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Epitaxial Ni-Mn-Ga and Fe-Pd films for magnetic shape memory applications: Growth, martensitic transformation and magnetic properties
|Sebastian Fähler , Jörg Buschbeck , Michael Thomas , Oleg Heczko , Ludwig Schultz|
Leibniz-Institute for Solid State and Materials Research, P.O.Box 270116, Dresden D-01171, Germany
Magnetic shape memory alloys like Ni-Mn-Ga and Fe-Pd can change their shape up to several percent by the reorientation of martensitic variants in a moderate magnetic field. However to date a significant strain is only obtained in single crystals bulk materials. It is expected that epitaxial films are most promising for observation of MSM effect in thin films.
In this presentation we show that Ni-Mn-Ga and Fe-Pd films can be grown epitaxially on MgO(100) substrates. Both material systems are compared with respect to their suitability for microactuator and sensor suitability.
Temperature depended X-ray diffraction and pole figure measurements are used to examine structure and its transformation and microstructural transitions (variant reorientation). Additionally temperature and orientation depended magnetic measurements are used to identify the anisotropic magnetic behavior of these films.These experiments are used to analyze in detail the effect of constraints imposed by the substrate. This can result in significantly different behavior compared to bulk single crystal, e. g., the reduced symmetry of a thin film reduces the number of variants observed. Additionally the stress in the deposited film modifies the martensitic transformation temperatures. Due to the low blocking stress of few MPa’s, no twin boundary motion of a thin Ni-Mn-Ga film on a thick, rigid substrate is expected. Contrary to this, magnetization and field dependent pole figure measurements indicate that - though constrained by the substrate - a reorientation of variants by a magnetic field can be possible in orthorhombic Ni-Mn-Ga films. A model explaining this unexpected behavior is proposed.
Presentation: Oral at E-MRS Fall Meeting 2007, Symposium E, by Sebastian Fähler
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-14 14:20 Revised: 2009-06-07 00:44