Synthesis And Magnetic Properties Of Two-dimensional Van Der Waals Magnetic Materials

Since the 19th century,with the great breakthrough of magnetic research,magnetic materials have entered all aspects of production and life.In the face of today's growing demand for information technology,the preparation of thinner,higher density,lower power consumption of magnetic devices has become the trend of the development of electronic industry.In recent years,two-dimensional van der Waals(vdW)magnetic materials have gradually become an excellent platform for people to study two-dimensional magnetism and realize low dimensional magnetic devices due to their cleavage layered structure and stable magnetism in the atomic layer thickness.However,the development of two-dimensional vdW magnetic materials is limited by the lower transition temperature and less application of magnetic control.In this paper,two-dimensional vdW magnetic materials are used as the research object,and two-dimensional extrinsic magnetic materials,intrinsic magnetic materials and magnetic heterostructure are prepared respectively.The origin,coupling and application of magnetic properties are studied by optical measurements.The main contents of this paper are as follows:(1)Preparation and magnetic study of graphite based magnetic nanosheets.Firstly,the scalable graphite nanosheets with room-temperature ferromagnetism were synthesized by one-step Wurtz reaction.The magnetic properties at room temperature reached 0.22 emu/g is two orders larger than the reported magnetic properties of defective graphite.The controllable experiments and first principles calculations show that the room temperature magnetic properties of the defective graphite come from the zigzag edge state of the graphite nanosheets.Further changing the preparation conditions,we can get the various crystal sizes and corresponding magnetic graphite nanometers.Secondly,the ferromagnetic graphite fluoride was obtained directly by fluorination of large-size graphite with fluorine gas under high temperature.The ferromagnetic graphite fluoride nanosheets were prepared by mechanical stripping.The magnetic properties of the nanosheets were characterized by the low temperature magneto-optical Kerr microscope,which was built by ourselves.It was found that the magnetic size of the graphite was related to the degree of fluorination.Finally,the synthesis of boron doped graphite nanosheets under magnetic field is tried.It is found that magnetic field can only slightly improve the doping content of boron,and cannot induce ferromagnetism.Further research on the displacement reaction under magnetic field is carried out.The factors influencing chemical reaction and material synthesis by magnetic field are given,which provides experimental basis for the synthesis of two-dimensional magnetic materials in the future.(2)Exchange bias and magnetic coupling of two-dimensional intrinsic ferromagnetic Fe3GeTe2.Ferromagnetic Fe3GeTe2 nanosheets were obtained by mechanical stripping of the vdW bulk of Fe3GeTe2,and then the exchange bias was realized in a single Fe3GeTe2 nanosheet by mechanical pressing.The bias phenomenon shows a certain thickness dependence.With the decrease of Fe3GeTe2 thickness,the maximum bias field of 1000 Oe can appear,which is the maximum bias value at the same test temperature in vdW system.Similar to the exchange bias of traditional thin film system,the exchange bias of Fe3GeTe2 also has an exercise effect,and the bias effect will disappear after a certain number of tests.In addition,the antiferromagnetic coupling of Fe3GeTe2 nanosheets with exchange bias is verified by the second harmonic generation.Further experiments show that the exchange bias and antiferromagnetic signal in Fe3GeTe2 have the same temperature and field dependence.Combined with theoretical calculation,it is believed that mechanical pressing can reduce the interlayer spacing of Fe3GeTe2 nanosheets and induce part of ferromagnetism to antiferromagnetism,resulting in exchange bias.The experimental results provide ideas and references for the realization of exchange bias and magnetic devices in two-dimensional vdW system.(3)Preparation and luminescence regulation of two-dimensional luminescent/magnetic heterostructure.First of all,WSe2/BN heterostructure was constructed by transfer stacking.The modulation of non-magnetic material BN on WSe2 luminescence was studied.It was found that BN stacking would introduce defect energy levels,which changed the luminescence mechanism of WSe2 from exciton luminescence dominated by direct band gap to defect luminescence dominated by indirect band gap.Further experiments showed that vacuum annealing could reduce or even eliminate the defects in the heterostructure.The trap makes WSe2 return to the intrinsic luminescence.Then,WSe2/Fe3GeTe2 heterostructure was constructed using Fe3eTe2 as magnetic layer.By increasing the thickness of Fe3GeTe2 nanosheets,the luminescence of WSe2 in the heterostructure can be controlled in a large range,and the transition from red shift to blue shift relative to the single WSe2.Further variable temperature experiments show that when the heterostructure temperature is lower than the transition temperature of Fe3GeTe2,the red shift and blue shift phenomenon will be enhanced.This phenomenon provides ideas and experimental basis for the application of magnetic devices based on two-dimensional vdW magnetic materials.