What's m-metal?


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.


Magnetic shielding

Shielding 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 with such as mu-metal. All these materials must be treated at very high temperatures (1100 ) after shaping. Some amorphous metals, which have the same magnetic properties of the nickel-iron alloys and are easier to use and less sensitive to plastic deformations, are available only in the form of very thin sheets ( ) of limited width ( ). These high-permeability materials have low saturation induction and therefore must be used in the presence of weak stray fields (smaller than a few mT). In the presence of stronger magnetic fields a supplementary shield, surrounding that made of the high-permeability material, must be used.

A selection of ferromagnetic materials which can be used for shielding is given in Table 1 as a function of the magnitude of the parasitic induction. is the ultimate shielding efficiency which can be obtained with the corresponding material and it is defined as the ratio of the magnetic induction outside the shield ( ) to that in the region surrounded by the screen ( ).

These 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.

Table 1: Selection of magnetic materials for shielding

In 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 , the shielding efficiency is given by:

where a and b are the inner and outer radii of the cylinder, respectively. In particular

where is the thickness of the shielding.

The shielding efficiency of a cylinder of finite length L > 8b is larger than 90 % of over a length of 4b.

The shielding efficiency can be enhanced by subdividing the magnetic material in layers separated by air gaps.