A temperature difference across a magnet gives rise to an electron spin current, which consists of electrons with opposite spins moving in opposite directions. Previously, this spin Seebeck effect was thought to exist only in magnetic materials. However, on page 210 of this issue, Jaworski et al.1 report the detection of a thermally driven spin current in a non-magnetic material subjected to a magnetic field.
In 1821, Thomas Seebeck gave a series of lectures in Berlin about his new finding — namely, that a temperature difference maintained between two junctions separating dissimilar metals in a closed circuit deflects a needle made of a magnet2. Seebeck initially named this effect thermomagnetism, but shortly thereafter it was discovered that magnets are not needed to induce it. Rather, the temperature difference creates an electric current that produces a magnetic field, which, in turn, affects the needle. The phenomenon is now known as the Seebeck effect. If the circuit is opened up, a voltage is produced that is proportional to the temperature difference. This thermoelectric Seebeck voltage is nowadays used in temperature sensors, and there is a major effort to try to exploit it to convert waste heat from various processes into electricity.
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