Sep 8 2015
A team of German and American physicists from the Johannes Gutenberg University Mainz (JGU), the University of Kaiserslautern, the University of Konstanz, and the Massachusetts Institute of Technology (MIT), have been able to trace the origin of the Spin Seebeck effect (SSE).
One of the challenging tasks that the current generation faces is the recovery of waste heat from various processes in order to increase their energy efficiency and make them more environmentally friendly. The SSE enables the conversion of a heat flux into electrical energy, even in materials that are electrically non-conducting. The SSE, being a new approach, has not yet been fully understood.
The team explored the temperature- and material- dependence of the effect and established that it shows a characteristic length scale, which can be associated with its magnetic origin. The connection made with the magnetic origin ends the long-term controversy associated with this effect and advances it towards first applications.
The research findings were published in Physical Review Letters – a scientific journal. The first author of the paper is a fellow of the JGU-based Graduate School of Excellence “Materials Science in Mainz” (MAINZ).
Spin-thermoelectric effect, which causes thermal energy to be converted into electrical energy, is the basis for the Spin Seebeck effect. Unlike the traditional thermoelectric effect, the spin-thermoelectric effect allows for heat recovery in magnetic insulators coupled to a thin metallic layer. This characteristic of the material led to the assumption that the origin of the SSE was thermally excited magnetic waves. Measurements are currently made using a second metallic layer that converts these magnetic waves into a detectable electric signal. However, this method does not facilitate clear-cut assignment of the signals detected during the experiment.
Scientists measured the effect under different conditions by varying the thickness of the material from a few nm up to several µm, and by varying the temperature, and determined a characteristic behavior. It was observed that in thin films, the amplitude of the signal increased with an increase in thickness of the material, and a saturation point was reached after the thickness exceeded a certain value. In conjunction with the detected improvement of this critical thickness at low temperatures, the researchers were able to demonstrate the concurrence with the theoretical model of thermally excited magnetic waves that was devised at Konztanz. Furthermore, the research team was able to correlate the effect with the assumed thermally excited magnetic waves.
This result provides us with an important building block of the puzzle of the comprehension of this new, complex effect, unambiguously demonstrating its existence.
Andreas Kehlberger, first author of the paper and a Ph.D. student at Johannes Gutenberg University Mainz.
"I am very pleased that this exciting result emerged in a cooperation of a doctoral candidate out of my group at the Graduate School of Excellence 'Materials Science in Mainz' together with co-workers from Kaiserslautern and our colleagues from Konstanz, with whom we collaborate within the Priority Program 'Spin Caloric Transport' funded by the German Research Foundation (DFG)," emphasized Professor Mathias Kläui, director of the MAINZ Graduate School of Excellence based at Mainz University. "It shows that complex research is only possible in teams, for instance with funding by the German Federal Ministry of Education and Research (BMBF) through the Mainz-MIT Seed Fund."
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