Adjustment And Improvement Of Magnetoelectric Properties Of Multiferroic Materials In Two Dimension

In 2004,the successful preparation of graphene set off a research upsurge of two-dimensional materials.On the one hand,with the decrease of material system dimension,quantum effect and correlation will be enhanced correspondingly,which will often lead to rich physical phenomena.On the other hand,two-dimensional materials are more easily controlled by the external field than the bulk,which is helpful for the application of high storage density and device miniaturization.In recent years,ferroelectric and ferromagnetic materials in two dimensional have been proved in experiments.It is a hot research direction to find two-dimensional multiferroic materials with both ferroelectric and ferromagnetic properties.There may be intrinsic magnetoelectric coupling between magnetism and ferroelectricity in multiferrous materials,which provides a basis for cross regulation of magnetoelectricity.Perovskite manganite oxides have intrinsic magnetic induced ferroelectric polarization,and R-P phase layered perovskite manganite oxides have octahedral rotation induced ferroelectric properties.These two kinds of materials may realize magnetic field controlled electrode and electric field controlled magnetism in two dimensions,which provides a good choice for the study of magnetoelectric control of two-dimensional multiferroic materials.In this paper,the strain effect and regulation of RMnO₃?R=La-Lu?series epitaxial thin films,the strain regulation and surface and interface effects of LuMnO₃two-dimensional thin films,and the multiferrousness of layered perovskite Ca₃Mn₂O₇two-dimensional double layers are studied by using the first principle,symmetry analysis and model analysis.The main contents are as follows:?1?The effects of epitaxial strain on the structure,electron,ferroelectric and magnetic properties of RMnO₃?R=La-Lu?films were studied.It is predicted that the substrate applied epitaxial strain can change the magnetic ground state of RMnO₃films into E-type order by the first principle calculation,which explains the reason why the manganese oxide films epitaxial grown on YAlO₃substrate has large ferroelectric polarization.?2?The strain effect of SrTiO₃on RMnO₃?R=La-Lu?films was studied.It is proved that the epitaxial strain applied to the substrate causes most of the manganese oxide magnetic phase to change from antiferromagnetic to ferromagnetic.It is revealed that the main reason for the magnetic phase transition is that the epitaxial strain restrains the Jahn-Teller distortion of Q₃type,which enhances the ferromagnetism in the nearest neighbor exchange and leads to the phase transition from antiferromagnetism to ferromagnetism.We propose a way to realize the coexistence and coupling of ferroelectric and ferromagnetic phases by adjusting the strain gradient.?3?The strain regulation of magnetic sequence and orbital sequence and the surface and interface effects of LuMnO₃films were studied.The results show that tensile strain can induce spiral and E-type multiferrous phases,while compressive strain can induce ferromagnetic semimetallic and antiferromagnetic polar metallic phases with high Curie temperature.For the heterostructure with SrTiO₃substrate,it is found that the electron accumulation on the interface decreases sharply with the increase of LuMnO₃layer thickness.The symmetry breaking of the surface crystal field leads to the appearance of d_3z2_-r2 type orbit and the phenomenon of surface metallization.?4?The structure,electron,ferroelectric and magnetic properties of Ca₃Mn₂O₇bilayer with R-P phase were studied.It is found that the structural reconstruction of Ca₃Mn₂O₇in two dimensions results in the suppression of spontaneous coupling of rotation and tilt and the disappearance of ferroelectric phase.By means of epitaxial strain strain,the ground state of ferroelectric phase can appear,that is to say,strain can drive the transformation from two-dimensional antiferroelectric to two-dimensional ferroelectric,and the DM mechanism is used to regulate the magnetization by electric field...