Micromagnetic study of Ni2MnGa under applied field (invited)

We present an experimental study of the domain and twin structure in the martensitic phase of the ferromagnetic shape memory alloy Ni2MnGa using magnetic force microscopy. After cooling through the austenite–martensite transformation under no field, a rectangular specimen with {100} faces consistently adopted a structure of fine-scale (101) twins with surface relief, due to demagnetization effects. After cooling under a [010] field of 2 kOe, a completely different twin structure consisting of (110) parallel bands with no surface relief was observed. Under increasing fields between 2 and 8.5 kOe, the following sequence of domain structures were observed: fir tree patterns in twin bands, fir tree patterns localized at twin boundaries, single domains with thick walls (approx. 0.5 μm) coincident with twin boundaries. The domain walls exhibit unusually high contrast, and there is evidence for a wall substructure with irregularly spaced nodes. These observations are consistent with a theoretical micromagnetic study (R. D. James, Q. Pan, R. Tickle, R. Kohn, and M. Wuttig, preprint) in which approximately homogeneous rotation of magnetization occurs in alternate bands at large fields; this rotation reduces the driving force on twin boundaries before the maximum strain is achieved in some tests.