Effects of composition and crystallographic ordering on the ferromagnetic transition in NiCoMnIn magnetic shape memory alloys

In the present study, the effects of composition and the level of crystallographic ordering were studied in Ni₄₅Co₅Mn_50-XIn_X magnetic shape memory alloys with X = 13.55, 13.71 and 13.92, in order to reveal the relationship between the Curie temperature (T_C) of the austenite phase and the degree of L2₁ ordering. Various samples were prepared using secondary heat treatments at different temperatures below the order-disorder transition temperature and for different durations, after solution heat treatment, in order to control the degree of ordering. T_Cs were determined using calorimetric and magnetometry measurements. The relationship between the T_C and the degree of order was used to predict the evolution of the order during selected heat treatments. It was shown that the T_C of austenite exhibits a monotonous non-linear dependence on the fraction of the L2₁ phase. This relationship was corroborated via ab initio numerical calculations within a mean field approximation. Dependence of the crystallographic order parameter on temperature and duration of secondary heat treatments showed that ordering kinetics is fast, the total degree of order depends only on the annealing temperature and cooling rate for long enough treatments, and maximum achievable degree of order is limited by the low thermal energy available for atomic diffusion at low temperatures. Finally, small changes in composition were observed to have a significant impact on martensitic transformation temperatures, and a significant but smaller effect on the ferromagnetic transition in these systems.