solenoid wound pickup coil next to a selection of bolts and a steel rod

The Barkhausen effect — named after German Physicist Heinrich Barkhausen — is the term given to the noise output produced by a ferromagnetic material due to the change in size and orientation of its discrete magnetic domains under the influence of an external magnetic field. The domains are small: smaller than the microcrystalline grains that form the magnetic material, but larger than the atomic scale. Barkausen discovered that as a magnetic field was brought close to a ferrous material, the local magnetic field would flip around randomly, as the magnetic domains rearranged themselves into a minimum energy configuration and that this magnetic field noise could be sensed with an appropriately arranged pickup coil and an amplifier. In the short demonstration video below, this Barkhausen noise can be fed into an audio amplifier, producing a very illustrative example of the effect.

One example of practical use for this effect is with non-destructive testing and qualification of magnetic structures which may be subject to damage in use, such as in the nuclear industry. Crystalline discontinuities or impurities within a part under examination result in increased localized mechanical stresses, which could result in unexpected failure. The Barkhausen noise effect can be easily leveraged to detect such discontinuities and give the evaluator a sense of the condition of the part in question. All in all, a useful technique to know about!

If you were thinking that the Barkhausen is a familiar name, you may well be thinking about the Barkhausen stability criterion, which is fundamental to describing some of the conditions necessary for a linear feedback circuit to oscillate. We’ve covered such circuits before, such as this dive into bridge oscillators.

Thanks to {Keith] for the tip!