First-principles Study Of The Magnetic Anisotropy Of Low-Dimensional Organometallic Compounds

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.