Martensitic transformation in as-grown and annealed near-stoichiometric epitaxial Ni2MnGa thin films

MACHAIN P.; A. CONDO; DOMENICHINI, P.; POZO LOPEZ G.; M. SIRENA; V. CORREA; N. HABERKORN

PHILOSOPHICAL MAGAZINE

TAYLOR & FRANCIS LTD

Lugar: Londres; Año: 2015 vol. 95

1478-6435

Magnetic shape memory nanostructures have a great potential in the field ofthe nanoactuators. The relationship between dimensionality, microstructureand magnetism characterizes the materials performance. Here, we study themartensitic transformation in supported and free-standing epitaxialNi47Mn24Ga29 films grown by sputtering on (0 0 1) MgO using a stoichiometricNi2MnGa target. The films have a Curie temperature of ~390 K and amartensitic transition temperature of ~120 K. Similar transition temperatureshave been observed in films with thicknesses of 1, 3 and 4 μm. Thicker films(with longer deposition time) present a wider martensitic transformation rangethat can be associated with small gradients in their chemical concentration dueto the high vapour pressure of Mn and Ga. The magnetic anisotropy of thefilms shows a strong change below the martensitic transformation temperature.No features associated with variant reorientation induced by magnetic fieldhave been observed. Annealed films in the presence of a Ni2MnGa bulk referencechange their chemical composition to Ni49Mn26Ga25. The change in thechemical composition increases the martensitic transformation temperature,being closer to the stoichiometric compound, and reduces the transformationhysteresis. In addition, sharper transformations are obtained, which indicatethat chemical inhomogeneities and defects are removed. Our results indicatethat the properties of Ni?Mn?Ga thin films grown by sputtering can be optimized(fixing the chemical concentration and removing crystalline defects) bythe annealing process, which is promising for the development of micromagneticshape memory devices.