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Magnetic nanoswitch for thermoelectric voltages

Especially interesting for
  • fundamental research
  • magnetic data storage
  • manufacturers of highly integrated circuits

The heat occuring in tiny computer processors could, in future, be used to switch these processors more easily or to store data more effi ciently. These are two of the several potential applications of a discovery made at PTB – the so-called thermoelectric voltage – which may well be very interesting, mainly for the use of nano-circuits, i. e. the use of small components based on magnetic tunnel structures.

A magnetic tunnel structure, consisting of two magnetic layers (red and blue), separated only by a thin insulation layer of approx. 1 nm (grey) – the so-called "tunnel barrier". Thermoelectric voltage VTh as a result of a temperature gradient ΔT.

Magnetic tunnel structures are already used as 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 cooperation with Bielefeld University and the Singulus company could, in the future, provide a new application: 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 infl uence on its electrical properties: if the magnetic moments of the two layers are parallel to each other, then 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 magnetization can amount to more than 100 %. It is therefore possible to control the electric current fl owing through the magnetic tunnel structure effi ciently by simply switching the magnetization.

The work carried out at PTB now shows that, besides the electric current, also the thermal current fl owing through the tunnel structure can be infl uenced by switching the magnetization. In their experiments, a temperature difference was generated between the two magnetic layers, and the resulting electric voltage (the so-called "thermoelectric voltage") was investigated. 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 magnetization, it is thus possible to control the thermoelectric voltage and, ultimately, also the thermal current fl owing through the specimen.

In future, this effect could be applied, for example, by using and converting the energy of waste heat occurring in integrated circuits in a targeted way. The discovery of this so-called "tunnel magneto thermoelectric voltage" is a milestone in the research fi eld "spin calorics" – a fi eld 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.

Contact

Hans Werner Schumacher
Department 2.5 Semiconductor Physics and Magnetism
phone: (0531) 592-2500
e-mail: hans.w.schumacher(at)ptb.de

Scientific publication

Liebing, N.; Serrano-Guisan, S.; Rott, K.; Reiss, G.; Langer, J.; Ocker, B.; Schumacher, H.W.: Tunneling magneto power in magnetic tunnel junction nanopillars. Physical Review Letters 107 (2011) 177201