Competing Magnetic Phases on a Kagomé Staircase

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...

Magnetic anisotropy in geometrically frustrated kagome staircase lattices

“…Of particular interest has been magnetism of the two-dimensional kagome staircase M 3 V 2 O 8 (M ¼ Ni, Co, Mn) because of the concurrent presence of both highly frustrated lattice and strong quantum fluctuations. This system displays a rich, highly anisotropic, phase diagram [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Even though these materials have identical crystal symmetry and similar structural parameters, their magnetic properties are quite different.…”

Magnetic structure of the Kagomé mixed compound (Co0.5Ni0.5)3V2O8

“…The magnetic structure can be described as the antiferromagnetic coupling of ferromagnetic blocks (Co 4 O 15 + 2 CoO 4 units). This kind of magnetic structure has been previously reported for 6H-Ba 6 Co 6 ClO 16 [4].…”

“…Due to the reduced symmetry of the Kagomé staircase geometry with respect to the ideal plane net the degree of frustration is lowered leading to interesting long range ordered magnetic structures. Magnetization and neutron diffraction experiments on a (Co 0.52 Ni 0.48 ) 3 V 2 O 8 powder sample [3] revealed only one magnetic phase transition into an antiferromagnetic ground state in contrast to the richness of magnetic phase transitions of its parent compounds [4,5]. The magnetic structure is modulated by a composition dependent propagation vector k=(δ, 0, 0) with δ being 0.491(4) for (Co 0.52 Ni 0.48 ) 3 V 2 O 8 where a similarity to the NVO type magnetic structure was assumed [3].…”

“…Neutron single crystal diffraction experiments followed by group theory analysis produced a more detailed picture. The magnetic structure of (Co 0.5 Ni 0.5 ) 3 V 2 O 8 exhibits features, which differ from the predominantly collinear alignment of its parent compounds NVO and CVO, which exhibit a variety of magnetic structures with magnetic moments mainly oriented along the a axis [4][5][6][7]. The averaged magnetic moments of the statistically distributed Ni 2+ and Co 2…”

Nuclear-Magnetic-Resonance Properties of the Staircase Kagomé Antiferromagnet PbCu 3 TeO 7

“…Staircase Kagome lattice with buckled Kagomé layers is one example, which has been realized in several materials, including A 3 V 2 O 8 (A= Cu,Co,Ni) [7][8][9] and PbCu 3 TeO 7 [10] . Consecutive magnetic transitions and magnetism induced ferroelectricity were reported in Ni 3 V 2 O 8 , characterizing a multiferroic material with strong magnetoelectric coupling [11,12] . The lattice structure of the newly discovered PbCu 3 TeO 7 is illustrated in Fig.1.…”