What is MuMetal?
Mu-metal
is a nickel-iron alloy (approximately 80% nickel, 15% iron, plus copper
and molybdenum) that has very high magnetic permeability. The high
permeability makes mu-metal very effective at screening static or
low-frequency magnetic fields, which cannot be attenuated by other
methods. The name came from the Greek letter mu (μ) which represents
permeability.
Mu-metal can have relative permeabilities of 80,000-100,000 compared to
several thousand for ordinary steel. In addition it has low coercivity
and magnetostriction resulting in low hysteresis loss. Its magnetic
properties are similar to other high permeability alloys such as
Permalloy, but it is more ductile and workable.
Mu-metal objects require heat treatment after they are in final form —
annealing in a magnetic field in hydrogen atmosphere, which reportedly
increases the magnetic permeability about 40 times. The annealing alters
the material's crystal structure, aligning the grains and removing some
impurities, especially carbon, which obstruct the free motion of the
magnetic domain boundaries. Bending or mechanical shock after annealing
may disrupt the material's grain alignment, leading to a drop in the
permeability of the affected areas, which can be restored by repeating
the hydrogen annealing step.
What is Magnetic
Shielding?
The high permeability of mu-metal provides a low reluctance path for
magnetic flux, leading to its major use, in magnetic shields against
static or slowly varying magnetic fields. Magnetic shielding made with
high permeability alloys like mu-metal works not by blocking magnetic
fields but by shunting them—providing a path for the magnetic field
lines around the shielded area. So the best shape for shields is a
closed container surrounding the shielded space. The effectiveness of mu-metal
shielding decreases with the alloy's permeability, which drops off at
both low field strengths and, due to saturation, at high field
strengths. So mu-metal shields are often made of several enclosures one
inside the other, each of which successively reduces the field inside
it. RF magnetic fields above about 100 kHz can be shielded by Faraday
shields, ordinary conductive metal sheets or screens which are used to
shield against electric fields.
History
Mu-metal was developed by scientists named Smith and Garnett and
patented in 1923 for inductive loading of submarine telegraph cables by
The Telegraph Construction and Maintenance Co. Ltd. (now Telcon Metals
Ltd.), a British firm which built the Atlantic undersea telegraph
cables.[3][4] The conductive seawater surrounding an undersea cable
added a great deal of capacitance to the cable, limiting the bandwidth
and slowing signaling speed to 10 - 12 words per minute. The bandwidth
could be increased by adding 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 by adding copper to the previous high permeability
alloy Permalloy to improve ductility. 50 miles of fine mu-metal wire was
needed for each mile of cable, creating a great demand for the alloy.
The first year of production Telcon was making 30 tons per week. In the
1930s this use for mu-metal declined, but by World War II many other
uses were found in the electronics industry (particularly shielding for
transformers and cathode ray tubes) as well as the fuzes inside magnetic
mines.
Uses and properties
Mu-metal is used to
shield equipment from magnetic fields. For example:
Other materials
with similar magnetic properties include
supermalloy,
supermumetal, nilomag, sanbold, Molybdenum
permalloy,
Sendust, M-1040,
Hipernom and HyMu-80.
