Symmetry Analysis for the Ruddlesden-Popper Systems, Ca3Mn2O7 and Ca3Ti2O7

TL;DR

This paper conducts a symmetry analysis of structural and magnetic phase transitions in Ca3Mn2O7 and Ca3Ti2O7, revealing coupled ferroelectric and magnetic orderings and domain structures.

Contribution

It introduces a comprehensive symmetry-based framework with order parameters and Landau free energy to understand phase transitions and coupled properties in these materials.

Findings

Coupled ferroelectric and weak ferromagnetic orderings.

Identification of domain structures and phase domains.

Symmetry relations elucidate structural and magnetic transitions.

Abstract

We perform a symmetry analysis of the zero-temperature instabilities of the tetragonal phase of Ca3Mn2O7 and Ca3Ti2O7 which is stable at high temperature. We introduce order parameters to characterize each of the possible lattice distortions in order to construct a Landau free energy which elucidates the proposed group-subgroup relations for structural transitions in these systems. We include the coupling between the unstable distortion modes and the macroscopic strain tensor. We also analyze the symmetry of the dominantly antiferromagnetic ordering which allows weak ferromagnetism. We show that in this phase the weak ferromagnetic moment and the spontaneous ferroelectric polarization are coupled, so that rotating one of these ordering by applying an external electric or magnetic field one can rotate the other ordering. We discuss the number of different domains (including phase domains) which exist in each of the phases and indicate how these may be observed.