If you tear into old TVs or recording equipment, you may see shields made from some exotic-looking metal. Old timers will tell you it’s called mu metal, and its purpose is to — sort of — shield things from magnetic fields. The qualification is important. Unlike a conductive RF shield, mu metal doesn’t really stop a magnetic field. Instead, it gives magnetic flux an easier path to follow around whatever you’re trying to protect.What’s In The Metal?Mu metal belongs to a family of soft magnetic nickel-iron alloys. A typical modern formulation is about 80% nickel and 15% iron, with molybdenum and a few other elements making up most of the remainder. What makes it useful is its extremely high magnetic permeability. Commercial material can have relative permeability around 100,000 or more, and some specialty alloys can reach even higher.You can think about reluctance as the magnetic equivalent of resistance. Put a high-permeability shell around something sensitive, and magnetic flux would much rather travel through the shell than through the space inside it, just like current tends to take the path of least resistance.This works particularly well for DC and low-frequency fields, exactly where your usual copper or aluminum EMI shield isn’t much help.You May Have Seen It BeforeA multilayer magnetic shield box. (Photo by [Zureks] CC-BY-SA-3.0)Classic applications included shielding CRTs, tape heads, transformers, photomultipliers, and sensitive analog instruments. Put a transformer too close to the wrong part of an old television or audio amplifier and 60 Hz magnetic fields could cause very visible — or audible — trouble. The disappearance of CRTs and magnetic tape might make mu metal sound like another material destined for the antique electronics cabinet.However, mu metal is still around. Modern applications include magnetometers, precision current sensors, electron microscopes, scientific instruments, and experiments that require extremely low magnetic fields. Commercial multi-layer mu-metal chambers are still sold for creating near-zero-field environments; with suitable construction and degaussing, some claim attenuation of static and low-frequency fields by factors approaching a million.Quantum and cryogenic instrumentation have also created some 21st-century magnetic shielding problems. Ordinary mu metal loses performance at very low temperatures, so related nickel-iron alloys are made specifically for operation at liquid-nitrogen and liquid-helium temperatures.Don’t Bend ItThere are a couple of catches. First, mu metal gets much of its impressive permeability from its metallurgical structure. Machining, stamping, welding, or even bending it can introduce stresses and seriously degrade its magnetic properties. High-performance shields are therefore commonly formed first and then hydrogen annealed to restore their permeability.So buying a sheet of wonderfully permeable material and folding it into a box isn’t necessarily the recipe for a wonderfully permeable box.The second surprise is saturation. Mu metal is superb with weak fields but isn’t necessarily what you want closest to a powerful magnet. Its saturation induction is only around 0.75 tesla. In strong fields, manufacturers recommend combining it with a lower-permeability material having higher saturation capability, letting that outer layer tame the field before the mu metal handles what’s left.HistoryBritish scientists Willoughby S. Smith and Henry J. Garnett patented mu metal in 1923 for inductive loading of submarine telegraph cables for a British company that built the Atlantic undersea telegraph cables. The seawater surrounding these cables added capacitance, requiring inductance to compensate. This was first done by wrapping the conductors with a helical wrapping of metal tape or wire of high magnetic permeability, which confined the magnetic field.Mu-metal was invented to directly compete with permalloy, the first high-permeability alloy used for cable compensation, but it belonged to competitor Western Electric. Mu-metal was developed by adding copper to permalloy to improve ductility. Each 1.6 km of cable needed about 80 km fine mu-metal wire so there was a great demand for the alloy.Other TricksMu metal isn’t the only way to fight magnetic interference, as you can see in [FesZ’s] video below. Ordinary steel and other high-saturation magnetic alloys can redirect stronger fields. At higher frequencies, conductive copper or aluminum shields become effective through induced eddy currents. Ferrite is good at high frequencies, too, but is not very ductile nor is it very conductive. When you really need a quiet magnetic environment, active compensation coils can measure the ambient field and generate an opposing one.But if the problem is a weak DC or low-frequency magnetic field, the basic trick hasn’t changed much. You just give the magnetic flux an easier path. Sometimes the old material in that 50-year-old television can be at home in a quantum computer, too.