Correlation of microstructure and magnetotransport in organic semiconductor spin valve structures.

Yoahua Liu, Johns Hopkins University

There is currently a rapidly increasing interest in spin-dependent electronic transport in organic semiconductors (OSC). At its heart, this is based on the expectation that weak spin-orbit coupling in these light-element-based materials will lead to long spin relaxation times and long spin coherence lengths that may ultimately enable their use in magnetoelectronic devices. However, while there are several reports of observation of magnetotransport effects in multilayer OSC spin valve structures, the origins are still under debate, and both spin polarized tunneling and spin-coherent diffusive transport mechanisms have been invoked to explain the observed results. We have studied magnetotransport in several Co/OSC/Fe systems, using Alq3, CuPc, PTCDA and CF3-NTCDI as the spin transport layers. Magnetoresistance (MR) was observed up to room temperature in Alq3 and CuPc based devices. Focusing on the Alq3 system, we studied the devices. microstructure by X-ray reflectometry, Auger electron spectroscopy, and polarized neutron reflectometry. Our study shows evidence for spin-coherent diffusive transport and reveals the correlation between microstructure and magnetotransport in these organic devices. In particular, larger MR effects are associated with smaller average roughness at both the Alq3/Co and Fe/Alq3 interfaces and also with a shaper magnetic boundary at the Alq3/Fe interface, indicating the importance of detailed control and understanding of interfaces in these systems.

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