Experimental Observations versus First‐Principles Calculations for Ni–Mn–Ga Ferromagnetic Shape Memory Alloys: A Review

Abstract

This review summarizes recent advances in first‐principles simulations of the structural, mechanical, and magnetomechanical properties of Ni–Mn–Ga‐based ferromagnetic shape memory alloys from the perspective of experimental characterization. It focuses on calculations that can capture the main features of the low‐temperature phases, particularly the appearance of spatial modulations, the occurrence of intermartensitic transitions, and the high mobility of the twin boundaries, which allows for magnetically induced reorientation. Although the calculations can only be used to interpret some of these features and are not yet ready to be used as a tool for designing new alloys with the required properties, the gap between experimental observations and simulations is narrowing. When experiments and theory are discussed together, new challenges arise for both. In addition, new results on first‐principles‐determined elastic constants of the 10M martensite of Ni–Mn–Ga are presented and discussed with respect to the high mobility of the twin boundaries.

Localization versus delocalization of d-states within the $$\hbox {Ni}_{{2}}$$MnGa Heusler alloy

“…The same behaviour was found for DFT +U calculations with U = 3 eV applied simultaneously on Ni and Mn. These tests suggest that the magnetic moment becomes the leading parameter when analysing the stability of the MnGa martensite structures, assumption that is supported by the experimental evidence since the paramagnetic austenite in off-stoichiometric alloys always transforms into the NM martensite 5 .…”

“…The MSM occurs in a low-temperature martensite phase arising from the structural reorientation of highly mobile twin boundaries. This is caused by the alignment of the local magnetic moments with the external field due to the large magnetocrystalline anisotropy 1 , 2 , 4 , 5 . The high temperature austenite phase of MnGa is characterized by the cubic structure 6 , typical of Heusler alloys (see Fig.…”

“…Understanding the structural and magnetic couplings in Heusler alloys is key to the ongoing research, where first-principles calculations have become an invaluable tool 5 . Density-functional theory (DFT) based codes traditionally use the exchange-correlation potential described by the local density approximation (LDA) 23,24 or the semi-local generalized gradient approximation (GGA) 14,25,26 .…”

Magnetic and transformation properties of Ni2MnGa combinatorically substituted with 5 at.% of transition elements from Cr to Cu – Experimental insight

“…In contrast to the computational approach, which can be done systematically, most of the experimental alloying was done by trial and error and somehow randomly [17][18][19][20][21][22][23][24][25][26][27][28][29]. On the other hand, the calculated systematic results are often widely off-mark for real compounds [30] and experimental corrective insight or validation is needed. The complex behavior of Heusler alloys and in particular their sensitivity on off-stoichiometry and a small amount of alloying or doping calls for thorough studies.…”

“…The thorough experimental systematic multidimensional mapping can be used for theoretical approaches using machine learning. We deliberately selected the smallest alloying amount suitable to be compared with ab-initio calculation using an extended unit cell [30]. In the paper, we report and discuss the fundamental properties of alloyed Heusler alloys relevant for MIR and partly for MCE and relate these to usual phenomenological criteria [33,34] used for predicting new alloys.…”