The crystal structure and phase transitions of the magnetic shape memory compound Ni₂MnGa

Abstract

High resolution neutron powder diffraction and single crystal measurements on the ferromagnetic shape memory compound Ni 2 MnGa have been carried out. They enabled the sequence of transformations which take place when the unstressed, stoichiometric compound is cooled from 400 to 20 K to be established. For the first time the crystallographic structure of each of the phases which occur has been determined. At 400 K the compound has the cubic L2 1 structure, and orders ferromagnetically at T C ≈ 365 K. On cooling below ∼260 K a super-structure, characterized by tripling of the repeat in one of the 110 cubic directions, forms. This phase, known as the pre-martensitic phase, persists down to the structural phase transition at T M ≈ 200 K and can be described by an orthorhombic unit cell with lattice parameters a ortho = 1 √ 2 a cubic , b ortho = 3 √ 2 a cubic , c ortho = a cubic and space group Pnnm. Below T M the compound has a related orthorhombic super-cell with b ortho ≈ 7 √ 2 a cubic , which can be described within the same space group. The new modulation appears abruptly at T M and remains stable down to at least 20 K.

Theoretical prediction and experimental study of a ferromagnetic shape memory alloy:Ga2MnNi

“…34 Atomic positions have not yet been determined for the monoclinic structure. Under such circumstances, our work is important because it shows that a new FSMA material can be predicted by computing the energy cost of formation of the martensitic phase in a simpler tetragonal structure.…”

Superelasticity in Polycrystalline Ni-Mn-Ga-Fe Microwires Fabricated by Melt-extraction

“…9 Mn 28.6 Ga 21.5 microwires, Qian et al reported that SIMT stress increased linearly with the temperature and could be described by Clausius-Clapeyron relationship 4 . The application of ternary Ni 2 MnGa SMAs is limited because of their low martensitic transformation temperature and high intergranular fracture tendency 5 . Thus, various options such as a fourth element doping and grain refinement have been used to improve the properties of Ni-Mn-Ga alloys.…”

Composition and temperature dependence of the thermoelectric power of Ni2+xMn1−xGa alloys

“…The crystal structure at high temperature is the cubic L2 1 and transforms into a modulated tetragonal state at low temperature [1,2]. An important feature of the NiMn-Ga alloys is the occurrence of differently modulated intermediate structures that are observed in-between the initial and the final phase with the variation of temperature [3].…”