At room temperature if you surround a region with mu-metal any external magnetic fields up to a thousand Oersteds (0.1 tesla) cannot penetrate the region. Mu metal's main characteristic is that it is very magnetically soft ...viz. it has a small coercive field...this is the field required to reverse its state of magnetisation.
is often required to protect electronic equipment installed near big
detector magnets from magnetic fields. Shielding is usually provided by
using very high-permeability materials, e.g. nickel-iron based alloys
selection of ferromagnetic materials which can be used for shielding is
given in Table 1 as a function of the magnitude of the parasitic
materials are formed in the shape of a completely or almost completely
closed box surrounding the equipment or of plates or cylinders placed
near the equipment. While in the first case the magnetic field inside
the box vanishes completely or reduces to negligible values, in the
second case an effective shielding is limited to a small region near the
shield and is proportional to its size.
the case of an infinitely long hollow cylinder placed in a uniform
magnetic field transverse to the axis of the cylinder, in the
non-realistic approximation of a uniform magnetic permeability
a and b are the inner and outer radii of the cylinder,
shielding efficiency of a cylinder of finite length L > 8b is
larger than 90 % of
shielding efficiency can be enhanced by subdividing the magnetic
material in layers separated by air gaps.