Barkhausen effect

Barkhausen's work in acoustics and magnetism 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 offered direct evidence for the existence of ferromagnetic domains, which previously had been postulated theoretically.

The energy loss due to the domain walls moving through these defects is responsible for the hysteresis curve of ferromagnetic materials.

Ferromagnetic materials with high coercivity 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.

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.

Replica of Barkhausen's original apparatus, consisting of an iron bar with a coil of wire around it (center) with the coil connected through a vacuum tube amplifier (left) to an earphone (not shown) . When the horseshoe magnet (right) is rotated, the magnetic field through the iron changes from one direction to the other, and the crackling Barkhausen noise is heard in the earphone.
Magnetization (J) or flux density (B) curve as a function of magnetic field intensity (H) in ferromagnetic material. The inset shows Barkhausen jumps.
Origin of the Barkhausen noise: as a domain wall moves it gets caught on a defect in the crystal lattice , then "snaps" past it, creating a sudden change in the magnetic field.
A set-up for non-destructive testing of ferromagnetic materials: green – magnetising yoke , red – inductive sensor , grey – sample under test.