Martensitic transformation ofNi2FeGaferromagnetic shape-memory alloy studied via transmission electron microscopy and electron energy-loss spectroscopy

Abstract

The structural properties of ${\text{Ni}}_{2}\text{FeGa}$ Heusler alloy synthesized by melt-spinning technique have been systematically studied by means of in situ heating and cooling transmission electron microscopy. It was found that the ${\text{Ni}}_{2}\text{FeGa}$ alloy was annealed into a well-defined $L{2}_{1}$ structure at around 980 K, and complex microstructural domains appeared along with lowering temperature. At room temperature (293 K), a rich variety of micromodulated domains were observed. The domain structures were aligned along the $⟨110⟩$ or $⟨100⟩$ directions resulting to complex tweed structures. Below martensitic transformation (MT) temperature $({M}_{s},\ensuremath{\sim}142\text{ }\text{K})$, the cubic parent phase transformed into unmodulated martensitic variants and modulated martensitic variants. The variants were alternated along the $⟨100⟩$ direction with various arrangements and steplike incommensurate boundaries. The modulated martensitic variants were composed of lamellar structures that have predominately a $5M$ modulation structure along the $⟨110⟩$ directions. The electron energy-loss spectroscopy analysis of the low-loss region and the electron energy-loss near-edge fine structure revealed a visible change of the electronic structure along with MT, which can be well interpreted by means of intra-atomic or intraband charge redistribution due to $spd$ orbital hybridization among the Ni-Fe-Ga atoms.