# Mu-metal

General
Surname Mu-metal
other names Permalloy, supermalloy
Components Mu-metal and permalloy: 76… 80% nickel , 15… 16% iron , 4… 5% copper , 2… 3% chromium or molybdenum ;
Supermalloy: 75… 79% nickel, 16… 20% iron, 3… 5% molybdenum

Other mixing ratios are also common, such as 80% nickel, 16% iron and 4% cobalt .

Brief description Material with high magnetic permeability
properties
density 8.7 g / cm³
Physical state firmly
Melting point 1454 ° C
Saturation magnetization 0.8 T
Permeability number 80,000 ... 500,000
Specific resistance 55 · 10 −6  Ω · cm
MR coefficient 2… 4%
upper application temperature 150 ° C
Expansion coefficient (20… 100 ° C) in 10 −6  K −1 13.5
E-module in kN / mm² 200

Mu-Metal (µ-metal, Mumetall, English Mu-metal or English permalloy ) belongs to a group of soft magnetic nickel - iron - alloy with 72 to 80% nickel and proportions of copper , molybdenum , cobalt or chromium with high magnetic permeability , the is used to shield low-frequency magnetic fields and to manufacture the magnetic cores of signal transmitters , magnetic current sensors and current transformers .

## properties

Mu-metal has a high permeability ( = 50,000–140,000), which causes the magnetic flux of low-frequency magnetic fields to concentrate in the material. This effect leads to considerable shielding attenuation when shielding low-frequency or static magnetic interference fields . ${\ displaystyle \ mu _ {r}}$

When mu-metal is bent, deformed or machined, the high permeability breaks down drastically. Values ​​down to = 150 are possible. For this reason, mu-metal should definitely be annealed again after mechanical stress in order to restore the high permeability by healing lattice defects. ${\ displaystyle \ mu _ {r}}$

Mu-Metal is available in the form of foils and sheets, starting with a thickness of approx. 0.1 to 5.0 mm in standard dimensions, as well as in plate form.

New measurements (as of July 2014) have shown that with a magnetic final annealing under protective gas, a permeability ( = 300,000) can be achieved. ${\ displaystyle \ mu _ {r}}$

## application areas

Shielding of an industrial building with panels made of mu-metal
Shielding of the magnetic stray field of a power transformer with three orthogonally arranged sleeves made of multi-layered mu-metal
• As a magnetoresistive element in hard disk heads. The resistance of the element can be influenced by changing the surrounding magnetic field. (see magnetoresistive effect )
• As core material for low frequency - transformer , current transformer , magnetic current sensors
• In powder form for the production of pressed powder cores
• In the form of thin sheet metal as a material for shielding magnetic interference fields in electronic devices or watch cases . Finished shields with typical wall thicknesses of 1 to 2 mm in standard shapes such as B. cups, tubes and hoses available. There are also shielding hoods for magnetic heads, monitor picture tubes , beakers for vehicle display instruments and shields for small electric motors in tape recorders
• Shielding cabins: Walk-in shielding cabins or rooms with multi-layer shielding have been built for examinations without magnetic fields or high-field laboratories.

## Manufacturing

After cooling, the melted alloy is processed into sheets , strips or wire . Mu-Metall can be punched , etched , deep-drawn , bent , soldered , welded and electroplated . Machining, drilling and grinding are also possible. After mechanical processing, the finished workpiece is subjected to a final annealing at 1000 to 1200 ° C and a subsequent tempering treatment at 400-600 ° C. These thermal treatments are carried out under vacuum or protective gas such as B. hydrogen. Particularly high permeabilities or other special magnetic properties can be achieved through special cooling processes or magnetic field annealing .

## History of Permalloy and Mu-Metal

Shielding of a submarine cable with wrapped permalloy metal tape.
Shielding of a submarine cable with coiled Mu metal wire (Krarup cable)

The development of the soft magnetic nickel-iron alloys Permalloy and Mu-metal is closely connected with the development of telegraphy , especially with the development of the transatlantic submarine cables . This development began around 1850. At that time, the system of Morse code developed by Samuel Morse and its recording using telegraphs had established itself worldwide. Overland and from Dover to Calais, a number of cable connections had already been established by 1850. The first attempt to lay a submarine cable between Ireland in Europe and Newfoundland in America was successful in 1858. The initial enthusiasm for this colossal achievement turned into a great disappointment, as it took 16 hours to transmit the British Queen's greeting to the American President although it was only 103 words. On this cable, it was found that the long lines caused distortion that limited the maximum signal speed to only 10-12 words per minute. A short time later, the cable was abandoned as unusable.

In 1887 Oliver Heaviside was able to prove that by increasing the inductance of the line by additionally inserted coils ( coiled line ), a distortion-free transmission of low-frequency signals through cables is possible even over longer distances. An increase in the impedance of the cable was achieved and thus a better adaptation to the signal sources.

The regular installation of coils in a submarine cable was not possible at that time. However, an increase in impedance for distortion-free signal transmission could be achieved by wrapping the line with an iron wire. The Danish telegraph engineer Carl Emil Krarup , who developed the Krarup cable named after him, did the first work around 1900 . However, the permeability of iron was insufficient to compensate for the distortion of a transatlantic cable over 4500 km without interruption. Therefore, targeted research was carried out into a material that has a greater permeability than iron. This search then led in 1914 to the discovery of a highly permeable nickel-iron alloy by Gustav Elmen at Bell Laboratories , USA.

This alloy originally consisted of 78.5% nickel and 21.5% iron and had a permeability of 90,000. It was thus 200 times more magnetically permeable than the best iron compound of that time. Elmen called this alloy "Permalloy", which means something like "Permeable Alloy". Later, in 1923, he also found out that the permeability could be further increased significantly by heat treatment.

Landing of the submarine cable on New York Beach, 1924. It crossed the Azores to Málaga, Spain. The total length of this connection was 4704 nautical miles.

The first application of a permalloy-wrapped and shielded submarine cable was the connection between New York and the Azores in 1924. It has been shown that the signal speed in this cable, whose conductor was wrapped with permalloy, was four times faster than in previous cables.

The patent rights for Permalloy were held by Western Electric , which was 100% owned by AT&T . However, the total manufacturing capacity of all US-based companies was not so large that they could meet the need for cables. The largest manufacturer of cables with around 70% market share was The Telegraph Construction and Maintenance Co. Ltd. , (now Telcon Metals Ltd. ) in Great Britain. Telcon found that permalloy was prone to breakage as it wrapped around the conductor. While searching for their own solution, the two Telcon scientists WS Smith and HJ Garnett found in 1923 that by adding copper , later chromium or molybdenum , the permalloy alloy could be more mechanically deformed without affecting the permeability . With a composition of 77% nickel, 16% iron, 5% copper and 2% chromium or molybdenum, American patent rights could also be circumvented with this new alloy. They named this new alloy mu-metal after the Greek symbol μ , which is used for permeability, and patented it. Since Telcon also had the experience and the machines for the Krarup cable, and a wire was even easier to process than a flat strip, the Telcon cables were provided with coiled Mu metal wire. 50 kilometers of the mu-metal as wire were required for every kilometer of the cable, creating a great demand for the alloy. In the first year of production Telcon produced 30 tons per week. During the Great Depression of the 1930s, that use for mu-metal declined, but many other uses were found in the electronics industry during World War II and after, particularly for shielding transformers , magnetometers, and MRI machines .

The alloy composition of the permalloy was adapted after the development of the mu-metal, so that today mu-metal and permalloy have equivalent properties.

## Individual evidence

1. ^ GA Berner: Illustrated specialist encyclopedia of watchmaking , keyword 'Permalloy', accessed on November 9, 2012.
2. Soft magnetic materials and semi-finished products . Brochure PHT-001, edition 2002, Vacuumschmelze , Hanau , p. 15.
3. a b c d Allen Green: 150 Years Of Industry & Enterprise At Enderby's Wharf . In: History of the Atlantic Cable and Undersea Communications . FTL design. 2004.
4. ^ History of the Atlantic Cable & Undersea Communications, Bill Glover, Cabot Strait Cable and 1857-58 Atlantic Cables, [1]
5. Oliver Heaviside: Electromagnetic Induction and its propagation. In: The Electrician. June 3, 1887
6. ^ Bragg, L. Electricity (London: G. Bell & Sons, 1943), pp. 212-213.
7. a b G. W. Elmen, HD Arnold, Permalloy, A New Magnetic Material of Very High Permeability, Bell System Tech., Volume 2, issue 3, pages101–111, publisher: American Tel. & Tel., USA, July 1923 [2 ]
8. ^ GW Elmen: Magnetic Alloys of Iron, Nickel, and Cobalt . In: American Tel. & Tel. (Ed.): Bell System Tech. J. . 15, No. 1, USA, January 1936, pp. 113-135.
9. ^ History of the Atlantic Cable & Undersea Communications, 1924 New York - Azores Cable, [3]
10. page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. Willoughby Statham Smith, Henry Joseph Garnett, New and improved magnetic alloys and their application in the manufacture of telegraphic and telephonic cables , granted July 27, 1926
11. US Patent 1582353 Willoughby Statham Smith, Henry Joseph Garnett, Magnetic Alloy , filed January 10, 1924, granted April 27, 1926
12. US Patent 1552769 Willoughby Statham Smith, Henry Joseph Garnett, Magnetic Alloy , filed January 10, 1924, granted September 8, 1925