Frontier Research Institute for Interdisciplinary Sciences
Tohoku University

In 1831, an English scientist Michael Faraday saw electric currents when he slid a bar magnet in and out of a coil of wires. This law of electromagnetic induction serves as a bedrock of modern civilization, offering the fundamental operating principle of many types of electrical generators and motors. Physically, this phenomenon can be understood as the kinetic energy of the moving magnet has been converted to an electromotive force, via classical electromagnetism.

In 2009, a new kind of electromotive force was experimentally reported. Now you don’t need to move a magnet. What is dynamical here is the magnetic nanostructure inside the magnet, which triggers a conversion of the internal energy of the magnet to an electromotive force. This effect is called spin-motive force (SMF). Interestingly, SMF turns out to have a deep connection with Faraday’s electromagnetic induction; from a modern quantum mechanical point of view, both phenomena can be attributed to the time-varying Berry phase of an electron wave function. The electromagnetic induction is observed when the Berry phase is accumulated by the electric charge degree of freedom of the electron. The Berry phase due to spin, another degree of freedom of the electron, on the other hand, leads to SMF, hence its name.

While SMF is still very young, it is as a basic and universal physical effect as the celebrated electromagnetic induction. SMF may provide a key element for next generation technologies.