• Dali Electronics

Super Muniperm, Mu Metal

Mu Metal is a nickel-iron soft magnetic alloy with very high permeability, which is used for shielding sensitive electronic equipment against static or low-frequency magnetic fields.

Mu-metal typically has relative permeability values of 80,000–100,000 compared to several thousand for ordinary steel. It is a “soft” magnetic material it also has low magnetic anisotropy and magnetostriction, giving it a low coercivity so that it saturates at low magnetic fields. This gives it low hysteresis and losses when used in AC magnetic circuits. Mu-metal has more advantages as it is more ductile and workable, allowing it to be easily formed into the thin sheets needed for magnetic shields.

Mu-metal objects require heat treatment after they are in the final form—annealing in a magnetic field in a hydrogen atmosphere, which 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.

Super Muniperm International standards: ASTM A 753, DIN 17405, IEC 404, JIS C 2531.

Magnetic shielding

The high permeability of Mu-metal provides a low reluctance path for magnetic flux, leading to its 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 providing a path for the magnetic field lines around the shielded area. Thus, 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. Thus, mu-metal shields are often made of several enclosures one inside the other, each of which successively reduces the field inside it. Because mu-metal saturates at such low fields, sometimes the outer layer in such multi-layer shields is made of ordinary steel. Its higher saturation value allows it to handle stronger magnetic fields, reducing them to a lower level that can be shielded effectively by the inner mu-metal layers.

Physical Properties

Physical PropertiesMetricEnglish
Density8.74 g/cc0.316 lb/in³

Mechanical Properties

Mechanical PropertiesMetricEnglish
Hardness, Brinell105 – 290105 – 290
Tensile Strength at Break530 – 900 MPa76900 – 131000 psi
Tensile Modulus190 – 221 GPa27600 – 32100 ksi
Izod Impact, Unnotched0.420 – 1.00 J/cm0.787 – 1.87 ft-lb/in

Electrical Properties

Electrical PropertiesMetricEnglish
Electrical Resistivity0.0000550 – 0.0000620 ohm-cm0.0000550 – 0.0000620 ohm-cm
Magnetic Permeabilitymin 60000min 60000
max 240000max 240000
Magnetic Coercive Force, Hc0.0126 Oe0.0126 Oe
Magnetic Remanence, Br3700 Gauss3700 Gauss
Curie Temperature380 °C716 °F

Thermal Properties

Thermal PropertiesMetricEnglish
CTE, linear13.0 µm/m-°C (@Temperature 20.0 – 100 °C)7.22 µin/in-°F (@Temperature 68.0 – 212 °F)
Thermal Conductivity30.0 – 35.0 W/m-K208 – 243 BTU-in/hr-ft²-°F
Component Elements PropertiesMetricEnglish
Iron, Fe14 %14 %
Molybdenum, Mo4-6 %4-6 %
Nickel, Ni79-81 %79-81 %

Descriptive Properties

Saturation Induction (T)       0.77

Popular Shielding Applications of MuMetal

  • Shielding of RF magnetic fields
  • Cryogenic Shields
  • Shielding from natural Earth Magnets
  • Cathode ray tubes (CRTs) used in oscilloscopes
  • Sensitive electronic equipment against the magnetic field
  • AC magnetic circuits
  • Shield against the electric field
  • Mobile Networks & Antennas Rays
  • Shielding of SQUID – Superconducting Quantum Interference Device –
  • telegraph cables
  • Electric power transformers, which are built with mu-metal shells to prevent them from affecting nearby circuitry. high quality but low noise audio frequency transformers.
  • Hard disks, which have mu-metal backings to the magnets found in the drive to keep the magnetic field away from the disk
  • Cathode-ray tubes used in analogue oscilloscopes, which have mu-metal shields to prevent stray magnetic fields from deflecting the electron beam
  • Magnetic phonograph cartridges, which have a mu-metal case to reduce interference when LP’s are played back
  • Magnetic resonance imaging equipment
  • The magnetometers used in magnetoencephalography and magnetocardiography
  • Photo-multiplier tubes
  • Vacuum chambers for experiments with low-energy electrons, for example, photoelectron spectroscopy.
  • Superconducting circuits and especially Josephson junction circuits
  • Fluxgate magnetometers and compasses as part of the sensor
  • Ground fault interrupter cores
  • Anti-shoplifting devices
  • Tape recorder head laminations
  • Magnetometer bobbin cores
  • Mu-metal is also used in Optical Cables.

For Any Inquiries Call 8928403611/12 or Email: sales@domadia.com 

Frequently Asked Questions (FAQ)

What is a magnetic field?

We are surrounded by magnetic fields (both AC and DC) from the earth’s magnetic field to man-made sources such as magnets, motors and transformers. When a piece of sensitive equipment is being affected by these fields we need to produce a shield. Examples that are affected are cathode ray tubes, photomultiplier tubes, audio transformers, scanning electron microscopes, position sensors.

How does a magnetic shield work? 

No known material that can block magnetic fields without itself being attracted to the magnetic  force. A magnetic shield acts as a kind of sponge redirecting the magnetic field around the shield instead of passing through the sensitive instrument which is being shielded. To be a good magnetic shielding material it must have a high permeability which means that the magnetic field lines are strongly attracted to the shielding material.

The most common shielding alloys are chosen based on the intensity of the magnetic field. If the magnetic field is too high for the material chosen it will saturate and become ineffective. In this case, you can use a multi-layer shield with a combination of the different alloys. Alloys should also have a very low remanence to prevent them from becoming permanently magnetized.

What is the best shape for a shield?

The most efficient shape is spherical but this is very difficult to produce and largely impractical in most shielding applications. The next best is a cylinder with closed ends. These endcaps significantly increase shielding attenuation. This is followed by a box shape but the corners need to have a large bend radius to minimize flux leakage. If possible do not use a flat sheet.

What is the difference between RF and Magnetic Shielding?

Radio Frequency shielding is required to stop high-frequency fields (> 100 kHz) and copper, aluminium, metalized plastics are normally used because they are conductive and have very little permeability. Magnetic shielding is typically found in the 30 – 300 Hz AC range.

What is the difference between DC and AC?

DC is a direct current that flows in one direction only such as the fields emitted from the Earth or produced by magnets and some motors. AC is alternating current that reverses its direction over a short period and these fields are generated by typical 50-60 Hz electric power equipment.  Magnetic shielding is effective for both of these types.

What is magnetic permeability?

It is a material’s ability to absorb magnetic flux. It is a ratio of flux density to field strength. The higher the permeability the better the magnetic shield attenuation performance.

What is field attenuation?

This is also known as the shielding  factor (S) and is a ratio of the magnetic field strength outside of the magnetic shield (Ha) and the resultant field on the inside of the shield ie Ha/Hi (no units) or S = 20 x log(Ha/Hi) (Db). There are various formulas based on the permeability of the material, the shape and size of the shield and the material thickness.  In most cases, these formulas are only approximate and are for DC fields only.

For a closed shielding can : 

S = 4/3 X (Mu x d/D)  where Mu :

The permeability(relative)

d :  material thickness

D:  Shielding Diameter

For a long hollow cylinder in a magnetic transverse field : 

S = Mu x d/D

For a cubic shielding box : 

S = 4/5 X (Mu x d/a)

a: box side length.

In the case of multiple layer shields with air gaps provided by insulating spacers, the shielding factors of the individual shields are multiplied  together resulting in excellent shielding factors.

For a double layer shield : 

S= S1 x { S2 x (2 x change in diameter  /diameter) }

Why are both, FeNi48 and MuMetal used together?

It is has got a very high level of permeability but a relatively low level of saturation whereas, FeNi48 has a lower level of permeability but a level of higher saturation. FeNi48 is used closest to the very strong field to protect the material according to Mumetal from saturation.

Why is a final heat treatment required for Mµ-Metal, FeNi48 and pure iron?

After plastic deformation, a high-temperature  heat treatment is required to rearrange the crystal structure as well as allowing the grains to grow. Without this final heat treatment, the magnetic properties and the shielding attenuation will be much reduced.

Do cryogenic temperatures affect the performance of MuMetal?

MuMetal is affected by cryogenic temperatures. The saturation induction remains the same but the permeability decreases. At cryogenic temperatures, we need to use a special cryogenic MuMetal, which we also supply.

Can you use magnetic shielding materials at a high vacuum?

MuMetal is similar to stainless steel so outgassing is minimal.

Can a shield be re-heat treated?

Yes it can be if it has received knocks or if there are concerns about the shield’s shielding ability.

Do you carry stocks of shielding alloys?

Yes, We carry and sell a large range of stock including sheets and coil from 0.1 mm to 5 mm in thickness in MuMetal quality.

Can you weld material  µ-Metal?

Yes without a problem but it must be fully heat-treated after welding.

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