The Heusler-type alloy ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ exhibits well defined shape memory properties in a ferromagnetic state, which means that the martensitic transition temperature is lower than the Curie point of this material. The change of composition makes these characteristic temperatures approach each other. To study this behavior, the measurements of specific heat, ac magnetic susceptibility, and dc resistivity were performed. The phase diagram of the cubic ferromagnet describing possible structural and magnetic transitions is obtained theoretically. This diagram is compared with experimental data on ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}.$ An estimate is given of the magnetic-field influence on the temperature of martensitic transformation in the studied alloys.

Ferromagnetic shape memory alloys ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ were studied in the range of compositions $0.16⩽x⩽0.36$. The experimental phase diagram, constructed from differential scanning calorimetry, transport, and magnetic measurements, exhibits distinctive feature in a compositional interval $0.18⩽x⩽0.27$, where martensitic and magnetic transitions merge in a first-order magnetostructural phase transition ferromagnetic $\text{martensite}\leftrightarrow \text{paramagnetic}$ austenite. Observed in this interval of compositions a nonmonotonous 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.

On the basis of Monte Carlo simulations using the Potts model Hamiltonian, we investigate the complex temperature dependence of magnetization of ferromagnetic $\text{Ni-Mn-}X$ $\left(X=\text{In},\text{Sn},\text{Sb}\right)$ Heusler alloys, in which part of the Mn atoms, which occupy the $X$ sites, interacts antiferromagnetically with the Mn atoms on the Mn sublattice. It is shown that this antiferromagnetic exchange is responsible for metamagnetic behavior and a series of magnetic phase transitions in the Heusler alloys. For an optimal choice of parameters of the model Hamiltonian, which have partially been obtained from ab initio calculations, we are able to describe the physics associated with the coupled martensitic-magnetic phase transition. The simulations lead to a qualitative agreement with the experimental magnetization curves and their dependence on temperature and composition.

A review is given of experimental and theoretical works concerning the investigation of magnetic and structural phase transitions and of the magnetocaloric effect in Ni-Mn- X ( X= Ga, In, Sn, Sb)...

We present ab initio calculations of magnetic exchange parameters of stoichiometric Heusler compound Ni${}_2$MnSn and a few nonstoichiometric Ni${}_2$Mn${}_{1+x}$Sn${}_{1-x}$ cases. Use of the exchange parameters in subsequent Monte Carlo simulations allows us to evaluate the magnetization curves as a function of temperature and composition as well as the critical temperatures of the magnetic phase transitions. The latter are compared to those obtained from a mean-field approximation using the Heisenberg model. We find that the variation of the experimental Curie temperatures of nonstoichiometric alloys can be explained theoretically if we assume that the main impact of disorder is the intermixing of manganese and tin on their corresponding sublattices and the simultaneous appearance of strong antiferromagnetic trends which originate from the nearest-neighbor Mn-Mn interactions on different sublattices. The Curie temperatures of the Ni-Mn-Sn alloys which have been obtained from the Monte Carlo simulations are in qualitative agreement with the experimental transition temperatures.