Magnetic materials are used in RF and power electronics to make inductors, transformers, ferrite beads, and circulators. Understanding their properties is essential for choosing the right core material.

Relative Permeability (\(\mu_r\))

Permeability \(\mu_r\) relates the magnetic flux density to the applied field: \(B = \mu_r\mu_0 H\). High \(\mu_r\) increases inductance but also limits useful frequency range (permeability drops at high frequencies).

Ferrites

Ferrites are ceramic magnetic materials with high resistivity (minimising eddy current losses at RF). They are used in:

• Ferrite beads — suppress high-frequency noise on power lines and signal traces by presenting high impedance above a target frequency.
• Ferrite cores — increase inductance of RF chokes and RF transformers, particularly below 50 MHz.
• Circulators and isolators — exploit the non-reciprocal properties of magnetised ferrite (Faraday rotation) to route signals directionally.

Core Saturation

At high flux densities, magnetic cores saturate — \(\mu_r\) collapses, inducing harmonics and power loss. Always verify that peak flux density is below the saturation flux density \(B_{sat}\) of the chosen material under worst-case conditions.

Core Loss

Core loss (hysteresis + eddy current losses) is characterised by the complex permeability \(\mu = \mu' - j\mu''\). The loss factor \(\mu''/\mu'\) (analogous to \(\tan\delta\)) increases with frequency. Ferrite materials are categorised by their intended frequency range (e.g. MnZn ferrites for <5 MHz, NiZn for 1–100 MHz).