Long spin diffusion lengths in doped conjugated polymers due to enhanced exchange coupling

Abstract

Carbon-based semiconductors such as conjugated organic polymers are of potential use in the development of spintronic devices and spin-based information processing. In particular, these materials offer a low spin–orbit coupling strength due to their relatively light constituent chemical elements, which should, in principle, favour long spin diffusion lengths. However, organic polymers are relatively disordered materials and typically have a carrier mobility that is orders of magnitude lower than crystalline inorganic materials. As a result, small spin diffusion lengths of around 50 nm have typically been measured using vertical organic spin valves. Here, we report measuring spin diffusion lengths in doped conjugated polymers using a lateral spin transport device architecture, which is based on spin pumping injection and inverse spin Hall detection. The approach suggests that long spin diffusion lengths of more than 1 μm and fast spin transit times of around 10 ns are possible in conjugated polymer systems when they have a sufficiently high spin density (around 1020 cm−3). We explain these results in terms of an exchange-based spin diffusion regime in which the exchange interactions decouple spin and charge transport.

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Fig. 1: Lateral spin pumping device architecture and measurement scheme.
Fig. 2: Observation of long-range spin transport in F4TCNQ-doped PBTTT.
Fig. 3: Carrier density dependence of spin current transport.
Fig. 4: Angular dependence of the ISHE signal.
Fig. 5: Theoretical modelling of spin transport in an exchange-mediated regime.

Data availability

The authors declare that all relevant data are included in the paper and in the accompanying Supplementary Information. Additional data are available from the corresponding authors upon reasonable request.

Change history

  • 16 May 2019

    Owing to a technical error, the ‘Published online’ date of this Article originally mistakenly appeared as ‘15 March 2019’, but should have been ‘18 March 2019’.

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Acknowledgements

The authors wish to thank L. Vila and S. Auffret from SPINTEC in France for support with the fabrication of Co/Al2O3 films used in the organic nonlocal spin valves. We also thank C. H. W. Barnes and S. J. Brennan from the Thin Film Magnetism group of the Cavendish Laboratory for support with metal deposition, and J. A. Haigh of Hitachi Cambridge Laboratories for discussions on the measurements. K. Müllen of MPI Mainz supplied the polymer CDT-BTZ, for which the authors are grateful. The inputs of C. Chen and J. Armitage on the doping of organic semiconductors is also gratefully acknowledged. D.V. is grateful to X.-J. She for discussions on orthogonal resists. G.S. acknowledges postdoctoral fellowship support from the Wiener-Anspach Foundation and the Leverhulme Trust (Early Career Fellowship supported by the Isaac Newton Trust). I.E.J. acknowledges funding from the Royal Society through a Newton International Fellowship. Finally, the authors are very grateful for the excellent technical support offered by R. Chakalov and R. Beadle during the course of the ERC Synergy grant. Funding from the ERC Synergy Grant SC2 (grant no. 610115) is gratefully acknowledged.

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Correspondence to Deepak Venkateshvaran or Henning Sirringhaus.

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S.-J.W., D.V., H.S. and J.W. developed the idea of probing spin transport in organic semiconductors using lateral spin pumping architectures. S.-J.W. and D.V. nanofabricated the devices and performed lateral spin pumping measurements. M.R.M., U.C., E.R.M., S.A.E., S.M., S.S. and J.S. developed the supporting theory. R.D.P. and A.W. set up the experimental facilities required to do the experiments. R.D.P., J.W. and D.V. conceived the ideas for control experiments to remove spurious artefacts. S.-J.W., G.S., K.K. and I.E.J. developed the doping techniques used. R.C. and S.S. performed ESR measurements. D.P.G.H.W. and S.S. performed the supporting Hanle simulations. C.J., M.L., A.M. and I.M. synthesized and characterized the polymers used. M.C., J.N.M.S., T.J.W., O.Z. and P.S. provided technical help with measurements. D.V. nanofabricated and measured organic nonlocal spin valves. R.O.A. and A.I. assisted with thin film deposition of ferromagnets and tunnel barriers for organic nonlocal spin valves. D.V., S.-J.W., E.R.M. and H.S. wrote the manuscript with inputs from the other authors. All authors within the ERC Synergy SC2 grant made significant contributions to discussions throughout the progression of the project.

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Wang, S., Venkateshvaran, D., Mahani, M.R. et al. Long spin diffusion lengths in doped conjugated polymers due to enhanced exchange coupling. Nat Electron 2, 98–107 (2019). https://doi.org/10.1038/s41928-019-0222-5

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