The Reseach Of Preparation,Doping And Properties For Magnetic Half-metal CrO₂ Films

CrO₂ only has one kind of spin up electrons around the Fermi surface which makes it has wide application prospects in spintronic devices.However,CrO₂ is metastable phase of thermodynamics,and there are still many problems to be overcome before it can be applied in spintronic devices.In this thesis,by using the theoretical calculation and experimental method,the effects of element doping and substrate the properties of CrO₂ film were investigated.The main contents of the thesis are as follows:1.After a variety of different N-source attempts,we found that the films synthesized in the nitrogen atmosphere was N doped CrO₂ film.Through XRD test analysis,N dopants does not change the lattice structure of CrO2 but makes its lattice constant larger.Meanwhile,in the nitrogen atmosphere,the growth temperature range of the film has been greatly expanded?from 20 K expanded to110 K?.The concentration of N can be changed by regulating the preparation of temperature or the proportion of O₂ and N₂ in the carrier gas.N doping reduces the magnetic moment of CrO₂,but enhance the the anisotropic energy and Curie temperature of CrO₂ significantly.By first principles calculation,we found that when N atoms substitute O atoms in CrO₂ supercell,the half metallicity of CrO₂ will keep intact,while the magnetic moment and magnetic anisotropy will increase as found experimentally.2.According to the results reported so far,purly phased CrO₂ epitaxial film can only be prepared on TiO₂ substates under enough high oxygen pressure.In this study,however,we found that at a certain temperature range,when the carrier gas is N₂ or Ar,CrO₂ films can be prepared successfully on TiO₂ substrates,indicating O rich environment is not necessary.The surface roughness and grain orientation of the films can be manipulated by the change of the carrier gas and the preparation temperature.Moreover,we found that CrO₂ films prepared under N₂ atmosphere are actually N doped CrO₂ film,and that prepared under Ar atmosphere contains a lot of oxygen vacancy.N doping and oxygen vacancy have great influence on the magnetic properties of thin films.3.Using VASP,we calculated the density of the state and anisotropy energy of CrO₂ films grown on TiO₂,SnO₂ and RuO₂ substrate.It was found that substrate has significant impacts on the electronic structure of CrO₂ film.The effects of substrate can be attributed to the interface type and stress induced by lattice mismatch.The study of the magnetocrystalline anisotropy?MCA?of CrO₂ on different substrates suggests that substrate material also have a great influence on MCA.The variation of substrates not only leads to a change of MCA but also may change the direction of easy axis of the films.