By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments we show how to manipulate the local spin-polarization of a ferromagnetic surface by creating a complex energy dependent magnetic structure. We demonstrate this novel effect by adsorbing organic molecules containing π(pz)-electrons onto a ferromagnetic surface, in which the hybridization of the out-of-plane pz 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 pz − d Zener exchange type mechanism. As a key result, we demonstrate that it is possible to selectively inject spin-up and spin-down electrons from the same ferromagnetic surface, an effect which can be exploited in future spintronic devices.PACS numbers: 68.43.Bc,71.15.Mb Combining molecular electronics with spintronics represents one of the most exciting avenues in building future nanoelectronic devices [1][2][3]. For example, widely used in spintronic applications, the spin valve [4] is a layered structure of two ferromagnetic electrodes separated by a nonmagnetic spacer to decouple the two electrodes and allows spin-polarized electrons to travel through it. The efficiency of a spin valve depends crucially on the spin injection into and spin transport throughout the nonmagnetic spacer. On one side, since organic molecules are made of light elements with weak spin-orbit coupling as C and H, their use as spacer materials is very promising for transport properties since the spin coherence over time and distance is much larger than in the conventional semiconductors present in today's devices [5][6][7]. On the other side, the spin injection is mostly controlled by the ferromagnetic-organic layer interface [8,9] which is responsible for the significant spin loss in devices [10]. Therefore, a large effort is made to control the electronic properties at the organic-magnetic interfaces and, in this context, the theoretical first-principles calculations represent an indispensable tool to understand and guide experiments toward more efficient devices.In this Letter we propose a simple way to manipulate the local spin-polarization of a ferromagnetic surface by flat adsorbing organic molecules containing π(p z )-electrons onto it. As a consequence, around the Fermi level an inversion of the local spin-polarization at the organic site occurs with respect to the ferromagnetic surface due to a complex energy-and spin-dependent electronic structure of the organic-metal interface. The interaction between the molecule and the ferromagnetic surface reveals a mechanism similar to the p z − d Zener exchange [11] and enables a selective control of electron injection with different spins [i.e. up(↑) or down(↓)] from the same ferromagnetic surface within a specific energy The pz atomic orbitals in the spin-up channel hybridize with the majority (spin-up) states of the Fe atoms forming bonding (at lower energies) and antibonding (at higher energies) states some of them being pushed above the Fermi level...
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 surface 3d states. The molecule is anionic due to a transfer of one electron, resulting in a non-magnetic (S= 0) state. Nevertheless, tunneling through the molecule exhibits a pronounced spin-dependence due to spin-split molecule-surface hybrid states. [4,5]. However, detailed and quantitative access to different constituents of a single molecule is desirable, though challenging. Scanning tunneling microscopy (STM) is well established as a probe of a local spin [6][7][8][9][10][11][12][13] in an atomically well defined environment.Iacovita et al. recently performed a spin-polarized STM (SP-STM) study of a CoPc in contact with a ferromagnetic cobalt nano-island [14]. Stacking contrast, spin-dependent scattering, edge states, mesoscopic relaxations as well as the adsorbate induced modification create a complex environment [15] toward understanding the influence of the substrate on molecular magnetism. After careful selection of electronically equivalent Co nanoislands a ferromagnetic exchange interaction between the molecular spin and the cobalt lead was successfully deduced, both theoretically and experimentally.In this letter we demonstrate a significant spinpolarization for a CoPc molecule in contact with a ferromagnetic Fe thin film due to molecule-substrate hybridization even though the molecule loses its net spin. As confirmed by SP-STM, an energy/site-dependent spin polarization from inversion to amplification is resolved on the sub-molecular scale. State-of-the-art density functional theory (DFT), which includes the decisive role of van-der-Waals (vdW) interactions, reveals both the magnetic and electronic nature of the molecule coupled to the ferromagnetic substrate. Even though the net spin of the molecule is lost due to a transfer of one electron, spin-splitting is recovered through the local bonding of molecular orbitals with Fe 3d bands.Simulations were carried out in the DFT [16] formalism with a plane wave implementation as provided by the VASP code [17]. Pseudopotentials used were generated with the projector augmented wave method [18] by using the PBE generalized-gradient exchange-correlation energy functional [19] (GGA). A slab consisting of two Fe and three W atomic layers, with a (5×7) in-plane surface unit cell modeled the molecule-surface system. The kinetic energy cutoff of the plane waves was set to 500 eV while the Brillouin zone was sampled by the Γ point. Optimized molecule-surface geometries were obtained by relaxing all molecular degrees of freedom and those of the Fe overlayers by including long-range vdW intera...
A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.
An important development in recent synthesis strategies is the formation of electronically coupled one and two-dimensional organic systems for potential applications in nanoscale molecule-based devices. Here, we assemble one-dimensional spin chains by covalently linking basic molecular building blocks on a Au(111) surface. Their structural properties are studied by scanning tunneling microscopy and the Kondo effect of the basic molecular blocks inside the chains is probed by scanning tunneling spectroscopy. Tunneling spectroscopic images reveal the existence of separate Kondo regions within the chains while density functional theory calculations unveil antiferromagnetic coupling between the spin centers.
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