Magnetoelectric correlations in BiMnO$_3$ whithin Landau theory: comparison with experiment

TL;DR

This paper develops a Landau theory model for magnetoelectric correlations in BiMnO$_3$, explaining experimental observations and predicting how coupled magnetic and ferroelectric order parameters behave near phase transitions.

Contribution

It introduces a coupled two-order-parameter Landau model for BiMnO$_3$ and compares its predictions with experimental data, including susceptibility and coherence length.

Findings

Magnetoelectric coupling significantly renormalizes the magnetic transition temperature.

The model predicts an additional anomaly in the ferroelectric subsystem.

Results show good agreement with experimental data for BiMnO$_3$.

Abstract

We discuss a simple phenomenological Landau theory of phase transitions with two coupled single-component order parameters and compare the results with available experimental data. The model corresponds to the case of a ferroic system, in which ferromagnetic and ferroelectric transitions originally occur at temperatures $T_M$ and $T_f$, respectively. For $T_f>T_M$ the magnetoelectric coupling strongly renormalizes the magnetic transition temperature, $T_M\to T_{RM}$ (with $T_{RM}>>T_M$), as well as generates an additional anomaly in ferroelectric subsystem $T_{RM}$. Full susceptibility tensor has also been determined. The concept of \textit{Arrot plot} is replaced by the \textit{Arrot planes} which appear when both types of order coexist. The results are in good overall agreement with experimental data for the ferroelectromagnetic BiMnO$_3$. We also estimate the contribution of Gaussian fluctuations of both order parameters, that lead to corrections to the mean-field specific heat. Those corrections are still insufficient even though other quantities agree quite well with experiment. We calculate the temperature dependence of the coherence length for both types of order as well.