Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

A spin triplet supercurrent through the half-metallic ferromagnet CrO2

Abstract

In general, conventional superconductivity should not occur in a ferromagnet, though it has been seen in iron under pressure1. Moreover, theory predicts that the current is always carried by pairs of electrons in a spin singlet state2, so conventional superconductivity decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. It has been predicted that this rapid spatial decay would not occur if spin triplet superconductivity could be induced in the ferromagnet3,4. Here we report a Josephson supercurrent through the strong ferromagnet CrO2, from which we infer that it is a spin triplet supercurrent. Our experimental set-up is different from those envisaged in the earlier predictions, but we conclude that the underlying physical explanation for our result is a conversion from spin singlet pairs to spin triplets at the interface. The supercurrent can be switched with the direction of the magnetization, analogous to spin valve transistors, and therefore could enable magnetization-controlled Josephson junctions.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.

$32.00

All prices are NET prices.

Figure 1: Basic aspects of the experimental system.
Figure 2: Observed superconducting transport properties of the superconductor–CrO 2 –superconductor system.
Figure 3: Control of the critical current by changing the magnetization orientation.

References

  1. Shimizu, K. et al. Superconductivity in the non-magnetic state of iron under pressure. Nature 412, 316–318 (2001)

    Article  ADS  CAS  Google Scholar 

  2. Tinkham, M. Introduction to Superconductivity 2nd edn (McGraw-Hill, New York, 1996)

    Google Scholar 

  3. Bergeret, F. S., Volkov, A. F. & Efetov, K. B. Long-range proximity effects in superconductor-ferromagnet structures. Phys. Rev. Lett. 86, 4096–4099 (2001)

    Article  ADS  CAS  Google Scholar 

  4. Bergeret, F. S., Volkov, A. F. & Efetov, K. B. Odd triplet superconductivity and related phenomena in superconductor-ferromagnet structures. Rev. Mod. Phys. 77, 1321–1373 (2005)

    Article  ADS  CAS  Google Scholar 

  5. Iosad, N. N. et al. Properties of (Nb0.35, Ti0.15)xN1-x thin films deposited on silicon wafers at ambient substrate temperature. J. Appl. Phys. 88, 5756–5759 (2000)

    Article  ADS  CAS  Google Scholar 

  6. Coey, J. M. D. & Venkatesan, M. Half-metallic ferromagnetism: example of CrO2 . J. Appl. Phys. 91, 8345–8350 (2002)

    Article  ADS  CAS  Google Scholar 

  7. Ryazanov, V. V. Josephson superconductor-ferromagnet-superconductor π-contact as an element of a quantum bit (experiment). Phys. Usp. 42, 825–827 (1999)

    Article  ADS  CAS  Google Scholar 

  8. Kontos, T., Aprili, M., Lesueur, J. & Grison, X. Inhomogeneous superconductivity induced in a ferromagnet by proximity effect. Phys. Rev. Lett. 86, 304–307 (2001)

    Article  ADS  CAS  Google Scholar 

  9. Korotin, M. A., Anisimov, V. I., Khomskii, D. I. & Sawatzky, G. A. CrO2: a self-doped double exchange ferromagnet. Phys. Rev. Lett. 80, 4305–4308 (1998)

    Article  ADS  CAS  Google Scholar 

  10. Gupta, A., Li, X. W. & Xiao, G. Magnetic and transport properties of epitaxial and polycrystalline chromium dioxide thin films (invited). J. Appl. Phys. 87, 6073–6078 (2001)

    Article  ADS  Google Scholar 

  11. Soulen, R. J. Jr et al. Measuring the spin polarization of a metal with a superconducting point contact. Science 282, 85–88 (1998)

    Article  ADS  CAS  Google Scholar 

  12. Parker, J. S., Watts, S. M., Ivanov, P. G. & Xiong, P. Spin polarization of CrO2 at and across an artificial barrier. Phys. Rev. Lett. 88, 196601 (2002)

    Article  ADS  CAS  Google Scholar 

  13. Spinu, L., Srikanth, H., Gupta, A., Li, X. W. & Xiao, G. Probing magnetic anisotropy effects in epitaxial CrO2 thin films. Phys. Rev. B 62, 8931–8934 (2000)

    Article  ADS  CAS  Google Scholar 

  14. Kadigrobov, A., Shekhter, R. I. & Jonson, M. Quantum spin fluctuations as a source of long-range proximity effects in diffusive ferromagnet-superconductor structures. Europhys. Lett. 54, 394–400 (2001)

    Article  ADS  CAS  Google Scholar 

  15. Leggett, A. J. A theoretical description of the new phases of liquid 3He. Rev. Mod. Phys. 47, 331–414 (1975)

    Article  ADS  CAS  Google Scholar 

  16. MacKenzie, A. P. & Maeno, Y. The superconductivity of Sr2RuO4 and the physics of spin-triplet pairing. Rev. Mod. Phys. 75, 657–712 (2003)

    Article  ADS  CAS  Google Scholar 

  17. Saxena, S. S. et al. Superconductivity on the border of itinerant-electron ferromagnetism in UGe2 . Nature 406, 587–592 (2000)

    Article  ADS  CAS  Google Scholar 

  18. Giroud, M., Courtois, H., Hasselbach, K., Mailly, D. & Pannetier, B. Superconducting proximity effect in a mesoscopic ferromagnetic wire. Phys. Rev. B 58, R11872–R11875 (1998)

    Article  ADS  CAS  Google Scholar 

  19. Petrashov, V. T., Sosnin, I. A., Cox, I., Parsons, A. & Troadec, C. Giant mutual proximity effects in ferromagnetic/superconducting nanostructures. Phys. Rev. Lett. 83, 3281–3284 (1999)

    Article  ADS  CAS  Google Scholar 

  20. Peña, V. et al. Coupling of superconductors through a half-metallic ferromagnet: Evidence for a long-range proximity effect. Phys. Rev. B 69, 224502 (2004)

    Article  ADS  Google Scholar 

  21. Wilhelm, F. K., Zaikin, A. D. & Schön, G. Supercurrent in a mesoscopic proximity wire. J. Low-Temp. Phys. 106, 305–310 (1997)

    Article  ADS  CAS  Google Scholar 

  22. Eschrig, M., Kopu, J., Cuevas, J. C. & Schön, G. Theory of half-metal/superconductor heterostructures. Phys. Rev. Lett. 90, 137003 (2003)

    Article  ADS  CAS  Google Scholar 

  23. Li, X. W., Gupta, A. & Xiao, G. Influence of strain on the magnetic properties of epitaxial (100) chromium dioxide (CrO2) films. Appl. Phys. Lett. 75, 713–715 (1999)

    Article  ADS  CAS  Google Scholar 

Download references

Acknowledgements

We thank I. van Dijk, M. G. Flokstra, H. T. Man and S. Russo for stimulating interactions. This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). The work at the University of Alabama was supported by the National Science Foundation as part of an MRSEC grant.

Author information

Authors and Affiliations

Authors

Corresponding authors

Correspondence to R. S. Keizer or T. M. Klapwijk.

Ethics declarations

Competing interests

Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Keizer, R., Goennenwein, S., Klapwijk, T. et al. A spin triplet supercurrent through the half-metallic ferromagnet CrO2. Nature 439, 825–827 (2006). https://doi.org/10.1038/nature04499

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature04499

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing