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SKM-SYDT: SKM-Symposium Diffusionless Transformations in Magnetic and Ferroelectric Bulk and Thin Films

SKM-SYDT 1: Diffusionless Transformations in Magnetic and Ferroelectric Bulk and Thin Films

SKM-SYDT 1.3: Hauptvortrag

Donnerstag, 17. März 2011, 11:30–12:00, TRE Ma

Adaptive martensite and giant strain effects in multiferroics — •Ulrich K. Rößler — IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany

Ferroelastic behavior in martensitic microstructures relies on the re-orientation of twin variants under applied external fields. Giant strains driven by multiferroic couplings, as in ferromagnetic shape-memory alloys (FSMA) and in ferroelectric perovskites, have been achieved in modulated lattice structures only. The concept of adaptive martensite introduced by Khachaturyan and co-workers explains these lattice structures by twinning at the unit cell level. The formation of such nanoscale microstructures is determined by lattice geometry and the compatibility at the habit plane. The martensite transformation creates metastable states with a maximum density of twin boundaries. For FSMAs as Ni₂MnGa or Co₂NbSn the modulated low symmetry phases can be identified as nanotwinned structures of a tetragonal lattice in accordance with results on the ground state from ab initio calculations. The adaptive nature of modulated structures and microstructure in the Heusler Ni₂MnGa alloys has been demonstrated in experiments. In epitaxial films the modulated structure at the habit plane is found to be rigidly connected to the coexisting tetragonal martensite by branching. In these constrained films, the habit plane acquires a fractal geometry which fixes the lengths in the microstructure from the lattice cells up to the coarsened twins. Metastability and hierarchical organization of adaptive martensites are suggested as essentials for the easy variant re-arrangement in mesoscale twinned microstructures.