Thermally driven spin injection from a ferromagnet into a non-magnetic metal

Creation, manipulation and detection of spin-polarized carriers are the key elements of spin-based electronics. Most practical devices use a perpendicular geometry in which the spin currents are accompanied by charge currents. In recent years, new sources of pure spin currents (that is, transport of spin angular momentum without charge currents) have been demonstrated and applied. Here we demonstrate a conceptually new source of pure spin current driven by the flow of heat across the interface between a ferromagnet and a non-magnetic metal. This spin current is generated because, in a ferromagnet, the Seebeck effect—which describes the generation of a voltage as a result of a temperature gradient—is spin dependent. We studied this new source of spin currents experimentally in a non-local lateral geometry and developed a three-dimensional model that describes the heat, charge and spin transport in this geometry, enabling us to quantify this process. We obtain a spin-dependent Seebeck coefficient for Permalloy of -3.8 µV K-1, suggesting that thermally driven spin injection is a feasible alternative for electrical spin injection in, for example, spin-transfer-torque experiments.