Permeability, Mumetal® and Alloys
What is magnetic permeability?
What do we mean by "magnetically 'soft'"?
Why are both NETIC S3-6 and CO-NETIC AA alloys offered?
Why is CO-NETIC AA alloy offered in both Perfection Annealed and Stress Annealed types?
Does cutting the alloy destroy its properties?
What effect does heating the magnetic material have on its magnetic shielding properties?
What are the issues using magnetic shielding material at high vacuum?
What is magnetic permeability?
Magnetic permeability refers to a material's ability to absorb magnetic flux. It is defined as the ratio of the intensity of induced magnetization in the material to the intensity of the inducing magnetic force field that produced it. For the most effective magnetic shield attenuation performance, the higher the magnetic permeability, the better. CO-NETIC AA® magnetic shielding alloys are made to our exacting specifications and, most importantly, fabricated and final annealed using processes optimized to achieve the highest practicable magnetic permeability.
What is Mumetal®?
Mumetal®, CO-NETIC AA and other shielding alloys, offer important electromagnetic field shielding characteristics, including very high magnetic permeability - its ability to absorb magnetic energy. The unique properties of this family of alloys results in the highest possible attenuation, making these shielding alloys the materials of choice for reducing low-frequency electromagnetic interference (EMI).
What do we mean by "magnetically 'soft'"?
Alloys useful for magnetic shielding are said to be magnetically "soft" because they can absorb magnetic energy without retaining it. Magnetically "hard" materials, on the other hand, make good magnets because they can retain a strong magnetic field even after the source of the magnetic energy has been removed. When a magnetic field is removed from a magnetic shield alloy, there is no remaining (residual) magnetic field. The measure of this property is called coercive force. For a magnetic shield, the lower the coercive force, the better. CO-NETIC AA® magnetic shielding alloy is formulated and processed to minimize coercive force.
Why are both NETIC S3-6 and CO-NETIC AA alloys offered?
The two primary magnetic shield alloys offered by Magnetic Shield Corporation have differing characteristics that sometimes suggest that one or the other (or sometimes both) be used. CO-NETIC AA alloy has the highest magnetic permeability and provides the highest attenuation in a magnetic shield. However, it has a relatively low saturation induction rating. NETIC S3-6 alloy, on the other hand, has a high saturation induction rating and is therefore better suited for strong magnetic fields. NETIC S3-6 alloy cannot achieve the high magnetic permeability of CO-NETIC AA, however, and so provides more modest attenuation factors. If very high attenuation ratios must be achieved in a very strong field, sometimes both alloys must be used. The NETIC S3-6 alloy is used closest to the source of the field to protect the CO-NETIC AA alloy from saturation.
Why is CO-NETIC AA alloy offered in both Perfection Annealed and Stress Annealed types?
CO-NETIC AA alloy always requires a special annealing process to develop its full magnetic properties. If fabrication involves severe drawing, forming or welding operations, the properties of the alloy will be adversely affected, and annealing must be done after the manufacturing operations are complete. For such applications, CO-NETIC AA alloy Stress Annealed is offered. It has better formability characteristics and costs less.
If the application does not involve severe drawing, forming or welding, then CO-NETIC AA alloy Perfection Annealed may be used, and the final annealing step may be avoided. This is because CO-NETIC AA alloy Perfection Annealed has been fully annealed before shipment and is ready to perform as a magnetic shield. CO-NETIC AA alloy Perfection Annealed has a large, open grain structure and must not be used when the shield fabrication involves severe drawing, forming or welding. CO-NETIC AA alloy Perfection Annealed is an economical choice for flat shields and some formed shields. Please contact us for help in determining which material is best for your specific application. All CO-NETIC AA alloy foil is Perfection Annealed.
Does cutting the alloy destroy its properties?
This concern about the magnetic shielding alloys arises because they do have a sensitivity to mechanical shock - bending, forming, even severe flexing may give some reduction in the alloy's magnetic permeability. Modern, vacuum-refined alloys have a lower sensitivity to shock, and normally withstand regular handling without significant loss of properties. Cutting by shearing, EDM, waterjet, photo-chemical etching, or blanking dies typically only affects that portion of the alloy immediately adjacent to the edge, and the shield will exhibit normal shielding performance. If the alloy had already received its final magnetic anneal, re-annealing should not be necessary.
What effect does heating the magnetic material have on its magnetic shielding properties?
CO-NETIC and NETIC magnetic shielding alloys have a positive coefficient of permeability with respect to temperature. This is a quick way of saying that the permeability increases as temperature increases. This occurs until operating temperature nears the Curie temperature, defined as the temperature where permeability goes to 1 and the alloy becomes magnetically transparent. The Curie temperatures are 850°F (454°C) for CO-NETIC and 840°F (449°C) for NETIC. At operating temperatures above the Curie temperature, the shield will not provide any attenuation of the magnetic field. Rolloff of permeability begins as operating temperature nears (within 25°F) the Curie temperature listed, so a safety factor for allowable operating temperature should be used.
Note: Temporary temperature excursions above curie temperature do not degrade the alloys. Shielding function is restored when the temperature again falls below Curie point.
What effect does cryogenically cooling the magnetic material have on its magnetic shielding properties?
CONETIC AA alloy is affected by operation at low temperatures. The saturation induction (which determines maximum field) remains unaffected, but the permeability decreases as the temperature drops, because of reduced molecular mobility.
The need for de-rating the attenuation highlights the importance of selecting the optimum shield size. Attenuation should be calculated for two sizes; a cryogenic atmosphere shield and a room temperature shield.
The chart below depicts the Low Temperature Performance of magnetic shielding material. To use the factors, multiply the room temperature attenuation by the performance factor shown for the operating temperature (T) of the shield.
AT = A x Performance factor at T / 100
AT = Attenuation ratio at temperature T.
What are the issues using magnetic shielding material at high vacuum?
CO-NETIC AA alloy is specifically formulated for maximum magnetic shielding properties and has some physical properties similar to stainless steel. Out-gassing is minimal and cleaning for high vacuum applications can follow the same procedures used for stainless steel. Handle annealed magnetic shields carefully to avoid severe deformation which causes reduced attenuation.
How can I learn more?
Magnetic Shield Corporation has a selection of product information brochures and engineering design guides which can help you solve problems with magnetic fields. Please see our literature page for details on the items available. Other sources include:
Bozorth, Richard M., FERROMAGNETISM, D. Van Nostrand Company, Inc., Princeton, NJ, 1951 (Reissued by IEEE Press in 1993)
White, Don, The 1998 EMC ENCYCLOPEDIA, emf-emi control, Inc., Gainesville, VA, 1998
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