Structural and magnetocaloric properties in hexagonal MnNiGa alloys with Co doping

Abstract

Hexagonal MnNiGe-based alloys are a series of novel functional materials with potential magnetostructural transitions (MSTs). Accordingly, it was investigated the magnetic features of bulk hexagonal MnNiGa alloy and attempted to partially substitute Mn by Co atoms to tailor its structural and magnetic properties. Nonetheless, the introduction of magnetic Co atom fails to bring about the first-order phase transition and gives rise to the emergence of second phase with cubic structure instead. For ternary MnNiGa parent alloy, the second-order nature of transition is confirmed by both the absence of thermal hysteresis and the standard Arrott plot. To the end, the values of isothermal entropy change are determined by Maxwell relation, and the maximal values follow the trend predicted by the mean-field theory. Its broad transition region (~53 K) leads to only a very small value of entropy change (~2.4 J·kg−1·K−1 at a field change of 3 T). In turn, the wide transition ensures a relative large refrigerant capacity (~89.4 J·kg−1), which is comparable to that of MnNiGe-based systems. Although the substitution of Co for Mn site is unsuccessful, the chemically modified MnNiGa is still a promising candidate for the application of magnetocaloric effect (MCE) with merits of higher magnetization and better mechanical performance than MnNiGe-based systems.