It’s important to know that magnetic shielding does not block a magnetic field, no material can stop the lines of flux from traveling from a magnet’s north to its south pole. However, it can be redirected.
Magnetic fields will pass through plastic, wood, aluminum, and even lead as if it was not there, however, ferrous materials such as iron, steel, or nickel can conduct magnetic fields and redirect magnetism.
All magnetic fields seek the shortest path from north to south and a piece of steel can provide a shortcut making the journey from north to south much easier than flowing through the air.
To remove magnetism from where you do not want it to be, you can use steel to provide the magnet with a shortcut to redirect the magnetism flow via an alternative route.
The simplest example of this would be putting a steel keeper across the poles of a horseshoe magnet, as the magnetic flow is still present but will flow through the steel so there is no external magnetic field.
A magnet in free space.
A magnet next to a steel wall.
A magnet in a steel enclosure.
Which Materials Will Work?
Any ferromagnetic metal. That is any metal-containing iron, nickel, or cobalt. The majority of steels are ferromagnetic metals and will work for redirecting magnetic shields. Steel is the most commonly used metal because it is cost-effective and widely available, however, some stainless steels are not ferromagnetic.
How Thick Should My Shield Be?
The magnetic field will be affected by the thickness of the metal being used to shield it, however, figuring out how thick the shield needs to be is dependent on several factors.
For example, you must consider the size of the magnetic field you are shielding, what is being shielded from, what is the shield’s shape, and finally, it makes sense to cover the magnet.
The thickness of your shield is important as if the shield is too thin it can become saturated and will not be able to hold any more lines of flux, so, therefore, you need the shield to be thick enough to hold as much flux as possible.
However, after a certain point adding steel thickness will not improve the shielding. In some cases where saturation becomes an issue, multiple layers of materials can be used.
Are There Other Materials That Can Be Used?
There are some specialized materials designed and made for magnetic shielding. The most common of these specialized materials is MuMetal or some other proprietary alloys.
Most of these will have a high nickel content, with either 50% or 80% nickel content.
These specialized magnetic shielding materials will have a higher relative permeability but a lower saturation point. Permeability is the degree of magnetization of a material responding linearly to an applied magnetic field.
When it comes to shielding the relative permeability is a measure of a material’s ability to absorb magnetic flux. The higher the number, the better the shield. For example, low carbon steels have a permeability of 1000-3000, whereas specially designed metals can be as high as 300,000-400,00.
Making A Final Decision
So how to decide which material is right for your specific shielding problem?
If the magnet being shielded has low field strength, then specially-made materials such as MuMetal can provide better shielding than steel.
However, for applications that involve large, powerful neodymium magnets then the higher saturation points of steel prove useful and serve better.
Overall, in many applications we come across and are asked about, a steel sheet-metal shield is usually the best solution.
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