Spin- and Energy-Dependent Tunneling through a Single Molecule with Intramolecular Spatial Resolution

We investigate the spin- and energy-dependent tunneling through a single organic molecule (CoPc) adsorbed on a ferromagnetic Fe thin film, spatially resolved by low-temperature spin-polarized scanning tunneling microscopy. Interestingly, the metal ion as well as the organic ligand show a significant spin dependence of tunneling current flow. State-of-the-art ab initio calculations including also van der Waals interactions reveal a strong hybridization of molecular orbitals and substrate $3d$ states. The molecule is anionic due to a transfer of one electron, resulting in a nonmagnetic ($S=0$) state. Nevertheless, tunneling through the molecule exhibits a pronounced spin dependence due to spin-split molecule-surface hybrid states.

Figure 1

(a) SP-STM image of CoPcs on magnetic iron domains ($\odot ,\otimes$). (b) Line profile as indicated in (a). (d)–(i) High-resolution images of CoPcs in all three orientations ($A$, $B$, $C$) on oppositely magnetized domains. (c) Adsorption configuration of a CoPc in orientation $A$. (a) and (d)–(i) 0.05 V, 200 pA; (a) $240\AA \times 280\AA$; (d)–(i) $25\AA \times 25\AA$. The color scale for (a) is nonlinear to illustrate the domain structure, while the color scale for (d)–(i) is linear.

Figure 2

The geometry and electronic structure for a free CoPc (a) and adsorbed on an Fe surface (b) without and (c) with vdW forces included during the relaxation. (d) Molecular deformation due to vdW forces. The ${\mathrm{C}}_6$ rings and two of the outer $N$ are twisted by up to 0.3 Å toward the surface and away from the plane defined by the Co ion and four inner $N$. All $H$ are pointing away from the surface.

Figure 3

Experimental and simulated SP-STM images at $U=0.05\mathrm{V}$ and $22\AA \times 22\AA$ for both spin directions. (a) Averaged experimental images. (b)–(d) Isocharge surfaces; (b) conventional DFT (GGA) (without vdW), (c) $\mathrm{DFT}+\mathrm{vdW}$, (d) $\mathrm{DFT}+\mathrm{vdW}$ and including SOC.

Figure 4

Local spin polarization at (a)–(c) three representative energies. Raw and average (over multiple images and according to symmetry [26]) experimental data are compared with DFT simulations incl. vdW and incl. spin-orbit Coupling. Line profiles follow high-symmetry directions as indicated in the sphere model inset in (a). Simulated data including SOC are only given in (b) as the SOC corrections for (a) and (c) are negligible. Insets: $22\AA \times 22\AA$.