Ni50Mn37.5Sn12.5 Heusler Alloy: Influence of Co Addition on the Structure, Martensitic Transition, and Magnetic Properties

The impact of Co-addition (x = 0, 2, 4, and 6 at. %) in the as-cast and annealed Ni50Mn37.5Sn12.5 Heusler alloy at 900 °C for 24 h on the microstructure, magnetic properties, and the martensitic transition was studied using X-ray diffraction (XRD), scanning electron microscopy, vibrating sample magnetometry, and differential scanning calorimetry. The crystal structure of as-cast samples consists of a 14M modulated martensite structure, a face-centered (FCC) γ phase, and a face-centered tetragonal (FCT) MnNi-type phase L10. The as-cast samples show a dendritic microstructure with different contrasts and non-uniform distribution. The annealed samples exhibit dual 14M and γ phases for the Co0 and Co2, but 14M + γ + MnNi for the Co4 and Co6. The appearance of the martensitic transformation in the annealed Co0 and Co2 samples can be due to the disappearance of the dendritic microstructure. The characteristic temperatures (martensite start, Ms; martensite finish, Mf; austenite start, As; and austenite finish, Af) decrease with Co addition. A ferromagnetic-like order exists at a lower temperature of 1.8 K for the as-cast and annealed samples and decreases at 300 K. The annealing increases the fraction of the AFM contributions at 300 K. The exchange bias values of the Co0, An-Co2, and An-Co6 are 146.7 Oe, 24 O2, and 32.6 Oe, respectively, at 300 K.