Ab initio design strategies for NiMn-based magnetocaloric materials

Tilmann Hickel, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Biswanath Dutta, Delft University of Technology, Delft, The Netherlands
Navid Shayanfar, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Fritz Körmann, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Jörg Neugebauer, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany

In the quest to enhance magnetocaloric properties in NiMn-based Heusler alloys, we have recently followed several strategies. The doping with ferromagnetic elements turned out to be most promising, as experimental measurements reported about a large magnetization drop also known as metamagnetic transition upon martensitic transformation in Co and Fe substituted NiMn-based Heusler alloys. To provide a theoretical foundation, we demonstrate the impact of Co and Fe doping on the properties in Mn-excess Ni-Mn-Al Heusler alloys by ab initio calculations. Magnetic exchange interactions as well as lattice vibrations are taken into account. The calculations reveal a delicate interplay of magnetic and chemical orders and the tetragonal distortion during the martensitic transformations, explaining the giant inverse magnetocaloric effect in these alloys. Inspired by these results, we went beyond perfect bulk materials, and include the impact of interfaces, in particular between Ni-Co-Mn-Al and a Fe/Co spacer layer. We predict the presence of magnetic and chemical ordering effects next to these layers. At the same time, we demonstrate that the elastic properties of thin coherently incorporated layers can have a strong impact on the martensitic transition temperature of the Heusler alloys. Within this presentation we try to summarize these phenomena by highlighting the coupling of magnetic and structural degrees of freedom as a major driving force for large magnetocaloric effects.

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