The electromotive force (e.m.f.) predicted by Faraday’s law reflects the forces acting on the charge, –e, of an electron moving through a device or circuit, and is proportional to the time derivative of the magnetic field. This conventional e.m.f. is usually absent for stationary circuits and static magnetic fields. There are also forces that act on the spin of an electron; it has been recently predicted1,2 that, for circuits that are in part composed of ferromagnetic materials, there arises an e.m.f. of spin origin even for a static magnetic field. This e.m.f. can be attributed to a time-varying magnetization of the host material, such as the motion of magnetic domains in a static magnetic field, and reflects the conversion of magnetic to electrical energy. Here we show that such an e.m.f. can indeed be induced by a static magnetic field in magnetic tunnel junctions containing zinc-blende-structured MnAs quantum nanomagnets. The observed e.m.f. operates on a timescale of approximately 102–103 seconds and results from the conversion of the magnetic energy of the superparamagnetic MnAs nanomagnets into electrical energy when these magnets undergo magnetic quantum tunnelling. As a consequence, a huge magnetoresistance of up to 100,000 per cent is observed for certain bias voltages. Our results strongly support the contention that, in magnetic nanostructures, Faraday’s law of induction must be generalized to account for forces of purely spin origin. The huge magnetoresistance and e.m.f. may find potential applications in high sensitivity magnetic sensors, as well as in new active devices such as ‘spin batteries’.
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This work was partly supported by Grant-in-Aids for Scientific Research No. 18106007, No. 19048018 and No. 20686002, the Special Coordination Programs for Promoting Science and Technology, and R&D for Next-Generation Information Technology by MEXT, PRESTO of JST, and EPSRC (UK). We thank B.-H. Yu for his help in the transport measurements. P.N.H. acknowledges a JSPS Research Fellowship for Young Scientists and the Global COE Program (CO4).
Author Contributions P.N.H. designed the experiment, fabricated the samples, collected most of data and performed analysis of data; S.O. set up measurement apparatuses and gave experimental advice; M.T. managed and planned the research and supervised the experiment; and S.E.B. and S.M. developed the theoretical explanation of the experiment. All authors discussed the results and commented on the manuscript.
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Hai, P., Ohya, S., Tanaka, M. et al. Electromotive force and huge magnetoresistance in magnetic tunnel junctions. Nature 458, 489–492 (2009). https://doi.org/10.1038/nature07879
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