Effect of Si on the field-induced anisotropy in Fe-rich nanocrystalline soft magnetic alloys

The field-induced magnetic anisotropy (K_u) and hyperfine interactions have been investigated for nanocrystalline Fe_bal.B_14∼14.8Si_xCu_1.5 (x = 0, 1, 2 and 4) alloys. ⁵⁷Fe Mössbauer spectroscopy was used to quantify the Si contents and volume fractions of the amorphous and bcc-Fe(Si) phases after nanocrystallisation. This information was then used to estimate the local K_u contribution from the bcc-Fe(Si) phase. The bulk K_u value of these nanocrystalline alloys showed an increase from 40 ± 5 J/m³ to 69 ± 5 J/m³ with increasing Si content from 0 to 4 at%. The local K_u of the bcc-Fe(Si) phase was found to increase in line with Si content, as predicted by the well-known directional pair ordering model and showed a large value of 172 ± 14 J/m³ for a local Si content of around 6 at%. The addition of Si to Fe-rich nanocrystalline alloys is widely perceived as an effective means for improving magnetic softness. However, the findings of this study imply that small additions of Si to Fe-rich nanocrystalline alloys may result in undesirable effects if the crystalline volume fraction is especially large.