【作者】 孙杰；

【导师】 王建利；

【作者基本信息】 中国矿业大学 ， 凝聚态物理， 2023， 硕士

【摘要】 单层二维磁性材料可以呈现长程磁序,这为二维极限下的磁性以及其他新奇物理效应的研究提供了理想的平台,成为国际上备受关注的前沿热点。磁性半导体可用于自旋产生和注入,以及自旋操纵和检测。且可通过施加外场的方式实现对居里温度、磁各向异性以及磁化强度等性质的调控,从而使其有望成为下一代低功耗自旋电子学/磁存储器件的核心材料。与其他自旋电子学材料相比,利用先进的半导体技术,磁性半导体可以应用于制备器件。但实验中发现的大多数磁性半导体的居里温度较低,阻碍了它们的实际应用。探索高居里温度的二维磁性半导体材料并进一步制备在室温下工作的新型二维自旋电子学器件,将是二维磁性材料迈向实际应用的重要一步,从而推动集成电路产业的发展。本论文寻找并理论上预测了本征室温钒基铁磁半导体单层VTe₂和单层Mg V₂S₄,进而设计了相应的自旋电子学器件。主要结论如下:通过第一性原理计算研究了二维In N与VTe₂所组建的异质结的电子和磁学性质。ln N/VTe₂范德瓦尔斯异质结和单层VTe₂均具有高居里温度和大的磁各向异性能。异质结是间接带隙半导体并呈现I型能带排列。不同于上自旋能带,下自旋能带在适当张力下可以实现从Ⅰ型带阶向Ⅱ型带阶的转变。应变可调的带阶类型,使得异质结可以应用于光电器件。此外,居里温度和磁各向异性能都对应变敏感。磁各向异性能随张力降低的主要原因是Te原子p_z轨道和p_y轨道自旋轨道耦合相互作用负贡献的增加。空穴掺杂可以使下自旋通道的电导率增至上自旋通道的电导率的10⁵倍,远远大于电子掺杂的作用。掺杂诱导的最低居里温度仍大于300K,实现了室温铁磁半金属态。这些特性表明ln N/VTe₂范德瓦尔斯异质结是一种很有前途的低维自旋电子材料。基于最近实验上合成的二维Mo Si₂N₄和WSi₂N₄,我们理论预测了单层Mg V₂S₄。铁磁超交换相互作用在总交换相互作用中起主要作用,最终导致单层Mg V₂S₄的铁磁基态和高居里温度。价带和导带都属于上自旋能带,表明单层Mg V₂S₄是一种本征室温铁磁半半导体。通过掺杂改变费米能级附近的电子态以及能级差的方式可以有效调节磁各向异性能。自旋翻转带隙使得半导体到半金属的转变也可以通过掺杂实现。与上自旋通道不同,下自旋通道的电导率在掺杂时一直为零。在没有掺杂的情况下,当磁化方向在相同温度下反平行时,电导率进一步减弱,使器件更接近完全闭合。此外,较小的掺杂浓度可以显着增强上自旋通道中的电导率。这些优异的特性使单层Mg V₂S₄成为自旋电子场效应晶体管的理想候选材料。更多还原

【Abstract】 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.更多还原