Preparation,Elctronic Structure And Magnetism Of Concentrated Magnetic Semiconductor Thin Films

The nature of spintronics is spin to participate in the conduction process.Magnetic semiconductor is one of the most ideal material having ferromagnetic and semiconductor at the same time.Curie temperature of traditional dilute magnetic semiconductor cannot reach room temperature.In recent years,a new type of antiferromagnetic spintronics based on tunneling anisotropic magnetoresistance?TAMR?is of growing attention.Different from traditional pining layer,antiferromagnetic layer spontaneously participate in conduction process related to spin,which extended the application of antiferromagnetic.Small magnetic stray field and Curie temperature generally higher than room temperature of antiferromagnetic material promote the development of room temperature spintronics devices.Antiferromagnetic semiconductor combines the advantages of the two,which can be both room temperature spin conductive and semiconductor electronic control.Limited by experimental conditions,the related research on antiferromagnetic is relatively rare.In this thesis,we used molecular beam epitaxy?MBE?and reflection high energy electron diffraction?RHEED?,and prepared a series of high quality concentrated magnetic semiconductor?chalcogenide of transition metal magnetic elements such as Mn,Fe and V?thin film;then using scanning tunneling microscope?STM?to characterize their electronic structure,morphology;at last,using spin polarized STM to measure the surface magnetism.The main research results of this thesis are as follows:1.By using the MBE monitoring by in-situ RHEED,we prepared magnetic elements?Mn,Fe,V and so on?selenide and antimonide in SrTiO₃?111?and SrTiO₃?100?substrates.We successfully prepared the high quality MnTe,VSe,FeTe,FeSe film by optimizing the growth parameters such as substrate temperature.Further using the STM topography characterization,combining RHEED lattice,the basic lattice structure of MnTe,VSe film were identified as the NiAs model.And on this basis,the basic characteristics of the surface electronic structure was measured by scanning tunneling spectrum?STS?.In addition,we have studied the growth of two kinds of lattice structures of Fe-Se and Fe-Te thin film,optimized preparation of substrate and growth parameters for different thickness of high quality hexagonal FeSe?FeTe?and tetragonal FeSe?FeTe?thin film.This series of film preparation work,helps to improve concentration magnetic semiconductor research.2.Electronic structure and atomic structure of hexagonal FeSe and tetragonal FeSe thin film was studied by STM.We confirmd hexagonal FeSe film grown on SrTiO₃?111?substrate was NiAs structure,whose surface has a lot of electronic refactoring?such as 2×2,2×1?associated with lattice extrusion in the process of growth.By the STS we found hexagonal FeSe was a good semiconductor with an energy gap of 0.45 eV.In Spin-Polarization STM magnetic characterization,we found clear hysteresis loop using a ferromagnetic tip,and found the obvious in-plane nonlinear antiferromagnetic super lattice without obvious surface magnetic signal.By comparing the positive and negative magnetic signal in plane,we obtained magnetic structure was approximate Néel antiferromagnetic through vector arithmetic.First principles calculation also found this antiferromagnetic structure in hexagonal FeSe was more stable,and density of state calculation results were consistent with the experiment.Hexagon FeSe as antiferromagnetic semiconductor,is of great significance for the antiferromagnetic spintronics.3.Electronic structure and atomic structure of hexagonal FeTe and tetragonal FeTe thin film was studied by STM.Hexagonal FeTe grown in SrTiO₃?001?substrate was NiAs island shaped structure,different island often have different height and different lattice orientation.Tetragonal FeTe grown in SrTiO₃?001?substrate was PbO layered structure,whose Fermi surface has a characteristics gap of approximately 15 mev from the STS,but not superconducting.After high temperature annealing we got the tetragonal FeTe surface with Te defects.Then we using spin polarized STM to study the surface magnetism,and found the obvious antiferromagnetic super-lattice structure,which was consistent with one-stripe antiferromagnetic periodicity of dissociative sample Fe_1+yTe?y<0.12?.This work helps to improve the Fe-Te system research,also has important significance for the magnetic research of FeTe superconducting parent material.