New study sheds light on hybrid improper ferroelectrics

Distortions in the crystal structure of ferroelectric materials give rise to a spontaneously formed polarization and electric field. Because of this unique property, ferroelectrics can be found in everything from ultrasound machines and diesel fuel injectors to computer memory. Ferroelectric materials are behind some of the most advanced technology available today.

The finding that ferroelectricity can be observed in materials that exhibit other spontaneous transitions, like ferromagnetism, has given rise to a new class of these materials, known as hybrid improper ferroelectrics. The properties of this new class of material are, however, still far from being fully understood. New findings published in a paper in Applied Physics Letters help to shine light on these materials and indicate their potential for new optoelectronic and storage applications.

A team of researchers from China has characterized one type of hybrid improper ferroelectric made up of calcium, manganese and oxygen (Ca₃Mn₂O₇), investigating its ferroelectric, magnetoelectric and optical properties. Not only were they able to demonstrate ferroelectricity in Ca₃Mn₂O₇, but also coupling between its magnetism and ferroelectricity, a key property that could lead to faster and more efficient bit operations in computers.

"Our work solves a long-term puzzle in this field, which could push forward the frontiers and enhance the confidence to continue the research in this field," said Shuai Dong from Southeast University in Nanjing.

Like batteries, ferroelectrics have positively and negatively charged poles. A major distinguishing feature of these materials, however, is that this polarization can be reversed by using an external electrical field.

"This can be useful because it can be used in devices to store information as ones and zeros," Dong explained. "Also, the switching of polarization can generate current, which can be used in sensors."

"Our work solves a long-term puzzle in this field, which could push forward the frontiers and enhance the confidence to continue the research in this field."Shuai Dong, Southeast University

Unlike traditional ferroelectrics, which directly derive their properties from polar distortions in the material's crystal lattice, hybrid improper ferroelectrics generate polarization from a combination of nonpolar distortions.

When hybrid improper ferroelectrics were first theorized in 2011, two materials were proposed. In the years since, nonmagnetic Ca₃Ti₂O₇ crystals have been demonstrated experimentally, but a full characterization of its magnetic counterpart, Ca₃Mn₂O₇, has remained elusive.

"Multiple transitions as well as phase separations were evidenced in Ca₃Mn₂O7, making it more complex than the early theoretical expectations," Dong said. "This material is complex, and the leakage is serious, which prevents the direct measurement of its ferroelectricity in high temperature."

To gain a better understanding of Ca₃Mn₂O₇, Dong and his collaborators confirmed the material's ferroelectricity using pyroelectric measurements that examined its electric properties across a range of temperatures. They also measured Ca₃Mn₂O₇'s ferroelectric hysteresis loops, a method that mitigates some extrinsic leakage. Further investigation showed that Ca₃Mn₂O₇ exhibits a weak ferromagnetism that can be modulated by an electric field.

They also found that Ca₃Mn₂O₇, which was long-rumored to have ferroelectric and magnetoelectric properties, exhibited strong visible light absorption in a band gap well suited for photoelectric devices. This feature of Ca₃Mn₂O₇ might pave the way for the material to be used for applications such as photovoltaic cells and light sensors, with the built-in electric field leading to a larger photogenerated voltage than can be produced by today's devices.

"The most surprising thing for us was that no one noticed its prominent light absorption before," Dong said. In the future, Dong hopes to explore Ca₃Mn₂O₇'s photoelectric properties, as well as investigate whether introducing iron to the crystal would enhance its magnetism.

This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.