Relative permeability of mu-metal
| Value | 80,000 — |
| Status | Measured: ± 30,000 — (0.38 relative) |
| Source | Manufacturer data — typical annealed mu-metal, low-field μᵣ |
| Categories | ElectromagneticMaterial Propertiesmagnetism |
| ratio | 80,000 — |
| percent | 8,000,000 % |
Learning zone
Mu-metal is roughly 80% nickel with iron, molybdenum and copper, developed at the Telegraph Construction Company around 1923 to improve the inductive loading of submarine telegraph cables. Its relative permeability in weak fields runs from 50,000 to well over 100,000 — but only after a hydrogen anneal at about 1100 °C that grows large, strain-free grains. Bend, drill or drop a finished shield and the permeability drops by an order of magnitude, so mu-metal parts are formed first and annealed last.
It shields by conduction rather than blocking: a mu-metal can offers magnetic flux a path of far lower reluctance than the air inside, so field lines detour around the enclosure. That is why it works on static and low-frequency fields, where copper shielding is useless, and why nested shells work better than one thick one. Photomultipliers, CRTs, atomic clocks, SQUID magnetometers and magnetically shielded rooms all depend on it. It saturates early, around 0.7 T, so a strong field must be attenuated in stages.