向红军 教授
复旦大学

时间：12月15日上午10点
地点：唐楼A313

内容摘要：Multiferroics, displaying magnetic, polar, and elastic order parameters simultaneously, have attracted numerous research interests. In order to speed up realistic applications of multiferroics, it is highly desirable to understand the magnetoelectriccoupling mechanism in multiferroics and predict new high-performance multiferroics.In recent years, we have theoretically studied the microscopic origin of ferroelectricity in different multiferroic systems. We proposed a unified model [1,2] which includes purely electronic and ion-displacement contributionsimultaneously to describe spin-order induced ferroelectricity. An efficient method [3] was developed to compute the model parameters from first-principles. On the basis of the unified model and density functional calculations, we explained the ferroelectricity induced by the proper-screw spin spiral [2], discovered a novel magnetoelectric coupling mechanism in which the magnitude of the polarization is governed by the exchange striction with the direction by the spin chirality [4], proposed that the ferroelectricity in the chiral-lattice magnet Cu2OSeO3 is due to the unusual single-spin site term [5], unraveled that the magnetoelectric effect observed in BiFeO3 originates from the exchange striction [2].Common multiferroics are ferroelectric. We have predicted that hexaferrite BaFe12O19 may be the first example of multiferroic materials that displays antiferroelectricity [6]. The antiferroelectricity in this system is "geometrically frustrated" by the underlying hexagonal structure, which may lead to the possibility of exotic electric-dipole glass. The antiferroelectric state can also be driven by an external electric field into a metastable ferroelectric state, which may be made stable at room temperature by appropriate element substitution or strain engineering. This property together with its room temperature ferrimagnetism may be exploited for applications as multiple-state memory devices. Besides, we propose the concept of a new type of multiferroics, namely, "asymmetric multiferroic" [7]. In asymmetric multiferroics, two locally stable ferroelectric states are not symmetrically equivalent, leading to different magnetic properties between them. We further predict that a Fe-Cr-Mo superlattice with the LiNbO3-type structure is an asymmetric multiferroic, which may be used to realize electric-field control of magnetism at room temperature. Our study suggests that asymmetric multiferroic may provide a new playground for voltage control of magnetism and find its applications in spintronics and quantum computing.
References
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