  We have studied magnetotransport in organic-inorganic hybrid multilayer
junctions. In these devices, the organic semiconductor (OSC) Alq$_3$
(tris(8-hydroxyquinoline) aluminum) formed a spacer layer between ferromagnetic
(FM) Co and Fe layers. The thickness of the Alq$_3$ layer was in the range of
50-150 nm. Positive magnetoresistance (MR) was observed at 4.2 K in a current
perpendicular to plane geometry, and these effects persisted up to room
temperature. The devices' microstructure was studied by X-ray reflectometry,
Auger electron spectroscopy and polarized neutron reflectometry (PNR). The
films show well-defined layers with modest average chemical roughness (3-5 nm)
at the interface between the Alq$_3$ and the surrounding FM layers.
Reflectometry shows that larger MR effects are associated with smaller
FM/Alq$_3$ interface width (both chemical and magnetic) and a magnetically dead
layer at the Alq$_3$/Fe interface. The PNR data also show that the Co layer,
which was deposited on top of the Alq$_3$, adopts a multi-domain magnetic
structure at low field and a perfect anti-parallel state is not obtained. The
origins of the observed MR are discussed and attributed to spin coherent
transport. A lower bound for the spin diffusion length in Alq$_3$ was estimated
as $43 \pm 5$ nm at 80 K. However, the subtle correlations between
microstructure and magnetotransport indicate the importance of interfacial
effects in these systems.
