We have investigated surface roughnesses and magnetic properties of Co ferromagnetic thin films evaporated on polyethylene naphtalate (PEN) organic substrates. As a result, the surface roughness decreases from 1.3 to 0.55 nm with increasing the Co thickness up to 55 nm, where a two-step smoothing phenomenon can be seen. As for magnetic properties, the coercive force and the squareness of the hysteresis loop show the maximum values at a Co thickness of 5.3 nm. This experimental result can be explained by the competition between the shape magnetic anisotropy and the induced magnetic anisotropy of Co ferromagnetic thin films.
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