Effect of Contact Geometry on Spin Transport in Amine-Ended Single-Molecule Magnetic Junctions

We employ the first-principles calculation with non-equilibrium Green's function method to comprehensively investigate the crucial role of interfacial geometry in spin transport properties of Co/1,4-benzenediamine (BDA)/Co single-molecule magnetic junctions (SMMJs). Two bonding mechanisms are proposed for the hard-hard Co-N coupling: (1) the covalent bonding between the H-dissociated amine linker and spin-polarized Co apex atoms and (2) the dative interaction between the H-non-dissociated (denoted by +H) amine linker and Co apex atoms. The former covalent contact dominates the π-resonance interfacial spin selection that can be well preserved in H-dissociated cases regardless of the choice of top, bridge, and hollow contact sites. From our detailed analyses of spin-polarized transmission spectra, local density of states, and molecular density of states, the underlying mechanism is that the strong hybridization between Co-d, N-p , and the π-orbital of the phenyl ring in dissociated cases renders the 2-fold HOMO (4-fold LUMO) of the central molecule closer to the Fermi energy. In contrast, the enlarged Co-N bond length of the latter dative contact in the H-non-dissociated case not only destroys the spinterface coupling but also blocks the spin injection. This theoretical work may provide vital and practical insights to illustrate the spin transport property in real amine-ended SMMJs since the contact geometries and interfacial bond mechanisms remain unclear during the breaking junction technique.