K. Yamauchi — Page 2

arXiv2011

Ferroelectricity due to orbital ordering in E-type undoped rare-earth manganites

Aiming at understanding the origin of the electronic contribution to ferroelectric polarization in undoped manganites, we evaluate the Berry phase of orbital-polarizable Bloch electrons as an orbital ordering (OO) establishes in the background of an antiferromagnetic E-type configuration. The onset of OO is tuned by the Jahn-Teller (JT) interaction in a tight-binding model for interacting electrons moving along zigzag chains. A finite polarization is found as soon as the JT coupling is strong enough to induce OO, supporting the large electronic contribution predicted from first principles.

cond-mat.str-el↗

arXiv2006

First-principles stabilization of an unconventional collinear magnetic ordering in distorted manganites

First-principles calculations have been performed for different collinear magnetic orderings in orthorhombic manganites, such as HoMnO3, TbMnO3 and YMnO3, showing large GdFeO3-like distortions. Our results suggest that the AFM-E type ordering, experimentally observed in HoMnO3 and recently proposed from model hamiltonian studies as a potentially novel phase, is indeed the magnetic ground state. Its stability is strongly connected with octahedral distortions and points to the relevance of structural more than chemical effects. The calculated exchange constants, extracted from a Heisenberg model used to fit the first-principles total energies, show that the ferromagnetic in-plane nearest-neighbour coupling is reduced compared to less-distorted manganites, such as LaMnO3. In parallel, the antiferromagnetic next-nearest-neighbour coupling along planar Mn-O-O-Mn paths in highly-distorted manganites plays a relevant role in the stabilization of the AFM-E spin configuration. In agreement with experiments, the density of states shows that this phase is insulating with an indirect band-gap of about 0.5 eV.

cond-mat.mtrl-sci↗