Design of the Local Spin Polarization at the Organic-Ferromagnetic Interface

By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments the creation of a complex energy dependent magnetic structure with a tailored spin-polarized interface is demonstrated. We show this novel effect by adsorbing organic molecules containing $\pi (p_z)$ electrons onto a magnetic surface. The hybridization of the out-of-plane $p_z$ atomic-type orbitals with the $d$ states of the metal leads to the inversion of the spin polarization at the organic site due to a $p_z\mathrm{\text{−}}d$ Zener exchange-type mechanism. As a key result, we demonstrate the possibility to selectively and efficiently inject spin-up and spin-down electrons from a ferromagnetic-organic interface, an effect which can be exploited in future spintronic devices.

Figure 1

(a) Spin-resolved local density of states of an Fe atom of the clean surface (upper panel) and an Fe atom below the C atom of the Cot molecule (lower panel); (b) adsorption geometries and the spin-resolved local density of states of the Bz, Cp and Cot molecules adsorbed on the 2ML $\mathrm{Fe}/\mathrm{W}(110)$; (c) the adsorption energies of the Bz (${\mathrm{C}}_6{\mathrm{H}}_6$), Cp (${\mathrm{C}}_5{\mathrm{H}}_5$) and Cot (${\mathrm{C}}_8{\mathrm{H}}_8$) are given in meV per CH group of atoms. Compared to Bz, Cp and Cot molecules interact strongly with the surface due to an effective hybridization between out-of-plane orbitals ($p_z$ of C and $d_{z^2}$, $d_{xz}$, $d_{yz}$ of Fe), while the in-plane orbitals are weakly interacting ($s$, $p_x$, $p_y$ of C and/or $d_{x^2+y^2}$, $d_{xy}$ of Fe). All adsorbed molecules show a general characteristic: the energy dependent spin polarization, i.e., in a given energy interval the number of spin-up and spin-down electrons is unbalanced. For this specific interval the molecule has a net magnetic moment delocalized over the molecular plane, although the total magnetic moment of the molecule is 0.0 ${\mu }_B$. Note also the increased weight of the states crossing the Fermi level situated in the spin-down channel at metal site and in the spin-up channel at molecular site. Therefore, at the molecular site, an inversion of the spin polarization occurs with respect to the ferromagnetic surface.

Figure 2

The spin polarization at 2.5 Å above the Bz, Cp and Cot molecules adsorbed on the 2ML $\mathrm{Fe}/\mathrm{W}(110)$ ($15.85\AA \times 13.45\AA$) surface plotted for occupied ($[-0.4,0.0]\mathrm{eV}$) and unoccupied ($[0.0,+0.4]\mathrm{eV}$) energy intervals around the Fermi level. All the organic molecules show a high, locally varying spin polarization ranging from attenuation to inversion with respect to the ferromagnetic Fe film. As compared to Bz, the strongly interacting Cp molecule shows an amplification of the inversion of the spin polarization below Fermi level, while the Cot molecule shows a strong amplification of the inversion of the spin polarization for both occupied and unoccupied states around the Fermi level due to a higher number of $p_z$ interacting electrons.

Figure 3

Overview SP-STM image of a multidomain (blue/yellow) 2 ML Fe stripe on W(110) with intact and few metalized ${\mathrm{H}}_2\mathrm{Pc}$ present in three distinct orientations. Experimental ($22\AA \times 22\AA$) SP-STM images for ${\mathrm{H}}_2\mathrm{Pc}$ adsorbed on 2 ML $\mathrm{Fe}/\mathrm{W}(110)$ at $U=+0.05\mathrm{V}$ for both spin channels [i.e., up ($\uparrow$) and down ($\downarrow$)] and local spin polarization. ${\mathrm{H}}_2\mathrm{Pc}$ molecules show a high, locally varying spin polarization ranging from attenuation to inversion with respect to the ferromagnetic Fe film.