Materials for Blocking Magnets: Understanding Magnetic Shielding

When it comes to shielding magnets, the common belief is that materials directly impede their magnetic fields. However, the truth is more complex. This article explores the actual mechanisms and materials used for blocking magnets, with a detailed breakdown of the space between magnets and the effectiveness of different materials, including mu metal and various metals.

Understanding the Basics of Magnetism

The magnetic field created by a magnet is confined by a material, such as steel, which acts as a keeper to hold an S pole and control the flux. However, it's important to note that this material isn't necessarily preventing the magnetic field from passing through; rather, it's creating a path that is more conductive than air or space.

The Role of Space in Magnetic Fields

When placing a non-magnetic material like thick cardboard or even a non-magnetic metal like cheeseburger between two magnets, the core issue remains the space between the magnets. The strength of the magnetic field between the magnets can be so weak that even a single sheet of paper can break the connection. Increasing the number of layers, say from one to ten or more, can prevent the magnets from sticking, depending on the strength of the magnets involved.

Effective Materials for Shielding Magnets

Materials that are not magnetic themselves can provide needed separation, but there are specific options that excel in magnetic shielding:

Mu Metal

One of the best materials for magnetic shielding is mu metal. Mu metal is designed to divert magnetic fields significantly. Properly formulated, a mu metal-cored double shielding with an air gap can completely isolate the magnetic field inside a space. In contrast, Faraday cages do not block static or slowly changing fields, hence they are not suitable for magnetic shielding.

Mu metal is expensive, with thin sheets costing approximately $100 per square foot. However, its effectiveness in magnetic shielding makes it a valuable material in many applications, from medical devices to research equipment.

Iron and Steel

Iron and steel are more commonly used and are excellent choices for magnetic shielding. While they are not as effective as mu metal, they are often the go-to materials for their cost-effectiveness and availability.

Other Metals

Other metals such as brass, copper, and aluminum can also be used to block or shield against magnetic fields, but their effectiveness is generally lower compared to iron and steel. These materials are useful in certain applications where magnetic fields need to be reduced or isolated.

Specialty Materials and Applications

In more specialized applications, superconductors are used. These materials can perfectly block magnetic fields when cooled to extremely low temperatures. For instance, mumetal alloys, which are used in standard magnetic shielding, can degrade when subjected to excessive mechanical stress, compromising their magnetic shielding properties.

Historically, shielding phototubes from magnetic fields was achieved using a combination of thick iron and mu metal. The calculations for the thickness of the iron are intricate, as it needs to be much thicker than expected due to its ability to attract nearby magnetic fields.

The understanding and application of these materials play a crucial role in various fields, from electronics and telecommunications to medical research and aerospace.