【作者】 张凯；

【导师】 武晓君；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2021， 博士

【摘要】 身处当下信息时代的二十一世纪,随着摩尔法则的逐渐失效,开发和设计用于信息读取、存储和传输的新型材料和电子器件显得尤为重要。近年来,自旋电子器件的提出为信息存储技术开辟了一条新的研究途径。相比于一般电子器件利用电子电荷来进行信息处理,自旋电子学材料致力于操控电子的自旋这一自由度。在理想情况下,由于在回路中仅存在自旋极化电流而不存在电荷电流,将不会在自旋器件中产热从而降低能耗。同时与基于电荷的传统半导体电子器件相比,这些新颖的器件还具有非易失性、更高的数据存储速度和集成密度。自旋电子学已在磁场传感、磁阻随机存取存储器、自旋场效应晶体管等获得应用。目前磁性材料是实现自旋器件最为常用的核心组成部分,其中而根据磁性材料电子结构特征的不同,可以将其分类为磁性金属、半金属、磁性半导体以及双极磁性半导体等。这些材料可以实现自旋电流的产生、注入、传输以及存储和外场调控。而为了实现器件的小型化,发展纳米尺度自旋器件是未来的趋势。所以,为满足这一需求,设计可合成的低维磁性材料具有科学和应用意义。随着计算方法和计算资源的快速发展,理论计算模拟在新材料开发中扮演越来越重要的角色。这是因为理论计算不仅能从原子尺度出发研究结构和性能的关系,同时和实验共同形成理论模拟加实验验证的新模式节约成本。所以,本文借助第一性原理密度泛函理论和结构全局搜索方法等设计一系列实验可行的低维磁性结构并系统探究电子结构特征及其性能调控。本文共分为七章。第一章介绍自旋电子器件的发展状况,低维磁性材料的研究状况和当前的挑战与目标。在第二章中我们简要介绍密度泛函理论的核心知识,包括理论基础、基本近似方法、Kohn-Sham方程、交换关联泛函以及基于密度泛函理论的软件包。第三章,我们利用第一性原理计算通过构建异质结和原子掺杂这两种策略分别在非磁材料中引入自旋极化。第一部分我们通过在非磁半导体二硫化钼(MoS2)中引入线型原子掺杂获得一系列具有局域自旋极化的复合结构。其中我们考虑采用硼、碳、氮和氟元素替换一排硫原子以及分别用过渡金属锰、铁、钴、镍取代一排钼原子来引入磁性。结果表明可以在缺陷附近引入自旋极化并且发生量子相变转变为铁磁金属或铁磁半导体,此外通过施加应力的方式还能调控不同电子态之间的转变。第二部分我们选取自然界存在的室温反铁磁半导体三氧化二铬(Cr2O3)作为衬底通过搭建异质结的方式对非磁的单层MoS2进行自旋注入,计算结果表明衬底表面的铬原子会与硫原子发生轨道杂化,电子从衬底转移至MoS2使其产生自旋极化。由于时间反演对称性的破坏以及自旋轨道耦合效应使得单层MoS2能谷在高对称点(±K)处简并性破坏从而引发23.4meV的塞曼分裂,此外分裂数值可以通过外加电场调控。第三部分是对实验合作工作的理论分析。实验上在Cr2O3衬底表面生长了少层石墨烯,仪器表征表明石墨烯是产生空穴掺杂。我们通过搭建简化模型,计算结果显示电子确实是从石墨烯向衬底转移,同时在石墨烯中引入自旋极化,这对于发展石墨烯基自旋器件具有重要意义。第四章,我们借助理论计算探讨分子尺度过渡金属硫属化物一维超细纳米线在自旋电子器件中的应用前景。首先我们构筑五种不同纳米线结构来确定不同元素组合下的基态构像,然后确定磁基态及稳定性。电子结构计算表明这些纳米线具有丰富的本征性质,如非磁、铁磁和反铁磁半导体,它们的磁矩以及磁耦合基态可以通过晶体场理论得以解释。此外电子或空穴注入能实现铁磁半导体向半金属的转变,而纳米线本身的扭曲程度也可以调整自身带隙的大小。这些有趣的电子和磁学性质及其可调控性使过渡金属硫属化物一维超细纳米线成为自旋电子学材料的一个新家族。第五章,由晶体场理论可知,中心金属离子的轨道能级占据取决于两个因素,即中心离子价态以及配体种类。所以我们考虑改变配位环境这一策略能否在非磁材料中引入孤对电子从而诱导本征磁性?基于近期实验合成Janus钼硫硒(MoSSe)这一崭新结构形式,我们在非磁单层钛硫族化物基础上搭建了一系列Janus TiXY(X=S,Se,Te;Y=H,F)结构来验证这一猜想。计算结果表明氢或氟原子替换一侧硫族原子后钛离子价态由正四价转变为正三价,在3d轨道产生一个未配对电子使得材料产生本征磁性。这些Janus单层纳米片都具有铁磁基态以及良好的结构稳定性,能带结构展示TiSH,TiSeH和TiTeF是自旋无带隙半导体,TiSF和TiSeF是铁磁半金属,特别的是TiTeH是双极磁性半导体。蒙特卡罗模拟给出居里转变温度都高于室温。此外在单层TiTeH中由于存在重元素碲具有强自旋轨道耦合作用,根据巨磁能带效应通过自旋取向翻转可以实现半导体到金属性的量子相变以及部分能带交叉和打开能隙的拓扑电子态转变。通过计算模拟我们不仅证实改变配位环境是诱导本征铁磁材料的有效策略,同时我们提出的Janus TiXY纳米片可作为应用于自旋电子器件的理想候选材料。第六章,我们基于铁氮化物设计具有超高自旋传输速率自旋电子学材料。首先我们将拓扑学和磁学结合在平面拓扑五元环铁氮化物(pp-Fe4N2)中获得Nodal-Loop(NL)半金属性并且展现室温铁磁性,全局结构搜索表明在二维受限条件下是所有平面结构中能量最有利的。此外这一结构可以通过沿体相Fe2N[001]方向剥离超薄原子层自发弛豫后获得,这意味着实验上可以通过选择合适的衬底外延生长制备。在全局结构搜索中我们也得到众多非平面铁氮化物,我们选取能量最低的三种结构,模拟结果表明都是具有狄拉克锥的铁磁金属,居里温度可达226～556 K。第七章,我们借助高通量这一新兴手段,以平面拓扑五元环和Lieb晶格为基础,考虑金属与非金属双元素组合,最终从2940种组合中获得58个稳定平面结构,未被报道的结构共有47个,其中平面拓扑五元环结构38个,具有Lieb晶格的平面拓扑四元环20个。这些新结构展示了丰富的本征性质,如磁性、半导体性、狄拉克、外尔以及NL半金属性。部分结构展示了 Lieb-like能带和局域的平带特征,为探究新型物理效应提供潜在的真实材料平台。我们通过高通量计算获得了平面拓扑五环、四环双元素金属化合物的小型数据库,也为进一步探究更为复杂的多元素平面结构提供了借鉴基础。更多还原

【Abstract】 The 21st century is the information age,with the gradual failure of Moore’s Law,it is particularly important to develop and design new materials and electronic devices for information reading,storage,and transmission.In recent years,the proposal of spintronic devices has opened up a new approach for information storage technology.Compared with traditional electronic devices that use electronic charges for information processing,spintronics materials are dedicated to manipulating the degree of freedom of electron spin.Ideally,since there is only a spin-polarized current in the circuit and no charge current,there will be no heat generation in the spin device,thereby reducing energy consumption.At the same time,these novel devices also have non-volatility,higher data storage speed and integration density compared with traditional semiconductor electronic devices based on electric charges.Nowadays,spintronics has alreadly been applied in magnetic field sensing,magnetoresistive random access memory,spin field-effect transistors,etc.At present,magnetic materials are the most important components to construct spin-based devices.According to the different electronic structure characteristics of magnetic materials,they can be classified into magnetic metals,half-metals,magnetic semiconductors,nodal-line semi-metals,etc.These materials can realize the spin current generation,injection,and transmission.For realizing the miniaturization of devices,fabricating spintronics in nanoscalekewang is desired.Therefore,the design of synthetic low-dimensional magnetic materials has both scientific and application significance.With the rapid improvement of computational methods and computing resources,theoretical calculation simulation plays an increasingly important role in the exploitation of new materials.This is because theoretical calculations can not only study the relationship between structure and performance from the atomic scale but also save costs by forming a new model with experimental verification together.Hence,in this paper,based on systematic first-principles density functional theory calculations and global structural search we designed a series of experimentally feasible low-dimensional magnetic structures and explore the characteristics of electronic structures and the tenability of their performance.This article is divided into six chapters.We introduce the development status of spintronic devices,the research status of low-dimensional magnetic materials,and current challenges and sesign goals.In the second chapter,we we briefly introduce the framework of density functional theory,including theoretical foundations,basic approximation methods,Kohn-Sham equations,exchange-correlation functionals,and software packages based on density functional theory.In Chapter 3,we use first-principles calculations to introduce spin polarization in non-magnetic materials through two strategies:heterojunction construction and atomic doping.In the first part,we obtained a series of hybrid structures with local spin polarization by introducing linear atom doping into the non-magnetic semiconductor molybdenum disulfide(MoS2).Among them,we consider replacing a row of sulfur atoms with boron,carbon,nitrogen,and fluorine elements,while replacing a row of molybdenum atoms with transition metal manganese,iron,cobalt,and nickel to induce magnetism.The results show that spin polarization can be introduced near the defect and a quantum phase transition from non-magnetic semiconductor to ferromagnetic metal or ferromagnetic semiconductor are confirmed.In addition,the transition between different electronic states can be controlled by applying external strain.In the second part,we select the room temperature antiferromagnetic semiconductor chromium trioxide(Cr2O3)that exists in nature as the substrate to inject spin polarization into non-magnetic monolayer MoS2 by constructing a heterojunction.The calculation results show that the orbitals of chromium on the substrate surface will hybridize with sulfur atoms,and electrons will be transferred from the substrate to MoS2.Due to the destruction of time-reversal symmetry and the strong spin-orbit coupling effect,the energy valleys of single-layer MoS2 are degenerately destroyed at the high symmetry point(±K),which triggers a Zeeman split of 23.4 meV.In addition,the split value can be tuned by a vertical electric field.The third part is the theoretical analysis of experimental work.In the experiment,a few layers of graphene were grown on the surface of the Cr2O3 substrate,and the instrumental characterization showed that the graphene was doped with holes.By building a simplified model,the calculation results show that electrons are indeed transferred from graphene to the substrate,and at the same time,spin polarization is generated in the graphene,which is of great significance for the development of graphene-based spintronics.In Chapter 4,based on theoretical calculations we explore the application prospects of one-dimensional transition metal chalcogenide molecular nanowires in spintronic devices.First,we construct five different nanowire structures to determine the ground state conformation under different element combinations and then estimate the magnetic ground state and evaluate structural stability.Electronic structure calculations show that these nanowires have diverse intrinsic properties,such as non-magnetic,ferromagnetic,and anti-ferromagnetic semiconductors.Their magnetic moment and magnetic coupling ground state can be explained by crystal field theory.In addition,electron or hole injection can realize the transformation of ferromagnetic semiconductor to half-metal,and the degree of distortion of the nanowire itself can also modulate the bandgap values.These interesting electronic and magnetic properties and their controllability make one-dimensional transition metal chalcogenide ultrafine nanowires a new family of spintronics materials.Chapter 5,according to the crystal field theory,the orbital energy level occupation of the central metal ion depends on two factors,namely the oxidation state of the central ion and the type of ligand.So,whether the strategy of changing the coordination environment can bring lone pairs of electrons in non-magnetic materials to induce intrinsic magnetism is an open question?Inspired by the recent synthesized Janus molybdenum sulfur selenium(MoSSe),a new structural form,we built a series of Janus TiXY(X=S,Se,Te;Y=H,F)on the basis of non-magnetic monolayer titanium chalcogenides to verify this conjecture.After hydrogen or fluorine atom replaces one side of the chalcogen atom,the valence state of titanium ion changes from positive tetravalent to positive trivalent,and unpaired electrons are generated in the 3d orbital leading to intrinsic magnetism.These Janus single-layer nanosheets all have a ferromagnetic ground state and structural stability.The band structure shows that TiSH.TiSeH,and TiTeF are spin-gapless semiconductors,TiSF and TiSeF are ferromagnetic half-metals,and especially TiTeH is a bipolar magnetic semiconductor.Monte Carlo simulation indicates that the Curie temperature of these Janus monolayers are higher than the room temperature.In addition,due to the presence of the heavy element tellurium in the single-layer TiTeH,it has a strong spin-orbit coupling effect.According to the giant magneto band-structure effect,the quantum phase transition from semiconductor to semi-metal is happened and the topological state transformation induced due to band crossing and opening gap via spin direction reversal.Through computational simulations,we not only confirmed that changing the coordination environment is an effective strategy for inducing intrinsic ferromagnetic in non-magnetic materials,but the Janus TiXY nanosheets we proposed can be used as ideal candidate materials for spintronic devices.In Chapter 6,we proposed a nodal loop half metal material which can be used in ultra-high spin-transfer speed spintronics based on iron nitrides.Here,topology and magnetism are compatible in an iron nitride with planar pentagonal topology(pp-Fe4N2)which displays nodal-loop(NL)half-metallicity and exhibits room-temperature ferromagnetism.This is the first time realizing an ideal NL in planar pentagonal topology,and the global structure search shows that the energy is the most favorable among all planar structures under two-dimensional restriction.In addition,this structure can be obtained by spontaneously relaxing the ultra-thin atomic layer that exfoliated from the bulk Fe2N along[001]direction,which means it can be prepared epitaxial growth on a suitable sunstrate in experiments.In the global structure search,we also obtained many non-planar iron nitrides.We selected the three most energy favourable structures to explore their properties.The simulation results demonsttate that they are all ferromagnetic metals with Dirac cones around feimi-level,and the Curie temperature can reach 226-556 K.In Chapter 7,we had performed high-throughput screening binary metal compounds which possess planar pentagonal topology or Lieb lattice,and finally obtained 58 stable planar structures from 2940 combinations,including 38 planar topological five-membered ring structures and 20 planar topological four-membered rings with Lieb lattice.Among them,a total of 47 structures unreported before.Varieties of intrinsic properties,such as magnetism,semiconductivity,Dirac,Weyl,and NL semi-metal are presented in these planar monolayers.Some structures reveal the Lieb-like energy band and local flat band,providing a potential real material platform for exploring new physical effects.Through high-throughput calculations,we obtained a database of planar binary compounds with pentagon topology or Lieb lattice,which also provided a reference for further exploration of more complex multi-element planar structures.更多还原