mathrm{BF}}_4$ (pyz = pyrazine $={\mathrm{N}}_2{\mathrm{C}}_4{\mathrm{H}}_4$) near the critical temperature. The Néel temperature $T_N$ and the order-parameter critical exponent $\beta$ were obtained from the NMR line broadening as a function of temperature. As the applied field strength ($H\parallel c$) was increased, $T_N$ increased and $\beta$ 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 $H\parallel c$ and $H\parallel ab$, where minimum features disappeared for ${\mu }_{0}H>2\mathrm{T}$.

• Received 31 January 2019
• Revised 9 April 2019

DOI:https://doi.org/10.1103/PhysRevB.99.214403