Soft magnetic properties of sputtered bcc Fe-Nb-B films with nanoscale grain size

A mostly single bcc phase with nanoscale grain size of about 10 nm was found to be formed by annealing Fe-Nb-B sputtered amorphous films. The bcc Fe-Nb-B alloy films with solute contents of 6.5 to 9.5 at%Nb and 9.5 to 15 at%B annealed at 923 K for 3.6 ks exhibit good soft magnetic properties. Especially, a high permeability (μ) of 4000 at 1 MHz combined with high saturation magnetization (B_s) of 1.55 T was obtained for a Fe_81.1Nb_7.5B_11.4 sputtered film. The good soft magnetic properties seem to result from the decrease in apparent magnetic anisotropy by formation of the nanocrystalline bcc structure and by reduction of magnetostriction due to dissolution of Nb and B into the bcc phase. Amorphous regions surrounding bcc grains were observed in the high-resolution electron micrograph for a Fe_79.5Nb_7.3B_13.2 film annealed for 3.6 ks at 923 K. It is presumed by analysis of the temperature dependence of the grain size and magnetostriction that the thermal stability of the residual amorphous phase increases with the increases of solute elements discharged from the bcc grains.