Research On Preparation Process And Magnetoresistance Effect Of CoFeB-MgO Based Magnetic Tunnel Junction

The research and development of the Spintronics are inseparable from magnetization nanophase materials,the growth of magnetic nanomaterials and the research of vertical magnetic film are the foundation of spintronics,and the spin electronics device based on magnetic nanomaterials is a bridge which builds the spin electronics to the information industry.Because of its wide potential applications,magnetic tunnel junction(MTJ)based on Mg O barrier becomes the most promising device in the field of spin electronics.Although researchers have acheived the considerable results for tunneling magnetoresistance(TMR)effect recent years,which also has been gradually applied into the actual device model,many physical mechanisms,such as MTJ,are not very clear.In this case,this dissertation focuses on the magnetic random access memory(RAM)with perpendicular magnetic anisotropy(PMA)magnetic multilayer film,in which the Magnetic Tunnel Junction based on Mg O Preparation Process and Magnetic Origin is studied,including the exploration on the growth of multilayer film,the crystallization mechanism of barrier and ferromagnetic layers,and the influence of bottom layers and magnetic field annealing on Co Fe B/Mg O/Co Fe B layers,to optimize the MTJ device structures.The origin of the perpendicular magnetic anisotropy(PMA)and the contribution of each film to perpendicular magnetic anisotropy are revealed by using vibrating sample magnetometer(VSM).In this dissertation,the main contributions are as follows.Firstly,the growth process of magnetic tunnel junction has been optimized.The each layer for MTJs was successively grown by magnetron sputtering technique with adjusting the conditions of the vacuum of chamber,the intensity of gas pressure,sputtering power and annealing temperature.Multi equipments,such as atomic force microscope(AFM),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and transmission electron microscope(TEM),were used to analyze the growth statuses of each layer and the interface.Finally,there was massive crystallization appeared with Co Fe B/Mg O/Co Fe B layers,and the process conditions were determined for each layer.Secondly,the perpendicular magnetic anisotropy and its origin have been studied.Structures of magnetic thin films with different design were prepared and measured by VSM.Experiment data show that in the Ta/Co Fe B/Mg O system,PMA stems from the interface of Co Fe B/Mg O and the interior Co Fe B crystal,and the interface of Ta/Co Fe B strengthens the PMA.Different thicknesses of base layers,ferromagnetic layer and barrier layer were designed to figure out the effect of thickness on perpendicular magnetic anisotropy.The results show that the increase of the thickness of base layer could enhance the surface anisotropy to a certain degree.With the thickness of ferromagnetic layer increased,the magnetic anisotropy is changed from in-plane to the perpendicular direction.The increase of barrier layer thickness could change the magnetic anisotropy from in-plane to perpendicular.The MTJs were fabricated and annealed in the magnetic field and vaccum environment.VSM data indicate that MTJs show perpendicular magnetic anisotropy and step phenomenon after annealing.Thirdly,the magnetic tunnel junction device have been prepared and tested.According to the optimal technological conditions which were obtained from early stage of the experiment,combine with the classic sandwich structure of magnetic tunnel junctions,the growth and fabrication of magnetic tunnel junctions,which sectional structure is Ta/Ru/Ta/Co Fe B/Mg O/Co Fe B/Ta/Ru,were completed by magnetron sputtering and photoetching.By measuring the transport characteristics,it was found that the resistance of the device was non-linear,the TMR value was measured of 167%.Fourthly,based on simulation,the magnetic tunnel junction device have been improved,and improvement has been obtained.To further enhance the TMR values,the multilayer structure of Co Fe B/Mg O/Co Fe B was analyzed by XPS,it was found that Fe-oxide was existed at the interface of Co Fe B/Mg O.The first-principles calculations were used to study the influence of Fe-oxide on TMR values.The simulation results indicate that the presence of Fe-oxide significantly reduce the TMR value of magnetic tunneljunction device.To eliminate the influence of interfacial oxide,the core structure of magnetic tunnel junction was improved for Co Fe B/Mg/Mg O.Based on the simulations before,the growth and fabrication of magnetic tunnel junctions,which sectional structure is Ta/Ru/Ta/Co Fe B/Mg/Mg O/Co Fe B/Ta/Ru,were completed and the TMR value of which was 183%.The simulation and experimental results between Co Fe B/Fe-O/Mg O/Co Fe B and Co Fe B/Mg/Mg O/Co Fe B structures showed that the inserted of Mg layer could significantly reduce the negative effects on TMR values caused by Fe-O,which means that the insertion of Mg layer could effectively avoid the interfacial oxidation between Co Fe B and Mg O layers and enhance the TMR values of magnetic tunnel junctions.