Terahertz spin current pulses controlled by magnetic heterostructures

Journal name:
Nature Nanotechnology
Volume:
8,
Pages:
256–260
Year published:
DOI:
doi:10.1038/nnano.2013.43
Received
Accepted
Published online

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.

At a glance

Figures

  1. Scheme for engineering and detecting ultrashort spin current bursts.
    Figure 1: Scheme for engineering and detecting ultrashort spin current bursts.

    a, A ferromagnetic iron film (magnetization parallel to the y-axis, perpendicular to the plane of the paper) is excited by an optical femtosecond pump pulse. b, The excitation transforms slow majority-spin d electrons (red) into fast sp electrons, thereby launching a spin current towards the gold or ruthenium cap layer. c, ISHE: the spin–orbit interaction deflects majority and minority electrons in different directions (a) and thus transforms the longitudinal spin current s into a transverse charge current c, giving rise to the emission of a terahertz electromagnetic transient. Note that the thick white arrows are only symbolic: spins point either out of or into the plane of the paper.

  2. Predicted spin trapping in a magnetic heterostructure.
    Figure 2: Predicted spin trapping in a magnetic heterostructure.

    a, Calculated magnetization change ΔMy(z,t) of a iron/ruthenium structure induced by a laser pulse (duration, 20 fs; absorbed pulse energy, 1.3 mJ cm−2). Here, z denotes the in-depth film position and t the time since sample excitation at t = 0. The laser intensity inside the film is nearly independent of z. b, As in a, but for a gold-capped iron film. Note the much faster dynamics and reduced spin trapping in the gold cap layer.

  3. Terahertz emission from magnetic heterostructures.
    Figure 3: Terahertz emission from magnetic heterostructures.

    a, Terahertz signal waveforms obtained from photoexcited ruthenium- and gold-capped iron thin films. The signal inverts with reversal of the sample magnetization (dark to light curves). Pump-pulse parameters are as in Fig. 2. b, Fourier spectra |Ex(ω)| of the transient terahertz electric field directly after the sample, as extracted from the raw data of a. Inset: emitted terahertz pulse energy versus absorbed pump-pulse energy per area.

  4. Terahertz charge and spin currents.
    Figure 4: Terahertz charge and spin currents.

    a, Experimentally determined charge current density left fencejc,xright fence (z-averaged, flowing in-plane). The sign of the sheet current in the iron/ruthenium sample has been reversed for comparison. b, Calculated spin current density left fencejs,zright fence (z-averaged, flowing perpendicular to the film plane).

References

  1. Stöhr, J. & Siegmann, H. C. Magnetism: From Fundamentals to Nanoscale Dynamics (Springer, 2006).
  2. Wolf, S. A. et al. Spintronics: a spin-based electronics vision for the future. Science 294, 14881495 (2001).
  3. Jansen, R. Silicon spintronics. Nature Mater. 11, 400408 (2012).
  4. Bader, S. D. & Parkin, S. S. P. Spintronics. Annu. Rev. Condens. Matter Phys. 1, 7188 (2010).
  5. Stiles, M. D. & Miltat, J. in Spin Dynamics in Confined Magnetic Structures III (eds Hillebrands, B. & Ounadjela, K.) 225–308 (Springer, 2006).
  6. Bauer, G. E. W., Saitoh, E. & van Wees, B. J. Spin caloritronics. Nature Mater. 11, 391399 (2012).
  7. Ferguson, B. & Zhang, X-C. Materials for terahertz science and technology. Nature Mater. 1, 2633 (2002).
  8. Tonouchi, M. Cutting-edge terahertz technology. Nature Photon. 1, 97105 (2007).
  9. Ganichev, S. D. & Prettl, W. Intense Terahertz Excitation of Semiconductors (Oxford Univ. Press, 2006).
  10. Ruzicka, B. A., Higley, K., Werake, L. K. & Zhao, H. All-optical generation and detection of subpicosecond ac spin-current pulses in GaAs. Phys. Rev. B 78, 045314 (2008).
  11. Melnikov, A. et al. Ultrafast transport of laser-excited spin polarized carriers in Au/Fe/MgO(001). Phys. Rev. Lett. 107, 076601 (2011).
  12. Rudolf, D. et al. Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current. Nature Commun. 3, 1037 (2012).
  13. Leitenstorfer, A., Hunsche, S., Shah, J., Nuss, M. C. & Knox, W. H. Femtosecond charge transport in polar semiconductors. Phys. Rev. Lett. 82, 51405143 (1999).
  14. Valenzuela, S. O. & Tinkham, M. Direct electronic measurement of the spin Hall effect. Nature 442, 176179 (2006).
  15. Saitoh, E., Ueda, M., Miyajima, H. & Tatara, G. Conversion of spin current into charge current at room temperature: inverse spin-Hall effect. Appl. Phys. Lett. 88, 182509 (2006).
  16. Bonn, M. et al. Ultrafast electron dynamics at metal surfaces: competition between electron–phonon coupling and hot-electron transport. Phys. Rev. B 61, 10011005 (2000).
  17. Malinowski, G. et al. Control of speed and efficiency of ultrafast demagnetization by direct transfer of spin angular momentum. Nature Phys. 4, 855858 (2008).
  18. Walter, M. et al. Seebeck effect in magnetic tunnel junctions. Nature Mater. 10, 742746 (2011).
  19. Zhukov, V. P., Chulkov, E. V. & Echenique, P. M. Lifetimes and inelastic mean free path of low-energy excited electrons in Fe, Ni, Pt, and Au: ab initio GW+T calculations. Phys. Rev. B 73, 125105 (2006).
  20. Battiato, M., Carva, K. & Oppeneer, P. M. Superdiffusive spin transport as a mechanism of ultrafast demagnetization. Phys. Rev. Lett. 105, 027203 (2010).
  21. Wiese, N. et al. Strong temperature dependence of antiferromagnetic coupling in CoFeB/Ru/CoFeB. Europhys. Lett. 78, 67002 (2007).
  22. Schmidt, A. B. et al. Ultrafast magnon generation in an Fe film on Cu(100). Phys. Rev. Lett. 105, 197401 (2010).
  23. Nagaosa, N., Sinova, J., Onoda, S., MacDonald, A. H. & Ong, N. P. Anomalous Hall effect. Rev. Mod. Phys. 82, 15391592 (2010).
  24. Mosendz, O. et al. Quantifying spin Hall angles from spin pumping: experiments and theory. Phys. Rev. Lett. 104, 046601 (2010).
  25. Freimuth, F., Blügel, S. & Mokrousov, Y. Anisotropic spin Hall effect from first principles. Phys. Rev. Lett. 105, 246602 (2010).
  26. Seki, T. et al. Giant spin Hall effect in perpendicularly spin-polarized FePt/Au devices. Nature Mater. 7, 125129 (2008).
  27. Kirilyuk, A., Kimel, A. V. & Rasing, T. Ultrafast optical manipulation of magnetic order. Rev. Mod. Phys. 82, 27312784 (2010).
  28. Beaurepaire, E. et al. Coherent terahertz emission from ferromagnetic films excited by femtosecond laser pulses. Appl. Phys. Lett. 84, 34653468 (2004).
  29. Hilton, D. J. et al. Terahertz emission via ultrashort-pulse excitation of magnetic metal films. Opt. Lett. 29, 18051807 (2004).
  30. Kanda, N. et al. The vectorial control of magnetization by light. Nature Commun. 2, 362 (2011).
  31. Nastos, F., Newson, R. W., Hübner, J., van Driel, H. M. & Sipe, J. E. Terahertz emission from ultrafast optical orientation of spins in semiconductors: experiment and theory. Phys. Rev. B 77, 195202 (2008).
  32. Watanabe, S., Komine, T., Kai, T. & Shiiki, K. First-principle band calculation of ruthenium for various phases. J. Magn. Magn. Mater. 220, 277284 (2000).
  33. Gradhand, M. et al. Perfect alloys for spin Hall current-induced magnetization switching. SPIN 2, 1250010 (2012).
  34. Renger, J., Quidant, R. & Novotny, L. Enhanced nonlinear response from metal surfaces. Opt. Express 19, 17771785 (2011).

Download references

Author information

Affiliations

  1. Department of Physical Chemistry, Fritz Haber Institute, Faradayweg 4-6, 14195 Berlin, Germany

    • T. Kampfrath,
    • J. Nötzold,
    • S. Mährlein &
    • M. Wolf
  2. Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

    • M. Battiato,
    • P. Maldonado &
    • P. M. Oppeneer
  3. I. Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

    • G. Eilers,
    • V. Zbarsky &
    • M. Münzenberg
  4. Peter-Grünberg-Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, 52425 Jülich, Germany

    • F. Freimuth,
    • Y. Mokrousov &
    • S. Blügel
  5. Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Straße 15, 12489 Berlin, Germany

    • I. Radu

Contributions

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.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Author details

Supplementary information

Additional data