Paper EM+NC-WeA11
Spin Polarized Electron Tunneling Through Conjugated Molecules

Wednesday, October 22, 2008, 5:00 pm, Room 210

Electron tunneling across organic/inorganic interfaces is important to the performance of organic based electronic devices, such as organic light emitting diodes and organic field effect transistors. However, spin dependent electron tunneling across organic/inorganic interfaces is not well understood.¹ In this presentation we will demonstrate the injection, transport, and detection of a spin-polarized current through pi-conjugated molecules. Specifically we highlight the results for tetraphenyl porphyrin (TPP), but the results are similar for other conjugated organic semiconductors.² Tunnel junctions were made by depositing TPP between La_0.7Sr_0.3MnO₃ (LSMO) and Co electrodes. At cryogenic temperatures the devices show a negative magnetoresistance of 15-25%. A tunneling model that explains the temperature, bias voltage and sign of the magnetoresistance will be presented. In addition, we have directly measured the spin polarization (~37%) for electrons tunneling from Co through TPP monolayers using superconductor Al films as the spin detector. Collectively these results clearly demonstrate that spin polarized electrons tunnel through conjugated organic semiconductors with negligible spin-flipping and suggest that such molecules may find use in spintronic applications. For comparison, we will also present recent results utilizing halogen-substituted benzoic acid self-assembled monolayers as a “model” system. In this case, the presence or abscence of spin-flip scattering during tunneling is highly dependent on the choice of the halogen substituent.

¹T. S. Santos, J. S. Lee, P. Migdal, I. C. Lekshmi, B. Satpati, and J. S. Moodera, Physical Review Letters 98, 016601 (2007).
²W. Xu, G. J. Szulczewski, P. LeClair, I. Navarrete, R. Schad, G. Miao, H. Guo, and A. Gupta, Applied Physics Letters 90,072506 (2007).