Maximum permeability

Maximum permeability μ_max - a maximum value of magnetic permeability, when measured as a function of changing amplitude of excitation, usually at a fixed frequency of excitation for a given test.¹⁾ For electrical steels and Ni-Fe alloys the point of maximum permeability is around 50-60% of magnetic saturation. For example, for grain-oriented electrical steel the maximum permeability can be around flux density B = 1.2 T, whereas the saturation is above B = 2.0 T.

Permeability of ferromagnetic materials is a non-linear function of the applied excitation and reduces when approaching saturation

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

Family of B-H loops of a typical grain-oriented electrical steel, the maximum permeability occurs for the steepest slope of the B/H ratio

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

The concept of maximum permeability is typically applied to relative amplitude permeability, as measured under alternating excitation for soft magnetic materials. Non-magnetic materials have magnetic permeability close to unity, typically without a significant non-linearity (flat response, no maximum).

Maximum permeability is useful for broad classification of performance of various soft magnetic materials. It can be also used as a figure of merit for comparison of performance of the same material, but exposed to different processing or treatment such as annealing.

The highest values of magnetic permeability are achieved for materials such as Co-based amorphous ribbon for which values of relative permeability μ_r above 1 million were obtained. Newest grades of nanocrystalline ribbon can also reach values exceeding those of Ni-Fe alloys.

However, for magnetic shielding the Ni-Fe alloys (such as Mumetal) are used because of the excellent mechanical properties, allowing for varied mechanical processing needed to make a shield with arbitrary shape. operations such as welding and deep drawing are possible, and the high value of maximum permeability can be recovered by appropriate annealing procedure. This is not possible for amorphous and nanocrystalline materials (regardless if they are based on Fe or Co).

Overview of properties of soft magnetic materials: saturation, permeability, frequency range, material cost²⁾

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

• Magnetic permeability
• Relative permeability
• Initial permeability