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]), and we supposed that lattice parameters change no with increasing degree of anisate disorder (y). Secondly, using the calculated exchange constants as a function of distance between interacting atoms, the MC simulations were performed. The model lattice that contains a real unit cell of Heusler alloy includes 3925 atoms with periodic boundary conditions. The configurations of excess Mn atoms (x + y) at the In sublattice and In atoms (y) at the Mn sublattice are chosen randomly and the total number of Mn and In atoms are fixed by a nominal composition Ni₄₃ Co₇ Mn_37+y In_13-y(y = 1.5, 3.25, and 6.5). The MC simulations were performed using the Metropolis algorithm and 500 000 MC steps. Note that all model parameters expect the magnetic moments and J_ij constants were fixed and taken from [7]. Let us discuss the effect of additional annealing in the framework of theoretical approach assuming the presence of antisite disorder between In and Mn sublattice in Ni₄₃ Co₇ Mn₃₇ In₁₃ alloy. In Fig. 1a we present calculated nearest exchange coupling constants for austenite of Ni₄₃ Co₇ Mn_37+y In_13-y as a function of antisite defect concentration. As can be seen that all interactions with Mn₁ (Mn₂) atoms decrease (increase) with increasing y, respectively. Here Mn₁₍₂₎ atoms denote Mn atoms located at Mn (In) sublattice, respectively. This observation is associated with the fact that the number of Mn₁ atoms becomes smaller with an increase of antisite disorder. MC simulations of thermomagnetization curves for Ni₄₃ Co₇ Mn_37+y In_13-y. (y = 0 and 1.5) in magnetic field of 2 T under the heating and cooling protocols are shown in Fig. 1b. The inset contains the M(T) curves for Ni₄₃ Co₇ Mn_37+y In_13-y under heating. Thermal hysteresis between M(T) curves around the magnetostructural phase transition for the heating and cooling processes is clearly seen. Moreover, both the magnetization and Curie temperature of austenite are found to decrease compared to the ordered case (y = 0) with increasing degree of structural disorder. Generally, theoretical magnetization trend as a function of antisite disorder is provided by the recent experiments for both ordered and disordered Ni₅₀ Mn_34.5 In_15.5 alloys [9], [10]. In the framework of theoretical approach involving first-principles and Monte Carlo methods, a set of magnetization curves for the studied system are calculated. It is shown that the account of structural disorder (anti-site defects) results to decrease the magnetization and magnetocaloric properties around the magnetostructural transformation.