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Barkhausen effect 1

Barkhausen effect

Magnetization (J) or flux density (B) curve as a function of magnetic field intensity

(H) in ferromagnetic material. The inset shows Barkhausen jumps.

Domain wall motion with a Barkhausen jump

The Barkhausen effect is a name given to

the noise in the magnetic output of a

ferromagnet when the magnetizing force

applied to it is changed. Discovered by

German physicist Heinrich Barkhausen in

1919, it is caused by rapid changes of size of

magnetic domains (similarly magnetically

oriented atoms in ferromagnetic materials).

Barkhausen's work in acoustics and

magnetism led to the discovery, which

provided evidence that magnetization affects

whole domains of a ferromagnetic material,

rather than individual atoms alone. TheBarkhausen effect is a series of sudden

changes in the size and orientation of

ferromagnetic domains, or microscopic

clusters of aligned atomic magnets (spins),

that occurs during a continuous process of

magnetization or demagnetization. The

Barkhausen effect offered direct evidence

for the existence of ferromagnetic domains,

which previously had been postulated

theoretically. Heinrich Barkhausendiscovered that a slow, smooth increase of a

magnetic field applied to a piece of

ferromagnetic material, such as iron, causes

it to become magnetized, not continuously

but in minute steps.

Barkhausen noise

A coil of wire wound on the ferromagnetic

material can demonstrate the sudden,

discontinuous jumps in magnetization. The

sudden transitions in the magnetization of

the material produce current pulses in the

coil. These can be amplified to produce a series of clicks in a loudspeaker. This sounds as crackle, complete with

skewed pulses which sounds like candy being unwrapped, Rice Krispies, or a pine log fire. Hence the name

Barkhausen noise. Similar effects can be observed by applying only mechanical stresses (e.g. bending) to the

material placed in the detecting coil.

These magnetization jumps are interpreted as discrete changes in the size or rotation of ferromagnetic domains.

Some microscopic clusters of atomic spins aligned with the external magnetizing field increase in size by a suddenreversal of neighbouring spins; and, especially as the magnetizing field becomes relatively strong, other whole

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Barkhausen effect 2

domains suddenly turn into the direction of the external field. Simultaneously, due to exchange interactions the spins

tend to align themselves with their neighbours. The tension between the various pulls creates avalanching, where a

group of neighbouring domains will flip in quick succession to align with the external field. So the material

magnetizes neither gradually nor all at once, but in fits and starts.

Practical use

A set-up for non-destructive testing of ferromagnetic materials: green -

magnetising yoke, red - inductive sensor, greysample under test.

The amount of Barkhausen noise for a given

material is linked with the amount of

impurities, crystal dislocations, etc. and can

be a good indication of mechanical

properties of such a material. Therefore, the

Barkhausen noise can be used as a method

of non-destructive evaluation of the

degradation of mechanical properties in

magnetic materials subjected to cyclic

mechanical stresses (e.g. in pipeline

transport) or high-energy particles (e.g.

nuclear reactor) or materials such as

high-strength steels which may be subjected to damage from grinding. Schematic diagram of a simple

non-destructive set-up for such a purpose is shown on the right.

Barkhausen noise can also indicate physical damage in a thin film structure due to various nanofabrication processes

such as reactive ion etching or using an ion milling machine.[1]

References

External links

Barkhausen Effect (http://www.youtube.com/watch?v=iWdbRX5Nyg4) Video demonstrating the effect

Barkhausen Noise grinding burn and heat treat defect monitoring (http://www.stresstechgroup.com/content/

en/1034/1113/Barkhausen Noise Analysis.html)

http://www.stresstechgroup.com/content/en/1034/1113/Barkhausen%20Noise%20Analysis.htmlhttp://www.stresstechgroup.com/content/en/1034/1113/Barkhausen%20Noise%20Analysis.htmlhttp://www.youtube.com/watch?v=iWdbRX5Nyg4http://en.wikipedia.org/w/index.php?title=Ion_milling_machinehttp://en.wikipedia.org/w/index.php?title=Reactive_ion_etchinghttp://en.wikipedia.org/w/index.php?title=Nanofabricationhttp://en.wikipedia.org/w/index.php?title=Thin_filmhttp://en.wikipedia.org/w/index.php?title=Nuclear_reactorhttp://en.wikipedia.org/w/index.php?title=High-energy_particlehttp://en.wikipedia.org/w/index.php?title=Pipeline_transporthttp://en.wikipedia.org/w/index.php?title=Pipeline_transporthttp://en.wikipedia.org/w/index.php?title=Non-destructive_evaluationhttp://en.wikipedia.org/w/index.php?title=Mechanical_propertieshttp://en.wikipedia.org/w/index.php?title=Mechanical_propertieshttp://en.wikipedia.org/w/index.php?title=Dislocationhttp://en.wikipedia.org/w/index.php?title=File%3ABarkhausen_sensor.svghttp://en.wikipedia.org/w/index.php?title=Sensorhttp://en.wikipedia.org/w/index.php?title=Yoke

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Article Sources and Contributors 3

Article Sources and ContributorsBarkhausen effect Source: http://en.wikipedia.org/w/index.php?oldid=561929401 Contributors: Andreasmperu, BWDuncan, Hydrargyrum, K Eliza Coyne, Karol Langner, LightYear, Meisam,

Mild Bill Hiccup, Mmarre, Oli Filth, Pearle, Pegua, Physicistjedi, Pieter Kuiper, Pinestone, Pixelface, Rayc, RockMagnetist, Rossiya, Slawojarek, Slimey.limey, Stannered, The wub, Tone,

Topbanana, Zureks, 19 anonymous edits

Image Sources, Licenses and ContributorsImage:Barkhausen jumps.svg Source: http://en.wikipedia.org/w/index.php?title=File:Barkhausen_jumps.svg License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors:

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Image:Barkhausensprung.gif Source: http://en.wikipedia.org/w/index.php?title=File:Barkhausensprung.gifLicense: Public Domain Contributors: Denkwrdig, Denniss, Kersti Nebelsiek, Kri,

Mattes, Pieter Kuiper

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