How do MU Metal block magnetic field?

Hi oem7110,

This is indeed mu-metal. It works by absorbing the magnetic field, due to it's low reluctance (magnetic flux flows more easily through mu-metal so it flows through the metal rather than through the air). It is used to shield against static fields and Aluminium is often used in combination in order to shield against high frequency fields. Aluminium works by absorbing the field, causing an electric field in the metal, which in turn creates an opposing magnetic field that cancels the original field. By the way, no one who knows about magnets claims that it is impossible to shield a magnet. However, it is impossible to completely remove a magnetic field. Rather magnetic fields are either diverted, or directed back on themselves in reverse, thus cancelling the original field. In the case of active shielding, a pick up coil converts a magnetic field to an electric field and produces a cancelling field in reply.

Mu-metal is a nickel–iron soft magnetic alloy with very high permeability suitable for shielding sensitive electronic equipment against static or low-frequency magnetic fields. It has several compositions. One such composition is approximately 77% nickel, 16% iron, 5% copper and 2% chromium or molybdenum.

The high permeability of mu-metal provides a low reluctance path for magnetic flux, leading to its use in magnetic shields against static or slowly varying magnetic fields. Magnetic shielding made with high-permeability alloys like mu-metal works not by blocking magnetic fields but by providing a path for the magnetic field lines around the shielded area. Thus, the best shape for shields is a closed container surrounding the shielded space. The effectiveness of mu-metal shielding decreases with the alloy's permeability, which drops off at both low field strengths and, due to saturation, at high field strengths. Thus, mu-metal shields are often made of several enclosures one inside the other, each of which successively reduces the field inside it. Because mu-metal saturates at such low fields, sometimes the outer layer in such multilayer shields is made of ordinary steel. Its higher saturation value allows it to handle stronger magnetic fields, reducing them to a lower level that can be shielded effectively by the inner mu-metal layers.