Initial magnetic permeability, typically denoted by μ_i - a value of relative permeability for a given magnetic material measured at very low magnetic field excitation, such that it tends to a fixed value for that material, under those conditions.¹⁾

In practice, for real materials there is not just one “true” value of initial permeability, because the actual performance of a magnetic core made from a magnetic material will depend on physical dimensions, which might significantly contribute for instance to skin effect.

Also, measurement of initial permeability requires application of small amplitude and thus detection of small signals is difficult from the viewpoint of accuracy and reproducibility. For this reason for some magnetic alloys the value of initial permeability can be instead approximated by measuring a value such as μ0.4 (which are relative permeability at magnetic field strength H = 0.4 A/m).²⁾

For magnetically soft ferrites such as N87³⁾ or 3C94⁴⁾ initial permeability can be measured at similarly low amplitudes but at defined magnetic flux density e.g. B_peak ≤ 0.25 mT, which for initial permeability of μ_i = 2200 corresponds to H_peak ≤ 0.09 A/m.

Experimentally measured relative amplitude permeability μ_r for a typical conventional grain-oriented electrical steel - as the amplitude reduces the permeability tends to a fixed value of the initial permeability, which in this example is below B < 0.3 mT or H < 0.1 A/m.

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Illustration of several types of permeability, calculated as a ratio (slope) of change of B to change of H: μ_a or μ_ampl - amplitude, μ_max - maximum, μ_i - initial, μ_Δ - incremental, μ_rev - reversible

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

• Types of magnetic permeability