| Magnetic anomaly detection has atracked more and more attentions in application of underwater target detections due to its advatages such as no source and strong anti-interference ability.Thri-axial magnetic field sensor with high performance is core device for underwater target detections.Traditional assembled tri-axial sensors have problems such as nonorthogonality,non-uniform sensor response and low sensitive abilities,these problems make it hard to exploit advantages of magnetic anomaly detection.Supported by the national natural science foundation and military equipment exploration foundation,this dissertation focused on these problems and did researches on integrated designes of tri-axial magnetic field sensor and graphene based new magnetic tunnel junctions.The main content and innovation of this dissertation are listed as follows.As to the nonorthogonality problem of tradictional assembled tri-axial magnetic field sensors,a new structure of integrated tri-axial magnetic field sensor is presented and cross slope symmetrical differential magnetic flux concentration structures are designed to change the space magnetic field into the sensor's plane.Based on these designs,the de-couple model for the three components of space magnetic field is proposed and in-plane measurement of space magnetic field is accomplished.Based on this method of in-plane measuring of out-of-plane magnetic field component,new tri-axial magnetic field sensing systems are proposed based on in-plane measurement method of space magnetic field and integrated magnetic field sensing designs.A theoretical solution for improving the orthogonality and miniaturization of tri-axial magnetic field sensor is presented.As to the fact that the existing magnetic sensitive elements are unable to meet the high performance demands,new solutions of sensitive elements are presented with graphene.The core structure of nickel/graphene/nickel of magnetic tunneling junction(MTJ),the shape of controlling the magnetic change,nano junction area avoiding wrinkles of CVD graphene and insulating layer depressing electrical bypass of graphene edges are designed.Based on the first principles method,the magnetic sensing mechanism of graphene based MTJ is systemly investigated from several aspects such as electronic band structures,spin density of states of the interface and spin transmission spectrum.The internal cause dominanted by hybridization at the interface and spin filtering effect of high magnetoresistance ratio and regulation and affection laws of graphene layers,bias voltage,magnetic anisotropy and non-ideal interface factors on magnetoresistance effect are discovered.These works provide theoretical guides for the design and fabrication of graphene magnetic field sensing element with high performance.As to the problems in the fabrication of grahene based MTJ such as interface oxidation and defects,fabrication routines with magnetic film deposited on double sides,the spin sloping deposition of magnetic film and high-precision silicon mask are presented,and systemic characterization methods for fabrication are established.Key technologies such as high qulity control of magnetic film/graphene interface,the nano junction area control and the controllable pattern of magnetic film on graphene are resolved and graphene based magnetic sensitive element is fabricated.The test method for graphene based MTJ is investigated and low noise four probes magnetoresistance test system is established.The fabricated graphene based magnetic sensing element is tested and analyzed.The magnetoresistance ratio of graphene based MTJ close to theoretical predition is obtained and these results verified that the theoretical analye is right,the designs of strcture and fabricaton routines are proper and the fabrication technologies are valid.The achievements in this thesis build a foundation for the development of high performance integrated tri-axial magnetic field sensor. |
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