The correlation between the antiferromagnetic (AF) grain volume, ν
AF and the exchange anisotropy is discussed for Ni-Fe/25 at% Ni-Mn system. In spite of various thickness combination of ferromagnetic (F) and AF layers, the unidirectional anisotropy constant,
Jk commonly became large and saturated with increasing ν
AF. The substantial coercivity originated from the AF layer decreased and became constant with increasing ν
AF. These experimental results were well explained by a simple model based on Meiklejohn's single spin model. We concluded that increasing ν
AF is key to induce the exchange anisotropy effectively in F/AF bilayers.
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