Heat, Charge, and Spin: Thermal Spintronics in Ferromagnetic Films and Nanostructures

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Non-technical Abstract:The motion of charges through metals and semiconductors produces heat. This fundamental result of the collisions of electrons in electrical currents means that modern electronic circuits, where currents flow rapidly in features with nanometer dimensions, suffer losses and other challenges due to high densities of heat. If information could instead be carried by angular momentum in the form of the electron's spin, much of this heat can be avoided. Furthermore, recent work suggests that application of heat to nanomagnetic systems could provide a long-sought source for such spin currents, in addition to allowing possible applications in energy harvesting based on heat, charge, and spin flow. In this project, the principal investigator and his research team carry out fundamental studies of the interactions between these flows. The essential technique is a novel thermal isolation structure that allows accurate measurements of tiny temperature changes of magnetic thin films and nanostructures. The results allow a new view into the phenomena arising from the coupling of heat, charge, and spin, and form the essential foundations for designing and optimizing new devices for nanoelectronics and energy harvesting. Technical Abstract:The generation of pure spin currents is an important requirement for future spintronic nanoelectronics models. However, reliable methods to generate such a flow of angular momentum without associated charge remain elusive. Recently some groups have reported that a spin current can be produced simply by applying a thermal gradient to a ferromagnetic material. This effect, called the spin Seebeck effect, has generated tremendous interest in the interaction of heat, charge and spin in ferromagnetic systems and has also raised the possibility of new thermoelectric devices based on magnetic materials. Such thermoelectric systems could offer breakthrough performance by decoupling heat and charge flow, one of the traditional challenges in thermoelectric materials. This project extends the principal investigator's recent work on thermoelectric and thermomagnetic effects in thin film metallic ferromagnets. His research team's measurements are enabled by a micromachined thermal isolation platform that removes potentially confounding effects introduced by the presence of a highly thermally conductive bulk substrate. This allows accurate measurements of the thermopower (traditional Seebeck effect), thermal and electrical conductivity, and Peltier effects in addition to a variety of less widely known thermoelectric and thermomagnetic effects. All these measurements can be performed on a single sample. These novel techniques provide the probe of the physics governing the interaction of heat, charge and spin currents in nanomagnetic systems required to understand and optimize their potential use for spintronics and energy generation.