  We report detailed investigations of hybrid organic-inorganic vertical spin
valves. Spin polarized injection in tris(8-hydroxyquinoline) aluminum (Alq3)
organic semiconductor (OS) was performed using La0.7Sr0.3MnO3 manganite as the
bottom electrode and Co as the top electrode. While manganite was directly
connected to the organic semiconductor layer, a thin tunnel barrier was placed
between the OS and the Co electrode. A clear negative spin valve effect - low
resistance for antiparallel electrodes configuration - was observed below 210 K
in various devices using two different tunnel barriers: LiF and Al2O3. The
magnetoresistance effect was found to be strongly asymmetric with respect to
the bias voltage. Photoelectron Spectroscopy (PES) investigation of the
interface between manganite and Alq3 revealed a strong interface dipole, which
leads to a better matching of the metal Fermi level with Alq3 LUMO (1.1 eV)
rather than with HOMO level (1.7 eV). This unequivocally indicates that the
current in these devices is dominated by the electron channel, and not by holes
as previously suggested. The knowledge of the energy diagram at the bottom
interface allowed us to work out a semi- quantitative model explaining both
negative spin valve effect and strong voltage asymmetry. This model involves a
sharp energy selection of the moving charges by the very narrow LUMO level of
the organic material leading to peculiar resonant effects.
