What’s Mu Metal?

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 …. 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. 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.