Structural and magnetic transitions in Fe-, Co-, and Al-substituted Ni-Mn-Ga ferromagnetic shape memory alloys

Quaternary alloys based on Ni50Mn30Ga20 (an off-stoichiometric Heusler alloy, Ni2MnGa), with an intended substitution of 10 at. pct Ni by Fe or Co and with a 5 at. pct substitution of Ga by Al, were prepared by vacuum arc melting and heat treated. The as-cast and heat-treated alloys were characterized for their structural, thermal, and magnetic properties. The saturation magnetization improved with the addition of ferromagnetic elements Fe or Co and decreased with the Al addition. All three of the substitutions lowered the martensite transformation temperature, due to a reduction in the electron-to-atom (e/a) ratio for the Fe/Co additions or due to the more open structure resulting from the lower atomic size, in the case of the Al substitutions. Martensite transformation studied through thermal and thermomagnetic analysis methods shows a single-stage transformation in the case of the Fe/Co substitutions, whereas the Al substitution resulted in a two-stage transformation.