Magnetic shielding

Magnetic shielding describes a process in which a magnetic field is excluded from a certain area by deliberately redirecting its field lines. For this purpose, permanent magnets are usually used, or so-called opposing fields are induced. In certain cases, a sufficiently large distance can also be sufficient to achieve a shielding effect.

How does the magnetic shield work?

The shielding of a magnetic field can be necessary, for example, to protect a certain object or also people from an externally acting magnetic field. A classic example of this are scientific experiments that must be carried out free from the influence of the earth's magnetic field and other magnetic fields to avoid falsifications.
Rooms in which such shielding takes place are called zero field chambers and are also used for the calibration of magnetic probes.

Furthermore, fields from certain magnetic systems must be enclosed in some cases to avoid undesired interactions and thus disturbances such as feedback or transmission problems. This is the case with, among others:

  • Amplifiers and microphones
  • CRT monitors
  • Cathode ray tube oscilloscopes
  • Networktransformers
  • Tape and turntable motors
  • MRI machines

Magnetic fields can also influence the human organism. However, to date it is only possible to a limited extent to define specific limit values above which the effect of the magnetic fields can potentially damage the organic molecular structure.
Statutory requirements and guide values can nevertheless be found in the catalog of guidelines of the ICNIRP (International Commission on Non-Ionizing Radiation Protection) as well as in the German ordinance on electromagnetic fields and in DGUV15 of the professional association for wood and metal.

In many cases, a highly permeable, soft magnetic hollow body is used to provide shielding. Highly permeable is ferromagnetic material that can conduct magnetic fields particularly well. All substances that are magnetized quickly but can be demagnetized just as quickly as soon as they are separated from the previously applied magnetic field are referred to as soft magnetic.

According to the second Maxwell equation, a magnetic field has no source point. Therefore, it is not possible to use a simple screen in the form of a wall to stop the flow of the field lines. Rather, they are offered a supposedly more advantageous, since it is easier to "walk" path. Depending on the material chosen for shielding the magnetic field and its shape, the strength of the effect varies. The more permeable and self-contained the ferromagnetic hollow body or the coil used, the better the shielding function.

If, for example, there are slits in the surrounding material, this can lead to stray fluxes - i.e., to field lines that are not completely passed through the shielding object. This effect, which can also be measured, can be caused by an incorrectly selected, too low-permeability material such as ordinary steel or by magnetic saturation of the object. The latter occurs when the selected body has already been magnetized and there is no longer enough space inside for the newly emerging magnetic field. Its field lines then accordingly run in a stray field outside the object.

Which substances can be used to shield a magnetic field?

To avoid that the insulation does not work or works only to a limited extent, materials from the following list of materials should ideally be used for magnetic field shielding:

  • MU metal
  • Permenorm 5000 H2
  • Soft iron
  • Silicon iron
  • Vacoflux 50
  • Vitrovac 6025 150X
  • Vitroperm

These alloys are materials with a high relative permeability, which enables the precise redirection of the respective magnetic field. For example, shielding a magnetic field with aluminum foil is not an ideal option, as the permeability of aluminum is only around 1 and thus hardly differs from the surrounding air.

MU metal consists mainly of nickel and is highly permeable, which is why it is used for research purposes, for machines and in medical technology. However, it is correspondingly expensive and tends to magnetic saturation quickly. It is therefore less suitable for particularly high field strengths. Here, Permenorm 5000 H2 made of iron and nickel, Vacoflux 50 made of iron and cobalt, or soft iron (a pure iron alloy) are used. Plastic-based composite materials that contain ferromagnetic particles are just as suitable for magnetic shielding as fabrics. This is because meshes, air bubbles and other interruptions significantly reduce permeability and can lead to the creation of stray fields.


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