The effect of post annealing on structure, microstructure and magnetic properties of thin Ni-Mn-Ga films

The magnetic shape memory (MSM) alloy Ni-Mn-Ga is an active material where large strains are obtained by magnetically induced reorientation (MIR) of martensitic variants. For the integration in microsystems, epitaxial thin films are in the centre of interest since the highest strains have only been obtained in single crystals. In order to minimize the technological effort, sputter deposition at low deposition temperatures is favoured. However, for obtaining high degree of order and thus a high Curie temperature, an additional post heat treatment at elevated temperatures is necessary. We report on the consequences of the post annealing process on thin epitaxial Ni-Mn-Ga films. In addition to increasing the Curie temperature, the annealed film shows a secondary Ni-rich Ni-3(Mn, Ga) phase. This phase has a well defined interface to the high temperature austenitic phase of Ni-Mn-Ga. Ni3Ga is formed due to evaporation losses of Mn and Ga. The formation of those precipitates can be avoided by preparing thin Ni-Mn-Ga films directly at elevated temperatures.