Theoretical Design Of New Two-dimensional Multiferroic Materials

In 2004,the successful preparation of graphene caused a research upsurge in the field of two-dimensional materials.Among them,two-dimensional ferroic order materials have become a frontier hotspot in the field of two-dimensional materials.In recent years,as two-dimensional ferroelectricity and ferromagnetism have been successively confirmed in van der Waals materials,Whether the existence of two-dimensional multiferroic materials with ferroelectricity and ferromagnetism coexists has become an urgent scientific problem in the study of two-dimensional materials.Multiferroic materials not only integrate the functional properties of ferroelectricity and ferromagnetism,Moreover,there may be a coupling effect between different ferroelectric sequences,which provides a basis for achieving cross-control of magnetism and ferroelectricity.making it a kind of multifunctional with broad application prospects in the fields of spintronic devices,sensors,and data storage material.The realization of two-dimensional multiferroic materials can improve the storage density of electronic devices,reduce their size and energy consumption,and contribute to the realization of miniaturization and low power consumption of multifunctional electronic devices.At present,the exploration of two-dimensional multiferroic materials is mainly concentrated in van der Waals materials,and rarely involves reports of magneto electric coupling effects,while the traditional three-dimensional multiferroic materials mainly exist in transition metal oxides.Recently,two-dimensional perovskite transition metal oxides that are reduced to a single layer and are stress-free have been successfully prepared experimentally,which provides a huge opportunity for designing two-dimensional functional properties based on perovskite oxides.Based on two-dimensional perovskite oxides,not only can we explore whether the traditional ferroelectric and magneto electric coupling mechanism can be maintained in the two-dimensional limit,but also we can find and design new multi-ferroic materials,so it is of great significance.This paper adopts first-principles calculations,combined with theoretical means such as symmetry analysis and model analysis,and proposes a design idea based on the perovskite oxide bilayer to realize the coexistence and coupling of polarization and magnetization.The structure,ferroelectric conversion,electronic and magnetic properties of the entire two-dimensional double perovskite system are studied,and some general conclusions are drawn.The main contents are as follows:(1)It is confirmed that a specific type of octahedral rotation twisted in the perovskite double layer can induce ferroelectricity,The combination of different magnetic ions at the B site(ie,magnetic double perovskite)can introduce ferromagnetism at the same time to realize the coexistence of two-dimensional ferroelectricity and ferromagnetism.It is revealed that the orthogonal twin mechanism through the multi-step flip tilt and twist is the lowest energy conversion path.It is proved that the reversal of the polarization in the ferroelectric conversion will cause the reversal of the in-plane magnetization.This work provides guidance for the design of two-dimensional multiferroic materials with intrinsic magnetoelectric coupling,which is helpful to realize the control of the electric field on the two-dimensional magnetization direction.(2)The general conclusions about the two-dimensional double-perovskite double-layer system structure,ferroelectric conversion,electronic and magnetic properties are revealed.It is confirmed that the change of magnetization direction depends on the path of ferroelectric conversion.Only the ferroelectric conversion in which the oblique twist of the octahedron is reversed can cause a change in the magnetization direction.This work shows that two-dimensional multiferroic materials widely exist in the bilayer structure system of Ca-based double perovskite materials,and it also provides ideas for the design of two-dimensional polar ferromagnetic metals and half-metals.