We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni3V2O8. The temperature and magnetic-field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.

No abstract

No abstract

We present thermodynamic and neutron data on Ni3V2O8, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T -one amplitude modulated, the other helical -plus a commensurate canted antiferromagnet for T → 0. The H − T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni3V2O8 thus provides an elegant example of order from sub-leading interactions in a highly frustrated system. PACS numbers: 75.10. Jm, 75.25.+z, 75.30.Kz Geometrical magnetic frustration leads to unusual low temperature spin order and dynamics and presents new challenges for the theoretical understanding of magnetic systems. Frustrated materials are often characterized by triangle-based lattices and short-range antiferromagnetic (AF) interactions. 1 Of particular interest has been magnetism on the two-dimensional (2D) kagomé lattice, which consists of corner-sharing triangles. While the Heisenberg spin-1/2 model appears to have short range spin correlations and a gap to free spinons, 2,3 the S → ∞ classical model has Néel order with a √ 3 × √ 3 unit cell at temperature T = 0. 4 Materials that approximate the kagomé AF can be expected to lie close to a quantum critical point, and indeed early work on the kagomé system SCGO exposed a spin liquid phase possessing a large fraction (15%) of the total spin entropy and short range √ 3 × √ 3 order. 5,6 Later work on jarosite systems showed different "q = 0" long range order apparently favored by interlayer interactions. 7Here we study Ni 3 V 2 O 8 (NVO) in which the S = 1 Ni 2+ spins form the orthorhombic kagomé staircase structure shown in Fig. 1(a). 8 This structure has the coordination and two-dimensionality of the regular kagomé lattice, but the kagomé planes are buckled. The system is particularly attractive because its complex magnetic phase diagram can be understood on the basis of an embellished kagomé spin hamiltonian. The model we introduce also applies to the isostructural compounds where Ni is replaced by Cu 9 or Co. 10 Although the symmetry of these compounds is the same as that of NVO, their phase diagrams are very different. As indicated below, this difference results from a small quantitative change in the parameters which dictate how frustration is relieved.A previous study of the magneto-thermal response in polycrystalline NVO revealed four zero field phase transitions with Θ W /T N > 5, where Θ W is the Weiss constant and T N the magnetic ordering temperature. 10 In this letter we report an unexpectedly rich anisotropic field-temperature (H − T ) phase diagram (Fig. 2), with high and low temperature incommensurate (IC) phases (HTI and LTI) and two commensurate (C and C') spin structures. These magnetic structures are determined via neutron diffraction. We also explain the salient features of NVO by a model, in which the spine (Ni s ) and cross-tie (Ni c ) spins interact via nearest neighbor (NN) and second nearest-neigh...

We present powder and single-crystal neutron diffraction and bulk measurements of the Kagoméstaircase compound Ni3V2O8 (NVO) in fields up to 8.5T applied along the c-direction. (The Kagomé plane is the a-c plane.) This system contains two types of Ni ions, which we call "spine" and "crosstie". Our neutron measurements can be described with the paramagnetic space group Cmca for T < 15K and each observed magnetically ordered phase is characterized by the appropriate irreducible representation(s). Our zero-field measurements show that at TPH = 9.1K NVO undergoes a transition to an incommensurate order which is dominated by a longitudinally-modulated structure with the spine spins mainly parallel to the a-axis. Upon further cooling, a transition is induced at THL = 6.3K to an elliptically polarized incommensurate structure with both spine and cross-tie moments in the a-b plane. At TLC = 4K the system undergoes a first-order phase transition, below which the magnetic structure is a commensurate antiferromagnet with the staggered magnetization primarily along the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat peak at T CC ′ = 2.3K indicates an additional transition, which we were however not able to relate to a change of the magnetic structure. Neutron, specific heat, and magnetization measurements produce a comprehensive temperature-field phase diagram. The symmetries of the two incommensurate magnetic phases are consistent with the observation that only one phase has a spontaneous ferroelectric polarization. All the observed magnetic structures are explained theoretically using a simplified model Hamiltonian, involving competing nearest-and next-nearest-neighbor exchange interactions, spin anisotropy, Dzyaloshinskii-Moriya and pseudo-dipolar interactions.