Heusler Ni-Mn-X (X = In, Sn, Sb) alloys have unique properties, such as the shape memory effect, the giant magnetocaloric effect and the large magnetoresistance. These properties are associated with the martensitic transition. In our work we present the modeling of thermomagnetization curves in relatively a low magnetic field for non-stoichiometric Heusler Ni₅₀Mn_25+xSb_25-x compounds by the help of the Monte Carlo method.

In the proposed model we use the three-dimensional cubic lattice with periodic boundary conditions. The sites of lattice only occupy the magnetic Mn atoms due to that the magnetic moment of Ni atoms is much more less then magnetic momentum of Mn atoms and Sb atoms have not the magnetic momentum. The whole system can be representing as two interacting parts – magnetic and structural subsystems. The magnetic part is described by the “q-state” Potts model for the magnetic phase transition from ferromagnetic to paramagnetic one [1]. The structural part is described by the degenerated three state BEG model for the structural transformation from the austenitic phase to the martensitic phase [2]. In the martensitic state we consider two types of the magnetic interaction between nearest sites of the lattice. One of part of Mn atoms interact with each other ferromagnetically and another part of Mn atom interact with other Mn atoms antiferromagnetically. In the austenitic phase we consider that each atom of Mn interact with each other ferromagnetically. The initial configuration of Mn atoms with antiferromagnetic interaction on the cubic lattice was set random and was determined from experimental data of Ni₅₀Mn_25+xSb_25-x alloys. By the help model the temperature dependences of the magnetization for different concentrations are obtained. It is shown that theory are in good agreement with experimental data.

[1] F.Y. Wu. Rev. Mod. Phys. 54 (1982) 235.

[2] T. Castan et al. Phys. Rev. B 60 (1999) 7071.

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