Abstract

The electronic ferroelectric material RFe₂O₄ possesses a layered structure in which rare-earth/oxygen layers (R-layers) and iron/oxygen double layers (W-layers) alternate; it was predicted to exhibit ferroelectricity through the formation of a charge ordered within the W-layers formed by Fe²⁺ and Fe³⁺ ions[1]. This charge order arises from the effects of charge frustration, where adjacent ions share the same valence state, leading to erexchange interactions. Furthermore, since the spins of these identical ions are also aligned, the system—incorporating spin exchange interactions, adopts a highly degenerate energy state. Consequently, as the temperature rises, this polar charge order gains entropy associated with this degeneracy and become stable. It is through the complex interplay of thermal, magnetic, and electric factors that the material becomes a room-temperature ferroelectric [2]. Such ferroelectric polarization was expected to exhibit various interesting properties. For instance, since the polarization is associated with charge ordering, it’s reversal should be able to achieve through the movement of electrons., which is lighter than moving atoms, that can be anticipated to have a very low coercive field. Furthermore, given the presence of charge ordering coupled with magnetism, it is conceivable that this system exhibits multiferroic polarization capable of being reversed by a magnetic field. However, because this system exhibits high electrical conductivity, standard techniques for evaluating ferroelectricity, such as pyroelectric current measurements cannot be applied, making experimental verification challenging. Hence, we developed a serials methods utilizing pulsed lasers that avoids current generation. By this approach, we demonstrated that RFe₂O₄ possesses a coercive field of 15 V/cm, which is four orders of magnitude lower than that of conventional ferroelectrics. Furthermore, we also demonstrated that ferroelectric polarization of RFe₂O₄ can be reversed by a magnetic field as low as 800 G. These results indicate the material's potential for application as a novel semiconductor in the development of energy efficient devices. We also conducted experiments demonstrating polarization control via ultrafast optical electric fields and the generation of reversible terahertz electric fields [3]. Such result we plan to present at this event.

Reference

[1] Sumio Ishihara, J. Phys. Soc. Jpn. 79, 011010 (2010).
[2] K. Fujiwara, H. Yu et al., Sci. Rep., 11, 4277 (2021).
[3] H. Yu et al., Materials 16, 1989 (2023).

Biography

Dr. Hongwu Yu obtained his Bachelor and Master degrees from Tokyo Institute of Technology, Japan, and obtained his Ph.D. from Tokyo Institute of Technology, Japan in 2024. From 2024 to 2025, he served as an assistant professor in the department of physics, Okayama University, Japan.  Currently, he is an assistant professor at the insittute of integrated research, Institute of Science Tokyo, Japan.  His research interests manily focused on synthesis, fabrication, and invesigating related magnetic, electric, and optical properties of inorganic materials.

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