Ligand-Field Modulation Of Magnetic Properties Of Single Transition Metal Atoms

The electronic and magnetic properties of single atoms are dictated by their surrounding environment.Accurate understanding the influence of organic ligands on the properties of central metal atoms in metal-organic complexes and metal-organic coordination networks is beneficial for designing metal-organic frameworks with specific functionalities.For example,easy-axis magnetic anisotropy separates two magnetic states with opposite magnetic moments,and single magnetic atoms and molecules with large easy-axis magnetic anisotropy are highly desired for future applications in high-density data storage and quantum computation.In this thesis,we used low-temperature scanning tunneling microscopy and spectroscopy(STM/STS)at a single-molecule level to systematically study the on-surface chemical reactions and self-assembled structures between tetra-pyridyl-porphyrin(TPyP)molecules and Fe,Ni atoms on Au(111)substrate.Artificial modulation of the magnetic properties of single Fe and Ni atoms is achieved by building appropriate ligand field via tuning the chemical reactions.By tuning the metalation reaction between TPyP molecules and Fe atoms on Au(111)substrate,we have firstly obtained an Fe-porphyrin complex which is in the intermediate state of the metalation reaction,and investigated the magnetic property of this complex.As revealed by inelastic electron tunneling spectroscopy in magnetic field,this Fe-porphyrin complex has magnetic anisotropy energy of more than 15 meV with its easy-axis perpendicular to the molecular plane.Two magnetic states with opposite spin directions are discriminated by the dependence of spin-flip excitation energy on magnetic field and are found to have long spin lifetimes.Our density functional theory calculations reveal a weak ligand field with elongated Fe-N bonds makes the Fe atom in this complex has a high-spin state S = 2 and a large orbital angular momentum L = 2,which give rise to easy-axis anisotropy perpendicular to the molecular plane and large magnetic anisotropy energy by spin-orbit coupling.In addition,the central Fe atom is decoupled from Au(111)substrate by the lifting of organic ligand,so that the influence from itinerant electrons is relatively weak.Since the Fe atom is protected by the molecular ligand,the complex can be processed at room or even higher temperatures.Compared with metal-organic complexes,the properties of metal atoms in metal-organic coordination networks are rarely revealed by STM.A hindrance in the study of metal atoms in coordination networks is that the metal atoms are usually embedded below the molecular plane,making these atoms invisible in the STM images.Here we investigated the coordination network formed by TPyP molecules and dinuclear Ni centers by cryogenic STM/STS,and revealed the correlations between the electronic and magnetic properties of above-plane Ni atoms and the interaction with surrounding organic ligands.Even in the same Ni-pyridyl coordination motif,three types of above-plane Ni atoms are identified at the single-atom level from their topographies in STM images,and electronic states in STS,which are determined by various hybridization between Ni atoms and surrounding pyridyl groups,as well resulting in different magnetic properties of Ni atoms.Further dI/dV spectra near the Fermi level in magnetic field reveal that the weaker the ligand field is,the higher spin state and the larger magnetic anisotropy energy of the above-plane Ni atom has.We also find the three types of above-plane Ni atoms do not show any scattering by itinerant electrons,indicating the above-plane Ni atoms are effectively decoupled from the metal substrate.In addition,the d I/dV maps also reveal the orbital distribution of the above-plane Ni atoms,and in the network,the orbitals of the coordinated Ni atoms correlate with each other via in-between TPyP molecules.Our works demonstrate on-surface reactions between metal atoms and organic molecules can be utilized to synthesize metal-organic complexes and metal-organic coordination frameworks with special magnetic properties.We also exemplify the possibility to achieve multiple electronic and magnetic properties by tuning the metal-ligand interaction in the same coordination motif.