barkhausen_noise

# Barkhausen noise

 Stan Zurek, Barkhausen noise, Encyclopedia Magnetica https://E-Magnetica.pl/barkhausen_noise

Barkhausen noise (BN) - the phenomenon of rapid changes of positions of domain walls during the process of magnetisation of a ferromagnetic material.1)2)3)

Barkhausen noise is caused by rapid changes of flux density B due to domain wall movements - this causes high-frequency noise-like changes in induced voltage V

These sudden jumps (also referred to as Barkhausen jumps) can be made audible by suppressing the large voltage induced in a search coil (with a high-pass filter) and amplifying the frequencies in the audible range (see the recording of audible noise with the animation).1)4)

Barkhausen noise was discovered by Heinrich Barkhausen in 1919.5)

A phenomenon similar to magnetic Barkhausen noise is also present in ferroelectric materials, which have ferroelectric domains and hence ferroelectric domain walls.6)7)

Recording of a real Barkhausen noise in grain-oriented electrical steel (with sound) (link to video file)
Barkhausen noise (BN) activity is greater at intervals during which the changes of flux density B are the most rapid

### Other types of analysis

Duration of Barkhausen events is correlated with their amplitude. One local Barkhausen jump can initiate others and the whole such sequence is sometimes referred to as Barkhausen avalanche.13)

The duration of avalanches can vary, and they can be analysed from the viewpoint of duration or frequency components. A whole range of analyses can be used even within the same study of the Barkhausen noise phenomenon.13)

Statistical analysis of Barkhausen avalanches in polycrystalline NiFe films of different thicknesses, from 20 to 1000 nm: a) distributions of avalanche sizes measured at 50 mHz, b) similar plot for the distributions of avalanche durations, c) average size as a function of the avalanche duration, d) power spectra.13) by F. Bohn, G. Durin, M.A. Correa, N.R. Machado, R.D. Della Pace, C. Chesman, R.L. Sommer, CC-BY-4.0

### Magneto-acoustic emissions

If the moving domain walls separate domains which are not in the opposing directions (0-180°), but for example at 90° to each other, then the changes in domain wall position can cause changes of dimensions of the material due to magnetostriction.

Such low-amplitude local vibrations of the material are known as magnetoacoustic emissions (MAE). The frequency spectrum for studying such phenomenon is similar to the Barkhausen noise, and also the type of analysis is similar, for example by plotting the power spectrum. However, the detection is carried out with a very sensitive microphone or acceleration sensor, rather than an inductive coil.4)

Simplified block diagram of signal processing for magneto-acoustic emissions4)

## Non-destructive testing

Barkhausen noise activity is affected by crystallographic structure and defects in the given material. Materials exposed to mechanical stress can deform thus increasing the number of internal defects. Also other processes such as neutron irradiation in nuclear plants can degrade the crystallographic arrangements in the steel exposed to such radiation.

The image below shows an example of Barkhausen noise activity in two samples exposed to different mechanical stress, so that the elastic deformation was ε=2.5% and 15%, respectively. In the sample with larger deformation the BN activity is visibly reduced, and this can be correlated with the amount of damage sustained by the given steel.14)

Reduced Barkhausen noise activity in material exposed to larger deformation14) by M. Pitoňák, M. Neslušan, P. Minárik, J. Čapek, K. Zgútová, M. Jurkovič, T. Kalina, CC-BY-4.0
System for measuring residual mechanical stresses in rails by means of Barkhausen noise15) by Y.-I. Hwang, Y.-I. Kim, D.-C. Seo, M.-K. Seo, W.-S. Lee, S. Kwon, K.-B. Kim, CC-BY-4.0

Detection of mechanical properties through measurement of Barkhausen noise is beneficial, because it can be carried out on the surface of the material, without the need for cutting out a sample - hence it belongs to the class of non-destructive testing. The applicability of the method is limited, because the Barkhausen noise cannot be measured or correlated to the material damage in an absolute way.

Nonetheless, there are commercial devices capable of performing non-destructive measurements in an automated way. The excitation is typically applied by a small U-shaped magnetising yoke, and the sensing is carried out by pick-up coils, with processing and filtering similar to as described above. Parameters such as degradation in strength, increase in hardness or embrittlement can be automatically quantified to some extent.

XYZ scanner with transducer: a) block diagram, b) photo, c) 3D view of the transducer16) by M. Maciusowicz, G. Psuj, CC-BY-4.0

However, the correlation between Barkhausen noise and the mechanical properties of a given magnetic sample is not strict, and cannot be quantified independently of a material. It is therefore not possible to calibrate such system for a generic measurement.

Instead, a comparative measurement has to be carried out, when a known “good” sample is available for calibration. For example, degradation of surface of gears made of magnetic steel can be detected.17) In such applications the quality and thermal pre-processing is well known for the “good” steel and degradation with the Barkhausen noise system can give reliable results.

The BN method can be used for assessment of a large surface area for example by employing the scanning methods.16) A small-size detection head can be automatically moved around a large surface to perform the “scanning” action, and a computerised system can collate, analyse and display all the data accordingly.