Mean field and Monte Carlo simulation studies of premartensitic effects in Ni₂MnGa

Abstract
We have extended the degenerate BEG model[l] to include magnetic degrees of freedom in order to study the premartensitic effects in Ni₂MnGa. The model is solved by using mean-field theory and Monte Carlo techniques [2]. The numerical simulations reveal the crucial importance of fluctuations in pretransitional effects. Moreover, we find that a large variety of premartensitic effects may appear due to the magnetoelastic coupling. For large values of the coupling parameter a first-order transition line ending in a critical point appears. 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 strength. In particular, the premartensitic transition occurs when such coupling is strong enough to prevent a complete softening of the involved phonon mode. The implication of the results in relation to the available experimental data is discussed.