Theoretical analysis of magnetic coupling in sandwich clusters Vn(C6H6)n+l

The mechanism of ferromagnetism stability in sandwich clusters V-n(C6H6)(n+1) has been studied by first-principles calculation and model analysis. It is found that each of the three types of bonds between V and benzene (Bz) plays different roles. V 3d(z)(2) orbital, extending along the molecular axis, is weakly hybridized with Bz's HOMO-1 orbital to form the sigma-bond. It is quite localized and singly occupied, which contributes 1 mu(B) to the magnetic moment but little to the magnetic coupling between neighboring V magnetic moments. The in-plane d(x)(-y)(2)(2), d(xy) orbitals are hybridized with the LUMO of Bz and constitute the delta-bond. This hybridization is medium and crucial to the magnetic coupling though the delta states have no net contribution to the total magnetic moment. d(xz), d(yz), and HOMO of Bz form a quite strong pi-bond to hold the molecular structure but they are inactive in magnetism because their energy levels are far away from the Fermi level. Based on the results of first-principles calculation, we point out that the ferromagnetism stability is closely related with the mechanism proposed by Kanamori and Terakura [J. Phys. Soc. Jpn. 70 (2001) 1433]. However, the presence of edge Bzs in the cluster introduces an important modification and suppresses significantly the ferromagnetism stability. A simple model is constructed to explain the essence of the physical picture.