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