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- ItemThickness dependent exchange bias in martensitic epitaxial Ni-Mn-Sn thin films(New York : American Institute of Physics, 2013) Behler, Anna; Teichert, Niclas; Dutta, Biswanath; Waske, Anja; Hickel, Tilmann; Auge, Alexander; Hütten, Andreas; Eckert, JürgenA thickness dependent exchange bias in the low temperature martensitic state of epitaxial Ni-Mn-Sn thin films is found. The effect can be retained down to very small thicknesses. For a Ni50Mn32Sn18 thin film, which does not undergo a martensitic transformation, no exchange bias is observed. Our results suggest that a significant interplay between ferromagnetic and antiferromagnetic regions, which is the origin for exchange bias, is only present in the martensite. The finding is supported by ab initio calculations showing that the antiferromagnetic order is stabilized in the phase.
- ItemReducing the nucleation barrier in magnetocaloric Heusler alloys by nanoindentation(New York : American Institute of Physics, 2016) Niemann, R.; Hahn, S.; Diestel, A.; Backen, A.; Schultz, L.; Nielsch, K.; Wagner, M.F.-X.; Fähler, S.Magnetocaloric materials are promising as solid state refrigerants for more efficient and environmentally friendly cooling devices. The highest effects have been observed in materials that exhibit a first-order phase transition. These transformations proceed by nucleation and growth which lead to a hysteresis. Such irreversible processes are undesired since they heat up the material and reduce the efficiency of any cooling application. In this article, we demonstrate an approach to decrease the hysteresis by locally changing the nucleation barrier. We created artificial nucleation sites and analyzed the nucleation and growth processes in their proximity. We use Ni-Mn-Ga, a shape memory alloy that exhibits a martensitic transformation. Epitaxial films serve as a model system, but their high surface-to-volume ratio also allows for a fast heat transfer which is beneficial for a magnetocaloric regenerator geometry. Nanoindentation is used to create a well-defined defect. We quantify the austenite phase fraction in its proximity as a function of temperature which allows us to determine the influence of the defect on the transformation.