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Ferromagnetic Shape Memory Thin Films
Shape memory effect (SME) results from a reversible diffusionless martenisitic transformation that occurs in certain materials. Conventional shape memory alloys (NiTi, for example) are typically driven thermally. In some ferromagnetic materials such as alloys of Fe containing approximately 30 at.% Pd the SME can be induced by the application of a magnetic field. In these materials the magnetic shape memory effect (MSME) is achieved by conversion of martensite variants through twin boundary motion so that the new crystal orientation allows the magnetic moment to be better aligned with the external magnetic field. MSME has already been demonstrated in the Ni-Mn-Ga systems, ordered Fe3Pt and disordered Fe-Pd alloys in bulk form. However, only a limited amount of research has been conducted on Fe-Pd thin films. In this project Fe-Pd thin films with a variation in Pd content are obtained by magnetron sputtering. The effect of post-deposition heat treatment on the crystal structure is investigated using x-ray diffraction (XRD). The shape memory effect in this material is demonstrated by using the substrate curvature method during thermal cycles.
Publications:
Y. Sugimura, I. Cohen-Karni, P. McCluskey and J. J. Vlassak, "Mechanical behavior of sputter-deposited Fe-Pd shape-memory thin films", J. Mater. Res. 20 (9), 2279-2287 (2005). (Download)
Y. Sugimura, T. Cohen-Karni, P. McCluskey and J. J. Vlassak, "Fabrication and characterization of Fe-Pd ferromagnetic shape-memory thin films", Mater. Res. Soc. Symp. Proc. 785, D7.4.1 (2004). (Download)
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Study of Ni-Mn-Ga Heusler Ferromagnetic Shape Memory Alloys
Shape memory materials are an interesting class of materials that undergo a reversible phase transformation characterized by a change in crystal structure. Typically the high-temperature phase (Austenite) has greater crystallographic symmetry than the low temperature phase (Martensite). As a result we have multiple symmetry-related variants of martensite. Our model shows the change symmetry between phases affects recoverable strain in polycrystalline samples. Polycrystalline thin film ferromagnetic shape memory alloys based on the Ni-Mn-Ga Heusler system demonstrate this concept. In this alloy system the symmetry of the martensite phase is a function of alloy composition. A combinatorial approach is used to identify compositions with low martensite symmetry and thus larger recoverable strain.
