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D7.2.1

Fabrication and characterization of freestanding NiMnGa films Holger Rumpf1, Corneliu Craciunescu1,2, Jürgen Feydt1, Achim Gilles1, Manfred Wuttig1,3, and Eckhard Quandt1 1 Center of Advanced European studies and Research, Bonn, Germany 2 Politehnica University of Timisoara, Romania 3 Department of Materials Science, University of Maryland, College Park, MD, U.S.A. ABSTRACT Freestanding Ni50Mn30Ga20 films of 13 µm thickness were fabricated by DC magnetron sputtering and analyzed. Magnetic measurements revealed the influence of the post deposition rapid thermal annealing process on the ferromagnetic hysteresis. Ferromagnetic properties evolved after annealing at 400°C. Thermal annealing of at least 600°C led to polycrystalline films that transformed reversibly and martensitically as shown by structural analysis and differential scanning calorimetry. Transformation temperatures and enthalpies of transformation of the martensitic transformation were strongly influenced by the temperature of the rapid annealing process. It is proposed that the annealing data reflect the evolution of the crystalline state.

INTRODUCTION The fabrication of ferromagnetic shape memory alloy (FSMA) films is a recent challenge that has its origin in the miniaturization of devices based on the attractive alloy properties [1-4]. While bulk single crystals of the Heusler-type alloys have shown good thermo- and magnetoelastic properties [5], polycrystals suffer from brittleness that seems to be difficult to overcome. Thin films made out of NiMnGa have been reported in the last years [6-8] and are very good candidates for MEMS applications, because they are more flexible (less brittle, etc.) than the bulk. They have shown an extrinsic two-way shape memory effect in bimorph architectures, when the film was attached to Si [7] or Mo [9] substrates. Freestanding NiMnGa films have also shown a two-way shape memory effect after being annealed at high temperature and then aged in a constrained shape [10]. The crystallization of the NiMnGa films has also been addressed. The films are partially crystalline when deposited on unheated substrates [7], and, as-expected, their crystallization temperature is influenced by the annealing program [11]. Recent investigations on irradiated NiTi freestanding films [12] have shown that the two-way shape memory effect can appear if part of the film is not transforming and this can become even more attractive if the untransformed part is nanocrystalline [13]. It thus appears interesting to study FSMA films that are annealed in the crystallization range, especially if the inhomogenity of the film can account for particular properties, like those shown in double layer films [14]. This paper reports on freestanding NiMnGa films deposited from powder metallurgically prepared targets and subjected to a rapid annealing process. Their properties in the vicinity of the crystallization temperature is investigated. It is shown that a post deposition thermal treatment is necessary to induce a ferromagnetic hysteresis. Highe