Have you ever wondered about how to shield a magnet? Can a magnetic field be blocked so a magnet only pulls on one side? Need to shield a sensitive device from magnetic fields?
First, one important point must be clear: Magnetic shielding does not block a magnetic field. No material can stop the lines of flux from traveling from a magnet's orth pole to it's south pole. The field can, however, be redirected.
In the series of pictures below, follow the lines of flux as paths from one pole of the magnet to the other. In the first, a magnet in free space is shown, with the field lines flowing through air. In the second, a wall of steel provides an "easier" path for the lines of flux to follow. These lines flow out from the magnet's pole, into the steel for some distance, and back out into the air to get back to the magnet's other pole. In the third picture, a steel enclosure reduces the ambient field strength inside by providing a path around either side of the space.
The short answer is: Any ferromagnetic metal. That is, anything containing iron, nickel or cobalt. Most steels are ferromagnetic metals, and work well for a redirecting shield. Steel is commonly used because it's inexpensive and widely available. Note that some stainless steels, especially the 300 series varieties, are not ferromagnetic.
This will depend on many factors. What is the size and nature of the magnetic field you're shielding? What are you shielding it from? Does it make sense to shield the magnet, or your magnetically sensitive device? Is your shield a perfect sphere, a closed cylinder, or some other shape?
The thickness of the shield matters, up to a point. When the shield is too thin, it becomes saturated, and can't "hold" any more lines of flux. You want it to be thick enough to hold as much flux as possible. However, once you reach a certain limit, adding steel thickness won't improve your shielding much.
In some cases where saturation is an issue, multiple layers of material are used.
See the animation at right, where the thickness of a steel wall is varied. Once it gets below a critical thickness, the material is saturated. It can't hold any more lines of flux. At that point, the flux pops out the far side, and travels through the air.
Yes, there are some specialized materials specifically made for magnetic shielding. The foremost of these is MuMetal, an industry reference material defined in Milspec 14411C. Companies that provide magnetic shielding materials typically offer a version of MuMetal, and some other proprietary alloys. Most of these have a high nickel content, with either 50% or 80% nickel in the mix.
Specialized magnetic shielding materials usually have a higher relative permeability, but a lower saturation point.
Permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. For shielding, Relative Permeability is the Permeability divided by the Permeability of free space, a constant. In more practical terms, Permeability is a measure of a material's ability to absorb magnetic flux. The higher the number, the better the shield.
Low carbon steels have a Permeability of 1000 - 3000, while MuMetal can have values as high as 300,000 - 400,000.
The saturation point is the flux density at which the material can not contain any more magnetic flux. Steel saturates around 22,000 Gauss, while MuMetal saturates at about 8,000 Gauss.
In lower flux density fields, such high permeability materials provide greater attenuation. In higher field densities, MuMetal becomes saturated, and loses its effectiveness. In these cases, steel provides good attenuation and a much higher saturation threshold.
Which material is right for you depends on your specific shielding problem. For low field strength, sensitive electronics, MuMetal can provide better shielding than steel. For many applications involving large, powerful neodymium magnets, the higher saturation point of steel serves better. In many specific cases we're asked about, a steel sheet-metal shield is often the best solution.