Electromagnetic testing emt-mft chapter 9b

Barkhausen noise
A coil 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. This sounds as crackle, complete with skewed
pulses which sounds like candy being unwrapped, Rice crispier, 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 spins aligned with the
external magnetizing field increase in size by a sudden reversal of neighbouring 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 neighbours. The
tension between the various pulls creates avalanching, where a group of neighbouring
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 starts.
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Barkhausen_effect