Spin order due to orbital fluctuations: Cubic vanadates

We investigate the highly frustrated spin and orbital superexchange interac tions in cubic vanadates. The fluctuations of t(2g) orbitals trigger a nove l mechanism of ferromagnetic interactions between spins S = 1 of V3+ ions a long one of the cubic directions which operates already in the absence of H und's rule exchange J(H), and leads to the C-type antiferromagnetic phase i n LaVO3. The Jahn-Teller effect can stabilize the orbital ordering and the G-type antiferromagnetic phase at low temperatures, but large entropy due t o orbital fluctuations favors again the C phase at higher temperatures, as observed in YVO3.