The Study Of Tailoring Magnetic Property On Two-dimensional Fe₃GeTe₂

With the rapid development of information technology,the traditional semiconductor technology based on the charge characteristics of electrons has encountered a development bottleneck.Spintronic devices can greatly improve the processing speed of the device and greatly reduce the power consumption of the device due to the simultaneous use of the two properties of electrons’spin and charge,and have great potential in the application of next-generation information technology.van der Waals materials exhibit great potential in spintronics,arising from their excellent spin transportation,large spin–orbit coupling.Compared with traditional three-dimensional materials,two-dimensional materials have a larger specific surface area,so they are more sensitive to external stimuli such as light and electricity.Two-dimensional magnetism can be controlled by various control methods such as electricity,mechanics,and optics.The magnetic phase transition temperature of the van der Waals magnetic materials found so far is mostly lower than room temperature,which greatly limits their practical application in spintronic devices.The Fe₃GeTe₂ material studied in this paper has a Curie temperature（T_C）of about 230 K in bulk,which is the highest magnetic phase transition temperature among existing van der Waals magnets,and is also the most promising material for regulating T_C to room temperature.This paper systematically studies Fe₃GeTe₂ and its magnetic control.The main work and research results are as follows:（1）Strain control of magnetic properties of Fe₃GeTe₂:This work utilizes the flexible polymer polyvinyl alcohol（PVA）to tune the ferromagnetic（FM）properties of mechanically exfoliated Fe₃GeTe₂ nanoflakes through substrate deformation.Fe₃GeTe₂nanoflakes embedded in PVA substrates can achieve up to 4.7%tensile strain.The Young’s modulus of the PVA substrate is compatible with Fe₃GeTe₂ nanosheets,ensuring efficient strain transfer.The ferromagnetic properties can be reversibly tuned by loading and unloading strain,and uniform phonon softening can be observed when strain is applied.When the strain is 4.7%,the comprehensive magnetic properties are optimal with a T_C of about 400 K,and a stable single-domain ferromagnetism appears.Furthermore,for the first time in this material,a vertical exchange bias image,ie the hysteresis loop is vertically shifted along the magnetization axis,has been observed.This phenomenon can be maintained up to 200 K with a maximum bias of 13%.The magnetic model of Fe₃GeTe₂ exhibits a three-dimensional Ising to two-dimensional Ising transition with the applied strain.（2）Study on Electron Doping Control of Magnetic Properties of Fe₃GeTe₂:This work realizes the large-scale fabrication of few-layer Fe₃GeTe₂ nanosheets by an organic cation-assisted electrochemical exfoliation-synchronized electron doping method.The insertion of organic ions in the van der Waals gap results in an expansion of the interlayer spacing to 0.8 nm.The introduction of extra electrons leads to strong in-plane electron oscillations,which enhance the electron-phonon interaction,which is beneficial to improve the spin-orbit coupling of Fe ions.The exfoliated nanosheets are ferromagnetic with perpendicular magnetic anisotropy,and the magnetic order can be maintained up to385 K.Due to the insertion of organic ions,almost every Fe₃GeTe₂ monolayer can be polarized by induced charges.From a theoretical point of view,this strong ferromagnetism in the two-dimensional limit arises from optimized electron occupancy states in the spin-splitting band and enhanced indirect double-exchange interactions.（3）Study on charge transfer control of Magnetic Properties of Fe₃GeTe₂:In this work,an electron acceptor-type organic molecule surface chemisorption method was used to realize the charge transfer of Fe₃GeTe₂,which ultimately led to the enhancement of the ferromagnetism of the complex.The rise in resistance shown by electrical testing indicates charge transfer between the organics and Fe₃GeTe₂.In addition,by comparing the molecular spreading states on the nanosheet surfaces under different preparation methods,we summarize the conditional parameters to achieve the optimal magnetization state.The peak position shift of the Raman spectrum indicates the existence of spin-phonon coupling.The magnetic order of the complex can be maintained up to about 395K,and the magnetic model can be explained by the three-dimensional Ising model.The large-scale magnetization effect of nanosheets shows that this method is an efficient and convenient magnetization measure.