【作者】 王鹏；

【导师】 赵纪军； 蒋雪；

【作者基本信息】 大连理工大学 ， 凝聚态物理， 2019， 博士

【摘要】 信息技术是促使当前社会进步的一个重要推动力,随着各种新技术的出现,每天产生的数据都在呈指数级增长,人们对信息存储的需求也随之快速增长。当前使用最广泛的信息存储技术是磁性存储,其成本低,存储密度高,稳定性高,在众多领域得到应用。磁性存储是利用磁性材料作为存储介质,当每个存储单元保持特定方向磁化的磁化状态时,信息被存储在其中。磁存储介质需要有较大的磁各向异性来对抗热扰动等因素的影响,保持其磁化方向,使信息能够保持长期稳定。要提高磁性存储的密度,需要我们不断提高存储介质的磁各向异性,缩小单个存储单元的尺寸。当前的磁性存储正在朝着原子、分子(或团簇)的水平发展,其终极目标是实现单原子存储。磁各向异性主要来源于原子的自旋轨道耦合效应,过渡金属因为其未满的d轨道,且自旋轨道耦合较强,在磁性材料设计上潜力巨大。材料由块体到低维时,配位场发生巨大的变化,有利于增强其自旋轨道耦合作用,找到大的磁各向异性。本文利用第一性原理计算研究了低维有机材料支撑的5d过渡金属原子的磁各向异性,为设计低维磁性材料提供一些新的思路。首先研究了 5d过渡金属修饰的二维酞菁聚合物的磁性。对于单个金属原子修饰的体系(TM@Pc),Re原子修饰二维酞菁聚合物的磁各向异性能(MAE)高达20.7 meV。再用一个同核金属原子修饰,形成二聚体结构后,体系的易磁化轴会在平行于二维衬底和垂直二维衬底的方向切换。同时,在Os2@Pc和Ir2@Pc中发现了40 meV以上的磁各向异性能。我们分析了其电子结构,并讨论了它们的磁各向异性的来源。同时还讨论了其结构稳定性和磁耦合行为。其中的Re@Pc和Ir2@Pc有较高的居里温度,在磁存储器件上有着较大的应用潜力。接着,又研究了5d过渡金属修饰的二维卟啉聚合物的磁性。在W和Re原子修饰的二维卟啉中发现了大约24 meV的MAE。首次提出使用化学官能团修饰来调控二维金属有机材料的磁各向异性。当卟啉分子边缘的H原子被羟基和氨基取代后,Re修饰的维卟啉的MAE可以被增强到60 meV。对电子结构的分析表明,磁各向异性的增强可以被归因于取代基团引起的电荷重新分布和Re原子5d轨道能级的移动。报据计算,该体系的居里温度在200 K左右,稳定性也较好。化学官能团修饰为调控金属有机材料的磁各向异性提供了一个全新的思路。在二维金属有机Kagome晶格中也发现了较大的磁各向异性,分别研究了M3C12N12H12和M3C12O12两种晶格,其中Re3C12N12H12的MAE达到27.6 meV/atom。此外,这些材料还表现出强的铁磁耦合行为。通过蒙特卡洛模拟计算出W3C12O12的居里温度达到458 K,Re3C12N12H12的也达到291K。W3C12012还是一种自旋半金属材料,可以用于未来的自旋电子学器件。除了二维有机材料,磁性金属有机分子也是未来自旋电子学器件的候选材料。我们研究了基于5d过渡金属的茂合物(MCp2和M2Cp3)的磁各向异性。它们是由茂环(Cp)和过渡金属原子形成的三明治结构分子,能够为金属原子提供一个特殊的配位场,产生新的磁学性质。HfCp2和WCp2表现出了倾向于垂直于茂环平面的磁各向异性,MAE达到10 meV左右。我们进一步研究了三层过渡金属茂合物(M2Cp3)的磁各向异性,在Ta2Cp3中发现了超过60 meV的MAE。茂合物组成的变化,导致Ta原子受到的配位场发生变化,能级劈裂并发生移动,体系的MAE相比双层增强了很多。本论文通过在低维金属有机材料中改变过渡金属原子的配位环境,增强其自旋轨道耦合作用,设计出具有较大的磁各向异性的稳定结构,为未来的高密度磁性存储和自旋电子学器件找到一些可能的候选材料,对于设计新型磁性材料也有一定的指导意义。更多还原

【Abstract】 Information technology is nowadays an important impetus to the development of human society.Along with the emergence of new technologies,the data volume is increasing exponentially boosting the demand for data storage.Magnetic storage is the most widely used information storage technology due to several advantages,for instance,low cost,high density and excellent stability.The magnetic bit is stored as the direction of magnetization in the magnetic storage medium.Large enough magnetic anisotropy of the recording medium is required to overcome thermal fluctuations and other quantum effects.In order to minimize the bit size or raise the density of magnetic storage,we need to improve the magnetic anisotropy of storage medium further.The magnetic storage technology is progressing towards the level of atom,molecule and cluster and the ultimate goal is to realize single atom storage.Magnetic anisotropy originates from spin-orbital coupling(SOC).Transition metal atoms have localized and partially-filled d orbitals,endowing them with remarkable SOC effects and tremendous potential for achieving large magnetic anisotropy.Additionally,from bulk materials to low－dimensional systems,the ligand field will be altered greatly and the SOC effect and magnetic anisotropy will be enhanced tremendously.In this thesis,by using first-principles calculations,we investigate the magnetic anisotropy of 5d transition metal contained 2D organic materials and metal-organic molecules and propose some new strategies for designing novel low-dimensional magnetic materials.In the first work,magnetic properties of a 5d transition-metal adatom decorated two dimensional(2D)polyphthalocyanine framework(TM@Pc)are systematically investigated by means of first-principles calculations.Giant perpendicular magnetic anisotropy with a MAE of up to 20.7 meV is found in Re@Pc.After decorating with a homonuclear transition metal adatom,overturning of the easy axis is demonstrated and the magnetic anisotropy energy can be increased to over 40 meV for Os2@Pc and Ir2@Pc.The origin of magnetic anisotropy,structural stability and magnetic coupling behavior are also discussed.All the results show that these 2D organic materials can serve as promising candidates for future magnetic storage devices.Then for the first time,we propose a method to chemically engineer the magnetic anisotropy of 2D metal-organic materials.We find large MAE of 24 meV in W or Re embedded 2D polyporphyrin frameworks.Interestingly,the MAE can be enhanced up to 60 meV,through replacing the hydrogen atoms on the edges of the Re based 2D polyporphyrin framework by hydroxyl and amino radicals.Analysis of the electronic structures reveals that the enhancement of MAE is mainly attributed to charge redistributions and energy shifts of Re 5d orbitals induced by the functional radicals.Then we investigate the magnetism of a sernes of 2D metal-organic Kagome lattice,M3C12N12H12 and M3C12O12.Among them,Re3C12N12H12 has a large MAE of 27.6 meV/atom.In addition,some of them exhibit strong ferromagnetic coupling behaviors.According to our Monte Carlo simulations,W3C12O12 possess very high Curie temperature of 458 K.Re3C12N12H12 also has a Curie temperature of 291 K,comparable to the room temperature.By examining their band structures,W3C12O122 is identified to be a half-metal,which is of great importance in spintronmcs.In the last work,the magnetic properties of a series of 5d metallocenes,namely,two cyclopentadienyl(Cp)rings sandwiched with a single 5d transition metal atom,are investigated.Our first-principles calculations reveal that the Cp rings not only provide a suitable ligand environment for metal atom,but also result in tunable magnetism depending on the transition metal element.Among them,HfCp2 and WCp2 show high preference for the magnetization axis perpendicular to the Cp plane,with large MAEs around 10 meV.We further consider the triple decker metallocenes(M2Cp3),and find a huge MAE of above 60 meV in Ta2Cp3.The orbital energy split and shifts induced by composition change in metallocenes is mainly responsible for the significant MAE enhancement.In brief,by constructing suitable ligand field for transition metal atoms,we find giant magnetic anisotropy in low-dimensional metal-organic materials.We pave a feasible pathway for designing promising building block of future magnetic storage devices.更多还原