Direct observation of ferroelectricity in Ca$_3$Mn$_2$O$_7$ and its prominent light absorption

TL;DR

This study confirms ferroelectricity and magnetoelectric coupling in Ca$_3$Mn$_2$O$_7$, a layered perovskite, and highlights its strong visible light absorption, making it promising for photoelectric applications.

Contribution

It provides the first direct measurement of switchable polarization and demonstrates magnetoelectric coupling in Ca$_3$Mn$_2$O$_7$, expanding understanding of magnetic ferroelectric materials.

Findings

Confirmed ferroelectricity in Ca$_3$Mn$_2$O$_7$

Observed magnetoelectric response

Identified strong visible light absorption

Abstract

Layered perovskites $A_3M_2$O$_7$ are known to exhibit the so-called hybrid improper ferroelectricity. Despite experimentally confirmed cases (e.g. nonmagnetic $M$=Ti and Sn), the ferroelectricity in magnetic Ca$_3$Mn$_2$O$_7$ remains a puzzle. Here, the structural, ferroelectric, magnetoelectric, and optical properties of Ca$_3$Mn$_2$O$_7$ are systematically investigated. Switchable polarization is directly measured, demonstrating its ferroelectricity. In addition, magnetoelectric response is also evidenced, implying the coupling between magnetism and ferroelectricity. Furthermore, strong visible light absorption is observed, which can be understood from its electronic structure. Its direct and appropriate band gap, as well as wide conducting bands, makes Ca$_3$Mn$_2$O$_7$ a potential candidate for ferroelectric photoelectric applications.