Magnetocaloric Effect in Heusler Family Alloys Ni_{2 – x}Co_xMn_1.25Ti_0.75 with Second-Order Phase Transition

The effect of Co additive on the magnetic and magnetocaloric properties of Heusler alloys Ni_{2 – x}Co_xMn_1.25Ti_0.75 with a second-order magnetic phase transition was studied using density functional theory and Monte Carlo simulations. It was shown that the ground magnetic state in the cubic austenitic phase is ferromagnetic ordering. An increase in the Co content leads to a decrease in the equilibrium lattice parameter and an increase in the ferromagnetic exchange interaction between Mn, Ni, and Co. As a result, an increase in the Curie temperature from ≈250 to 395 K is observed with a change in the Co content in the range 0.375 ≤ x ≤ 0.875. Using a set of exchange integrals and the Heisenberg Hamiltonian, the Monte Carlo method is used to simulate the temperature behavior of the magnetization and the isothermal change in the magnetic part of the entropy, ∆S_mag, in various magnetic fields from 0 to 2 T. For each of the compositions, the value of ∆S_mag is close to 1 J/(kg K). The maximum effect (∆S_mag ≈ 1.11 J/(kg K)) is observed for the composition with x = 0.375 at a temperature of ≈255 K. In the Ni_1.5Co_0.5Mn_1.25Ti_0.75 and Ni_1.375Co_0.625Mn_1.25Ti_0.75 compounds, the magnetocaloric effect ∆S_mag = 1.05 J/(kg K) is manifested in the vicinity of room temperature. In general, the presented theoretical results qualitatively and quantitatively reproduce the experimental data available in the literature for Ni–Co–Mn–Ti alloys with similar compositions.