Effect of Ni-Mn ratio on structural, martensitic and magnetic properties of Ni-Mn-Co-Ti ferromagnetic shape memory alloys

In this work, we study the structural, martensitic and magnetic characteristics of all-d-metal Ni35-xMn35+xCo15Ti15 (x = 0, 1, 2, 3) alloys. The x-ray diffraction analysis confirms that the alloy system crystallizes as 5 M modulated and L1(0) non-modulated martensite with a small fraction of B2 austenite phase for x = 0. The proportion of austenite phase increases while the martensite phase decreases with increase in x, and B2 austenite phase becomes dominant for x = 3. Large endothermic/exothermic peaks with distinct thermal hysteresis in differential scanning calorimetry analysis indicate the martensitic/reverse martensitic transformations. The martensitic transformation temperature decreases considerably with increase in x. Thermal magnetization (M-T) curves show an abrupt variation in magnetization from weak magnetic martensite to ferromagnetic austenite near martensitic transformation temperature. M(H) loops measured at room temperature confirm the ferromagnetic behaviour of all the prepared samples. Isothermal magnetization (M-H) curves of Ni34Mn36Co15Ti15 at various temperatures show that the magnetic field can induce the magnetostructural transformation in this alloy. A magnetic entropy change of 11.3 J K-1 kg(-1) is observed at room temperature (290 K) in Ni34Mn36Co15Ti15 alloy.