A joint research team from the National Institute for Materials Science (NIMS) and the University of Tokyo has created a new composite material that could improve thermoelectric conversion. The composite is made by coating magnetic-insulator powders with a metal and sintering them to form a three-dimensional network of nano-interfaces. This design allows the researchers to observe a thermoelectric effect driven by spin currents in an insulator—a phenomenon previously only seen in thin-film structures—now demonstrated in a larger, bulk material. Their findings were published in Nature Communications. Thermoelectric conversion is a process that turns waste heat into electricity, which is crucial for improving energy efficiency and achieving carbon neutrality. One specific thermoelectric effect, the spin Seebeck effect, was discovered in Japan in 2008. It involves using a temperature gradient in a magnetic material to generate electricity through spin currents. However, traditional devices that use this effect rely on layered structures of magnetic materials and thin metal films, which limit the potential for increasing power output by adding more layers or thickness. In their study, the researchers used yttrium iron garnet (YIG), a magnetic insulator, and coated its powder with platinum (Pt). They then sintered the coated powder at low temperatures under high pressure to create a bulk composite with YIG/Pt interfaces spread throughout the material in three dimensions. This structure differs from conventional layered devices, as it allows thermoelectric conversion through the spin Seebeck effect using interfaces distributed throughout the material, rather than just at thin-film interfaces. The team demonstrated that the spin Seebeck effect occurs in the YIG-Pt composite and that the metal channels within the material allow for scaling in thickness. This work introduces the concept of a "trans-scale spin Seebeck effect," extending a nanoscale phenomenon to macroscale energy conversion. By embedding nanoscale interfaces throughout the material in three dimensions, the researchers have enabled a spin-based thermoelectric effect, previously limited to thin-film interfaces, to occur in bulk materials. Looking ahead, the researchers believe that optimizing the design and materials used for the three-dimensional interface structure could further enhance thermoelectric performance. Their work opens the door to new thermal management devices by demonstrating the conversion of heat into electricity in a macroscopic insulator—a feat difficult to achieve with traditional thermoelectric technologies.