Various magnetic domain structures in a Ni–Mn–Ga martensite exhibiting magnetic shape memory effect

Abstract

Magnetic domain structures of the Ni–Mn–Ga martensite were observed by means of type I and type II magnetic contrast in scanning electron microscope. The different configuration of magnetic domain patterns coupled together with the twin structures were studied in multivariant, two-variant, and single-variant martensite. The martensitic band contains broad stripelike magnetic domains following the easy axis of magnetization, i.e., the crystallographic c axis. These stripe domains are connected by 90° domain walls creating a staircaselike structure in the adjoining bands. It is found that the internal twins, substructures of the martensite twin domains, are distorted into a zig–zag shape in order to accommodate the main band magnetization. Furthermore, the dagger-shaped stripe domains occur only when the internal twins are present. When the sample exhibits the single-variant state, the internal twins disappear totally and the stripe magnetic domains spread over the whole specimen. The configuration observed here for the magnetic microstructure together with the crystallographic microstructure can help in understanding the magnetic shape memory effect.