The magnetic shield is made of magnetic materials. The parameter to measure the magnetic permeability of the material is magnetic permeability, and the relative size is usually expressed by numbers. The permeability of the vacuum is 1, and the permeability of the shielding material is from 200 to 350,000; another important parameter of the magnetic shielding material is the saturation magnetization. Magnetic shielding materials are generally divided into three categories, namely high permeability materials, medium permeability materials and high saturation materials.
The permeability of high saturation permeability materials is between 80,000 and 350,000, and the saturation field can reach 7500 Gs after heat treatment; medium permeability materials are usually used together with high permeability materials, and their permeability values are from 12,500 to 150,000. The saturation field is 15500Gs; the permeability of the high saturation field is 200-50000, and the saturation field can reach 18000-21000Gs.
For the shield, the type of material selected has a great influence on its performance and cost. When designing a shield, it is important to understand the characteristics of different shielding alloys commonly used. The understanding of these different properties will enable you to choose the right material to meet the target requirements. Magnetic shielding materials should be selected according to their respective characteristics, especially magnetic permeability and magnetic saturation performance. Due to the effectiveness of changing the direction of the low frequency magnetic field, high permeability materials (such as 80% nickel alloy Mumetal, which is a high permeability iron-nickel alloy) are often used as shielding materials. These alloys can meet the requirements of MIL-N-14411C Part 1 and ASTM A753-97 Style 4. The relatively thin thickness available is 0.002 to 0.125 inches, and it can be easily processed by experienced shield processors. When the magnetic field needs to be reduced in a very small space, these alloys are typically used. These materials are often selected when it is necessary to provide higher shielding than required, or when the magnetic field strength (more typical at higher field strengths) requires materials with higher saturation values. When the shielding target only needs to reduce the field strength slightly (reduction by 1 to 1/4), or when the field strength is sufficient to saturate the high permeability shield, ultra-low carbon steel (ULCS) may be the best choice. The carbon content of these lower-cost materials is typically less than 0.01%; compared with other steels, they have higher magnetic permeability and excellent saturation performance. These materials have less flexibility and are easier to manufacture than silicon steel, which allows easy installation and processing of small components in large-area shielding projects. ULCS can be used with high permeability materials to create the best shielding body that requires high saturation protection and high attenuation levels. Cryoperm10 (a registered trademark of Vaccumschmelze GmbH, Germany) is the best choice for low temperature shields. Like Mumetal, Cryoperm10 is also a high permeability nickel-iron alloy, which is specially processed to provide an increase in permeability when the temperature is reduced. Standard shielding alloys (such as Mumetal) lose most of their permeability at low temperatures. But Cryoperm10 can increase the permeability by 10 times at 77.3 to 4.2°K.