Role of Fe and Co addition in the phase stabilization and magnetic properties of Ni-Mn-Ga magnetic shape memory alloys

This research focuses on the impact of Co and Fe dopants on the crystal structure and magnetic properties of a Ni-Mn-Ga alloy. The aim is to enable control over the possible magnetostrictive actuation temperature range through tuning the Curie temperature, T_C, and the martensitic transformation temperature, T_M. As these parameters are strongly influenced by the structural characteristics, Powder Neutron Diffraction was used to unravel the crystal structure and atomic site occupancies. As a result, a clear correlation was observed between the dopant content and the transition temperatures as well as the magnetic behavior. The Fe and Co dopants induce atomic disorder in the structure, which enhances ferromagnetic interactions as evidenced by noticeable higher values of T_C and saturating magnetic moment, μ_sat. Unlike Co, which remarkably increases T_M, Fe doping leads to slightly lower martensitic transformation temperatures. Furthermore, the crystal structure of the martensitic phase only changes upon Co doping, becoming tetragonal and non-modulated, whereas the structure remains monoclinic 7 M modulated, similar to the non-doped alloy, upon Fe doping.