Characterizing Hybrid Improper Ferroelectrics for Novel Applications

The distortion of crystal structure in ferroelectric materials results in a spontaneously formed polarization and electric field. Due to this distinctive property, ferroelectrics could be found in anything from diesel fuel injectors and ultrasound machines to computer memory.

Some of the most sophisticated technologies that exist today are based on ferroelectric materials. Discoveries showing that it is possible to observe ferroelectricity in materials that demonstrate other spontaneous transitions, such as ferromagnetism, have resulted in an innovative category of such materials, called hybrid improper ferroelectrics.

However, the characteristics of such materials have not yet been completely understood. New findings reported in Applied Physics Letters, from AIP Publishing, help elucidate the properties of these materials and indicate the prospects for innovative storage and optoelectronic applications.

A group of scientists from China has characterized a specific type of hybrid improper ferroelectric, Ca₃Mn₂O₇. The scientists analyzed the magnetoelectric, ferroelectric, and optical properties of the material.

They could not only demonstrate ferroelectricity in Ca₃Mn₂O₇ but also coupling between its ferroelectricity and magnetism—an important property that has the ability to enable more efficient and more rapid 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."

Shuai Dong, Co-Author

For example, similar to batteries, ferroelectrics include positively and negatively charged poles. However, a key differentiating aspect of these materials is that it is possible to reverse this polarization by applying an external electric field.

In contrast to conventional ferroelectrics, the properties of which are directly derived from polar distortions in the crystal lattice of the material, hybrid improper ferroelectrics form polarization from a combination of non-polar distortions.

In 2011, when, for the first time, hybrid improper ferroelectrics were theorized, two materials were proposed. From that time, non-magnetic Ca₃Ti₂O₇ crystals were experimentally demonstrated; however, a complete characterization of Ca₃Mn₂O₇, its magnetic counterpart, has been pending.

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

Shuai Dong, Co-Author

In order to gain further knowledge about Ca₃Mn₂O₇, Dong and his colleagues confirmed the ferroelectricity of the material with the help of pyroelectric measurements that investigate its electric properties over an array of temperatures and also measured the ferroelectric hysteresis loops of Ca₃Mn₂O₇, a technique that prevents some extrinsic leakage.

Subsequent analysis demonstrated that Ca₃Mn₂O₇ exhibits a weak ferromagnetism that can be regulated by applying an electric field.

They discovered that Ca₃Mn₂O₇, a material that was long-considered to have magnetoelectric and ferroelectric properties, also demonstrated powerful visible light absorption in a band gap best suited for photoelectric devices.

This property of Ca₃Mn₂O₇ might open the door for the material to be used in anything from light sensors to photovoltaic cells with the intrinsic electric field resulting in larger photogenerated voltage compared to existing devices.

As a next step, Dong aspires to investigate the photoelectric properties of Ca₃Mn₂O₇ and also test whether the introduction of iron into the crystal would improve its magnetism.