What is magnetic shielding?

What material is suitable for magnetic shielding? Answer: High magnetic susceptibility materials.
Why can high magnetic susceptibility materials be magnetically isolated? Let's first explain a more common question: What materials are magnetically conductive?
Any medium responds to an external magnetic field. Some materials have a weaker response, which we call weak magnetism, such as: paramagnetic, antiferromagnetic substances; Magnetic (negative magnetic susceptibility). The well-known iron, cobalt, nickel, etc. are all strong magnetic substances, which have a strong response to magnetic fields and can be attracted by magnets. Of course, vacuum also has magnetic susceptibility, which has a special name: vacuum magnetic susceptibility.
We can think of a magnetic field as consisting of individual lines of force. Magnetic force lines like high magnetic susceptibility substances, and when they encounter them, they will go into them at once. This kind of intimacy decreases with the decrease of magnetic susceptibility. On the other hand, the number of magnetic force lines that can be accommodated per unit volume of each substance is also limited, and the number of magnetic force lines that can be accommodated by a high Ms material per unit volume is more, and the number of magnetic force lines that can be accommodated with the decrease of Ms is also smaller.
Therefore, for some iron-based materials with high magnetic susceptibility and high saturation magnetization, the magnetic field lines are very favorable. When the magnetic force line encounters this kind of material during the conduction process, it will drill into it at once, and because of the high saturation magnetization, the material can accommodate a lot of magnetic force lines. This kind of material conducts magnetism very well. This kind of material can also be magnetically isolated, because the magnetic field lines have gone to the inside of this material, and they are unwilling to go to other places, so they are also magnetically isolated. Mu metal is used in the laboratory, which has a high magnetic permeability. mu-metal is more expensive and can be replaced by other materials, such as metals with strong ferromagnetism.
At present, ordinary iron plates are usually used to shield the magnetic field. Magnetic shielding requires materials with high magnetic permeability. The material that meets this requirement is iron-nickel alloy, which has high magnetic permeability. Please note that there needs to be an air gap between the iron plate and the magnet, and any non-magnetic permeability material can be filled in the air gap for support. For larger magnets, it is difficult to meet the requirements of complete shielding with only a single layer of shielding material. At this time, one method is to increase the thickness of the iron plate, but a more effective method is to use combined shielding. In another shield, there is a certain air gap between them. The shielding effect of the combined shield is much better than that of a single shield, so the combined shield can attenuate the magnetic field to a very low level.