Fabrication and Characterization of Fe-Pd Ferromagnetic Shape-Memory Thin Films

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Fabrication and Characterization of Fe-Pd Ferromagnetic Shape-Memory Thin Films

Yuki Sugimura, Tzahi Cohen-Karni, Patrick McCluskey and Joost Vlassak Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, U.S.A. ABSTRACT Fe-Pd thin films with approximately 30 at.% Pd have been produced by magnetron sputtering. Various heat treatment conditions were studied in order to obtain the face-centered tetragonal (fct) martensitic phase at room temperature. X-ray diffractometry was used to identify the various phases present at room temperature and the substrate curvature technique was employed to measure film stress as a function of temperature. The shape memory effect was demonstrated in samples containing the fct martensite phase at room temperature.

INTRODUCTION Ferromagnetic shape memory materials have received attention recently because large strains can be achieved by the application of an external magnetic field. The mechanism by which strain is obtained in the magnetic shape memory effect (MSME) differs from that in magnetostriction. In magnetostriction strain is induced upon rotation of magnetic moment in each magnetic domain. On the other hand, the magnetic shape memory effect 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 [1-4], ordered Fe3Pt [4,5], and disordered Fe-Pd alloys [4,6-10] in bulk form. However, only a limited amount of research has been conducted on Fe-Pd thin films [11-14]. In bulk Fex-1Pdx alloys the transformation from the parent face-centered cubic (fcc) phase to the face-centered tetragonal (fct) martensite phase occurs at x ~0.3. The martensite start temperature, Ms, is typically below room temperature and decreases rapidly with increasing Pd content [7]. Inoue et al. [13,14], however, have shown that the Ms of Fe-Pd alloys increases to above room temperature in thin film form, and that fct martensite can be obtained at room temperature at x ~0.28. In this study 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. The shape memory effect is demonstrated in films with fct martensite by using the substrate curvature method during thermal cycles.

EXPERIMENTAL DETAILS Fe-Pd thin films were deposited by magnetron sputtering in a high vacuum chamber equipped with multiple confocal guns (ATC 1800, AJA International). The sputtering configuration is schematically shown in Figure 1. Previous studies by other researchers on Fe-Pd

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thin films sputtered from Fe-30 at.% Pd alloy targets consistently showed a deficiency in the Pd content of the sputtered film [11-13]. Therefore, elemental Fe and elemental Pd targets were selected in order to avoid this problem. The use of elemental targets also provides a simple

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