1. Kavli Institute of NanoScience, Faculty of Applied Sciences, Delft University of Technology, 2628 CJ, Delft, The Netherlands
2. MINT Center, University of Alabama, Tuscaloosa, Alabama 35487, USA
3. Physics Department, Brown University, Providence, Rhode Island 02912, USA
4. †Present address: Walther-Meiner-Institut, Bayerische Akademie der Wissenschaften,Walther-Meiner-Strae 8, D-85748, Garching, Germany

Correspondence to: R. S. Keizer¹T. M. Klapwijk¹ Correspondence and requests for materials should be addressed to R.S.K. (Email: r.s.keizer@tnw.tudelft.nl) or T.M.K. (Email: t.m.klapwijk@tnw.tudelft.nl).

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 ferromagnet^{3, 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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