Theoretical Study On Magnetic Anisotropy Of The Organometallic Molecules

In recent years,the rapid development of spintronics has accelerated the continuous upgrading of information technology,especially the important progress of molecularscale spin regulation has put the information society on its wings.The current era of big data puts high demands on magnetic storage media.Although the development of spintronics is remarkable,when the storage medium is as small as the molecular scale,it usually produces a strong quantum effect.This greatly limits the practical application of molecular-scale magnetic storage media.The main reason is that the physical mechanism of the magnetic-structure correlation at the molecular scale is not yet perfect and needs to be further explored.The main work of this thesis is to combine highprecision quantum chemical methods,such as the complete active space self-consistent field method(CASSCF),density functional theory(DFT),and systematic theoretically study on the magnetic properties of organic transition metal molecules and their devices.We gradually reveal the physical mechanism behind these magnetic phenomena to help people in the design of molecular magnetic devices in the future.In the first chapter,we reviewed the history of the generation and development of single-molecule magnets,and introduced important concepts related to single-molecule magnets,such as ground state spin,magnetic anisotropy,and magnetic flipping energy barrier.We introduced some experimental studies on spin control in magnetic molecular devices in recent years.The more representative work is mechanically controllable break junction(MCBJ)and scanning tunneling microscope(STM)tip devices.These experiments have achieved precise control of magnetic anisotropy at the molecular scale,which has provided great inspiration for current sdudy.This provides a realistic reference for us to carry out theoretical study.In Chapter 2,we briefly introduced some methods used in our theoretical work.These methods mainly include the widely used density functional theory(DFT),and the high-precision fully activated spatial self-consistent field method(CASSCF)used to process strongly correlated electrons.Both methods have their own advantages and disadvantages,but they can fully combine their advantages when dealing with molecular devices.In Chapter 3,we conducted a theoretical study on the evolution of the Au-[Co(tpySH)2]--Aumolecular junction under mechanical stretching conditions.The DFT calculation fully shows that neither the GGA functional nor the hybrid functional can describe the magnetic anisotropy(MA)of the system well in this type of molecular junction system.When combined with the two methods of CASSCF and DFT,the embedded method achieves an accurate simulation of the MA evolution process of this composite molecular junction system during the stretching process.The calculation results reproduce the experimental observations,and further prove that the distortion of the octahedral CoN6 structure played an important role in the evolution of the molecular junction MA.In this work,we fully demonstrated the two-fold role played by the environment(that is,the electrodes around the molecule)in the entire stretching process which shows the complexity in the magnetic control of molecular devices.In Chapter 4,we focused on the MA of isolated transition metal molecules.Firstly,the MA of the mononuclear[Ni(MDABCO)2Cl3]ClO4 molecule reported in the current experiment has been studied.This molecule has the largest MA in nickel-based complex molecules.By fine-tuning its structure,we found that it is difficult to further improve the MA of this molecule through a single structure control.The reason is that this structure is multi-coordinated and it is difficult to control multiple bond lengths at the same time.Secondly,CASSCF calculations were performed on low-coordination nickel-based molecules selected in the Cambridge Crystal Structure Database(CCDC)of the United Kingdom.Four-coordinated molecules may also have obvious magnetic anisotropy.However,although the planar structure of the molecule can achieve a single structure control,the planar structure of the nickel-based molecule is usually low-spin and cannot produce magnetic anisotropy.We turned to study nickel-based molecules with a tetrahedral structure.Studies have shown that bond angles play an important role in the regulation of MA,and the adjustment of bond length will also affect magnetic anisotropy.These two aspects influence each other.,Which together determine the final MA.In the fifth chapter,we summarized and looked forward to all the studies.This chapter explains the value and significance of our theoretical work,and also predicts the possible development direction of single-molecule magnets and devices in the future.In the next step,we will further explore the wonderful magnetic-structure correlation on the basis of these research work.