Two-dimensional Ferromagnetism And Electric Field Controlled Magnetism Based On Perovskite

With the development of two-dimensional materials,two-dimensional ferromagnets have attracted more and more attention because of their important applications in spintronics devices.However,due to Mermin-Wagner theorem,two-dimensional long-range magnetic sequence is not suitable for isotropic Heisenberg model at non-zero temperature,so the existence of two-dimensional ferromagnetic materials has been a long-term unsolved problem.Until 2017,because the magnetic anisotropy in the materials eliminated this limitation,two-dimensional ferromagnetism was realized for the first time in the single layers of van der Waals materials Cr I₃and Cr₂Ce₂Te₆.Subsequently,many two-dimensional ferromagnets were found in experimental and theoretical research.However,this series of research hotspots are mainly concerned about van der Waals materials.Compared with van der Waals materials,perovskite materials show richer associated electronic and magnetic properties,such as giant magnetoresistance,superconductivity and multiferroic properties,due to the competition and cooperation of lattice,charge,orbit and spin.However,unlike van der Waals materials,which still maintain their structural and chemical bond characteristics when they are reduced to a single layer,perovskite materials may undergo structural reconstruction when they are reduced to two dimensions.Therefore,whether there are some related electronic effects and novel evolution phenomena in two-dimensional perovskite is still a key scientific problem to be solved urgently.Recently,stress-free single-layer perovskite has been prepared by experiments,and it can be transferred to the substrate that can realize the functional properties of perovskite materials as required,which provides an opportunity to explore more regulation of two-dimensional perovskite compared with van der Waals materials.In addition,using the possible coexistence and coupling between two-dimensional ferromagnetic orders,it is expected to realize the cross-regulation effect of external fields on ferromagnetic orders,thus providing a good choice for designing new ways to manipulate electrons.In this thesis,the structure,electronic,magnetic and electrically controlled magnetic effects of some two-dimensional perovskite are studied by using first principles and Monte Carlo simulation.The main research contents are as follows:（1）The structure,electronics,magnetism and the control effect of electric field of two-dimensional perovskite single layer Sr₂Ru O₄were deeply studied by first principles.Through the distortion of oxygen octahedron structure,combined with phonon spectrum and energy gain,the ground state of the structure is determined to be rotational distortion mode.Due to the complex magnetic properties of Ru atoms,we discussed the effects of different U values on the structure,electronic properties and magnetic properties.The control of electric field on electronic properties and magnetism was studied.The Curie temperature near room temperature was simulated by Monte Carlo method,and it was confirmed that the two-dimensional single-layer Sr2Ru O4 was ferromagnetic semi-metal.It was confirmed that the electric field could transform the semi-metal properties of the two-dimensional single-layer Sr₂Ru O₄into metal,and the electric field could control the change of Curie temperature.（2）The structure,electron,strain,ferroelectricity and magnetism of two-dimensional single-layer perovskite Eu₂Ti O₄are studied,and its ground state structure and magnetic configuration are revealed.The ground state structure is determined by combining phonon spectrum,energy gain and molecular dynamics simulation,so the influence of electric field on ground state structure and magnetic ground state,the regulation of electric field on electronic properties and the influence of strain on ferroelectricity are studied.The Curie temperature of Eu₂Ti O₄is simulated by Monte Carlo method,and the Curie temperature can be changed by electric field,which makes it clear.The thesis has 36 figures,4 tables and 120 references.