PI: Roberto C. Myers, Co-PI Joseph P. Heremans
Like thermoelectricity is the study of the direct conversion of heat into electricity, the new field of spin caloric transport (SPINCATS) that will be studied here is the direct conversion of heat into magnetism, more specifically into a magnetic polarization. Experiments will be carried out to understand how this effect occurs by exploring its properties in various types of materials. Particularly important questions that will be addressed are: (1) Can this effect be reversed (i.e. is there a heat pumping activity associated with magnetic polarization)? (2) Is the effect is accompanied by an additional dissipation of waste heat?
The spin-Seebeck effect is a spin-polarization induced by a temperature gradient, and was recently discovered in ferromagnetic metals, semiconductors, and insulators. This is a new thermal transport phenomenon, the first mixed effect that involves spins rather than charge. While the effect is real, it is not understood. The project will lead to the fundamental understanding of the classical and irreversible thermodynamic properties of the transport of magnetic polarization. Three material classes will be studied, MnAs/GaMnAs, EuO and GdGaN. Spin-Seebeck measurements as a function of position, temperature, thermal gradient, and longitudinal conductivity will be carried out.
The broad technological impacts will come from the fact that these questions have a direct technological impact on the use of spin polarization as the basis for computer logic (spintronics). Indeed, one of the main limitations on the further miniaturization of computer logic circuits is the fact that they heat up, and waste heat management becomes overwhelming at small dimensions. While spin-polarization indeed can be the basis of logic circuits, it is crucial to know how much waste heat they will produce, if any. Other technological applications may be in the recovery of waste heat or in spin-caloric heat ?engines? and refrigerators. Further broad impact arises from the training and research experience for undergraduates and graduate students in the field of fundamental and irreversible thermodynamics, molecular beam epitaxy growth of compound semiconductor heterostructures, lithographic processing, electronic, magnetic, and thermoelectric / thermomagnetic characterization. Students will be recruited into a strongly interdisciplinary research environment spanning Mechanical Engineering, Materials Science, and Physics.
At the outset of this project, the aim was to understand the origin and mechanism of the spin-Seebeck effect, which consists of a thermally driven spin current. This effect provides a new means to convert of thermal energy into electricity, but in 2011, when the project began, the effect was still very new and had only been observed in a few different materials. The three year project expanded our understanding of spin Seebeck by studying it in a variety of new classes of materials including magnetic metallic glasses and non-magnetic semiconductors. The project led to the discovery of the giant spin-Seebeck effect, published by our team in Nature (2012). This discovery proves that spin-thermal effects are not necessarily weak effects, where, in fact, the giant spin-Seebeck signal can reach magnitudes of up to ~8mV/K, which is comparable to the largest charged based thermal voltages. That publication is now cited more than 50 times in just two years, which reveals its strong impact on the field of spin caloritronics, which might lead to heat engines based on spin and magnetism. Additionally, progress has been made in understanding magneto-transport effects in metallic glasses and materials with defect-induced ferromagnetism. In addition to the scientific accomplishments, the project has provided hands-on education and technical training for the next generation of practicing scientists and engineers. The multidisciplinary team included students from mechanical engineering, materials engineering, physics, and electrical engineering. Students on this research team have learned advanced materials synthesis techniques, thermal transport and magnetic materials characterization, and cleanroom fabrication methods. Besides having conducted the technical aspects of the research, students also wrote publications and delivered presentations at international conferences, which not only boosted the impact of the research, but additionally provided valuable communications training and greatly increased job prospects for the students.