Electronic,Magnetic And Transport Properties Of Vanadium-based Low-dimensional Room-temperature Ferromagnetic Materials

Monolayer two-dimensional magnetic materials can maintain a long range magnetic order in a single layer,which provides an ideal platform for the study of magnetic and other novel physical effects in the two-dimensional limit,and has become a frontier hot spot of international attention.Magnetic semiconductors can be used for spin generation and injection,as well as spin manipulation and detection.Moreover,Curie temperature,magnetic anisotropy and magnetization can be controlled by applying external field,so it is expected to become the core material of the next generation of low power spintronics/magnetic memory devices.Compared with other spintronics materials,magnetic semiconductors can be applied to fabricate devices using advanced semiconductor technology.But most of the magnetic semiconductors found in experiments have low Curie temperature,which hinder their practical application.The exploration of two-dimensional magnetic semiconductor materials with high Curie temperature and the further preparation of novel two-dimensional spintronics devices operating at room temperature will be an important step towards practical application of two-dimensional magnetic materials,thus driving the development of integrated circuit industry.Therefore,this thesis seeks and theoretically predicts the monolayer VTe₂ and monolayer Mg V₂S₄ of intrinsic room temperature vanad-based ferromagnetic semiconductor,and then designs the corresponding spintronics devices.The main conclusions are as followsThe electronic and magnetic properties of the heterostructure formed by monolayer In N and VTe₂ are studied by first principles calculation.Both ln N/VTe₂ vd W heterostructure and monolayer VTe₂ have high Curie temperature and large magnetic anisotropy energy.The heterostructure is an indirect band gap semiconductor and exhibits a type I band alignment.Different from the spin up band,the spin band band can transform from typeⅠto typeⅡband alignment under proper tension.The strain-adjustable band alignment type allows heterostructure to be used in optoelectronic devices.In addition,both Curie temperature and magnetic anisotropy energy are sensitive to strain.The main reason for the decrease of magnetic anisotropy energy with tension is the increase of the negative contribution of the spin orbit coupling interaction between the p_z and p_y orbitals.Hole doping can increase the conductivity of the spin down channel to 10⁵ times that of the spin up channel,which is far greater than the effect of electron doping.The lowest Curie temperature induced by doping is still greater than 300K,and the ferromagnetic half-metallic state at room temperature is realized.These properties indicate that ln N/VTe₂ vd W heterostructure is a promising low dimensional spintronics material.Based on two-dimensional Mo Si₂N₄ and WSi₂N₄ synthesized in recent experiments,we theoretically predict monolayer Mg V₂S₄.The ferromagnetic superexchange interaction plays a major role in the total exchange interaction,resulting in the ferromagnetic ground state and high Curie temperature of monolayer Mg V₂S₄.Both valence and conduction bands belong to the spin up band,indicating that monolayer Mg V₂S₄ is an intrinsic room temperature ferromagnetic half-semiconductor.Doping can effectively regulate the magnetic anisotropy energy by changing the electronic state and energy level difference near the Fermi level.Spin-flip band gap allows the conversion of semiconductors to half-metal through doping.Unlike the spin up channel,the conductivity of the spin down channel is always zero when doped.In the absence of doping,when the direction of magnetization is antiparallel at the same temperature,the conductivity weakens further,bringing the device closer to complete closure.In addition,a smaller doping concentration can significantly enhance the conductivity in the spin up channel.These excellent properties make monolayer Mg V₂S₄ an ideal candidate material for spintronic field effect transistor.