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) µB 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 squarelattice planes lead to an increase of the antiferromagnetically ordered moment to m0 = 0.93(5) µB at µ0H = 13.5 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 xy 1 = 1.563(13) meV and J z 1 = 0.9979(2)J xy 1 .

Magnetism has been predicted to occur in systems where dipolar interactions dominate exchange. We present neutron scattering, specific heat and magnetic susceptibility data for LiErF 4 , establishing it as a model dipolar-coupled antiferromagnet with planar spin-anisotropy and a quantum phase transition 1

We have studied the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)_{2}(ClO4)_{2} (where pz denotes pyrazine), using neutron inelastic scattering and series expansion calculations. We show that the presence of antiferromagnetic next-nearest-neighbor interactions enhances quantum fluctuations associated with resonating valence bonds. Intermediate magnetic fields lead to a selective tuning of resonating valence bonds and a spectacular inversion of the zone-boundary dispersion, providing novel insight into 2D antiferromagnetism in the quantum limit.

We study the strongly anisotropic quasi-one-dimensional S = 1 quantum magnet NiCl2·4SC(NH2)2 using elastic and inelastic neutron scattering. We demonstrate that a magnetic field splits the excited doublet state and drives the lower doublet state to zero energy at a critical field Hc1. For Hc1 < H < Hc2, where Hc2 indicates the transition to a fully magnetized state, three-dimensional magnetic order is established with the AF moment perpendicular to the magnetic field. We mapped the temperature/magnetic field phase diagram, and we find that the total ordered magnetic moment reaches m(tot) = 2.1 μB at the field μ(0)H = 6 T and is thus close to the saturation value of the fully ordered moment. We study the magnetic spin dynamics in the fully magnetized state for H > Hc2, and we demonstrate the presence of an AF interaction between Ni(2+) on the two interpenetrating sublattices. In the antiferromagnetically ordered phase, the spin-waves that develop from the lower-energy doublet are split into two modes. This is most likely the result of the presence of the AF interaction between the interpenetrating lattices.