Researchers at the University of Groningen, TU Delft, Tohoku University and the Foundation for Fundamental Research on Matter (FOM) have designed a nano heat exchanger that they can turn on and off using magnetism. In the future, these minuscule switches could be used to transport surplus heat from individual transistors in chips. The researchers published their results on 8 September 2013 in the online edition of Nature Physics.
The functioning of the switch is based on the spin of the electrons – a fundamental property that causes the magnetic moment of particles. Thus far physicists have thought that the temperature of an electron did not depend on the direction of its spin, but the researchers have now shown that sometimes it does. They created a difference in temperature on the interface between a non-magnetic and a magnetic metal. Depending on the sign of the temperature difference, it turned out that it was either the electrons with a spin parallel to the magnetization or the electrons with an anti-parallel spin that adopted the higher temperature. The electrons with the opposite spin acquired the lower temperature. The difference in temperature is caused by the fact that the conduction in the magnetic layer is different for the two spin directions.
Using this knowledge, the researchers built a nano pillar consisting of two magnetic layers, with a non-magnetic layer in between. In the pillar they could separately switch the two magnetic layers on or off and thus influence the conduction. The pillar is only 80 nanometers wide – 1000 times smaller than the width of a human hair.
When the magnetization in the outermost layers of the pillar is switched to the same direction, the electrons with the same spin direction in both layers will develop higher conductivity, and thus develop a higher temperature. Heat can then be easily transported from one side of the pillar to the other. In this case we can refer to a high conductivity.
If the magnetization in the two layers is opposing, electrons with a high conductivity in one magnetic layer will have the opposite spin to the electrons in the second magnetic layer. This means it will be more difficult to transport heat through the pillar, thus suppressing the conductivity. In this way it is possible to turn the amount of heat flowing through the pillar on and off.
The results are the next step in spin caloritronics, a young research field that studies the role of the magnetic moment of electrons in conduction.
Because the switches are so unbelievably small, we can used them to regulate heat transport very precisely. This could be very useful in chips that sometimes get far too hot at localized hotspots
prof. dr. ir Van Wees
This research was partly funded by FOM, NanoLab NL, JSPS, the German Research Foundation DFG and the Zernike Institute for Advanced Materials.
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