Distinguishing between tunneling and injection regimes of ferromagnet/organic semiconductor/ferromagnet junctions

Magnetoresistance effects in organic semiconductor spin valves have recently been reported, and have been variously interpreted as being due to tunneling magnetoresistance or giant magnetoresistance. We introduce a criterion for distinguishing between tunneling and injection conductivity necessary for properly analyzing organic spin-valve phenomena. We measure current-voltage (I-V) characteristics in Co/AlOx/rubrene/Fe junctions with a rubrene layer thickness, d, ranging from 5 to 50 nm. For d <= 10 nm, the I-V traces are typical of tunnel junctions. At d > 15 nm, the tunneling current becomes negligibly small. At larger biases, however, a second type of conductivity sets in. In this regime, the I-V curves are strongly nonlinear and temperature dependent. By comparing these to I-V curves measured in organic light-emitting diodes, we assign the latter mode to injection into the organic layer followed by hopping transport. We observe a spin-valve effect only in the tunneling regime.