Study of structure, phase transformations, magnetocaloric effect, and critical behavior in Ni51Fe Mn28Ga21 magnetic shape memory alloys

This study investigates the magnetostructural properties, magnetocaloric effect (MCE), and critical behavior of Ni 51 -x Fe x Mn 28 Ga 21 ( x = 0, 1, 2, and 3 at.%) magnetic shape memory alloys. Room-temperature X-ray diffraction reveals that the x = 0 and x = 1 compositions crystallize in a seven-layered orthorhombic martensitic structure. In contrast, the x = 2 and x = 3 alloys adopt a five-layered modulated tetragonal structure. Fe substitution causes moderate composition-dependent changes in the lattice parameters and shifts the martensitic transformation (MT) toward room temperature. These changes correlate with variations in the electron-to-atom ratio, local structural/compositional disorder, and unit-cell volume. Fe substitution also leads to reduced antiferromagnetic interactions, stabilization of the martensitic phase, pronounced magnetization jumps, and a slight broadening of the MT hysteresis. Among the Fe-doped alloys, the x = 2 composition exhibits the most favorable low-field MCE at 1.5 T. Critical-behavior analysis based on Arrott plots, modified Arrott analysis, the Kouvel-Fisher method, and the critical isotherm indicates predominantly second-order magnetic phase transitions for the x = 2 and x = 3 alloys. The corresponding effective critical exponents remain broadly close to mean-field values. • Fe doping changes the martensitic structure in Ni 51 -x Fe x Mn 28 Ga 21 alloys from 7 M ( x = 0, 1) to 5 M ( x = 2, 3). • Fe doping shifts the martensitic transformation (MT) temperature toward room temperature. • Fe doping modifies the magnetization jump, while the x = 2 alloy exhibits the most favorable low-field magnetocaloric response at the MT among the Fe-doped alloys. • Critical-behavior analysis indicates effective critical exponents broadly close to mean-field values for the x = 2 and x = 3 alloys.