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Determination of spin injection and transport in a ferromagnet/organic semiconductor heterojunction by two-photon photoemission


A fundamental prerequisite for the implementation of organic semiconductors (OSCs) in spintronics devices is the still missing basic knowledge about spin injection and transport in OSCs. Here, we consider a model system consisting of a high-quality interface between the ferromagnet cobalt and the OSC copper phthalocyanine (CuPc). We focus on interfacial effects on spin injection and on the spin transport properties of CuPc. Using spin-resolved two-photon photoemission, we have measured directly and in situ the efficiency of spin injection at the cobalt–CuPc interface. We report a spin injection efficiency of 85–90% for injection into unoccupied molecular orbitals of CuPc. Moreover, we estimate an electron inelastic mean free path in CuPc in the range of 1 nm and a 10–30 times higher quasi-elastic spin-flip length. We demonstrate that quasi-elastic spin-flip processes with energy loss 200 meV are the dominant microscopic mechanism limiting the spin diffusion length in CuPc.

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Figure 1: Conceptual principle of the experiments.
Figure 2: Schematic representation of the PSI and CSI regions, together with 2PPE spectra recorded for different CuPc coverage and shown as a function of the injection energy above EF (Ei).
Figure 3: Results of the spin-polarized measurements in the PSI region (2.6 eV<Ei<2.8 eV).
Figure 4: Representation of the quantities N+=Nup+Ndown and N=NupNdown, used to extract the inelastic mean free path λinel and the quasi-elastic spin-flip length λel,flip of electrons in CuPc orbitals lying (1.8±0.1) eV above the LUMO onset.
Figure 5: Results of the spin-polarized measurements in the CSI region (1.4 eV<Ei<2.2 eV).

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This work was supported in part by the NSF under Grant No. DMR-0602870 and by DFG Research Grant No. AE 19/8-2.

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Correspondence to Mirko Cinchetti.

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Cinchetti, M., Heimer, K., Wüstenberg, JP. et al. Determination of spin injection and transport in a ferromagnet/organic semiconductor heterojunction by two-photon photoemission. Nature Mater 8, 115–119 (2009).

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