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Ferromagnetic NiMnGa and CoNiGa Shape Memory Alloy Films C. Craciunescu, Y. Kishi, L. Saraf, R. Ramesh and M. Wuttig Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742 ABSTRACT This paper reports results on Heusler-type NiMnGa and CoNiGa ferromagnetic shape memory alloy films deposited by magnetron sputtering and pulsed laser deposition respectively, on silicon cantilever-type substrates. NiMnGa FMSMA films deposited by magnetron sputtering on silicon substrates show good ferromagnetic and shape memory properties if the substrate is heated above 350 °C or if the films deposited at RT are annealed above 600 °C. The growth of the film is columnar. TEM examination shows typical martensitic variants while the SAED pattern suggests an orthorhombic structure. CoNiGa FMSMA films deposited at 550 °C by PLD are crystalline. Mechanical spectroscopy data proves the presence of a typical martensitic transformation that develops below room temperature: the phase transitions manifest themselves by typical changes of the modulus defect and displacement vs. temperature characteristics similar to those associated with the martensitic transformation in known shape memory alloy films. The change in the elastic modulus as a function of the magnetic field suggests that in both NiMnGa and CoNiGa films the spontaneous magnetization is clamped by the magnetic anisotropy field.

INTRODUCTION Ferromagnetic Shape Memory Alloys (FMSMAs) are of interest in the fabrication of microactuators controlled by both thermal energy and magnetic field. At this time, two binary – FePd and FePt [1,2] - and two ternary (Heusler type) – NiMnGa [3-12] and CoNiGa [13] - alloys are known to exhibit ferromagnetic shape memory behavior. Since the field is relatively new, most of the attention has been given to the characterization of the bulk properties. Only recent work reported results on the fabrication of thin films made out of NiMnGa and showing shape memory effect and ferromagnetic behavior at the same time [14, 15]. The recently discovered Co2NiGa alloys are promising candidates for the successful fabrication of FMSMA films because they show higher saturation magnetization compared to NiMnGa and lower price compared to FePd and FePt. This paper reports experimental results on the fabrication of NiMnGa and CoNiGa FMSMA films deposited on heated silicon cantilever-type substrates by magnetron sputtering and plasma laser deposition (PLD) respectively. It is shown that the films show the shape memory effect while the spontaneous magnetization seems to be clamped in a direction parallel to the plane of the film.

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EXPERIMENT NiMnGa films with a thickness of 1µm were sputter-deposited on oxidized Si (100) cantilever shaped substrates using a D.C. magnetron sputtering system. The nominal composition of the targets used for the depositions was Ni50MnxGa(50-x) with 25 x 30. Depositions were made on substrates at room temperature (RT) and heated at 385°C (the limit of the deposition system). The base vacuum was 26