Print this page 9 September 2013 2013/29

Researchers develop a magnetic nanoscale thermal switch

Researchers from the University of Groningen, Delft University of Technology, Tohoku University and the FOM Foundation have designed a nanoscale thermal switch that they can switch on and off magnetically. In the future, these miniscule switches could be used to transport excess heat away from individual transistors and chips. The researchers published their findings on 8 September 2013 in Nature Physics.

The mechanism of the switch is based on the spin of the electrons: a fundamental property that causes the magnetic moment of particles. Up until now physicists thought that the temperature of an electron is independent of the direction of its spin. The researchers have now demonstrated that this is not always the case. They applied a temperature difference to the interface between a non-magnetic and a magnetic metal. The sign of the temperature difference determined whether the electrons parallel to the magnetisation or anti-parallel to it assumed a higher temperature. The electrons with the opposite spin direction assumed a lower temperature. The difference arose because the heat conduction for the two spin directions is not the same.

Heat in magnetic nanopillar
The researchers used this knowledge to construct a nanopillar that consists of two magnetic layers with a non-magnetic layer in between. In this pillar the magnetisation of both magnetic layers could be switched on or off independently to influence the heat conduction. The pillar is just 80 nanometres wide – 1000 times smaller than the thickness of a human hair.

If the magnetisation in the outermost layers of the pillar is in the same direction, electrons with the same spin direction in both layers will have a higher heat conduction, and therefore assume a higher temperature. This means heat can easily be transported from one side of the pillar to the other and so a high level of heat conduction occurs.

Yet if the magnetisation in the two layers is in opposite directions then electrons with a high heat conduction in the one magnetic layer have an opposite spin direction from the electrons in the second magnetic layer. This makes it harder to transport heat through the pillar and so the heat conduction is suppressed. Consequently the quantity of heat flowing through the pillar can be switched on and off.

Spin caloritronics
The results are a following step in 'spin caloritronics', a young research field that studies the role of the magnetic movement of electrons in heat transport.

As the switches can be so incredibly small, we can use these to regulate local heat supply or removal. That could be useful in chips that sometimes become far too hot at localised hotspots.

Contact
Bart van Wees, +31 (0)50 363 49 33
Fasil Dejene, +31 (0)50 363 48 80
Joost Flipse, +31 (0)50 363 49 19
Gerrit Bauer theoretical questions

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This research was jointly funded by the FOM Foundation, NanoLab NL, JSPS, the Deutsche Forschungsgemeinschaft and the Zernike Institute for Advanced Materials.

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Figure 1. Nanopillar

(a) On this image, made with a scanning electron microscope, the nanopillar is at the location marked by the dotted square. A platinum heating element (the rearmost of the three grey coloured strips) causes a temperature difference. A nano-thermometer, made up of a nickel-copper alloy (purple) and platinum (foremost grey strip) detects the difference in heat conduction. The middlemost grey strip is a platinum contact. The remaining coloured parts are gold contacts.
(b) Schematic representation of the spin temperatures in the nanopillar. The left sketch represents the situation when the magnetisations in the two magnetic layers are anti-parallel. The temperature of the spins on the heated side (top) is higher than that of the second magnetic layer on the cool side (bottom) and a temperature difference builds up between the two different spin directions. As spins can scarcely enter a magnetic material where the magnetisation is opposite to their own spin direction, the heat conduction in the antiparallel situation is hindered.
In the parallel situation (right) warm electrons can easily move from top to bottom and cold electrons from bottom to top and this ensures a high level of heat conduction.