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KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

KFM 13: Focus Session: (Multi-)Ferroic States III

KFM 13.4: Vortrag

Mittwoch, 20. März 2024, 10:40–11:00, EMH 225

Unraveling Coupled Martensitic and Magnetic Microstructure of Freestanding Multiferroic Ni-Mn-based Heusler Films — •Satyakam Kar^{1, 2, 3}, Yuki Ikeda⁴, Kornelius Nielsch^{1, 2}, Heiko Reith¹, Robert Maaß^{4, 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