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Discovery Enables Thermoelectric Voltages in Electronic Junctions to be Controlled

Due to a recently discovered effect in magnetic tunnel structures, thermoelectric voltages in nano-electronic junctions can be controlled.

The heat which occurs in tiny computer processors might soon be no longer useless or even a problem. On the contrary: It could be used to switch these processors more easily or to store data more efficiently!

A magnetic tunnel structure consists of two magnetic layers (red and blue) separated only by a thin insulation layer of approx. 1 nm (grey) ¡V the so-called "tunnel barrier". If a temperature difference ƒ´T is generated via the barrier, then the thermoelectric voltage VTh drops between the hot (red) layer and the cold (blue) layer. If the magnetic orientation, e.g. of the hot layer compared to that of the cold layer (arrows), is changed, this leads to a strong change in the measured thermoelectric voltage. (fig. by Schumacher/PTB)

These are two of the several potential applications made possible by a discovery made at the Physikalisch-Technische Bundesanstalt (PTB). This so-called "thermoelectric voltage" may well be very interesting - mainly for the use of nano-junctions, i.e. small components based on magnetic tunnel structures. The results obtained by the researchers have been published in the current issue of the renowned specialised journal Physical Review Letters.

Today, magnetic tunnel structures already occur in various areas of information technology. They are used, for example, as magnetic storage cells in non-volatile magnetic memory chips (the so-called "MRAMs" - Magnetic Random Access Memories) or as highly sensitive magnetic sensors to read out the data stored on hard disks. The new effect discovered at PTB within the scope of a research collaboration with Bielefeld University and the Singulus company could, in the future, add a new application to the existing ones: monitoring and controlling thermoelectric voltages and currents in highly integrated electronic circuits.

Magnetic tunnel structures consist of two magnetic layers separated only by a thin insulation layer of approx. 1 nm - the so-called "tunnel barrier". The magnetic orientation of the two layers inside the tunnel structure has a great influence on its electrical properties: if the magnetic moments of the two layers are parallel to each other, the resistance is low; if, on the contrary, they are opposed to each other, the resistance is high. The change in the resistance when switching the magnetisation can amount to more than 100 %. It is therefore possible to control the electric current flowing through the magnetic tunnel structure efficiently by simply switching the magnetisation.

The work carried out by the PTB researchers now shows that, besides the electric current, also the thermal current flowing through the tunnel structure can be influenced by switching the magnetisation. In their experiments, the scientists generated a temperature difference between the two magnetic layers and investigated the electric voltage (the so-called "thermoelectric voltage") generated hereby. It turned out that the thermoelectric voltage depends on the magnetic orientation of the two layers nearly as strongly as the electric resistance. By switching the magnetisation, it is therefore possible to control the thermoelectric voltage and, ultimately, also the thermal current flowing through the specimen.

In future, this new effect could be applied, for example, by using and converting the energy of waste heat occurring in integrated circuits in a targeted way. Furthermore, the discovery of this so-called "tunnel magneto thermoelectric voltage" is a milestone in the research field "spin calorics" - a field developing at a fast pace - which is currently promoted by the Deutsche Forschungsgemeinschaft (DFG) within the scope of a large-scale, 6-year priority programme.

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