Magnetic Materials

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The property that defines how a material responds to a magnetic field is its permeability. Permeability has a big effect on skin depth, metals with high permeability are poor conductors for RF signals.

The definition of permeability is the ratio of applied magnetic induction to the applied magnetic field:


(units are Gauss/Oersted)

Materials are divided into four categories, depending on their permeability.

Diamagnetic: Magnetic MaterialsR<1

Nonmagnetic: Magnetic MaterialsR=1 (air is a good example, but most metals are very close to nonmagnetic, close enough so you can round off Magnetic MaterialsR to 1.)

Paramagnetic: Magnetic MaterialsR>1

Ferromagnetic: Magnetic MaterialsR>>1

Material Type Relative permeability
Bismuth Diamagnetic 0.99983
Silver Diamagnetic 0.99998
Copper Diamagnetic 0.999991
Lead Diamagnetic 0.999983
Water Diamagnetic 0.999991
Vacuum Nonmagnetic 1
Air Paramagnetic 1.0000004
Aluminum Paramagnetic 1.00002
Palladium Paramagnetic 1.0008
Cobalt Ferromagnetic 250
Nickel Ferromagnetic 600
Mild Steel (0.2 C) Ferromagnetic 2,000
Iron (0.2 impurity) Ferromagnetic 5,000
Silicon Iron Ferromagnetic 7,000
Mumetal Ferromagnetic 100,000
Purified iron (0.05 impurity) Ferromagnetic 200,000
Supermalloy Ferromagnetic 1,000,000

Part of the table above came from Microwave Tubes by A.S.Gilmour. Order it from our book page!

Here's a table of Magnetic Susceptibilities of Paramagnetic and Diamagnetic Materials at 20°C on another web site, thanks to Faaron:

Note that the permeability of a material is not always a fixed bulk property, such as density. Permeability can be affected by how a material is treated, for example, the grain size of a metal can affect it.

Permeability of ferromagnetic materials can be quite nonlinear. Usually the values given in tables are for the maximum permeability, the slope at very low applied magnetic fields.

For these two reasons, you may very well see quite different relative permeability data for the same material, if you compare data from two suppliers or textbooks. Heck, you might even see two different numbers reported for the same material on this web site!

More to come!



Author : Unknown Editor