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Magnetic Shielding Foils
Thorlabs' Magnetic Shielding Foil, offered in high and low permeability versions, provides effective protection for application setups with sensitive electronic equipment whose normal operation can be affected by electromagnetic interference (EMI) and strong external magnetic fields. The high-permeability foil (MSFHP) works by easily absorbing external magnetic fields and redirecting them through the foil. Due to its high saturation induction characteristics, the low-permeability foil (MSFLP) is ideal for attenuating very strong, high-flux external fields. Combine both the high- and low-permeability foils to create one shield that provides the widest range of magnetic shielding. Please see the Graphs tab for hysteresis plots, permeability plots, and information about selecting a foil based on the expected magnetic field strengths.
The graphs above show the DC hysteresis loop for both the MSFHP and MSFLP magnetic shielding foils. The high permeability of the MSFHP foils produces a compressed hysteresis from ~ -1 to 1 Oe. In contrast, the high saturation induction of the MSFLP foils yields a hysteresis over a much larger range of field strengths (~ -10 to 10 Oe). The hysteresis loops demonstrate the range of field strengths where the foil is best used. To understand which foil is best used for a particular application, we must first consider the attenuation of the foil.
When designing a magnetic shield, the most important parameter to consider is the attenuation (g) of the magnetic field. This is just the ratio of the magnetic field intensity outside the shield (Ho) to that inside the shield (Hi). When creating a cylindrical shield, the attenuation can be expressed as the following.
In this equation Rin is the inside radius of the shield, Rout is the outside radius of the shield, T is the material thickness (Rout - Rin), and µ is the permeability of the foil. The attenuation of the magnetic field, therefore, is a factor of the foil's thickness and permeability. The equation above holds true for cylinders with a Length/Radius ≥ 4.
The graph to the left shows the permeability of the MSFHP and MSFLP foils. Permeability is a measure of a material's ability to conduct magnetic fields through its volume. High permeable materials have a strong response to external magnetic fields and will also produce higher attenuation. The relationship B=µH shows that the permeability is the ratio of the magnetic flux density (B) to the magnetic field intensity (H) and is often expressed in units of Gauss/Oersteds (Gs/Oe). From the graph to the left we see that for low magnetic field intensities (0 Oe< H <1 Oe) the MSFHP foil's permeability is much greater than that of the MSFLP foil, and thus serves as the better shielding material. For larger magnetic field intensity (H>1 Oe), the MSFLP foil will have the higher permeability and thus offers greater attenuation at these higher field intensities.