G-type Antiferromagnetism and Orbital Ordering due to the Crystal Field from the Rare-Earth Ions Induced by the GdFeO_3-type Distortion in RTiO_3 Where R = La, Pr, Nd and Sm(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Propert

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The origin of the antiferromagnetic order and puzzling properties of LaTiO_3 as well as the magnetic phase diagram of the perovskite titanates are studied theoretically. We show that in LaTiO_3, the t_<2g> degeneracy is eventually lifted by the La cations in the GdFeOs-type structure, which generates a crystal field. This allows the description of the low-energy structure of LaTiO_3 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 LaTiO_3 obtained by the neutron scattering experiment. The orbital-spin structures for RTiO_3 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 GdFeO_3-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 GdFeO_3-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.
社団法人日本物理学会の論文
2004-07-15