Direct exchange in the edge-sharing spin-1/2 compound MgVO_3

Bandstructure calculations with different spin arrangement for the spin-chain compound MgVO_3 have been performed, and paramagnetic as well as magnetic solutions with ferro- and antiferromagnetically ordered chains are found, the magnetic solutions being by 0.22 eV per formula unit lower than the paramagnetic one. The orbital analysis of the narrow band crossing the Fermi level in the paramagnetic solution reveals that the band has almost pure vanadium 3d character, the lobes of the relevant d-orbitals at the neighboring in-chain sites being directed towards each other, which suggests direct exchange. The tight-binding analysis of the band confirms the strong exchange transfer between neighboring in-chain V-ions. Besides, some additional superexchange transfer terms are found, which give rise both to in-plane coupling between the chains and to frustration, the dominant frustration occuring due to the interchain interaction.