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

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 nonfrustrated model of weakly coupled spin chains with intrachain coupling J similar or equal to 38 K and Neel temperature T-N similar or equal to 6 K, both in excellent agreement with experiment. The intrachain coupling between spin-1/2 V+4 ions takes place via two contiguous V+5 O-4 tetrahedra forming an extended superexchange pathway with the V+4-V+4 distance of 7.44 angstrom. 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 nonmagnetic, 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.