Role of Fe and Co addition in the phase stabilization and magnetic properties of NiMnGa Magnetic Shape Memory Alloys*

Magnetic Shape Memory Alloys (MSMAs) are a group of active materials that undergo phase transitions (resulting in large recoverable mechanical deformations) induced by temperature, stress and/or magnetic fields. Their superfast response and high energy density makes them ideal candidates to be implemented in the field of sensors and actuators. The magnetic properties of MSMAs depend on the magnetic coupling between atoms that, in turn, depends on the atomic positions within the lattice. We have performed powder neutron diffraction experiments in order to elucidate the crystal structure of the martensite phases of Ni₅₁Mn_28-xGa₂₁Y_x MSMAs, where the doping element is Y=Co,Fe and x=0,1,3,5. The analysis of the diffractograms obtained in the experiment shows that the x=0 alloy has a mixture of modulated orthorhombic and tetragonal phases; the Co-rich samples tend to stabilize the tetragonal phase towards a non-modulated one; and the Fe-rich samples tend to stabilize the orthorhombic phase. We have determined the atomic site occupancies in both the martensite and austenite phases of all the alloys studied, and explored their relation with the changes on the magnetic properties of the alloys.