Spin Transport and Magneto-Resistance in Organic Semiconductors
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Spin Transport and Magneto-Resistance in Organic Semiconductors Mohammad Yunus and P.Paul Ruden University of Minnesota, Minneapolis, Minnesota 55455 Darryl L. Smith Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ABSTRACT Calculated results for spin injection, transport, and magneto-resistance (MR) in organic semiconductors sandwiched between two ferromagnetic contacts are presented. The carrier transport is modeled by spin dependent device equations in drift-diffusion approximation. In agreement with earlier results, spin injection from ferromagnetic contacts into organic semiconductors can be greatly enhanced if (spin-selective) tunneling is the limiting process for carrier injection. Modeling the tunnel processes with linear contact resistances yields spin currents and MR that tend to increase with increasing bias. We also explore the possibility of bias dependent contact resistances and show that this effect may limit MR to low bias. INTRODUCTION π-conjugated organic semiconductors are materials of choice for optoelectronic and photovoltaic devices due to their potential for low-cost and large-area fabrication. Displays based on organic light emitting diodes (OLEDs) are already seeing commercial use.1 So-called spintronic devices have shown considerable potential for a great extension of device performance and functionality.2 Commercial successes of metal-based spintronic devices have been achieved in magnetic recording heads and memories that use the giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) effects.3 Intense research efforts are now focused on extending spintronics into the realm of semiconductor devices. However, spin injection from a ferromagnetic (FM) contact into a semiconductor is a challenging task.4 A tunnel barrier with spin selective transmission probability between the FM contact and the semiconductor can greatly enhanced spin injection5,6,7,8 and there has been some success with spin injection and detection using inorganic semiconductors.9 Organic semiconductors also appear to be promising materials for spin transport. The weak spin-orbit and hyperfine interaction make the spin coherence time long,10 and the fabrication of organic semiconductors with strongly spinpolarized La0.7Sr0.3MnO3 (LSMO) contacts provides additional potential for organic semiconductor spintronics. LSMO is to form tunnel contacts to organic semiconductors with a suitable contact resistance.11 Spin injection due to a tunnel barrier at the injecting contact by itself does not cause measurable MR. Effective spin valves need also a similarly spin-selective collecting contact. MR effects in organic spin valve structures have been reported in the literature.12,13 In some cases these devices have FM contacts made from LSMO. Since the organic semiconductor layer thicknesses are much larger than reasonable tunnel lengths, carrier transport in the semiconductor is expected to be diffusive, and the observed MR is not attributed to tunneling from one metal contact to the other (TMR). In
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