Reversibly Controlled Ternary Polar States and Ferroelectric Bias Promoted by Boosting Square-Tensile-Strain

Abstract
Interaction between dipoles often emerges intriguing physical phenomena, such as exchange bias in the magnetic heterostructures and magnetoelectric effect in multiferroics, which lead to advances in multifunctional heterostructures. However, the defect-dipole tends to be considered the undesired to deteriorate the electronic functionality. Here, deterministic switching between the ferroelectric and the pinched states by exploiting a new substrate of cubic perovskite, BaZrO3 is reported, which boosts the square-tensile-strain to BaTiO3 and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation. First-principles calculations propose a complex of an oxygen vacancy and two Ti3+ ions coins a charge-neutral defect-dipole. Cooperative control of the defect-dipole and the spontaneous polarization reveals ternary in-plane polar states characterized by biased/pinched hysteresis loops. Furthermore, it is experimentally demonstrated that three electrically controlled polar-ordering states lead to switchable and nonvolatile dielectric states for application of nondestructive electro-dielectric memory. This discovery opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates.

A research team, affiliated with UNIST has unveiled a novel method of storing data using the dielectric constant, rather than the electrical resistivity. According to the research team, their findings are expected to opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates.

This breakthrough has been carried out by Professor Yoon Seok Oh and his research team in the Department of Physics at UNIST, in collaboration with Professor Tae Heon Kim from the University of Ulsan.

In this study, the research team developed a new substrate of cubic perovskite, BaZrO3, which boosts the square-tensile-strain to BaTiO3 and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation.

The fourfold symmetric square lattice and strain on the surface of the cubic perovskite BaZrO3 substrate produce four variants of 100 in-plane electric polarization and the domain structures. The large tensile strain also induces built-in oxygen vacancies and defect-dipoles due to the unit cell expansion. As the strain-driven built-in defect-dipole cooperates with the four-variant domains of the in-plane polarization, we have found that the electrical poling process reversibly control the orientation of the built-in defect-dipoles and the ternary polar states, characterized by the biased/pinched hysteresis loops.

The development of four-variants polar domains of the BaTiO3 on the BaZrO3 substrate presents that the large isotropic surface lattice can spawn a novel ground state and physical phenomena in other inaccessible heterostructures, such as 2D topological phases of honeycomb superlattices on sixfold symmetric surface. The BaZrO3 substrate will be harnessed as a new platform for artificial design to a conceptual material system via heteroepitaxy inevitably combined with strain engineering. In addition, the switchable dielectric states inspire that the dielectric constant, rather than the electrical resistivity, can be considered a low-energy-consumption memory information.

The findings of this research have been published in the October 2022 issue of Advanced Materials, a top journal in the field of condensed matter physics.

Journal Reference
Jun Han Lee,Nguyen Xuan Duong, et al., “Reversibly Controlled Ternary Polar States and Ferroelectric Bias Promoted by Boosting Square-Tensile-Strain,” Adv. Mater., (2022).