The magnetization distribution in the L21 phase of a Ni 2Mn(Al,Ga) alloy, which is a type of ferromagnetic shape memory alloy, has been studied by electron holography and Lorentz microscopy. This alloy contains many antiphase boundaries (APBs), which are introduced by the chemical ordering from the B2 state to the L21 state. In situ Lorentz microscopy observations have revealed that APBs behave as strong pinning sites for domain wall motion. The width of a 180° wall, which was formed at the position of an APB, was estimated to be approximately 10 nm. This narrow domain wall was rationalized by the depression of ferromagnetism within APBs.
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