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 in the size of

magnetic domain A magnetic domain is a region within a magnetic material in which the magnetization is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction. When c ...

s (similarly magnetically oriented atoms in ferromagnetic materials). Barkhausen's work in

acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician ...

and

magnetism Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, ...

led to the discovery, which became the main piece of experimental evidence supporting the domain theory of ferromagnetism proposed in 1906 by Pierre-Ernest Weiss. The Barkhausen effect is a series of sudden changes in the size and orientation of ferromagnetic domains, or microscopic clusters of aligned atomic magnets ( spins), that occur during a continuous process of

magnetization In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Accordingly, physicists and engineers usually define magnetization as the quanti ...

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

When an external magnetizing field through a piece of

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagne ...

material is changed, for example by moving a magnet toward or away from an

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

bar, the magnetization of the material changes in a series of discontinuous changes, causing "jumps" in the magnetic flux through the iron. These can be detected by winding a coil of wire around the bar, attached to an amplifier and loudspeaker. The sudden transitions in the magnetization of the material produce current pulses in the coil, which when amplified produce a sound in the

loudspeaker A loudspeaker (commonly referred to as a speaker or, more fully, a speaker system) is a combination of one or more speaker drivers, an enclosure, and electrical connections (possibly including a crossover network). The speaker driver is an ...

. This makes a crackling sound, which has been compared to candy being unwrapped, Rice Krispies, or the sound of a log fire. This sound, first discovered by German physicist Heinrich Barkhausen, is called 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 caused by discrete changes in the size or rotation of ferromagnetic domains. Domains change size by the domain walls moving within the crystal lattice in response to changes in the magnetic field, by the process of dipoles near the wall changing spin to align with spins in the neighboring domain. In a perfect crystal lattice this can be a continuous process, but in actual crystals local defects in the lattice, such as impurity atoms or dislocations in the structure form temporary barriers to the change of spin, causing the domain wall to be hung up on the defect. When the change in magnetic field becomes strong enough to overcome the local energy barrier at the defect, it causes a group of atoms to flip their spin at once, as the domain wall "snaps" past the defect. This sudden change in magnetization causes a transient change in magnetic flux through the bar, which is picked up by the coil as a "click" in the earphone. The energy loss due to the domain walls moving through these defects is responsible for the

hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...

curve of ferromagnetic materials. Ferromagnetic materials with high

coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming Magnetization, demagnetized. Coercivity is usual ...

often have more of these defects, so they produce more Barkhausen noise for a given magnetic flux change, while materials with low coercivity, such as silicon steel transformer laminations, are processed to eliminate defects, so they produce little Barkhausen noise.

Practical use

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

dislocation In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to sli ...

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 A pipeline is a system of Pipe (fluid conveyance), pipes for long-distance transportation of a liquid or gas, typically to a market area for consumption. The latest data from 2014 gives a total of slightly less than of pipeline in 120 countries ...

) or high-energy particles (e.g.

nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...

) 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. The Wiegand effect is a macroscopic extension of the Barkhausen effect, as the special treatment of the Wiegand wire causes the wire to act macroscopically as a single large magnetic domain. The numerous small high-coercivity domains in the Wiegand wire outer shell switch in an avalanche, generating the Wiegand effect's rapid magnetic field change.

References

External links

Barkhausen Effect
Video demonstrating the effect

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What is Barkhausen noise
Ferromagnetism 1919 in science