A New Principle Of
"Electromotive Force"

High-sensitivity magnetic sensors as well as new active devices such as "spin batteries" are seen being spawned as a result of a discovery made by a research group led by Professor Tanaka Masaaki at the University of Tokyo Department of Electronic Engineering. By working together with research teams under Professor Maekawa Sadamichi at the Tohoku University Institute for Materials Research in Sendai and Professor S.E. Barnes at the University of Miami Physics Department in the United States, Tanaka and his team, in particular for spin batteries, foresee an energy revolution where everyone can obtain electricity readily just by finding a magnet. Spin-derived forces applied to electronics thus promises development of all types of novel "spintronics" devices (like vehicle-use spin batteries) in the future.

The combined efforts found that an electromotive force (e.m.f.) can be induced by a static magnetic field in magnetic tunnel junctions which contain zinc-blende-structured MnAs quantum nanomagnets. The research results were announced as a paper concerning "electromotive force and huge magnetoresistance in magnetic tunnel junctions" in the academic publication Nature, in March of 2009. In 2007, the Nobel Prize in Physics was awarded to two scientists, Albert Fert and Peter Grünberg, who had found that giant magnetoresistance (GMR) is a change in electrical resistance which occurs, upon exposure to high magnetic field, in materials consisting of alternating ferromagnetic and non-magnetic metal layers. However, the "huge magnetoresistance in magnetic tunnel junctions" at the nanostructure level far surpassed the possibilities foreseen by the two Nobel Laureates.

The experimental structure used by the Tokyo/Tohoku/Miami researchers produced a much larger amount of electricity for much longer than had been thought possible. GMR is applied for information storage systems today, as a magnetic sensor, but the new finding promises development of an even more sensitive sensor. 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, and for circuits partly composed of ferromagnetic materials it had been predicted that an e.m.f. of spin origin can arise 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.

Regarding spin batteries, since a huge magnetoresistance of up to 100,000 percent was observed for certain bias voltage by the researchers, results strongly supported the concept that in magnetic nanostructures Faraday's law of induction must be generalized to account for forces of purely spin origin. Classical Faraday's law reflects the force acting on the charge (-e) of an electron moving through a device or circuit in a magnetic field, but this study indicates that it must also reflect the force acting on the spin of an electron, which is the origin of the new e.m.f. and huge magnetoresistance. In a nutshell, such batteries—instead of using chemical reactions as has been the case for earlier types of electrical batteries—enable energy storage using magnets. The spin batteries can therefore potentially be recharged upon placement within any large magnetic field.

The fact is, planet Earth itself is a gigantic magnet. As such, it may be possible to recharge spin batteries for example by finding natural conditions where the magnetic field is sufficient (apparently almost everywhere on Earth's surface) for "charging" said batteries. In the future, it may be possible to drive an electrical vehicle into such locations and then drive off fully charged. Not only can the spin batteries result in a revolution by letting people tap an unlimited supply of electrical power but they could mean humankind being able to gain clean as well as inexpensive sources of energy.