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

Heinrich Barkhausen

Heinrich Georg Barkhausen , born at Bremen, was a German physicist.Born into a patrician family in Bremen, he showed interest in natural sciences from an early age...

 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

Acoustics

Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...

 and magnetism

Magnetism

Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...

 led to the discovery, which provided evidence that magnetization affects whole domains of a ferromagnetic material, rather than individual atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s alone. The Barkhausen effect is a series of sudden changes in the size and orientation of ferromagnetic domains, or microscopic clusters of aligned atomic magnets (spin

Spin (physics)

In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

s), that occurs during a continuous process of magnetization

Magnetization

In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material...

 or demagnetization. The Barkhausen effect offered direct evidence for the existence of ferromagnetic domains, which previously had been postulated theoretically. Heinrich Barkhausen discovered 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

Coil

A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...

 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

Loudspeaker

A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

. This sounds as crackle, complete with skewed pulses which sounds like candy being unwrapped, Rice Krispies

Rice Krispies

Rice Krispies is a breakfast cereal that was created by Clayton Rindlisbacher for the Kellogg company, and later marketed by Kellogg's in 1927 and released to the public in 1928...

, 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 spin

Spin (physics)

In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

s aligned with the external magnetizing field increase in size by a sudden reversal of neighboring spins; and, especially as the magnetizing field becomes relatively strong, other whole domains suddenly turn into the direction of the external field. Simultaneously, due to exchange interactions the spins tend to align themselves with their neighbors. The tension between the various pulls creates avalanching, where a group of neighboring 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 ends

Practical use

The amount of Barkhausen noise for a given material is linked with the amount of impurities, crystal dislocation

Dislocation

In materials science, a dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials...

s, 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

Pipeline transport

Pipeline transport is the transportation of goods through a pipe. Most commonly, liquids and gases are sent, but pneumatic tubes that transport solid capsules using compressed air are also used....

) or high-energy particles (e.g. nuclear reactor

Nuclear reactor

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

) 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.

External links

• Barkhausen Effect Video demonstrating the effect