Magnetic domain structures of soft magnetic alloys Fe73.5Cu1Nb3Si13.5 B9 with various heat treatments are studied by electron holography. Under the existence of the external magnetic field of about 0.2mT perpendicular to the thin films of about several tens nm in thickness, the magnetic lines of force not only inside the specimens but also outside the specimens are visualized by phase reconstruction from electron holograms with the Fourier transform. While the circular magnetic domains form in the specimen annealed at 823K, the magnetic domains become smaller and the magnetic lines of force fluctuate significantly with the increase of the grain size of constituent crystalline phases. Furthermore, by tilting the specimens and increasing the external magnetic field in the specimens, the magnetic induction process of the films with different heat treatments is compared. It is clarified that the change of the magnetic domain structure due to the tilting sensitively depends on the microstructure or the heat treatment, well corresponding to the coercive force and permeability of Fe73.5Cu1Nb3Si13.5 B9 alloys.
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