Magnetic And Transport Properties Of Van Der Waals Heterojunction

With the progress of science and technology,the iteration of electronic products is faster and faster,and people have higher and higher requirements for high performance,low power consumption and high portability of electronic products.However,with the nano size of electronic devices such as chips becoming smaller and smaller,the Moore law based on silicon seems to end gradually,so it is more and more urgent to replace silicon based materials with new materials.With the continuous progress of science and technology and the continuous research of researchers,more and more new materials have been discovered,among which two-dimensional materials are widely studied because of their unique physical properties,ultra thin thickness and easy to prepare.However,most of the two-dimensional materials have no magnetism,which greatly limits their application in the field of information storage.At present,using different two-dimensional materials stacking to construct van der Waals heterojunction can introduce magnetism to the two-dimensional materials,and the magnetic van der Waals heterojunction can also be used to make electronic devices,such as spin valve,magnetic tunnel junction,etc.The magnetic tunnel junction is composed of two poles and the insulator between them.Previous studies have shown that many strange spin correlation phenomena have been found in these tunnel junctions,such as spin filtering,tunneling magnetoresistance and near path effect.Tunneling magnetoresistance is an important physical phenomenon,which has a great economic impact on the storage of magnetic information,including reluctance random access memory,microwave generator,RF sensor and neural network.In this paper,the magnetic properties of the new two-dimensional van der Waals MnBi₂Te₄ heterojunction and the electronic transport properties of the magnetic tunnel junction based on MnBi₂Te₄heterojunction are studied by the first principle calculation method.The main contents and results are as follows:1.Since the double-layer MnBi₂Te₄ exhibits antiferromagnetic properties between layers,which is a great obstacle to the construction of its magnetic tunnel junction,how to change it from antiferromagnetic to ferromagnetic is of great significance to the realization of topology.Based on first principles,this chapter studies the magnetic changes of heterojunction constructed by double-layer MnBi₂Te₄ and three kinds of topological insulators Sb₂Te₃,Bi₂Te₃and Bi₂Se₃ with different number of layers(1QL-5QL).Specifically,the energy difference between ferromagnetism and anti-ferromagnetism after inserting topological insulator is calculated by first principles software VASP to judge whether it changes from antiferromagnetic state to ferromagnetic state.The results show that when three kinds of topological insulators with different layers are inserted into the double-layer MnBi₂Te₄,only the 5QL Sb₂Te₃heterojunction changes its magnetism from anti-ferromagnetism(AFM)to Ferromagnetism(FM).Not all topological insulators can induce the change of magnetic properties,which also shows that magnetic properties can not be induced only by carriers.This study provides an effective theoretical guidance for inducing the magnetic properties of even number layer MnBi₂Te₄.2.On the basis of the previous chapter,we replace the topological insulators with graphene and bilayer Graphene,boron nitride(h-BN),molybdenum disulfide(Mo S₂)and tungsten selenide(WSe₂)and other common two-dimensional materials,through the same method as the previous chapter,continue to explore the interlayer magnetic conversion.The results show that:to achieve the interlayer ferromagnetic flip,it needs high carriers and the coupling between carriers and non-magnetic atoms.3.We have designed a new type of spin valves Cu(111)/MnBi₂Te₄/MnBi₂Te₄/Cu(111)and Cu(111)/MnBi₂Te₄/h-BN/n·MnBi₂Te₄/Cu(111)(n=1,2,3)based on van der Waals heterojunction,and studied the structure by density functional theory and non-equilibrium Green's function.The results show that the Cu(111)/MnBi₂Te₄/h-BN/n·MnBi₂Te₄/Cu(111)system is stable under bias voltage.When n=2 and 3,there is a switching effect within a certain voltage range,and when n=3,the anti-parallel structure has a negative differential resistance effect.The maximum TMR of MnBi₂Te₄/h-BN/3·MnBi₂Te₄ can reaches 162.6%.The results provide an effective theoretical guidance for the fabrication of novel spin valves using two-dimensional magnetic materials.