Study On The High Magnetic Sensitivity Mechanism And Key Technology Of Graphene Magnetic Tunnel Junctions

Graphene magnetic tunnel junction(MTJ)has an extremely broad application prospect in miniaturized and high-resolution magnetic sensors.However,the experimental TMR values of graphene MTJs reported so far are still far lower than the theoretical prediction.Although the researchers took a series of measures to avoid possible problems in the preparation process,the improvement effect was not significant.On the one hand,the influence rules of material,structure and interface etc.are not clear,lacking systematic theoretical research.On the other hand,there is still no mature preparation process to realize the graphene MTJ with high TMR value.Therefore,this paper carries out research on graphene MTJs from the aspects of high magnetic sensitivity mechanism analysis,structure and process scheme design,preparation of singlecrystalline ferromagnetic film and double epitaxial interface,as well as micro/nano fabrication and test analysis of devices.The main contents and results are as follows:1.The influence rules of material system,ferromagnetic layer lattice orientation and non-ideal epitaxial interface on magnetic sensitivity effect were revealed,and a layer-by-layer epitaxial growth scheme for magnetic multilayers called “epitaxial preparation of single-crystalline Ni(111)film + epitaxial growth of nickel-based graphene + epitaxial deposition of nickel film across the barrier layer” was proposed.The spin transport characteristics of graphene MTJs were simulated and analyzed using first-principle simulation software ATK,and the influence rules of material system,ferromagnetic layer lattice orientation and non-ideal epitaxial interface on the magnetic sensitivity effect were systematically studied from the aspects of energy band structure,electronic density of states and transmission spectrum:(1)Compared with Ni-Co and Cu intercalated systems,the band structures of Ni-Ni and Co-Co systems are highly overlapped with that of graphene near the K point,showing strong spin-dependent tunneling in the K space,which makes they have high spin filtering effect and magnetoresistance effect;(2)Compared with Ni(100)and Ni(110),Ni(111)has the best lattice match with graphene,which is the guarantee for realizing high spin filtering effect at the interface and high magnetoresistance value of magnetic tunnel junctions;(3)Compared with the non-ideal epitaxial interface,the magnetoresistance value of the MTJ with perfect epitaxial interface is about two orders of magnitude higher,which proves the importance of the double epitaxial interface.Based on the above theoretical research and the graphene growth conditions,Ni(111)/graphene/Ni(111)was determined as the structure of graphene MTJs.Aiming at the problems of interface oxidation and non-epitaxy in the traditional graphene MTJ preparation process,a layer-by-layer epitaxial growth scheme for magnetic multilayers called “epitaxial preparation of single-crystalline Ni(111)film + epitaxial growth of nickel-based graphene + epitaxial deposition of nickel film across the barrier layer” was proposed.2.The preparation method of Ni(111)single-crystalline film with “substrate pretreatment,electron beam evaporation and high-temperature annealing in ultrahigh vacuum(UHV)” as the core was proposed.The intrinsic mechanism of singlecrystalline growth of Ni thin films under the combined action of anisotropic surface energy and interface energy was revealed,and the preparation of wafer-scale Ni(111)single-crystalline films was realized.A high-temperature annealing pretreatment process for the sapphire substrate in oxygen atmosphere was established to eliminate atomic defects and adsorbed impurities on the substrate surface,and a substrate with good consistency of atomic step height,terrace width and step orientation was obtained;The effects of electron beam evaporation rate,substrate temperature and UHV hightemperature annealing process on the morphology of nickel films were grasped,and then the film growth process was systematically designed.Based on the anisotropic surface energy of nickel on sapphire substrate,the nickel film was induced to form the(111)twin structures,and then under the effect of UHV high-temperature annealing,the abnormal grain growth was realized driven by the interface energy difference between the twin structures and sapphire,which eliminated the grain boundary and twin structures of nickel films and realized the preparation of wafer-scale Ni(111)single-crystalline films.3.The CVD growth method for monolayer graphene based on the “combination of growth process regulation and self growth termination” was proposed,which solved the problem of nonuniform growth of nickel-based graphene affected by the segregation of a large number of carbon atoms,and realized the preparation of highquality Ni(111)/graphene epitaxial interface.Aiming at the problem that carbon precipitation in the growth process of nickel-based graphene is difficult to control and easy to lead to nonuniform graphene,a set of CVD growth process for monolayer graphene at atmospheric pressure was designed.Through the optimization of growth parameters such as component ratio of carbon and hydrogen,growth temperature and cooling rate,the process of carbon atom diffusion,precipitation and nucleation was controlled to construct the self growth termination at nickel catalytic surface,which controlled the massive precipitation of carbon atoms.As a result,a uniform monolayer graphene was obtained,and the high-quality Ni(111)/graphene epitaxial interface was prepared.4.The cross-graphene epitaxial deposition scheme of nickel films was proposed and the heteroepitaxial multilayers with double-epitaxial interface characteristics were prepared by optimizing the parameters of film deposition and annealing.In view of the poor wettability between graphene intrinsic surface and metal films,it is difficult to realize the epitaxial preparation of top nickel.Therefore,the behaviors of nickel atoms on the Ni(111)/graphene substrate like adsorption,migration and relaxation were simulated by the molecular dynamics,which revealed the mechanism of epitaxial growth through inducing the recombination of top nickel atoms across graphene by the underlying nickel lattice potential field.The cross-graphene epitaxial deposition scheme of “low-temperature deposition + high-temperature annealing” was proposed,which by optimizing the process of film deposition rate,substrate temperature and hightemperature post annealing,suppressed the island growth of nickel films caused by low nucleation density,improved the degree of single crystal,and realized the preparation of double epitaxial interface of Ni(111)/graphene/Ni(111)multilayers.5.The preparation process of graphene magnetic nano-junctions was broken through,which eliminated the influence of AMR effect and effectively improved the magnetoresistance value.The developed MTJ devices were comprehensively tested and analyzed under variable magnetic fields and biases:(1)The linear I-V characteristic of graphene MTJ is consistent with the ohmic contact behavior caused by Ni-C atomic orbital hybridization at the Ni/graphene interface;(2)The coexistence of TMR and AMR was observed experimentally and an MTJ bypass model was proposed to explain this phenomenon and the weakening of TMR value by AMR effect;(3)A two-step IBE inclined etching process was proposed,which eliminated the influence of AMR effect and improved the magnetoresistance value.The TMR value obtained at room temperature is higher than the reported level of monolayer graphene MTJs that can be prepared on a large scale.