The Two-Dimensional Square-Lattice S=1/2 Antiferromagnet Cu(pz)2(ClO4)2
Abstract
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 m0=0.47(5)muB 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 m0=0.93(5)muB at mu0H=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 J1xy=1.563(13)meV and J1z=0.9979(2)J1xy.