The Mu-metal reference article from the English Wikipedia on 24-Apr-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Mu-metal

Have you considered sponsoring a child
Mu-metal is a nickel-iron alloy (77% nickel, 15% iron, plus copper and molybdenum) that is very efficient for screening magnetic fields. It is used to shield vacuum chambers for experiments with low-energy electrons.

The name of the material refers to the Greek letter &mu, which is the symbol for magnetic permeability. Mu-metal has a high value of μ.