Ni-Mn-Ga films in the austenite and the martensite structures at room temperature: Uniaxial texturation and epitaxial gr
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1200-G08-03
Ni-Mn-Ga films in the austenite and the martensite structures at room temperature: Uniaxial texturation and epitaxial growth
Jérémy Tillier1, Antinéa Einig2, Daniel Bourgault1, Philippe Odier1, Luc Ortega1, Sébastien Pairis1, Laureline Porcar1, Paul Chaumeton1, Nathalie Caillault2, Laurent Carbone2 1 Institut Néel/CRETA, CNRS, 25 avenue des martyrs, BP166, 38042 Grenoble Cedex 9, France 2 Schneider Electric France, 37 quai Paul Louis Merlin, 38050 Grenoble Cedex 9, France
ABSTRACT Ni-Mn-Ga films in the austenite and the martensite structures at room temperature have been obtained using the DC magnetron sputtering technique. Two elaboration processes were studied. A first batch of samples was deposited using a resist sacrificial layer in order to release the film from the substrate before vacuum annealing. This process leads to polycrystalline films with a strong (022) fiber texture. The martensitic phase transformation of such polycrystalline freestanding films has been studied by optical and scanning electron microscopy. A second batch of samples was grown epitaxially on (100)MgO substrates using different deposition temperatures. The texture has been analyzed with four-circle X-ray diffraction. Epitaxial films crystallized both in the austenite and the martensite structures at room temperature have been studied. INTRODUCTION The ternary intermetallic compound Ni2MnGa has gained considerable attention, as it exhibits an interesting combination of thermoelastic and magnetic properties. In addition to the conventional shape memory effect, this system shows a magnetic field induced rearrangement of martensitic twinned structures commonly called as magnetic shape memory (MSM) effect [1,2]. Recent investigations in Ni2MnGa bulk single crystals revealed considerable shape changes up to 10% and an interesting response time below 1 ms [3-6]. So Ni-Mn-Ga alloys have a great potential of application as microactuators or microsensors for micro-electro-mechanical systems. However, the bulk alloy is too brittle to be machined in a required shape and Ni-Mn-Ga alloys have to be prepared directly in film shape [7]. So far, Ni-Mn-Ga films have been grown by means of various techniques such as molecular beam epitaxy [8], sputtering [9-11] and pulsed laser deposition [12]. The texturation of the film is a key parameter to maximize intensity of the MSM effect. Recent studies on epitaxially grown Ni-Mn-Ga films showing a biaxially textured 7M martensite structure exhibit MSM effect at room temperature [13]. In the first part of the discussion, the texture of a freestanding Ni-Mn-Ga film austenitic at room temperature is analyzed. The martensitic phase transformation is characterized both by optical and scanning electron microscopy. The second part focuses on the epitaxial growth of Ni-Mn-Ga austenite at high temperature. The crystallographic structure and texture of a martensitic film at room temperature is also discussed.
EXPERIMENT The freestanding films have been deposited on photoresist (Shipley S1818) sacrificial
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