A magnetic tunnel structure consists of two magnetic layers (red and blue) separated only by a thin insulation layer of approx. 1 nm (grey) - the so-called "tunnel barrier". If a temperature difference 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.