Modeling premartensitic effects in Ni2MnGa: A mean-field and Monte Carlo simulation study

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 paid to th e effect of the magnetoelastic coupling in Ni2MnGa. The microscopic model i s constructed and justified based on the analysis of the experimentally obs erved strain variables and precursor phenomena. The description includes th e (local) tetragonal distortion, the amplitude of the plane-modulating stra in, and the magnetization. The model is solved by means of mean-field theor y and Monte Carlo simulations. This last technique reveals the crucial impo rtance of fluctuations in pretransitional effects. The results show that a variety of premartensitic effects may appear due to the magnetoelastic coup ling. In the mean-held formulation this coupling is quadratic in both the m odulation amplitude and the magnetization. For large values of the magnetoe lastic coupling parameter we find a premartensitic first-order transition l ine ending in a critical point. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peak s in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interpl ay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling. In particular, the premartensitic transition o ccurs when such coupling is strong enough to freeze the involved mode phono n. The implication of the results in relation to the available experimental data is discussed. [S0163-1829(99)12933-3].