Exchange Bias Effect And Ionic Liquid Tunability Of The Two-dimensional Magnetic Materials

With the rapid development of the electronic information industry,electronic devices are becoming increasingly integrated and intelligent.However,the size of conventional silicon-based semiconductors following"Moore’s Law"is approaching its physical limit.If the size of the device continues to be reduced,the performance of the device will be seriously affected by the occurrence of quantum tunneling effect and the heating problem caused by high integration.There is an urgent need to find new materials that can replace traditional semiconductors.In 2004,the discovery of graphene brought two-dimensional materials into the public view.Two-dimensional magnetic materials,as a member of two-dimensional materials,have weak interlayer van der Waals force and can still maintain spontaneous magnetization at the atomic layer thickness.These properties make 2D magnetic materials easy to regulate by means of electric field,magnetic field,and interface,thereby broadening the application field of spintronic devices.In addition,spintronic devices by changing electron spin polarization have higher operating efficiency and lower power consumption than conventional transistors by changing charge motion.How to control the intrinsic properties of two-dimensional magnetic materials is still an important research direction.This paper mainly focuses on the basic structure characterization,device preparation and two effective methods of regulation of two-dimensional magnetic materials.The following are the key research contents of this paper:（1）The exchange bias effect is an effective method for regulating two-dimensional magnetic materials and plays an indispensable role in the field of magnetic data storage.In this paper,the exchange bias effect of two-dimensional Fe₃GeTe₂/Cr PS₄ heterostructure is investigated.The structural characterization of Fe₃GeTe₂ and Cr PS₄ single crystals confirms that the crystal quality meets the experimental requirements.The bulk Cr PS₄ is tested with a superconducting quantum interference device,which confirms its Neel temperature of 38K and find spin-flop transitions at different temperatures.The electrode structure is designed to test the intrinsic region and the heterostructure region on the same Fe₃GeTe₂ flake.The Fe₃GeTe₂/Cr PS₄ heterostructure structure is constructed,and its Curie temperature is determined to be 210K through its low-temperature magnetoelectric transport and the data of the two regions.It is found that the exchange bias fields of-98Oe and+107Oe exist in the heterostructure with the change of the field cooling direction when the temperature is 2K.After increasing the field cooling,it is found that the spin-flop transition of Cr PS₄ leads to the disappearance of the exchange bias effect at the interface.When changing the thickness of the ferromagnetic layer Fe₃GeTe₂,it is found from the experimental results that the exchange bias field has no relationship with the thickness of the ferromagnetic layer.This study discovers the exchange bias effect in Fe₃GeTe₂/Cr PS₄ heterostructure system for the first time and lays the foundation for the application of next-generation magnetic memory.（2）In two-dimensional magnetic materials,it is a very effective method to change the material carrier concentration through ionic liquid regulation.In this paper,the two-dimensional magnetic semiconductor material CrSiTe₃ is regulated by electrostatic field.Studying the air stability of CrSiTe₃,it is found that the oxidation rate of the thin layer is faster and the resistance will increase quickly.An electric double layer device composed of ionic liquid and CrSiTe₃ flakes is constructed.The room temperature electrical measurements of the device confirms that it is an ohmic contact,and the change of device resistance with temperature shows obvious semiconductor characteristics.When a positive gate voltage is applied to the device,the resistance increases sharply;when a negative gate voltage is applied,the resistance decreases.This study provides two specific solutions to the problems of easy oxidation and too large resistance of the sample,and confirms the effective regulation of CrSiTe₃ resistance by ionic liquids,which provides a new idea for the intrinsic regulation of two-dimensional magnetic materials.In this paper,two methods of construction of vdW heterostructure and electrostatic field are mainly used to regulate two-dimensional magnetic materials.The vdW heterostructure has a more controllable thickness and better flexibility,and a higher-performance magnetic memory device can be realized by pinning the ferromagnetic layer to control the parallel or anti-parallel magnetization directions to appear high and low resistance states.Electrostatic regulation can change the carrier concentration,symmetry and band gap of the material on the basis of keeping the original structure of the material,which further leads to the change of magnetism,Curie temperature and magnetic anisotropy.