Combinatorial Thin Film Synthesis of NiMnAl Magnetic Shape Memory Alloys Using MBE Technique
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Combinatorial Thin Film Synthesis of NiMnAl Magnetic Shape Memory Alloys Using MBE Technique R. Hassdorf 1, J. Feydt1, S. Thienhaus1, R. Borowski1, M. Boese2, T. Walther1, and M. Moske1 Center of Advanced European Studies and Research (caesar), D-53175 Bonn, Germany 2 Universität Bonn, Institut für Anorganische Chemie, D-53117 Bonn, Germany 1
ABSTRACT Composition spreads close to the Heusler alloy Ni2MnAl were grown onto 4-inch wafer substrates using molecular beam epitaxy. Compositional variations of up to 10 at.% relative to each constituent enable a direct comparison of the chemical-structural relationship with respect to martensitic transformation and to magnetic ordering as well as an efficient identification of the emerging phase stability regions. In this study, we set the primary focus on the structural aspects of the transformation behavior as observed by X-ray microdiffraction in combination with a specially designed heating stage. Notably, cross-sectional HRTEM imaging of the respective composition areas reveals a laminated two-phase martensitic structure inside the single grains, identified as a sequence of 2M and 14M variants. Stress relief upon transformation as observed by mechanical stress measurements reaches to 400 MPa depending on the composition. Magnetization measurements so far indicate field-induced ordering to occur at low temperatures, here, below 50 K which is assumed to be closely related to a high degree of structural disorder on the Mn-Al sublattice. Single-crystal thin films were realized by means of an epitaxial relationship to MgO (001). INTRODUCTION Ferromagnetic shape memory alloys have attracted much attention as potential highperformance magnetically controlled actuator materials due to the fact that they show large fieldinduced strains accompanied by the rearrangement of twin variants in the martensitic state [1]. So far, numerous candidate systems have been investigated in this respect including binary alloys like FePd or FePt as well as the Heusler-type alloys Ni2MnGa, Ni2MnAl, or very recently Co2NiAl and Co2NiGa [2–7]. In bulk form, these materials come up with high magnetostriction values comparable or even superior to the value of the giant-magnetostrictive Terfenol-D. The real challenge by now is to fabricate thin films of these materials while overcoming metallurgical constraints and to provide them with the same order of functionality as in the bulk counterparts. In this context, we make use of the advantage of combinatorial synthesis of multicomponent alloy thin films by means of molecular beam epitaxy (MBE). This approach is combined with a screening characterization technique, namely X-ray microdiffraction which enables an efficient identification of the phase stability regions. Here, stoichiometries close to the Heusler alloy Ni2MnAl are probed into their chemical-structural relationship with respect to the martensitic transformation. This is of particular interest as the bulk material in this composition range is known to be rich in modulated martensite var
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