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 with R=La, Pr, Nd and Sm

TL;DR

This paper explains how GdFeO_3-type distortions in RTiO_3 perovskites lift t_{2g} degeneracy, leading to orbital ordering and antiferromagnetism, with implications for understanding magnetic properties across similar compounds.

Contribution

It demonstrates that GdFeO_3-type distortion universally influences orbital and magnetic structures in perovskite titanates, highlighting a mechanism previously overlooked in the literature.

Findings

GdFeO_3-type distortion lifts t_{2g} degeneracy in LaTiO_3.

The crystal field stabilizes the AFM(G) state, matching neutron scattering data.

The mechanism applies to RTiO_3 with R=Pr, Nd, Sm, affecting their magnetic structures.

Abstract

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 GdFeO_3-type structure, which generates a crystal field with nearly trigonal symmetry. 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 with 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.