Discovered by German physicist Heinrich Barkhausen in 1919, the Barkhausen effect is a name given to the noise which a ferromagnetic material makes when the magnetic force applied to it is changed. To demonstrate this a coil of wire wound on the ferromagnetic material is affected by sudden, discontinuous jumps in magnetization using a hand held magnet. The sudden alterations in the magnetization of the material produces current pulse in the coil. This is amplified to produce a series of clicks in a loudspeaker; this is also called as Barkhausen noise. Similar effects can be observed by applying only mechanical stresses (e.g. bending) to the material placed in the detecting coil.
Barkhausen effect has many important applications today. For example, 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, it is used as a method of non-destructive testing for the degradation of mechanical properties in magnetic materials. It can also indicate physical damage in a thin film structure due to various nano-fabrication processes such as reactive ion etching.