Preparation And Properties Of Van Der Waals Magnetic Materials And Heterostructure Devices

With the increasing demand for information processing,there is a growing trend towards higher levels of chip integration.The reduction in transistor size has created a pressing need for the development of novel materials and technologies to address issues such as the escalation of static power consumption and leakage current.The emergence of graphene provides a direction for the development of new two-dimension devices.At present,two-dimensional materials have developed into a large family which includes metals,semiconductors,insulators,semimetal and topological materials with exceptional magnetoelectric and photoelectric properties.Furthermore,the discovery of magnetic van der Waals materials provides opportunities for the development of low-power spintronic devices.The smooth surface and absence of dangling bonds in two-dimensional materials enable them to be assembled into van der Waals heterojunction devices via van der Waals forces,which can realize more applications beyond the scope of traditional electronic and optoelectronic devices.Therefore,using the structural and performance advantages of two-dimensional materials to develop new device is expected to overcome the limitations of traditional silicon-based semiconductors.However,most two-dimensional materials exhibit relatively simple properties,such as a lack of simultaneous excellent electrical and magnetic properties,which limits their further application.Therefore,expanding the properties of two-dimensional materials and developing novel two-dimensional materials remains the focal point of current researche efforts.This paper investigates two-dimensional materials from two aspects.(Ⅰ)Preparation of graphene-based two-dimensional van der Waals heterojunction devices.Proximity effect and the dielectric shielding effect are utilized to supplement and enhance the performance of graphene,thereby improving its application value in the field of two-dimensional electronic devices.(Ⅱ)Designing and synthesizing new van der Waals ferromagnetic material CrVI6,which expands the kind of two-dimensional van der Waals materials.Furthermore,the magneto-optical sign of skyrmion is established by magneto-optical Kerr characterization,which has a wide-ranging impact on two-dimensional insulation systems.The specific research work is as follows:(1)The MnPS3/graphene van der Waals heterojunction device was constructed,which successfully introduced magnetism into graphene through the magnetic proximity effect of antiferromagnetic MnPS3.Electrical transports measurements reveal that Hall resistivity curve exhibits nonlinear behavior.The nonlinear behavior remains unchanged at different gate voltages,which suggests that graphene has been magnetized and the anomalous Hall effect has occurred.The temperature-dependent anomalous Hall resistivity shows that the transition temperature of anomalous Hall effect is consistent with the Neel temperature of MnPS3,confirming that the magnetism of graphene is derived from MnPS3.Theoretical calculations also show that the magnetic order of MnPS3 improves the spin orbit coupling of graphene and breaks the symmetry of the time inversion of graphene through the magnetic proximity effect,resulting in non-zero Berry curvature and nonlinear Hall resistivity.This research provides an opportunity to realize graphene-based 2D spintronic devices.(2)The CrOCl/monolayer graphene van der Waals heterojunction device was designed,and the carrier mobility of graphene was successfully improved through the dielectric shielding effect of CrOCl.Low temperature electrical transport measurements show a Shubnikov-de Haas(SdH)oscillation in the longitudinal resistance,indicating that the graphene carrier mobility is significantly improved.Carrier mobility of the monolayer graphene can be effectively increased up to 35000 cm2 V-1 s-1,as confirmed by back gate scanning.The temperature-dependent electrical transport measurements show that the SdH quantum oscillation and the Hall resistance platform can also be observed at 100 K due to the increased carrier mobility.The experimental results show that CrOCl with high dielectric constant can provide dielectric shielding effect,which effectively reduces the coulomb interaction between electrons and charge impurities,leading to the reduction of coulomb scattering of electrons and the improvement of carrier mobility.This facilitates the observation of SdH oscillation.Our work opens the door to improve the carrier mobility of graphene and realizes quantum transport at high temperature,which offers new opportunitiies for the application of low power devices.(3)Two-dimensional van der Waals ferromagnetic single crystal CrVI6 was synthesized by chemical vapor transport method for the first time,and the skyrmion was detected by magneto-optical Kerr.Analysis of the phase and magnetic properties of CrVI6 revealed that it is a ferromagnetic single crystal with an out-of-plane easy magnetization axis and a Curie temperature Tc of 51 K.The thickness-dependent magneto-optical Kerr characterizations demonstrated that the hysteresis loops of few layers CrVI6 exhibit anti-symmetric peaks within a thickness range of 62-115 nm.Additionally,the anti-symmetric peaks were observed only below 17 K.Based on theoretical calculations,we identify the Kerr bumps as direct manifestations of the topological charge of the skyrmions,which is called topological Kerr effect(TKE).This work finds a ferromagnetic insulating material with a novel spin order and establishes the magneto-optical signature of skyrmions by magneto-optical Kerr characterization,which provides a feasible scheme for detecting skyrmions in insulation syste m.