Microscopic Model and Phase Diagrams of the Multiferroic Perovskite Manganites

TL;DR

This paper develops a microscopic spin model for orthorhombic RMnO3 manganites, reproducing their complex phase diagrams and multiferroic states, and explaining how lattice distortions influence phase transitions and spin structures.

Contribution

It introduces a comprehensive microscopic model including superexchange, SIA, and DMI, accurately reproducing experimental phase diagrams and elucidating the role of lattice distortions in phase competition.

Findings

Reproduces experimental phase diagrams with Monte Carlo simulations.

Identifies two multiferroic states: ab-plane and bc-plane spin cycloids.

Shows lattice distortion controls phase reorientation and spin structures.

Abstract

Orthorhombically distorted perovskite manganites, RMnO3 with R being a trivalent rare-earth ion, exhibit a variety of magnetic and electric phases including multiferroic (i.e. concurrently magnetic and ferroelectric) phases and fascinating magnetoelectric phenomena. We theoretically study the phase diagram of RMnO3 by constructing a microscopic spin model, which includes not only the superexchange interaction but also the single-ion anisotropy (SIA) and the Dzyaloshinsky-Moriya interaction (DMI). Analysis of this model using the Monte-Carlo method reproduces the experimental phase diagrams as functions of the R-ion radius, which contain two different multiferroic states, i.e. the ab-plane spin cycloid with ferroelectric polarization P//a and the bc-plane spin cycloid with P//c. The orthorhombic lattice distortion or the second-neighbor spin exchanges enhanced by this distortion exquisitely controls the keen competition between these two phases through tuning the SIA and DMI energies. This leads to a lattice-distortion-induced reorientation of P from a to c in agreement with the experiments. We also discuss spin structures in the A-type antiferromagnetic state, those in the cycloidal spin states, origin and nature of the sinusoidal collinear spin state, and many other issues.