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Novel mechanism of spin current generation and application
Novel currents of elementary spin, termed spin currents, may provide a future alternative to electrical currents in data processing devices. Potentially they can be realized free of dissipation, which makes them attractive for spintronics, which is a new kind of electronics.
Scheme of the experimental set-up: A temperature gradient results in the diffusion of phonons in a sapphire layer (insulating material). The phonons are transformed into spinwaves at the ferromagnet-sapphire interface which are then detected in the platinum layer.
The Japanese teams of Prof. Eiji Saitoh, Sendai, and Prof. Sadamichi Maekawa, Tokai, jointly with Prof. Burkard Hillebrands from the TU Kaiserslautern (Department of Physics, State Research Center OPTIMAS) have shown that spin currents can be generated by sound waves. Furthermore spin currents can be generated on isolating, nonmagnetic substrates by another new effect termed long-range spin Seebeck effect: assume such a substrate with double layer stripes made of a nickel-iron alloy and platinum mounted on the substrate (see figure), heated at one end, so that a spatial temperature variation is generated across the substrate. Temperature generates quanta of sound waves, termed phonons, and, as experimentally found, their travel to or from the stripes due to the temperature gradient may generate a spin current. This fundamental discovery opens new potential for the design of novel devices for information processing. For example, the measurement of a spin current in a double layer stripe on a substrate with temperature gradient allows for the determination of the position of the stripe.
Now, what is a spin current? Next to the negative charge, electrons rotate about themselves, they spin, and this spin can be considered as a small, elementary magnet. Since we are here in the world of quantum mechanics, this spin can only be aligned parallel or antiparallel to an external magnetic field. It is straightforward, that these two alignment states are excellently suited to represent the logic levels “zero” and “one” of a digital logic. Spin currents are formed by moving spins, and they can be used to transmit and process logic data.
Currently very strong efforts are undertaken to generate and detect spin currents and to make them useful for applications. The discovery reported here represents a major milestone. The highly reputed journal “Nature Materials” reports on these findings in its online edition of August 21.1
1Long-range spin Seebeck effect and acoustic spin pumping
K Uchida, H. Adachi, T. An, T. Ota, M. Toda, B. Hillebrands, S. Maekawa, E. Saitoh
Nature Materials Advanced Online Publication, 21. August 2011 (DOI: 10.1038/NMAT3099).
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