G-type antiferromagnetism and orbital ordering due to the crystal field from the rare-earth ions induced by the GdFeO3-type distortion in RTiO3 where R = La, Pr, Nd and Sm

The origin of the antiferromagnetic order and puzzling properties of LaTiO3 as well as the magnetic phase diagram of the perovskite titanates are studied theoretically. We show that in LaTiO3, the t(2g) degeneracy is eventually lifted by the La cations in the GdFeO3-type structure, which generates a crystal field. This allows the description of the low-energy structure of LaTiO3 by a single-band Hubbard model as a good starting point. The lowest-orbital occupation in this crystal field stabilizes the AFM(G) state, and well explains the spin-wave spectrum of LaTiO3 obtained by the neutron scattering experiment. The orbital-spin structures for RTiO3 where R = Pr, Nd and Sm are also accounted for by the same mechanism. We point out that through generating the R crystal field, the GdFeO3-type distortion has a universal relevance in determining the orbital-spin structure of the perovskite compounds in competition with the Jahn-Teller mechanism, which has been overlooked in the literature. Since the GdFeO3-type distortion is a universal phenomenon as is seen in a large number of perovskite compounds, this mechanism may also play important roles in other compounds of this type.