Modeling Premartensitic effects in Ni2MnGa: A Mean Field and Monte Carlo Simulation Study

TL;DR

This study models premartensitic effects in Ni2MnGa using an extended Blume-Emery-Griffiths model with mean-field and Monte Carlo methods, highlighting the role of magnetoelastic coupling in precursor phenomena and phase transitions.

Contribution

It introduces a microscopic model incorporating structural and magnetic degrees of freedom to explain premartensitic effects in Ni2MnGa, emphasizing magnetoelastic interactions.

Findings

Premartensitic effects arise from magnetoelastic coupling.

A premartensitic first-order transition line ends at a critical point.

Large premartensitic fluctuations manifest as broad specific heat peaks.

Abstract

The degenerate Blume-Emery-Griffiths model for martensitic transformations is extended by including both structural and magnetic degrees of freedom in order to elucidate premartensitic effects. Special attention is paied to the effect of the magnetoelastic coupling in Ni2MnGa. The microscopic model is constructed and justified based on the analysis of the experimentally observed strain variables and precursor phenomena. The description includes the (local) tetragonal distortion, the amplitude of the plane-modulating strain, and the magnetization. The model is solved by means of mean-field theory and Monte Carlo simulations. The results show that a variety of premartensitic effects may appear due to the magnetoelastic coupling. For large values of the magnetoelastic coupling parameter we find a premartensitic first-order transition line ending in a critical point. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peaks in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interplay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling. In particular, the premartensitic transition occurs when such coupling is strong enough to freeze the involved mode phonon.