3. Choice of core material and format to suit the application

When trying to use a metallic magnetic material (Fe, FeSi, FeNi alloys, amorphous or nanocrystalline) at high frequency, we are confronted with problems inherent in the low resistivities of these materials:

• penetration depthδ: the useful electromagnetic wave is carried by only part of the material. This depth is proportional to the square root of the electrical resistivity ρ (Ω · m), and inversely proportional to the absolute permeability μ ₀ μ' and the frequency (§ 8