Magnetic and structural properties of non-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys

Structural and magnetic transition temperatures of ferromagnetic shape memory alloys present a strong dependence on slight departures from the stoichiometry, as does the mobility of twin boundaries responsible for the large magnetic field induced strains. In this work we study four non stoichiometric Ni-Mn-Ga polycrystalline alloys with compositions of 43–52 at.% nickel, excess manganese and deficient in gallium, and a single crystal of composition Ni₅₂Mn₂₆Ga₂₂. Those compounds are of technical interest due to the observed large room temperature magnetic field induced strains. Calorimetric and magnetic measurements determined the martensitic transition and Curie temperatures of the alloys (A_S = 331 K and T_Curie = 366 K for 52 at.% nickel alloy). Nickel defective alloys present a martensitic transition region broader than excess nickel ones. Neutron powder diffraction analysis confirmed orthorhombic martensitic structures for nickel defective alloys, and tetragonal for excess nickel ones. In the 52 atomic % nickel alloys case the crystallographic structure of the martensitic phase was also obtained on a single crystal with the same composition, trained to get a single variant in agreement with determined in the powder sample.