Composition and temperature dependence of the crystal structure of Ni-Mn-Ga alloys

The crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered (5M) and seven-layered (7M) martensitic phases, exhibit the martensitic transformation temperature (T-M) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, T-M and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or T-M has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni-Mn-Ga system has been confirmed. (C) 2004 American Institute of Physics.