The Exploration Of New Two-dimensional Magnetic Materials And Physical Properties Under Extreme Conditions

The emergence and development of spintronics has enabled Moore’s Law to continue.The use of electron spin degrees of freedom has promoted the integration of miniaturization of new-generation electronic devices into a new development trend.The evolution of two-dimensional materials such as graphene provides new opportunities for the development of spintronics,which is an indispensable part of integrated miniaturization.Two-dimensional materials have rich optical,electrical and thermal properties and broad application prospects.The van der Waals heterostructure constructed by stacking of two-dimensional materials provides a new direction for the development of the new generation of electronic devices,which has applications in spintronics,optoelectronics,sensors and memory.However,there are still two-dimensional magnetic vacancies in spintronics.It was not until 2017 that the existence of long-range magnetic order was first reported in the two-dimensional van der Waals magnetic materials Crl3 and Cr2Ge2Te6 monolayers.A large amount of exploration and research on two-dimensional magnetic materials has since become an opportunity for the further development of spintronics.The current two-dimensional magnetic materials have low Tc and a considerable part of them are unstable in air.Besides,the study of physical properties has not been completely clear.Thus,exploring new two-dimensional magnetic materials and studying the inherent mechanism properties are the two main directions of current research.Relying on the extreme experimental conditions such as high magnetic field facilities and high pressure technology,we have studied the physical properties of two-dimensional magnetic materials.In addition,we have also explored new types of two-dimensional magnetic materials.The main contents of this article are as follows:Chapter 1 is the introduction part and the first section introduces the general situation of the development of spintronics.Then the second section introduces the concept of magnetism and the classification of magnetic materials.Several kinds of typical two-dimensional magnetic materials are briefly reviewed in section three.The section four introduces heterostructure and spin devices based on two-dimensional magnetic materials.The fifth section is about the background and research content of this paper.The second chapter introduces the experimental methods used in this paper in detail.First the chemical vapor transport method and flux method used for sample growth are briefly introduced,followed by X-ray diffraction,X-ray energy dispersive spectroscopy,low-field magnetism measurement,specific heat measurement,high-pressure technique,pulsed high-field magnetization measurement and steady high-field magnetic torque measurement.In Chapter 3,we used high-pressure technique to study the lattice dynamics and transport properties in Fe3GeTe2.According to the experimental results,we found that the itinerant ferromagnetism of Fe3GeTe2 disappeared around the critical pressure point Pc≈10 GPa,and the fitting parameters of the resistance in the low temperature region were also observed to be abnormal near Pc.Meanwhile,we also observed the abnormal lattice softening in the Raman scattering spectrum near the quantum critical point Pc.At the pressure-induced quantum triple critical point Pc,spin phonon coupling and spin orbit interaction lead to the enhanced spin fluctuations,making it sufficient to completely suppress the ferromagnetic exchange interaction J that weakened with the increasing pressure.Thus,the magnetic state is completely suppressed,FM-PM transition occurs and Tc drops to 0.In Chapter 4,we study the high-field magnetic behavior of CrOCl by using the high magnetic field facilities.The results of pulsed high field magnetization measurement show that the anisotropy of CrOCl is very significant in the low field,while the anisotropy gradually degrades in the high field and tends to be isotropic.The magnetization reaches saturation at 29,5T.Further,combining the results of the pulsed high field magnetization,the steady high field magnetic torque and the low field specific heat,it is found that there is a new phase between 10T and 20T.The analysis shows that this may due to the magnetocrystalline anisotropy change caused by the structural phase transition.The fifth chapter mainly presents the exploration and physical properties of the new two-dimensional magnetic material VCl3.We obtained VCl3 in the process of exploring new types of two-dimensional magnetic material.The magnetic measurement of the sample showed that VCl3 exhibited obvious antiferromagnetism when the external magnetic field H//c under the temperature of the antiferromagnetic-paramagnetic phase transition at 29K.We found that VCl3 has an obvious λ-type peak at 22K in the curve of specific heat.With the increase of the applied magnetic field,the peak position of specific heat has a slight displacement.We assumed that VCl3 underwent microstructure changes under the external magnetic field.In Chapter 6,we carried out the mechanical exfoliation of Fe3GeTe2 and CrOCl and tried to construct heterostructures.On the basis of obtaining thin layers of Fe3GeTe2 and CrOCl by tape tearing method,we completed the hBN-Fe3GeTe2-hBN sandwich structure junction and hBN-graphene-CrOCl-graphene-hBN tunneling heterostructure.In the seventh chapter,we summarized the research content of this paper,and put forward several directions for further research work.