Coupling phenomena in magnetocaloric materials: From thin layers to composites

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In this joint project, we aim to understand and improve the magnetocaloric and physical properties of a Heusler alloy system such that the step from model systems to applications can be performed. As a point of departure we will use Ni-Co-Mn-Al, which has been identified as a very promising magnetocaloric Heusler alloy during the first funding period of the SPP. Based on the already achieved knowledge about this material system, the understanding and improvement will now continue on two length scales. On the length scale of thin films, where the chemistry, crystal structure, and microstructure can be readily controlled, and the step from perfect crystals to materials with imperfections, i.e. composition changes, interfaces, point defects, grain boundaries will be performed. Here, it is our goal to generalize our previously developed knowledge about the coupling of lattice and spin degree of freedom to these situations, and to increase the magnetocaloric performance, in particular RCP, with the help of experimental and theoretical concepts. On the length scale of devices we will construct novel composite architectures in order to achieve an optimum heat transfer within the magnetocaloric regenerator. The concept of choice are layered composites based on melt spun NiCoMnAl-type ribbons with different transition temperatures in order to achieve graded regenerators with large span of operating temperature. Particular interest is on achieving enhanced heat flow towards the heat transfer medium by preparing composites with additional components with high thermal conductivity. The charm of this project is a multifold knowledge transfer: First, the previously obtained knowledge about the coupling of Heusler materials as well as the coupling of entropy contributions, is applied to the challenges of the recently identified system. Second, the knowledge developed for thin films as model systems is transferred to novel demonstrator and device concepts and used to characterize their performance and long-term stability. Third, the knowledge of DFT-based simulations of the mechanisms of the magnetocaloric effect and their relation to the chemistry, structure and thermal conductivity will be used as a guideline for experimental investigations. In addition the strong collaboration between the experimental groups of which one is responsible for the thin film preparation and a variety of characterization techniques, and the other for the preparation and characterization of the layered composites will accelerate the use of Heusler alloys in magnetocaloric regenerators and devices.