The Influence of Martensitic Intercalations in Magnetic Shape Memory NiCoMnAl Multilayered Films

Abstract

Hysteresis and transformation behavior were studied in epitaxial NiCoMnAl magnetic shape memory alloy thin films with varying number martensitic intercalations (MIs) placed in between. MIs consists of a different NiCoMnAl composition with a martensitic transformation occurring at much higher temperature than the host composition. With increasing number of intercalations, we find a decrease in hysteresis width from 17 K to 10 K. For a large difference in the layers thicknesses this is accompanied by a larger amount of residual austenite. If the thicknesses become comparable, strain coupling between them dominates the transformation process, which manifests in a shift of the hysteresis to higher temperatures, splitting of the hysteresis in sub hysteresis and a decrease in residual austenite to almost 0%. A long-range ordering of martensite and austenite regions in the shape of a 3D checker board pattern is formed at almost equal thicknesses.

Nano Scaled Checkerboards: A Long Range Ordering in NiCoMnAl Magnetic Shape Memory Alloy Thin Films with Martensitic Intercalations

“…The stoichiometry of the compound in these two layers is chosen in such a way that their individual thermal hysteresis does not overlap, as depicted in Figure 1a. More details about the transition temperatures and the measured thermal hysteresis can be found in our previous study [19]. An illustration of the initially sputtered multilayer system is given in Figure 1b.…”

“…In a previous study, we have demonstrated that the thermal hysteresis could be decreased by preparing off-stoichiometric NiCoMnAl multilayer systems, which consist of alternating grown martensite intercalations (MIs) and active transforming austenitic layers (ALs) [19]. This MIs have shown to be beneficial for transforming films, because the formation energy during martensite nucleation is reduced.…”

“…The temperature dependent magnetization measurements have shown a significant decrease in hysteresis width as a function of the number of MIs. If the austenite active layers have a similar thickness compared to the martensite intercalations an extraordinary microstructure has been visualized in cross section images [19].…”