Importance of In-Plane Anisotropy in the Quasi-Two-Dimensional AntiferromagnetBaNi2V2O8

Abstract

The phase diagram of the quasi two-dimensional antiferromagnet BaNi2V2O8 is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to c. At µ0H * ≃ 1.5 T, a crossover to a high-field state, where TN (H) increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of TN and H * are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.PACS numbers: 75.30. Gw, 75.30.Kz, 75.50.Ee The study of quasi two-dimensional (2D) magnetic systems [1] continues to be a focus of theoretical and experimental investigations, motivated in large part by the discovery of high-temperature superconductivity in the quasi 2D cuprates. Further, the search for a magnetic system exhibiting true Berezinskii-Kosterlitz-Thouless (BKT) behavior, initially proposed for 2D XY magnetic systems [2], has been elusive and has only been seen in superfluid and superconducting films [3]. Theoretical studies indicate that BKT behavior can also be expected for 2D Heisenberg systems with a small easy-plane XY anisotropy [4]. Two recent experimental papers suggest that BaNi 2 V 2 O 8 may in fact be a physical realization of such a system [5,6]. BaNi 2 V 2 O 8 has a rhombohedral structure (space group R3) and its magnetic properties arise from a honeycomb-layered arrangement of spins S = 1 at the Ni 2+ sites. The quasi 2D properties are due to a strong antiferromagnetic Heisenberg superexchange J in the NiO honeycomb layers. Long range antiferromagnetic ordering, which would be precluded in a purely 2D Heisenberg system, sets in below the Néel temperature T N ≃ 50 K [5] because of small additional energy scales, which we include in the following Hamiltonian:The planar XY anisotropy D XY ≃ 1 meV is a factor of 10 smaller than J [7] and confines the spins to lie within the honeycomb layers (easy plane). If this were the only additional term in Eq.(1), a true BKT transition could be expected within each 2D layer [4]. However, a real crystal is always three-dimensional (3D) and a very small interlayer exchange J ′ ultimately leads to a crossover from 2D to 3D correlations, and then to a 3D ordering transition [4]. The value of J ′ is unknown for the present case [7]; however, the extremely small signal at T N in the specific heat [5] suggests that J ′ /J is very small (typically, J ′ /J is in the range 10 −2 -10 −6 in quasi 2D systems [1]). The in-plane anisotropy D IP is estimated by 4*10 −3 meV [7] and acts to align the spins along one of the three equivalent hexagonal easy a-axes [5]. The last term in Eq. (1) includes the effect of a magnetic field H, which also acts as an effective anisotropy [1,8].In this Letter, we study the (T, H) phase diagram of BaNi 2 V 2 O 8 for magnetic fields applied within the honeycomb planes. The combination of specific he...

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