Phase transitions in Ni2+xMn1-xGa with a high Ni excess

TL;DR

This study investigates phase transitions in Ni$_{2+x}$Mn$_{1-x}$Ga alloys with high Ni excess, revealing a unique magnetostructural transition where martensitic and magnetic phases merge, influenced by exchange interactions and magnetoelastic effects.

Contribution

It provides the first detailed phase diagram for Ni$_{2+x}$Mn$_{1-x}$Ga alloys in this composition range, highlighting the magnetostructural transition and its underlying mechanisms.

Findings

Identification of a compositional interval with merged martensitic and magnetic transitions.

Non-monotonous behavior of the magnetostructural transition temperature.

Estimation of Curie temperatures for martensite and austenite phases.

Abstract

Ferromagnetic shape memory alloys Ni$_{2+x}$Mn$_{1-x}$Ga were studied in the range of compositions $0.16 \le x \le 0.36$. Experimental phase diagram, constructed from differential scanning calorimetry, transport and magnetic measurements, exhibits distinctive feature in a compositional interval $0.18 \le x \le 0.27$, where martensitic and magnetic transitions merge in a first-order magnetostructural phase transition ferromagnetic martensite $\leftrightarrow$ paramagnetic austenite. Observed in this interval of compositions a non-monotonous behavior of the magnetostructural phase transition temperature was ascribed to the difference in the exchange interactions of martensitic and austenitic phase and to the competition between increasing number of valence electron and progressive dilution of the magnetic subsystem which occur in the presence of a strong magnetoelastic interaction. Based on the experimental phase diagram, the difference between Curie temperature of martensite $T_C^M$ and Curie temperature of austenite $T_C^A$ was estimated. Influence of volume magnetostriction was considered in theoretical modeling in order to account for the existence of the magnetostructural phase transition over a wide range of compositions.