Local random magnetocrystalline and macroscopic induced anisotropies in magnetic nanostructures

To clarify the influence of the coherent field-induced anisotropies (Ku) on the exchange softening effect in magnetic nanostructures, the soft magnetic properties of both bcc-Fe and D03-Fe(Si)-based nanocrystalline alloys annealed under static and rotating fields were surveyed. The random anisotropy model with Ku predicts that the grain size (D) dependence of the coercivity changes from D6 to D3 when the ratio of the random magnetocrystalline anisotropy and Ku is approximately 1 to 2. The D3 dependence is seen in the bcc-Fe-based alloys where Ku is ∼100 J/m3, while such a dependence is absent from the D03-Fe(Si)-based alloys simply because their Ku is as small as ∼10 J/m3 and the D3 dependence is completely overshadowed by other extrinsic coercivity mechanisms. The soft magnetic properties of the bcc-Fe-based alloys are improved dramatically by rotating field annealing, implying that the exchange softening in the bcc-Fe-based alloys is promoted by suppressing the strong Ku.