Correlation between microstructure and magnetotransport in organic semiconductor spin valve structures

TL;DR

This study investigates how the microstructure of organic-inorganic hybrid multilayer junctions influences magnetotransport properties, revealing that interface quality and magnetic structure significantly affect magnetoresistance and spin transport in organic semiconductor spin valves.

Contribution

It provides new insights into the relationship between microstructure, interface quality, and magnetotransport in organic semiconductor spin valves, including a lower bound estimate of the spin diffusion length in Alq3.

Findings

Positive MR observed up to room temperature

Smaller interface width correlates with larger MR effects

Estimated spin diffusion length in Alq3 is 43 ± 5 nm at 80 K

Abstract

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.