Specific Changes in the Magnetoresistance of Ni–Fe–Ga Heusler Alloys Induced by Cu, Co, and Al Substitutions

In this paper, we investigate the influence of Cu, Co, and Al substitutions on the transport properties, and in particular, the magnetoresistive effect in Ni–Fe–Ga ferromagnetic shape memory alloys (FSMAs) prepared as ribbons by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetometry, and magnetoresistive characterizations were performed. In the range of the martensitic transformation (MT), different FSMA compositions show a rich spectrum of different behaviors. For one of the compositions (Ni 52 Fe 20 Co 2 Ga 23 Al 3 ), the magnetoresistance (MR) showed a local minimum or, on the contrary, a local maxima of reduced amplitude on cooling, in the range of the MT, depending on the performed thermal treatments. In the same composition, by replacing one Al atom with a Co one, no local extremes are seen, the alloy having a concomitant magneto-structural transition. When the magnetic field was varied, the MR showed a nonmonotonic variation in the martensite phase for some compounds, possibly due to the dynamics of the martensite variants realignment. From the studied compositions, the highest MR found on the MT of −9% for 5 T is for Ni 50 Fe 20 Ga 27 Cu 3 .