The effect of antisite disorder on magnetic and magnetocaloric properties of Ni-Co-Mn-In alloys: ab initio and Monte Carlo studies

摘要

Nowadays, the Ni-Co-Mn-In Heusler alloys have drawn a much attention due to a series of functional properties such as the shape memory effect, giant magnetoresistance and magnetocaloric effects etc., which are promising for future technologies [1]-[3]. Usually, the most of unique properties are associated with the martensitic transformation between the martensite with complex magnetic order and the austenite with ferromagnetic order. Moreover, there are strong competing magnetic interactions in the vicinity of magnetostructural phase transition, which are responsible for the change in magnetization. Evidently, the manipulation of magnetic interactions in both martensite and austenite leads to change the magnitude of the magnetization drop and to achieve the better magnetocaloric properties across the martensitic transformation. The present theoretical study is addressed the question of effect of competing magnetic interactions on magnetic and magnetocaloric properties of Ni-Co-Mn-In alloys through the addition of structural defects. Evidently, samples prepared experimentally without an additional annealing can contain many impurities and defects. Opposite, the influence of additional annealing can result to a highly ordered structure with minimum defect concentration. In this connection for compositions without additional annealing, we focused attention on the formation of structural (antisite) disorder between In and Mn atoms on the corresponding In and Mn sublattices with concentration y which can be described by Ni₂ Mn_1+x In_1-x = Ni₂Mn_1-ySn_yMn_x+y In_1-x-y, where x is the Mn excess concentration and y is the degree of antisite disorder) [4]. While for the samples upon additional annealing, the ordered structure (without defects, y = 0 is proposed. Our methodology consists of ab initio calculations and Monte Carlo (MC) simulations based on the Potts-Blume-Emery-Griffiths model allowing to simulate austenite-martensite transformation as well as magnetic and magnetocaloric properties [5]-[7]. Two subsequent steps were used in our calculations. Firstly, we calculated the exchange coupling constants (J_ij) using the SPR-KKR package [8] within the general gradient approximation in the form of Perdew-Burke-Ernzerhof. The chemical and structural disorders were simulated in the coherent potential approximation. The Jij calculations were done for cubic austenite (c/a = 1) and tetragonal martensite (c/a = 1.21) of Ni₄₃ Co₇ Mn_37+y In_13-y. Here, y takes values as follows: 1.5, 3.25, and 6.5. We note that the equilibrium lattice parameters for austenite and martensite were taken from our previous calculations (See Ref. [7]), an