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Terahertz spin current pulses controlled by magnetic heterostructures

Abstract

In spin-based electronics, information is encoded by the spin state of electron bunches1,2,3,4. Processing this information requires the controlled transport of spin angular momentum through a solid5,6, preferably at frequencies reaching the so far unexplored terahertz regime7,8,9. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins10,11,12 from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter13 based on the inverse spin Hall effect14,15, which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states16. Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters7,8,9.

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Figure 1: Scheme for engineering and detecting ultrashort spin current bursts.
Figure 2: Predicted spin trapping in a magnetic heterostructure.
Figure 3: Terahertz emission from magnetic heterostructures.
Figure 4: Terahertz charge and spin currents.

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Acknowledgements

The authors thank R.K. Campen, K. Carva, F. Giesen, A. Hoffmann and A. Melnikov for stimulating discussions. T.K., S.M. and M.W. acknowledge funding by the EU-FP7 project CRONOS (grant no. 280879). M.M., G.E., F.F., Y.M. and S.B. thank the German Science Foundation for financial support through SFB 602 and SPP 1538 (SpinCaT). F.F. and Y.M. acknowledge funding by the HGF-YIG programme VH-NG-513 and computer time at the Jülich Supercomputing Centre. M.B., P.M. and P.M.O. acknowledge support from the Swedish Research Council, EU-FP7 projects Fantomas (grant no. 214810) and FemtoSpin (grant no. 281043), and the Swedish National Infrastructure for Computing (SNIC). 

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T.K., M.W., I.R. and M.M. initiated the project. T.K., J.N. and S.M. designed the experiment, performed the terahertz measurements and analysed the data. G.E., V.Z., I.R. and M.M. fabricated and characterized the samples. M.B., P.M. and P.M.O. conducted the spin transport calculations. F.F. performed the spin Hall conductivity calculations. T.K., M.B., F.F., Y.M., M.W., I.R., P.M.O. and M.M. co-wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to T. Kampfrath.

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Kampfrath, T., Battiato, M., Maldonado, P. et al. Terahertz spin current pulses controlled by magnetic heterostructures. Nature Nanotech 8, 256–260 (2013). https://doi.org/10.1038/nnano.2013.43

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