We present an experimental study of the two-dimensional $S=\frac{1}{2}$ square-lattice antiferromagnet $\text{Cu}{\left(\text{pz}\right)}_2{\left({\text{ClO}}_4\right)}_2$ (pz denotes pyrazine-${\text{C}}_4{\text{H}}_4{\text{N}}_2$) using specific-heat measurements, neutron diffraction, and cold-neutron spectroscopy. The magnetic field dependence of the magnetic ordering temperature was determined from specific-heat measurements for fields perpendicular and parallel to the square-lattice planes, showing identical field-temperature phase diagrams. This suggest that spin anisotropies in $\text{Cu}{\left(\text{pz}\right)}_2{\left({\text{ClO}}_4\right)}_2$ are small. The ordered antiferromagnetic structure is a collinear arrangement with the magnetic moments along either the crystallographic $b$ or $c$ axis. The estimated ordered magnetic moment at zero field is $m_0=0.47\left(5\right){\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}}_{\text{B}}$ and thus much smaller than the available single-ion magnetic moment. This is evidence for strong quantum fluctuations in the ordered magnetic phase of $\text{Cu}{\left(\text{pz}\right)}_2{\left({\text{ClO}}_4\right)}_2$. Magnetic fields applied perpendicular to the square-lattice planes lead to an increase in the antiferromagnetically ordered moment to $m_0=0.93\left(5\right){\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}}_{\text{B}}$ at ${\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}}_{0}H=13.5\text{T}$ evidence that magnetic fields quench quantum fluctuations. Neutron spectroscopy reveals the presence of a gapped spin excitations at the antiferromagnetic zone center and it can be explained with a slightly anisotropic nearest-neighbor exchange coupling described by $J_1^{\text{xy}}=1.563\left(13\right)\text{meV}$ and $J_1^{\text{z}}=0.9979\left(2\right)J_1^{\text{xy}}$.

• Received 29 May 2009

DOI:https://doi.org/10.1103/PhysRevB.81.134409