The theoretical approach for description of magnetic properties and magnetocaloric effect in all-d-metal Heusler alloys Ni2−xCoxMn1.25Ti0.75

This paper investigates the influence of Co addition and atomic ordering on the magnetic and magnetocaloric properties of all-d-metal Heusler alloys Ni 2 − xCo xMn 1.25Ti 0.75, which exhibit a second-order magnetic phase transition. The modeling approach employed is based on the density functional theory and Monte Carlo method. The atomic ordering is considered with reference to the fully ordered structure, L2 1, and the partially ordered structure, B2. It is demonstrated that in both structures, the predominant magnetic state within the cubic austenitic phase is characterized by ferromagnetic ordering. An increase in the Co content results in the strengthening of the ferromagnetic exchange interactions between Mn, Ni, and Co, as well as an increase in the Curie temperature. For the ground state L2 1 structure, the largest Curie temperature values are observed, exceeding those of the B2 structure by almost 100 K. The temperature dependencies of the magnetization and magnetocaloric effect ( Δ S m a g) in magnetic fields up to 2 T are calculated using the Heisenberg Hamiltonian by the Monte Carlo method. The largest effect ( Δ S m a g ≈ 1.4 J/kg K) is observed for the compound with x = 0.375 and B2 structure at temperature ≈ 150 K, whereas for L2 1-Ni 1.5Co 0.5Mn 1.25Ti 0.75 and B2-Ni 1.25Co 0.75Mn 1.25Ti 0.75, Δ S m a g of 1.05 J/kg K appears in the vicinity of room temperature.