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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:
μR=ΔB/ΔH
(units are Gauss/Oersted)
Materials are divided into four categories, depending on their permeability.
Diamagnetic: 
R<1
Nonmagnetic: R=1 (air is a good example, but most metals are very close to nonmagnetic, close enough so you can round off R to 1.)
Paramagnetic: R>1
Ferromagnetic: R>>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:
http://hyperphysics.phy-astr.gsu.edu/HBASE/tables/magprop.html
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!