The ‘Barkhausen effect’ is termed for any kind of noise (disturbances in the field of physics) in the magnetic output of any ferromagnet when it is subjected to any kind of change in the magnetizing force. The Barkhausen effect was discovered by a German physicist named Heinrich Barkhausen in the year 1919 and thus was named after him. This effect is said to be caused by the rapid changes in the size of magnetic domains and the atoms which are magnetically oriented inside the ferromagnetic materials.

HISTORY

Barkhausen’s great and widely approached work in the field of magnetism led to the discovery, which later became the main part of the experimental evidence in support of the domain theory of ferromagnetism that was proposed by Pierre-Ernest Weiss in the year 1906. The Barkhausen effect is studied to be a series of sudden changes in the size and orientation of some ferromagnetic domains, or small microscopic groups of aligned atomic magnets which are also called spin. This sudden change is the one that occurs during a continuous process of magnetization (being magnetized) or demagnetization (removing the magnetic charge).

The Barkhausen effect offered good direct evidence for the existence of the ferromagnetic domains, which previously had been just written theoretically. Heinrich Barkhausen while on the way of his discovery, discovered that a slow and smooth increase in a magnetic field when applied to a piece of any ferromagnetic material (such as iron) causes it to become magnetized. This magnetization is not at all continuous but in minute small steps.

When the external magnetizing field through the given piece of a ferromagnetic material is to be changed it can be changed by moving a magnet towards or away from an iron bar. This way the magnetization of the material changes in a series of some discontinuous changes. This in turn causes jumps in the magnetic flux through the iron rod or bar. These can be detected by winding a small coil of wire around the bar which is then attached to an amplifier and also a loudspeaker. The sudden transitions in the magnetization of the material would produce a wave kind of current pulses in the coil, which when amplified (connected to an amplifier) produces a sound and we can easily hear the sound with the help of the loudspeaker. This sound that we hear usually is a crackling sound, which has been compared to be produced when candy is being unwrapped, or of the sound of a log fire. This sound was also first discovered by the same person Heinrich Barkhausen and hence this sound is called the Barkhausen noise. Similar effects can be observed by applying only some kind of mechanical stresses like bending and shaking to the material placed in the detecting coil.

SCIENTIFIC REFERENCE

These magnetization jumps are said to be the discrete, and not continuous, changes in the size or rotation of ferromagnetic domains. Some small groups of atomic spins that are aligned with the external magnetizing field increase in size with the help of a sudden reversal in the surrounding spins. In case when the magnetizing field becomes sufficiently strong, other whole domains suddenly turn into the direction of the external magnetic field. Simultaneously due to exchange interactions of the domains the spins tend to align themselves with their corresponding neighbors.

The amount of Barkhausen noise that is produced for a given material is generally linked with the number of impurities present in it, crystal dislocations, etc. and can be a good indication of some of the very important mechanical properties of such material. Therefore, the Barkhausen noise can be used as a good and effective method of non-destructive evaluation of the degraded mechanical properties in magnetic materials which are subjected to cyclic mechanical stresses or particles with high energy or materials such as high strength steels which may be subjected to damage to some extend while grinding.

PRACTICAL INTERVENTION

Barkhausen noise is also said to indicate physical damage in a thin film structure due to various small and large processes. The Barkhausen effect makes the concept of magnetic domains much easier for us to get them noticed. During the experiments that were carried out in 1919 that involved magnetism and acoustics, Heinrich Barkhausen also gave satisfactory evidence that iron and other ferromagnetic materials are magnetized in small and fairly distinct intervals rather than smoothly and continuously, as had been given in theory.

It consisted of a coil, amplifier, and speaker as already stated. Any signal that is picked up by the amplifier was sent to a speaker, which aided Barkhausen to hear a progression of clicking noises whenever he used to move the magnet. The sound results by the shifting of what is known as the magnetic domains in the iron bar. Magnetic domains have atoms and each has its tiny magnetic field where are all are said to be aligned in the same direction. When the bar magnet is moved near or away from the core, those domains inside the iron gradually realign themselves with the field of the magnet.

Due to the effect of electromagnetic induction, the shifting of a domain creates some kind of change in the magnetic field around the iron, and that changing the magnetic field causes a current in the surrounding coil which can be easily detected by the amplifier. The measurement of Barkhausen noise is a kind of non-destructive testing method that makes the light to shed on the microstructure of ferromagnetic materials. These materials also contain permanent atomic magnetic dipoles that align themselves to point in the same direction. This effect usually comes into use when we want to detect any importance in any ferromagnetic substance for any kind of disturbance in such waves.

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