The amorphous Co–Fe–P and crystalline Co fine needles with diameter 0.1–0.2 μm aspect ratio more than 100 were prepared successfully by applying the dc electrodeposition to the anodization technique and their demagnetizing factors were measured. It was found that the demagnetizing factor of film with needles decreases with increasing aspect ratio and tends to take low constant values around 0.07–0.08. To obtain the low demagnetizing factor by using the shape anisotropy of needle, it was estimated that the aspect ratio more than 100 is needed. The equivalent demagnetizing factor defined by a ratio of a field needed to achieve saturation to the averaged flux density is estimated around 0.6 for the film with Co–Fe–P amorphous needles with 0.1 μm diameter.

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