Magnetic shielding refers to the attempt to isolate or block the magnetic field of the MRI magnet. This can be done to prevent unwanted interference from the MRI magnet on nearby electronic devices. Industries Served. Although our magnetic field shielding materials and products are used for low field shielding applications across a broad range of industries, Magnetic Shield Corporation provides industry specific expertise, technical engineering know-how, and design consultations to help solve increasingly high-tech advanced EMI challenges.

Magnetic shielding is a process that limits the coupling of a magnetic field between two locations. This can be done with a number of materials, including sheet metal, metal mesh, ionized gas, or plasma. The purpose is most often to prevent magnetic fields from interfering with electrical devices.

Unlike electricity, magnetic fields cannot be blocked or insulated, which makes shielding necessary. This is explained in one of Maxwell’s Equations, del dot B = 0, which means that there are no magnetic monopoles. Therefore, magnetic field lines must terminate on the opposite pole. There is no way to block these field lines; nature will find a path to return the magnetic field lines back to an opposite pole. This means that even if a nonmagnetic object — for example, glass — is placed between the poles of a horseshoe magnet, the magnetic field will not change.

Instead of attempting to stop these magnetic field lines, magnetic shielding re-routes them around an object. This is done by surrounding the device to be shielded with a magnetic material. Magnetic permeability describes the ability of a material to be magnetized. If the material used has a greater permeability than the object inside, the magnetic field will tend to flow along this material, avoiding the objects inside. Thus, the magnetic field lines are allowed to terminate on opposite poles, but are merely redirected.

While the materials used in magnetic shielding must have a high permeability, it is important that they themselves do not develop permanent magnetization. The most effective shielding material available is mu-metal — an alloy of 77% nickel, 16% iron, 5% copper, and 2% chromium — which is then annealed in a hydrogen atmosphere to increase its permeability. As mu-metal is extremely expensive, other alloys with similar compositions are sold for shielding purposes, usually in rolls of foil.

Magnetic shielding is often employed in hospitals, where devices such as magnetic resonance imaging (MRI) equipment generate powerful magnetic flux. Shielded rooms are constructed to prevent this equipment from interfering with surrounding instruments or meters. Similar rooms are used in electron beam exposure rooms where semiconductors are made, or in research facilities using magnetic flux.

Smaller applications of magnetic shielding are common in home theater systems. Speaker magnets can distort a cathode ray tube (CRT) television picture when placed close to the set, so speakers intended for that purpose are shielded. It is also used to counter similar distortion on computer monitors.

A number of companies will custom build magnetic shields from a diagram for home or commercial applications. Shielding using superconducting magnets is being researched as a means of shielding spacecraft from cosmic radiation.

Magnetic fields can pose a problem for electronic equipment, and attempting to shield for magnetic fields is not as simple and straightforward as for electric fields. No material is actually able to block magnetic fields without itself being attracted to the magnetic force. Unlike electricity, magnetic fields cannot technically be blocked or insulated, but can only be redirected. In general, high permeability materials, or those with the ability to support the formation of a magnetic field within themselves, are used for this purpose. When using a high permeability shielding enclosure to protect electrical components in the presence of a magnetic field, the shield works by diverting the magnetic flux and drawing the magnetic field lines into the shielding material rather than them passing into the protected space. The electromagnetic energy within the material is then dissipated and converted into heat, thus creating ohmic losses; this phenomenon is known as absorption loss.

Absorption loss is the primary shielding mechanism to shield low frequency magnetic fields. The absorption loss achieved by a magnetic shield is directly proportional to the shield’s material thickness, the permeability of the material, the conductivity of the material, and the frequency of the incident wave. Therefore, the ideal material choice for magnetic shielding would be a thicker, high permeability and electrically conductive material.

There are some overlooked aspects that need to be considered when choosing a magnetic material. The permeability of a material will in fact decrease as the frequency of the incident wave increases. For example, at 100 kHz the permeability of HyMu 80 is no better than cold-rolled steel. As a general rule of thumb, high permeability materials are ideal when dealing with frequencies below 10kHz. Another important factor is that the material’s effectiveness also depends on the magnetic field strength which it is being exposed to. High magnetic strengths can cause a material to become saturated, which will vary based on the thickness and type of the material being used.

Magnetic Shielding Metal

Magnetic Shielding Tape

The most popular material being used today in the magnetic shielding industry based on its superior characteristics with respect to permeability and saturation is an 80 wt% nickel-iron alloy that conforms to MIL-N-14411C, Composition 1 and/or ASTM A753, Type 4 such as HyMu 80. Leader Tech maintains stock of this material in varying thicknesses, and can help assist with the design of your enclosure and offer recommendations for your specific magnetic shielding application.

Magnetic Shielding

We at Leader Tech understand the need for magnetic shielding, and can help provide you with an appropriate and cost-effective solution for your unique application. In a future article, we will further explore the design considerations that need to be accounted for when designing magnetic shielding enclosures.