Research On Graphene EMR Type Magnetic Sensors

In recent years,the rapid development of space science research has put forward new requirements on space exploration technology.For the development of magnetom-eter,there is an urgent need to minimize the mass and volume under the premise of ensuring accuracy,improve reliability and shorten the delivery cycle.One important approach to realize this goal is to development novel magnetic sensors based on new material or new principle.Graphene has excellent electronic and mechanical properties,its EMR effect and Hall effect can be used to measure magnetic field in room tempera-ture.But up to now,the research on graphene for high precision magnetic field meas-urement is still scarce.In this article,we investigate the potential of graphene for weak magnetic field measurement.Theoretical analysis and finite element simulation of the performance of graphene EMR type devices（including EMR element,Hall element）are carried out,and device geometry is designed.Based on this,graphene magnetic sensors are fabricated.Their magnetic response is tested and analyzed theoretically.A low noise magnetic measurement system is further constructed to explore the feasibility of gra-phene for weak magnetic field measurement.The thesis analyzes the physical principle of graphene EMR effect theoretically,and builds finite element model of graphene EMR element.We propose eccentric metal shunt design,and analyze the effect of eccentric distance and angle on device sensitivity by fnite element simulation.Optimal structure of graphene EMR element is designed according to simulation results.Then the eccentric van der Pauw shaped graphene EMR element is fabricated and its magnetic response is tested.The EMR element shows a sensitivity of 2.6 V/（AT）.The output voltage changes monotonously with magnetic field.Test results and simulation results are consistent with each other,which is an ex-cellent verfiaction that eccentric distance and angle can alter the EMR element’s sensi-tivity.The eccentric metal shunt design show a strong advantage over concentric one,which has extremely low sensitity in±0.1 T magnetic field and can only measure sca-lar magnetic field.The thesis design and fabricate three sizes of graphene Hall elements.In order to accurately characterize graphene Hall noise,the performanace of three noise measure-ment techniques is compared comprehensively and systematically.Their advantages,drawbacks and applicable situations are summarized.According to this and the meas-ured graphene Hall noise level,noise test system is built.Graphene Hall noise as a function of back gate voltage is measured.It is ensured by various means that the meas-ured noise originates from graphene itself.At last,the generation mechanisom of gra-phene Hall noise is analyzed theoretically.According to the test results,graphene Hall noise shows‘Λ’shape dependence on back gate voltage which gets its maximum at Dirac point and decreases monotonously with increasingV_g-V_Dirac.This characteristic is quite different from graphene longitudinal noise which often shows‘M’or‘V’shape dependence on back gate voltage.Theoretical analysis shows that graphene Hall noise is consistent with McWhorter model.This leads to the conclusion that graphene Hall noise comes from carrier number fluctuation caused by tunneling of the carriers from the channel to the traps in the oxide.To realized low noise magnetic measurement system based on graphene Hall ele-ment,we apply spinning current modulation technique to graphene Hall element for the first time.Based on the theoretical analysis of spinning current modulation technique,test circuit is built and its accuracy is verified.Then the low noise magnetic field detec-tion system is built and its noise reduction performance is tested.With 3.9 kHz modu-lation frequency,the noise of the measurement system is lowered by 40 dB,reaching4.70×10^-5 T/Hz^0.5@1Hz.In our test,graphene Hall element is fabricated by CVD method and photolithograpy,but the resolution achieved is close to those exfoliated,electron beam lithography fabricated,boron niride substrated graphene Hall elements.The spinning current modulation technique can greatly reduce technological require-ments in graphene Hall element fabrication process.In order to further analyze the prin-ciple of spinning current modulation,the noise reduction technique in magnetoresistive sensor is also analyzed in this thesis.Though the realization methods of noise reduction in Hall sensor and magnetoresistive sensor are different,their essence is to modulate magnetic field induced voltage to high frequency,so as to separate the signal from the sensor 1/f noise and test circuit 1/f noiseThe research of this thesis provide a new perspective for the design and optimiza-tion of van der Pauw shaped graphene EMR device,provide a theoretical basis for re-ducing graphene Hall noise,point out the direction for optimizing fabrication process of graphene Hall elements and lay the foundation for the application of graphene EMR type devices in high precision magnetic field measurement.