Correlation between microstructure and magnetotransport in organic semiconductor spin-valve structures

Abstract

We have studied magnetotransport in organic-inorganic hybrid multilayer junctions. In these devices, the organic semiconductor tris(8-hydroxyquinoline) aluminum $({\text{Alq}}_{3})$ formed a spacer layer between ferromagnetic (FM) Co and Fe layers. The thickness of the ${\text{Alq}}_{3}$ 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 this effect 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 ${\text{Alq}}_{3}$ and the surrounding FM layers. Reflectometry shows that larger MR effects are associated with smaller $\text{FM}/{\text{Alq}}_{3}$ interface width (both chemical and magnetic) and a magnetically dead layer at the ${\text{Alq}}_{3}/\text{Fe}$ interface. The PNR data also show that the Co layer, which was deposited on top of the ${\text{Alq}}_{3}$, adopts a multidomain magnetic structure at low field and a perfect antiparallel 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 ${\text{Alq}}_{3}$ was estimated as $43\ifmmode\pm\else\textpm\fi{}5\text{ }\text{nm}$ at 80 K. However, the subtle correlations between microstructure and magnetotransport indicate the importance of interfacial effects in these systems.