Perfect spin filtering by symmetry in molecular junctions

Obtaining highly spin-polarized currents in molecular junctions is crucial and important for nanoscale spintronics devices. Motivated by our recent symmetry-based theoretical argument for complete blocking of one spin conductance channel in model molecular junctions [A. Smogunov and Y. J. Dappe, Nano Lett. 15, 3552 (2015)], we explore the generality of the proposed mechanism and the degree of achieved spin-polarized current for realistic molecular junctions made of various ferromagnetic electrodes (Ni, Co, Fe) connected by different molecules (quaterthiophene or $p$-quaterphenyl). A simple analysis of the spin-resolved local density of states of a free electrode allowed us to identify the Fe(110) as the most optimal electrode, providing perfect spin filtering and high conductance at the same time. These results are confirmed by ab initio quantum transport calculations and are similar to those reported previously for model junctions. It is found, moreover, that the distortion of the $p$-quaterphenyl molecule plays an important role, reducing significantly the overall conductance.

Figure 1

(a) Model geometry of the $\mathrm{ferromagnetic}\left|\mathrm{molecule}\right|\mathrm{ferromagnetic}$ nanojunction used. Spin-polarized projected density of states (PDOS) on $d_{zx,zy}$ orbitals of one of (equivalent) apex atoms of the electrodes for (b) fcc Ni, of (001) (top) and (111) (bottom) orientations; (c) hcp Co, of fcc (top) and hcp (bottom) positions of the pyramidlike tip; and (d) bcc Fe, of (001) (top) and (110) (bottom) orientations. The insets show the atomic structures of the corresponding electrodes which are terminated with only one apex atom. The pyramids contain either four [Ni(111), hcp Co, and Fe(110)] or five [Ni(001) and Fe(001)] atoms. The $d_{zx}$ and $d_{zy}$ orbitals are degenerate for all presented electrodes, except for Fe(110), due to a symmetry reason. Spin-up (spin-down) results are plotted by solid (dashed) lines.

Figure 2

Electronic and transport properties of the Fe(001)$\left|\mathrm{quaterthiophene}\right|\mathrm{Fe}$(001) nanojunction: (a) Spin-dependent PDOS on $d_{zx}$ orbitals of the Fe apex atom, (b) spin-resolved density of states on the quaterthiophene molecule, and (c) spin-resolved transmission function $T\left(E\right)$ as a function of the electron energy $E$ in the parallel magnetic configuration. Spin-up (spin-down) results are plotted by solid (dashed) lines.

Figure 3

Electronic and transport properties of the $p$-tetraphenyl molecule suspended between two Fe(110) electrodes. Three molecular conformations were considered: (a) a flat molecule, (b) a distorted molecule with ${30}^{\circ }$ rotation between adjacent phenyl cycles, and (c) a distorted molecule with ${90}^{\circ }$ rotation. Top panels present model geometries of molecular junctions. Middle panels show the charge-density isosurface plots for the HOMO of the free molecule. Note that the isosurfaces of positive and negative isovalues are shown in red and blue, respectively. Lower panels report the spin-resolved PDOS on the molecule, with the spin-up (spin-down) curves presented by solid (dashed) lines. At the bottom, the conductances for both spin channels are indicated.