Multiferroics in which giant ferroelectric polarization and magnetism coexist are of tremendous potential for engineering disruptive applications in information storage and energy conversion. Yet the functional properties of multiferroics are thought to be affected detrimentally by the presence of point defects, which may be abundant due to the volatile nature of some constituent atoms and the high temperatures involved in the synthesis of materials. Here, we demonstrate with theoretical methods that oxygen vacancies may enhance the functionality of multiferroics by radically changing their magnetic interactions in thin films. Specifically, oxygen vacancies may restore missing magnetic super-exchange interactions in large axial ratio phases, leading to full antiferromagnetic spin ordering, and induce the stabilization of ferrimagnetic states with considerable net magnetizations. Our theoretical study should help to clarify the origins of long-standing controversies in bismuth ferrite and improve the design of technological applications based on multiferroics.

巨大铁电极化和磁性共存的多铁磁材料在信息存储和能量转换的工程破坏性应用中具有巨大的潜力。然而，据认为，多铁化合物的功能特性会受到点缺陷的不利影响，由于某些组成原子的易挥发性质和材料合成中涉及的高温，点缺陷可能会很丰富。在这里，我们用理论方法论证了氧空位可通过从根本上改变薄膜中的磁性相互作用来增强多铁性化合物的功能。具体来说，氧空位可能会在大的轴向比率相中恢复缺失的磁性超交换相互作用，从而导致完全反铁磁自旋有序，并以相当大的净磁化强度诱导亚铁磁态的稳定。我们的理论研究应有助于弄清铋铁氧体长期存在争议的根源，并改善基于多铁性物的技术应用设计。