| In this dissertation, the injection, transport, and detection of a spin-polarized current through 7r-conjugated molecules is demonstrated. Specifically, the results for tetraphenyl porphyrin (TPP) and aluminum tris(8-hydoxyquinoline) (Alq3) are presented in detail. Tunnel junctions were made by depositing organic molecules between La0.7Sr0.3MnO3 (LSMO) and Co electrodes. At cryogenic temperatures, the devices showed a negative magnetoresistance of 15--25%. A tunneling model that explains the temperature, bias voltage, and sign of the magnetoresistance was presented. In addition, the spin polarization (∼37%) for electrons tunneling from Co through TPP monolayers using superconductor Al films as the spin detector was measured. Collectively, these results clearly demonstrated that spin-polarized electrons tunnel through conjugated organic semiconductors with negligible spin-scattering.;To better control the molecules present in the junctions and to elucidate the role of specific molecular features in transport, well-characterized and tailorable self-assembled monolayers of benzoic acid derivatives were selected. The results for the Al/Al2O3/4-iodo-benzoic acid/Co structure displayed transport dominated by spin-dependent scattering, while the Al/Al2O3/4-cyano-benzoic acid/Co structure showed evidence of only elastic and vibrationally-assisted tunneling, which was measured using inelastic electron tunneling spectroscopy (IETS). Since the only difference in these structures was the substitution of iodine for a cyano group, spin-dependent scattering which is evidenced in transport was attributed to the presence of organic radicals formed at metal-organic interfaces. These results demonstrated the choice of molecules present in the tunnel junction can significantly impact the nature of spin-polarized transport through organic layers. |
