The spin Seebeck effect, a form of thermoelectric conversion that converts heat into electrical energy, manifests on the surface of nanoscale thin films. However, because conventional spin Seebeck devices possess a laminated structure of a magnetic material and a metallic thin film, there was a limit to improving output power by increasing device thickness or multilayering.
A research team including Postdoctoral Researcher Sang-Jun Park and Distinguished Group Leader Ken-ichi Uchida (concurrently Professor at the Graduate School of Frontier Sciences, the University of Tokyo) at the Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, along with Assistant Professor Keisuke Hirata at the Graduate School of Frontier Sciences, the University of Tokyo, developed a new composite in which three-dimensional nano-interfaces are distributed throughout the interior of the material by coating the surface of magnetic insulator powder with metal and compacting it. Using this structure, they succeeded in observing thermoelectric conversion driven by spins in an insulator, which was conventionally observed only at nanoscale thin-film interfaces, in a macroscale material. This provides new material design guidelines for effectively utilizing heat energy in insulators. The findings were published in Nature Communications.
Provided by the University of Tokyo
The research group fabricated a nanostructured bulk composite combining yttrium iron garnet (YIG), a magnetic insulator, and platinum (Pt), a non-magnetic metal. YIG is a representative magnetic insulator widely used in research on the spin Seebeck effect. The surface of YIG crystal powder was coated with a Pt thin film using a dynamic powder sputtering method.
In this method, sputtering is performed while stirring the powder, allowing a metallic layer to be formed over the entire surface of individual powder particles. Transmission electron microscopy observation confirmed that a nanometer-scale Pt thin film was formed on the YIG powder surface.
Subsequently, the Pt-coated YIG powder was compacted under low-temperature and high-pressure conditions to fabricate a bulk composite. Normally, compacting YIG powder, an oxide, into a bulk form requires high-temperature processing at 900℃ or higher, but high-temperature processing degrades the continuity of the thin Pt layer and interface quality.
In this study, by utilizing the ductility and adhesion of the Pt layer, they succeeded in synthesizing the bulk composite via high-pressure pressing at much lower temperatures, such as 300℃ or room temperature. In the resulting composite, Pt was distributed around YIG particles, confirming that metallic conduction pathways were formed inside the material.
When the fabricated millimeter-scale YIG-Pt bulk composite was magnetized and subjected to a temperature gradient, a thermoelectromotive force that increased in proportion to the temperature gradient was observed, and its sign reversed according to the direction of magnetization (N-pole to S-pole direction). Systematic experiments showed that the observed thermoelectromotive force was generated by spin currents, demonstrating that the spin Seebeck effect manifests in a bulk composite composed of magnetic insulators.
In conventional thin-film spin Seebeck devices, the region contributing to thermoelectric conversion is limited to the vicinity of the interface. The output power per temperature gradient saturates beyond a certain thickness. On the other hand, in the three-dimensional bulk composite developed in this study, nano-interfaces capable of converting spin currents into electrical signals are formed three-dimensionally within the bulk material and connected as a metallic network. The output power per temperature gradient increases in proportion to the thickness (volume) of the device. As a result, it was demonstrated that nanoscale spin-current phenomena can be utilized in macroscale materials.
Through this research the material design concept has been demonstrated. There remains ample room to significantly improve spin thermoelectric conversion performance through optimization of the magnetic insulator and metallic layer, enhancement of interface quality, and control of the density and distribution of the three-dimensionally formed nano-interfaces. Because many functional magnetic materials, including permanent magnets, possess complex microstructures on the nano- to microscale, this achievement suggests that designing interfaces and microstructures from the perspective of spin thermoelectric conversion can manifest the spin Seebeck effect even in practical functional materials.
Moving forward, advancing trans-scale magnetic materials science is expected to lead to the development of new energy materials that contribute to the utilization of unutilized heat and heat-flow sensing.
Journal Information
Publication: Nature Communications
Title: Trans-scale spin Seebeck effect in nanostructured bulk composites based on magnetic insulator
DOI: 10.1038/s41467-026-75232-0
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