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The J1-J2 model on a square lattice exhibits a rich variety of different forms of magnetic order that depend sensitively on the ratio of exchange constants J2/J1. We use bulk magnetometry and polarized neutron scattering to determine J1 and J2 unambiguously for two materials in a new family of vanadium phosphates, Pb2VO(PO 4 ) 2 and SrZnVO(PO 4 ) 2 , and we find that they have ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground state is reduced, and the diffuse magnetic scattering is enhanced, as the predicted bond-nematic region of the phase diagram is approached.

The magnetically ordered phases and spin dynamics of magnetoelectric LiNiPO4 have been studied in fields up to 17.3 T along the c axis. Using neutron diffraction, we show that a previously proposed linearly polarized incommensurate (IC) structure exists only for temperatures just below the Néel temperature T N . The ordered IC structure at the lowest temperatures is shown instead to be an elliptically polarized canted spiral for fields larger than 12 T. The transition between the two IC phases is of second order and takes place about 2 K below T N . For µ 0 H > 16 T and temperatures below 10 K, the spiral structure is found to lock in to a period of five crystallographic unit cells along the b axis. Based on the neutron diffraction data, combined with detailed magnetization measurements along all three crystallographic axes, we establish the magnetic phase diagrams for fields up to 17.3 T along c and for fields up to 16 T along a and b. The spin excitations in the highfield IC spiral phase have been studied in detail by inelastic neutron scattering. A mean-field analysis shows that the spin Hamiltonian derived previously from the low-temperature spin waves at zero field predicts the transition between the linear and elliptical polarization of the IC structure, and that a generalization of the spin-wave theory, assuming the random-phase-approximation, accounts for the inelastic scattering data obtained in the commensurable uniform phase at fields below 12 T as well as those obtained in the high-field IC spiral phase.

We report thermodynamic properties, magnetic ground state, and microscopic magnetic model of the spin-1 frustrated antiferromaget Li2NiW2O8 showing successive transitions at TN1 18 K and TN2 12.5 K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector k = ( 1 2 , 0, 1 2 ) below TN2. The ordered moment of 1.8 µB at 1.5 K is directed along [0.89(9), −0.10(5), −0.49 (6)] and matches the magnetic easy axis of spin-1 Ni 2+ ions, which is determined by the scissor-like distortion of the NiO6 octahedra. Incommensurate magnetic order, presumably of spin-density-wave type, is observed in the region between TN2 and TN1. Density-functional band-structure calculations put forward a three-dimensional spin lattice with spin-1 chains running along the [011] direction and stacked on a spatially anisotropic triangular lattice in the ab plane. We show that the collinear magnetic order in Li2NiW2O8 is incompatible with the triangular lattice geometry and thus driven by a pronounced easy-axis single-ion anisotropy of Ni 2+ . a O3 O2 O1 O2 ' c b J 011 J 100 x z J 010 J 110