Role of oxygen vacancies in colossal polarization in SmFeO_3-δ thin films

The orthorhombic rare-earth manganates and ferrites multiferroics are promising candidates for the next generation multistate spintronic devices. However, their ferroelectric polarization is small, and transition temperature is far below room temperature (RT). The improvement of ferroelectricity remains challenging. Here, through the subtle strain and defect engineering, an RT colossal polarization of 4.14 μC/cm² is achieved in SmFeO_3-δ films, which is two orders of magnitude larger than its bulk and is also the largest one among the orthorhombic rare-earth manganite and ferrite family. Meanwhile, its RT magnetism is uniformly distributed in the film. Combining the integrated differential phase-contrast imaging and density functional theory calculations, we reveal the origin of this superior ferroelectricity in which the purposely introduced oxygen vacancies in the Fe-O layer distorts the FeO₆ octahedral cage and drives the Fe ion away from its high-symmetry position. The present approach can be applied to improve ferroelectric properties for multiferroics.