Microstructure dependence of magnetic properties of soft magnetic Fe-Si-B-P-Cu nanocrystalline alloys were studied by using in-situ Lorentz microscopy in a transmission electron microscope equipped with a magnetizing system. In particular, we investigated in detail motion of magnetic domain walls in heat-treated Fe85Si12B6P4Cu1 amorphous ribbons. Smooth motion of domain walls was observed for the optimally heat-treated (at 430 °C) nano-crystalline alloy. Pinning of domain walls was observed for higher-temperature-heat-treated (470°C) ribbons. Both ribbons showed a nanocrystalline structure containing α-Fe crystallites of about 15 nm in size. Electron diffraction patterns indicated that the higher-temperature-heat-treated samples contained boride precipitates, which is considered to cause less smooth domain wall motion.
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