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

Abstract

We employ the first-principles calculation with nonequilibrium 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 spinpolarized 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 Hdissociated 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 y , 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.

Exchange bias toggling in amine-ended single-molecule magnetic junctions by contact geometry

“…This is because CH 2 unit well separates N-p x,y orbital and π orbital of central phenyl ring near Fermi energy as shown in Figure 3 of Ref. [23].…”

“…Because of the covalent bonding between H-dissociated amine linker and Co apex atoms as shown in Figure 2 of Ref. [23], all three cases exhibit pronounced πresonant spin-up transmission near Fermi energy. Such spinterface effect assists strong enhancement of μ 0 H (0)…”

“…Here we adopt three possible contact geometries, i.e., top (T-III′), bridge (B-II), and hollow (H-III) contact sites proposed in Figure 1 of Ref. [23], as long as the hydrogen ion is dissociated to form a covalent bond between Co-d and N-p x,y orbitals. On the one hand the N ion bonds to the central phenyl ring, and on the other hand the amine linker tends to dissociate one (two) H ion to form one (two/three) bonding with Co adatom in T (B/H) case to fulfill the octet rule.…”

“…Frontiers in Physics frontiersin.org the two-probe structure with DFT and non-equilibrium Green's function (NEGF) formalism implemented in Nanodcal transport package [30][31][32] to investigate the spinpolarized transmission spectum and spinterface effect of SMMJs [23]. The double-ζ double-polarized basis set of local numerical orbitals are applied to all ions, the cut-off energy of real space grid density is 150 Hartree, and the k-point samplings are 1 × 1 × 100 and Γ point for semi-infinite Co electrode and central device, respectively.…”

“…In this study, we propose the prototypical Co/1,4-benzenediamine (BDA)/ Co SMMJs with three kinds of contact geometries for top (BDA-T), bridge (BDA-B), and hollow (BDA-H) contact sites of the N ion bonding to one, two, and three Co apex atoms, respectively. Since the hybridization between Co-d, N-p y , and π-orbital of the phenyl ring preserve the spin-up pronounced resonance channel [23], the DFT + JUNPY + LLG calculation reveals exchange bias toggling via the interplay between spinterface enhanced equilibrium field-like spin torque (FLST) and coercive field of Co electrode. We further use the nonequilibrium Keldysh formalism to clarify the crucial role of multi-reflection processes at interfaces in the non-sinusoidal angular dependence of equilibrium FLST, which may pave the way for unexplored magnetotransport properties of organic-based spintronics.…”