Which ferrite for which application?

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

• penetration depth δ: the useful electromagnetic wave is carried by only part of the material. This depth is inversely proportional to the square root of the electrical conductivity σ (Ω ^-1 · m ^-1 ), the permeability µ' and the frequency f (Hz) : $\delta \left(\text{m}\right)=\sqrt{\frac{1}{\text{π}\cdot \sigma \cdot f\cdot {\mu }_0\cdot {\mu }^{\prime }}};$

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