Theoretical Investigation On The Mechanical Control Of Magnetic Molecular Junctions

In the development of spintronic and quantum storage devices single molecular magnets have attained eminence because of their tunable functionalities.One of the promising means to fine-tune the spin and charge state of the molecule is by manipulating its local environment through some external stimuli,such as by using some mechanical handle.Scanning tunneling microscope(STM)tip is one of the promising ways to influence the local molecular magnetism mechanically.The STM tip forms a nano-junction with the transition metal confined within the molecule.Typically,a molecular-magnet requires characterization in a junction to determine the intrinsic electronic peculiarities before their fabrication in any spintronic or electronic device.Experimentally,it is quite demanding to attain a complete physical insight of the magnetic characteristics' whole evolution process,molecular functionalities,as well as arrangements of the magnetic-molecule in the nano-junction.In particular,the challenge is in finding the prime cause of charge transfer in the spin-changing molecules.Yet,it makes insights attained from ab-initio methods invaluable to develop the profound understanding of the local structural,electronic as well as magnetic characteristics and quantum transport elucidations,in molecular-magnetic-devices.This dissertation will study the mechanically controlled junctions made by STM tip and molecules embedded on the surface(111),tip/molecule/substrate(111)by using ab-initio methods.It is found that an STM-tip can controllably modulate the magneticanisotropy of the transition metal atom iron(Fe)confined in octaethylporphyrin(OEP)ring and axial Cl-ligand provoked by squeezing or retracting the junction maintained by an STM-tip.The tip environment affects the magneto-structure as well as the coordination ligand field of the molecule leading to the fundamental source of variation in magnetic anisotropy in the junction.By the first-principles calculations,hence it provided the general geometric,electronic,and magnetic characteristics evolution of FeOEPCl molecule embedded on various substrates,namely,Pb,Au,and Ag controlled by an STM-tip.The three substrates indicate distinct magnetic and structural evolution trends.It is found that spin-orbit coupling of Fe-ion to the surface and variation of Fe to five coordination ligand lengths can substantially influence the basic characteristics of the molecule.Even may significantly manipulate the ligand field splitting of the molecule resulting in the tunability of magnetic anisotropy of the molecule merely by using an STM tip.This gives mechanical control over the spin/charge state of the molecule.Such ability is beneficial in the development of high-density storage devices.Whereas the understandings achieved by our ab-initio methodologies in this study are on-demand for environmental modulation in the manufacturing of molecular spintronic and electronic devices.Chapter 1 covers the overview of the control of single molecular magnetism specifically metal-organic molecules when they are embedded in nano-junctions made by an STM tip.The influence of STM tip on the local environment of transition metal atom when it is adsorbed on a surface is elaborated in conjunction with effective ways to control Kondo screening and spin-orbit coupling as described in the literature is discussed.Chapters 2 focuses on the basics and background of ab-initio methodologies from density functional theory(DFT)to complete active space self-consistent field(CASSCF).Here,our computational methodology adopted in DFT and CASSCF is also described in this chapter.In chapter 3,initially describe the reference molecule of interest FeOEPCl,as well as,the choice of Hubbard-like correction adopted here is also described.In this chapter,the possibility of fine-tuning the magnetic anisotropy energy by using an STM tip is elucidated.An experimental study of FeOEPCl/Pb(111)observed a sudden jump in spin excitation energy with the tuning of the junction gap,though the prime cause of jump remained obscured.Additionally,this chapter elucidates the prime cause of the origin of a sudden spin excitation energy jump by using an embedding method,DFT+U+CASSCF.Finally,the STM tip influence in the FeOEPCl/Pb(111)junction is elaborated in conjunction with the role of the axial ligand confinement effect.The role of the axial Cl ligand is clarified by comparing the evolutions under the STM tip control in Pb/FeOEPCl/Pb(111)with the dechlorinated Pb/FeOEP/Pb(111)junction.In chapter 3 studies the case when Cl-ligand maintained the distance from tip and substrate,herein chapter 4,a distinct case of Au/FeOEPCl/Au(111)junction is studied in which the axial ligand makes a contact with the STM tip.Structural,magnetic characteristics evolutions when the tip comes in contact with the axial chloride ligand coordinated above the Fe are studied.The unique local magnetic evolution dependence on structural distortion is discussed in conjunction with Pb and Ag substrates.The determination of bistability in magnetic anisotropy of the system with the tip advancement to the contact regime is also analyzed.The roleplay of different tip/substrate environmental modulations are compared for Ag,Au,and Pb tip/molecule/substrate junctions.Chapter 5 is the conclusion and perspective of the dissertation.