We present an experimental study of the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)2(ClO4)2 (pz denotes pyrazine - C4H4N2) 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 Cu(pz)2(ClO4)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(5)mu_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 Cu(pz)2(ClO4)2. Magnetic fields applied perpendicular to the square-lattice planes lead to an increase of the antiferromagnetically ordered moment to m_0=0.93(5)mu_B at mu_0H=13.5T - 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^{xy}=1.563(13)meV and J_1^z=0.9979(2)J_1^{xy}.
@article{arxiv.1007.2279,
title = {The Two-Dimensional Square-Lattice S=1/2 Antiferromagnet Cu(pz)$_2$(ClO$_4$)$_2$},
author = {N. Tsyrulin and F. Xiao and A. Schneidewind and P. Link and H. M. Rønnow and J. Gavilano and C. P. Landee and M. M. Turnbull and M. Kenzelmann},
journal= {arXiv preprint arXiv:1007.2279},
year = {2010}
}