Orbital ordering induced ferromagnetism in Lu2V2O7

A V4+ pyrochlore Lu2V2O7 is a ferromagnetic Mott insulator with Tc = 73 K. The electronic configuration of the V4+ ions is (t2g)1, and the orbital ordering pattern was determined by the polarized neutron diffraction measurements. To discuss the stability of the ferromagnetic state in this compound, we treat the Hubbard Hamiltonian with 3d orbitals in the small hopping amplitude limit. We found that the hopping from t2g orbitals to eg orbitals, which is normally neglected in t2g electron system, plays an important role to stabilize the ferromagnetic state for observed orbital ordering structure. Spin wave spectrum has been calculated and the effects of the anisotropy are also discussed.