Magnetic And Magnetoelectric Transport Properties Of Fe_xGeTe₂（x=3,5） And Fe₃GeTe₂/PtTe₂ Heterostructures

In recent years,the research of two-dimensional van der Waals（2D vd W）magnetic materials has made breakthrough progress.Specially,the 2D vd W magnetic material Fex GeTe₂（x=3,4,5）has shown promising applications in magnetic storage and 2D spintronic devices for its Curie temperature（T_c）up to room temperature,strong magneto-crystalline anisotropy and metallic conductivity.However,the demagnetization process of Fe_xGeTe₂（x=3,4,5）materials has not been clearly studied,whether magnetic regulation can be achieved by simple interface modification,the spin orbital moment can be observed on the monolayer material are still need to be investigated.As for the 2D vd W Dirac semimetallic material Pt Te₂,it can be a good source of spin flow because of its strong spin-orbit coupling（SOC）and chiral spin structure.However,whether the anisotropic magnetoresistance and planar Hall resistance of Pt Te₂are contributed by chiral anomalies has not been studied.Meanwhile,spin orbit torque and the use of spin orbit torque to manipulate magnetic states are very important research topics in spintronics,and most of the research in this field has focused on heavy metal/ferromagnet heterojunction system.So the search for new material systems with strong spin-orbit coupling in all-2D vd W heterojunctions is crucial for the development of new 2D spintronic devices.Therefore,this dissertation investigates the magnetic and magnetoelectric transport properties of Fex GeTe₂（x=3,5）,Pt Te₂,and Fe₃GeTe₂/Pt Te₂ heterojunctions,and the main research contents and conclusions are as follows:1.The magnetic and magnetoelectric transport properties of Fe₃GeTe₂single crystal were investigated.It is demonstrated that:（1）Fe₃GeTe₂ has strong vertical magnetic crystal anisotropy with a T_c of about 200 K.The demagnetization process and coercivity（H_c）of Fe₃GeTe₂ exhibit significant thickness and temperature dependence.For Fe₃GeTe₂ bulk samples,the H_c and remanence are small,and the demagnetization process is the formation of multi-domain structure and the evolution of magnetic field.For Fe₃GeTe₂ nanosheet samples,the hysteresis loops have a rectangular shape at low temperatures,and the demagnetization process is a single-domain flipping process with H_c determined by the nucleation field,while the demagnetization process gradually changes from a single-domain process to a multi-domain flipping process with H_c determined by the pinning mechanism of the domain walls at increasing temperatures.The H_c of Fe₃GeTe₂ nanosheets increases rapidly with decreasing thickness due to the change of magnetization reversal mechanism.（2）The polyvinyl alcohol（PVA）electrolyte has a significant modulation effect on the H_c and anomalous Hall resistance（R_H）of Fe₃GeTe₂ nanosheets,and PVA-covered Fe₃GeTe₂ nanosheets reduce the H_c by about 20%and increase the R_H by about 40%.2.The magnetic properties and spin-orbit moment of Fe₅GeTe₂ single crystal were investigated.It is demonstrated that:（1）Fe₅GeTe₂ single crystal bulk has vertical magnetic crystal anisotropy with the T_c exceeding room temperature.The spin reorientation phase transition,which changes the direction of easy magnetization from perpendicular to in-plane with decreasing temperature,originates from the competition between magnetic dipole interaction energy and perpendicular magnetic crystal anisotropy energy.（2）A novel phenomenon is observed on the R_H of Fe₅GeTe₂ as the temperature changes.R_H increases first with decreasing temperature and reaches a maximum around 110 K,and then R_H decreases with further decreasing temperature.The uncommon phenomenon of R_H variation with temperature originates from the ferromagnetic-subferromagnetic phase transition of Fe₅GeTe₂ near 110 K.（3）The second harmonic measurement reveal that the Fe₅GeTe₂ monolayer nanosheet has a spin-orbit moment and the sign of the spin-orbit moment coefficient changes around 110 K.3.The anisotropic magnetoresistance and planar Hall resistance of type-II Dirac semimetals Pt Te₂ were investigated,and the magnetic and magnetoelectric transport properties of Fe₃GeTe₂/Pt Te₂ heterostructures were investigated.It is demonstrated that:（1）Pt Te₂ has positive magnetoresistance and the contribution of chiral anomalies to the magnetoresistance is excluded.The first-principles calculation results suggest that the Dirac point of Pt Te₂ is about 0.67 e V below the Fermi energy level,and thus the contribution of chiral anomaly to magnetoresistance is weak and the Fermi surface has strong anisotropy.Therefore,the strong anisotropic magnetoresistance and planar Hall resistance of Pt Te₂ originate from the anisotropic orbital magnetoresistance.（2）The H_c of the Fe₃GeTe₂/Pt Te₂ heterostructure decreases with increasing current,which is due to the undesirable contact between the two layers of Pt Te₂ and Fe₃GeTe₂ in the heterostructure,the high interfacial resistance and the low thermal conductivity of the 2D material.Therefore,the Joule heat generated by the current accumulates at the Fe₃GeTe₂/Pt Te₂ interface and cannot be removed in time,causing the material temperature to increase,the magnetic properties of Fe₃GeTe₂ to change,and the H_c to decrease.（3）When investigating the spin-orbit moment of Fe₃GeTe₂/Pt Te₂ heterostructure by the second harmonic,a harmonic signal similar to SOT is observed,and the damping-like field and field-like field are calculated based on the heterostructure with out-of-plane anisotropy.