Fabrication and soft magnetic properties of FeSiB based flakes with insulating surface layer suitable for high frequency power applications

FeSiB based soft magnetic flakes (SMFs; ϕ: 63-106 µm, t: 2-5 µm) coated with optimally thick (∼10 nm) SiO2 were fabricated as the potential magnetic material for pressed cores suitable for use in power inductors operating beyond 10 MHz. Such thin SMFs were produced by mechanical ball milling of commercially available thick (∼27 µm) ribbons followed by an optimized sol-gel coating in presence of a silane coupling agent. Milling helped to reduce ribbon thickness below the skin depth to minimize eddy current loss at high frequencies. The effect of milling (medium, duration, rotation speed, and ball-to-powder ratio) and coating conditions on the soft magnetic properties of the SMFs were investigated. An optimally tiny fraction of the milled flakes was allowed to become nanocrystalline, which augments saturation flux density (BS) and permeability (µ) without incurring additional losses. The results from X-Ray photoelectron spectroscopy and magnetization measurement confirm that a stress-releasing post-annealing process reduced coercivity from ∼62 Oe to ∼6 Oe and enhanced the magnetic saturation moment from 126 emu/g to 142 emu/g without affecting the thin yet robust insulating SiO2 layer.