Information capacitance in Ni2MnGa Heusler alloy: A study of the martensitic transformation

Abstract

Information capacitance is a new measure of complexity that is applicable, in principle, to an arbitrary physical system, which makes it interesting for the study of systems near phase transitions. In this context, the Ni${}_2$MnGa Heusler alloy is of special interest as it exhibits two coupled phase transitions (structural and magnetic). Thus questions arise about how each of them contributes to the complexity of the system; i.e., to the information capacitance. To answer this question, we employ a Hamiltonian model together with a Monte Carlo-Metropolis procedure to calculate the magnetization, the tetragonal (structural) distortion and, mainly, the entropy of the system, since the latter quantity is associated with the information capacitance. The Hamiltonian has three parts: magnetic, elastic (where the Blume–Emery–Griffiths model is used to describe the degree of freedom of the atomic displacements in the lattice), and the magnetoelastic part (which accounts for the interdependence of the magnetic and elastic subsystems). Additionally, entropies (total and partial) were calculated using the interrelation between the thermal entropy, calculated by the specific heat, and the Gibbs definition to study the effects of correlation on the phase transformations that are included in the Monte Carlo calculations. The results show that each phase transition contributes differentially to the correlation, depending on the temperature. This allows us to analyze, for example, the degree of coupling between the magnetic and structural subsystems during different stages of the martensitic phase transitions present in the Ni${}_2$MnGa system. By extension, this richness of analysis is inherited by the information capacitance. Furthermore, this measure of complexity, along with its marginal form (C${}_{sum}$), highlights the distinct phase transitions within the considered magneto-structural model.