Comparison of different methods for studying magnetic domains in Ni–Mn–Ga martensites

The relationship between magnetic domains and martensite twin structure was studied in selected Ni–Mn–Ga alloys. Optical microscopy, scanning electron microscopy (SEM) and magnetic force microscopy were used for studying the magnetic domains and the morphology of the martensitic microstructure. SEM Type-I magnetic domain contrast was consistent with the contrast obtained from magneto-optical indicator, as both contrasts originate from the stray field above the sample surface. SEM Type-II magnetic contrast in the backscattered electron (BE) image can display, apart from the martensitic twinning structure, both the magnetic domain contrast and the domain wall contrast. Therefore, the coupling of magnetic and twin structures can be evaluated from the BE image. Surprisingly, optical non-polarizing microscopy can reveal the domain contrast, too. This contrast is due to the surface relief associated with the magnetic domains.