Mu metal, found in old televisions and recording devices, does not completely block magnetic fields; instead, it provides an easier path for magnetic flux, allowing it to flow around sensitive components. A modern formulation consisting of nickel-iron-based soft magnetic alloys contains approximately 80% nickel and 15% iron; the remainder consists of molybdenum and other elements. Its high magnetic permeability can reach approximately 100,000 in commercial products and even higher levels in specialized alloys.
Mu metal is used particularly in direct-current and low-frequency fields, where copper or aluminum EMI shields are ineffective. Once used to protect CRTs, tape heads, transformers, photomultiplier tubes and sensitive analog devices, the material is now also found in magnetometers, precision current sensors, electron microscopes and scientific instruments. Multilayer rooms can reduce fields by up to approximately one million times through appropriate manufacturing and demagnetization. Since its performance declines at low temperatures, special alloys have been developed for liquid nitrogen and helium conditions.
Because machining, welding, stamping or bending can create stresses that degrade permeability, high-performance shields are subsequently subjected to hydrogen annealing. It also has a saturation value of approximately 0.75 tesla, so alongside strong magnets it can be used together with outer layers that have higher saturation capacity. Mu metal was patented in 1923 by Willoughby S. Smith and Henry J. Garnett for the inductive loading of submarine telegraph cables.
Why it matters
The importance of Mu metal stems from its ability to help protect sensitive components in direct-current and low-frequency magnetic fields, where copper and aluminum shields are inadequate. For this reason, the material has a wide range of applications, extending from older imaging and recording devices to modern magnetometers, current sensors, electron microscopes and other scientific instruments. However, high permeability depends on how the material is processed; because bending, stamping, welding and similar processes can impair performance, high-performance shields may require subsequent hydrogen annealing and demagnetization. In addition, the degradation of its properties at low temperatures and its saturation limit of approximately 0.75 tesla increase the importance of alloy selection and multilayer structures in design.