First-principles Investigation On Magnetic Properties And Magnetoelectric Effect In Low-dimensional Materials

In the age of big data,the information storage and security are closely related with human being's life.However,some big challenges in this field still remain.We should consider how to improve the performance and storage density,to reduce the size,to lower the power consumption,and to enhance stability of device.Magnetic nanoparticles are distinguished candidates for fabricating the data memory with low power and nonvolatility.In this context,the thesis will focus the research on magnetism in low-dimensional systems including magnetic properties,magnetic anisotropy,and magnetoelectricity,etc.Combined with the first-principles calculations and analytical models,we further study the physical origin of these phenomena and effects,which provides guidance for designing the advanced information storage.From the perspective of the research background and status,Chapter I presents a development of some important techniques in Spintronics,such as voltage control of magnetism,single atomic and spin system,and magnetoelectric effect,to elucidate the research significance and general topic.Then,the first-principles calculations based on density functional theory are introduced in Chapter II through retrospecting to the evolution from many-body problem to single-electron solution.This calculation method is used in the whole thesis as a major tool.In Chapter III,the efforts are devoted to the manipulation of the magnetic properties of the transitional-metal atoms?Mn,Fe and Co,etc.?doped WS₂ monolayer.By performing first-principles calculations,it is found that doping Co atom on the W vacancy can achieve a giant perpendicular magnetic anisotropy.This enhanced magnetic anisotropy can be attributed to the rearrangement of the energy levels ordering caused by the exchange splitting energy and the spin-orbit coupling strength.Additionally,the influence on the magnetic properties and magnetic anisotropy of WSe₂,WTe₂,MoSe₂ and MoTe₂ is also be studied.Our findings will establish the theoretical foundation for designing the single-atomic spin storage.With development of the electrical means,the voltage control of magnetism has been one of the significant research direction in the field of information storage.In Chapter IV,utilizing the simple electrodeposition method,we fabricate electric-field control of the magnetism in Ni/Cu coaxial cylinders in an electric double layer configuration.Our first-principles calculations,together with the results from experimental characterization,confirm that the magnetization change in this capacitor is mainly due to the surface magnetoelectric effect,which can be enhanced by employing a thinner ferromagnetic layer and a more numbers of Ni/Cu wires.Differently from the traditional capacitor,the proposed Ni/Cu coaxial cylinder capacitor can store both charges and spins,and can work at room temperature,which indicates that it has potential applications in the research field of Spintronics.Surprisingly,through the process of simulation,we found that there also exists the interface magnetoelectric effect,though it is a little weak.Therefore,taking Ni/Cu structure as an example,we present a fundamental study in the composite structure of ferromagnetic and non-magnetic metal in Chapter V.Through the micromechanisms of Ni/Cu based on the simple modeling,it is elucidated that why there exists the interface magnetoelectric effect,and how the charge carriers and spins are changing under the applied electric field,which confirm that there indeed exists a complex magnetic response.Furthermore,to enhance the interface magnetoelectric effect should be deemed critical to design the future advanced storage devices.This work widens the scope of the research on spin capacitor,and meanwhile,enriches the physical mechanisms of the voltage control of magnetism in magnetoelectric multilayers,which bring new ideas for generating novel storage applications.At the end of the thesis,the main conclusions and bright prospects are given in Chapter VI.Through the above study of magnetic properties,magnetic anisotropy and magnetoelectric effect in low-dimensional materials,we have a comprehensive understanding of their manipulating methods,magnitude of related factors,and physical properties.All these efforts would accelerate the development of Spintronics and next-generation storage applications,which will ultimately achieve the goal of higher density and lower power consumption.