Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry

TL;DR

This paper investigates how crystal chemistry influences magnetoelectric ordering in BiFeO3, emphasizing the role of the Bi lone pair activity and its temperature-dependent effects on phase transitions, magnetic couplings, and spin structures.

Contribution

It reveals the significance of the Bi lone pair's stereochemical activity in phase transitions and magnetic ordering, providing new insights into the mechanisms behind magnetoelectric phenomena in BiFeO3.

Findings

Lone pair activity increases as temperature decreases.

Magnetic couplings switch from antiferromagnetic to ferromagnetic with temperature.

Doping with La3+ suppresses the spatially modulated spin structure.

Abstract

In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic beta-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity - its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO3 in the range from 945 C down to 25 C and found the couplings, which undergo the antiferromagnetic-ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La3+.