Abstract
We derive kinetic equations describing injection and transport of spin-polarized carriers in organic semiconductors with hopping conductivity via an impurity level. The model predicts a strongly voltage dependent magnetoresistance, defined as resistance variation between devices with parallel and antiparallel electrode magnetizations (spin-valve effect). The voltage dependence of the magnetoresistance splits into three distinct regimes. The first regime matches well-known inorganic spintronic regimes, corresponding to barrier-controlled spin injection or the well-known conductivity mismatch case. The second regime at intermediate voltages corresponds to strongly suppressed magnetoresistance. The third regime develops at higher voltages and accounts for a novel paradigm. It is promoted by the strong nonlinearity in the charge transport whose strength is characterized by the dimensionless parameter eU/kBT. This nonlinearity, depending on device conditions, can lead to both significant enhancement or to exponential suppression of the spin-valve effect in organic devices. We believe that these predictions are valid beyond the case of organic semiconductors and should be considered for any material characterized by strongly nonlinear charge transport.
Highlights
Magnetoresistance in organic spintronic devices is an easy effect to detect [1]
We derived a set of kinetic equations describing non linear effects related to injection and transport of spin polarized carriers in organic semiconductors with hopping conductivity
The model predicts a strongly voltage dependent magnetoresistance splitted into three distinct regimes
Summary
Magnetoresistance in organic spintronic devices is an easy effect to detect [1]. Vertical multilayered devices combining manganite (La0.7Sr0.3MnO3) and cobalt external magnetic electrodes with an Alq transport interlayer have perhaps never failed (no negative reports available) to exhibit a clearly measurable magnetoresistance [2,3,4,5,6]. The question whether OSC are free of conductivity mismatch or the MR in OSC may not be caused by real spin injection into the organic electronic states has raised The latter doubt was strongly enhanced by the absence of Hanle effect in all the tested organic spintronic devices[23, 24]. It means that contrary to the case of ordinary semiconductor with diffusive transport the conductivity mismatch arguments are applicable for organic semiconductors only for some limited voltage range, while for higher voltages strong nonlinear effects in organic semiconductor enable the detection of the magnetoresistance caused by spin polarized injection. We show in our work that for such levels the non-linearity can both suppress and enhance spin valve effect in some cases
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