Effect of alloying Ni-Mn-Ga with Cobalt on thermal and structural properties

Materials showing a large magnetic field induced strain can potentially substitute giant magnetostrictive materials as well as piezoelectrical ceramics in actuating devices. However the magnetic shape memory alloys face several problems for an industrial application. Besides the well known brittleness the temperature-range is still limited due to the structural and magnetic phase transition around 75 degrees C. By alloying one of the most common systems Ni-Mn-Ga with 4.8 at% and 6 at% Cobalt the Martensite temperature was increased up to 422K and the Curie temperature up to 432K. Neutron diffraction experiments were performed on these Ni-Mn-Ga-Co single crystalline samples at room temperature, which show the existence of two different non-modulated martensitic structures. Dependent on the composition, a tetragonal and an orthorhombic structure in single crystalline samples were verified. Furthermore intermartenstic phase transitions above room temperature were determined in several samples alloyed with 6 at% cobalt. A temperature dependent measurement of the structure of Ni44.6Mn29.1Ga20.1Co6.2 showed an orthorhombic phase from room temperature up to 379K changing to a tetragonal intermartensitic phase before reaching the austenitic phase above 389K.