1. Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan


2. Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi-shi 332-0012, Japan


3. Physics Department, University of Miami, Coral Gables, Florida 33124, USA


4. TCM, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK


5. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan


6. CREST, Japan Science and Technology Agency, Tokyo 100-0075, Japan

Correspondence to: Masaaki Tanaka^1,2 Correspondence and requests for materials should be addressed to M.T. (Email: masaaki@ee.t.u-tokyo.ac.jp).

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 predicted^{1, 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'.