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KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur
KFM 13: Focus Session: (Multi-)Ferroic States III
KFM 13.4: Talk
Wednesday, March 20, 2024, 10:40–11:00, EMH 225
Unraveling Coupled Martensitic and Magnetic Microstructure of Freestanding Multiferroic Ni-Mn-based Heusler Films — •Satyakam Kar1, 2, 3, Yuki Ikeda4, Kornelius Nielsch1, 2, Heiko Reith1, Robert Maaß4, 5, and Sebastian Fähler3 — 1Leibniz IFW Dresden, Dresden, Germany — 2TU Dresden, Dresden, Germany — 3Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany — 4Federal Institute of Materials Research and Testing, Berlin, Germany — 5University of Illinois Urbana-Champaign, Illinois, USA
Ni-Mn-based Heusler alloys combine ferroelasticity and ferromagnetism to achieve multifunctional applications like high stroke actuation, multicaloric effects, and thermomagnetic energy harvesting. The underlying principle in these applications is a reversible martensitic phase transformation. In bulk, this first-order transformation is governed by an invariant plane, connecting both phases: austenite and martensite. Theory predicts that this plane should converge to a line for ultrathin films, but experimental evidence is missing. Here, we examine 500 nm thick freestanding epitaxial Ni-Mn-Ga films using electron microscopy and magnetic force microscopy techniques and demonstrate that a line constraint controls the martensitic microstructure. This line constraint results in a complex martensitic and magnetic microstructure, differing from the bulk and the constrained film microstructures. Using a simple phenomenological model, the martensite microstructure can be deduced from a line constraint. Our findings show that finite size effects on martensitic transformation are accessible at a film thickness suitable for microsystem technologies.
Keywords: Martensitic transformation; Epitaxial films; Magnetic shape memory alloys; Heusler alloys