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 (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 ± 5 nm at 80 K. However, the subtle correlations between microstructure and magnetotransport indicate the importance of interfacial effects in these systems.

Observation of a large spin-dependent transport length in organic spin valves at room temperature

“…Meanwhile, the energy redistribution may also change the interactions between the C 60 energy levels and electrodes at the interfaces, leading to lower spin injection and/or detection efficiencies. This study has shown that the microstructure in organic spin valve devices is closely correlated with magnetotransport, in agreement with the literature 44 . On the basis of an optimized evaporation temperature for the C 60 layer at 723 K, we deduced that an efficient SDT pathway exists in a spin valve device with a C 60 layer of 80 nm for achieving a high MR ratio in which spin electrons are efficiently injected 17,45 into the C 60 layer and hopping transport occurs with very a small spin flip, as illustrated in Fig.…”

The role of interfaces in organic spin valves revealed through spectroscopic and transport measurements

“…For example, there are reports of spin valves made from nominally the same materials and structure having quite different properties. For Fe/Alq 3 /Co based devices, there have been reports of no MR [39], positive MR [40], and negative MR [41]. As yet, there is no consensus that explains this.…”

Giant magnetoresistance in ferromagnet/organic semiconductor/ferromagnet heterojunctions

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] The early report of spin valve effects utilizing a thick layer ͑Ͼ100 nm͒ of tris͑8-hydroxyquinolinato͒aluminum ͑Alq 3 ͒ as a spacer in vertical devices 3 has brought considerable attention to organic semiconductor spintronics. But, low device resistance, weak temperature ͑T͒ dependence of I-V curves, and low-bias ͑V b ͒ MR have brought critiques [9][10][11] arguing that the reported MR ͑Ref.…”