Mu metal is a nickel-iron alloy that is 75% nickel and 15% iron with a little copper and molybdenum. It is a soft metal that has the ability to shunt magnetic fields around it, effectively shielding whatever is behind it from a magnetic field. It is extremely ductile and can be bent or torn with enough force, as seen when Twilight rips a chunk in half for use as a shield.
Mu metal was used by the Chaw of Chaws during their destruction of the Devil's Triangle that held Ezylryb prisoner. They used a shield made of mu metal so as not to have their navigation disrupted by the flecks and succumb to the Triangle. Soren and Bubo used mu metal helmets to sheild themselves from the effects of the flecks in the Triangles they were destroying around the Pure Ones' camp in the St. Aggie's canyons. Metal Beak also wore a mu metal helmet to cover his ruined face so that he was not affected by the flecks' magnetic power.