Spin-chain magnetism and uniform Dzyaloshinsky-Moriya anisotropy in BaV₃O₈

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We report on the microscopic magnetic model of a spin-1/2 magnet BaV₃O₈. In contrast to earlier phenomenological analysis, our density-functional band-structure calculations combined with quantum Monte-Carlo simulations establish a relatively simple and non-frustrated model of weakly coupled spin chains with the intrachain coupling of J 38 K and the Neel temperature of Tₙ 6 K, both in excellent agreement with the experiment. The intrachain coupling between the spin-1/2 V⁺⁴ ions takes place via two contiguous V⁺⁵O₄ tetrahedra forming an extended superexchange pathway with the V⁺⁴--V⁺⁴ distance of 7.44 A. Surprisingly, this pathway is preferred over shorter V⁺⁴--V⁺⁴ connections, owing to peculiarities in the interacting orbitals of the magnetic V⁺⁴ ions and V⁺⁵ ions that are non-magnetic, but feature low-lying 3d states contributing to the superexchange process. We also note that the crystal structure of BaV₃O₈ supports the long-sought uniform arrangement of Dzyaloshinsky-Moriya (DM) couplings on a spin-1/2 chain. While our calculations yield only a weak DM anisotropy in BaV₃O₈, the crystal structure of this compound provides a suitable framework for the search of spin chains with the uniform DM anisotropy in other compounds of the vanadate family.