3.2. Magnetic Barkhausen noise
Different from the MFL technique developed for inspecting 
the macro-defects, the magnetic Barkhausen noise (MBN) 
technique is usually used to determine the magnetic easy axis [66], 
the residual and applied stresses [67] and thegrainsize [68] of
ferro magnetic materials.
In 1919 Barkhausen [69] found that when a ferro magnetic
material was magnetized by an increasing field, the noise in the
form of voltage pulses was generated in a coil placed near 
the material. Fig. 5a shows a schematic of the Barkhausen noise
during the magnetization process.The test system is presented
in Fig. 5b. From the microstructural point of view, the development 
of Barkhausen noise is due to the abrupt motion of 180
domains across local pinning sites [70–72].
The parameter commonly used in the analysis of the detected
MBN signal voltage V is the MBN energy defined as
MBNenergy ¼ A
X
events
Z
V2 dt: