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This is basically a physics question. If a sophisticated answer is needed, please go for it.

The standard explanation of magnetic permeability is something like this:

In a ferromagnetic material, there are many small regions of the material called domains, and each domain has its own magnetic field. When an external magnetic field is applied to the material, the magnetic moments of the domains tend to align in the direction of the applied field. However, in the absence of an external magnetic field, the magnetic moments of the domains tend to be randomly oriented, resulting in a net magnetic moment of zero.

So in other words, the domains are flipping to align with the external field, thus reinforcing it. Presumably, this has some quantum-mechanical-like behavior, because flipping each group of atoms will have a threshold energy. So they should flip in little groups of varying size and not in an orderly atom-by-atom manner (presumably).

This noisy domain flipping process is called the Barkhausen Effect.

So I have some questions:

  1. I have used some very aggressive LC filters before, with moderately strong signals (perhaps up to 25% of the inductor saturation point). Why have I never seen a measurable increase in noise due to Barkhausen Effect?

  2. For very weak signals, does the inductance of an inductor drop to a lower value since I'm not supplying a strong enough external field to flip the domains? If so, then is this a source of harmonic distortion in LC filters for example?

If the answer to #2 is NO - i.e. there is no lower limit that results in the collapse of permeability, then here's my followup question:

  1. What is the physical mechanism that allows inductors to operate in a very stable and linear manner when the applied signal is very weak and far below the coercivity?

Thanks for your thoughts!

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  • \$\begingroup\$ Here's a quote from 4th edition of Matter & Interactions, pg. 697: "However, in a few materials, notably iron, nickel, cobalt, and some alloys of these elements, the orbital and spin motions in neighboring atoms line up with each other and therefore produce a sizable magnetic field. These unusual materials are called “ferromagnetic.” The reason for the alignment can be adequately discussed only within the framework of quantum mechanics; basically the alignment is due to electric interactions between the atoms, not to the much weaker magnetic interactions." \$\endgroup\$ May 4, 2023 at 0:03

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The key here is to understand that domains do not flip. Domains shrink or grow as individual atoms at the edges of the domain flip.

The very wikipedia page you linked has a gif showing how the Barkhausen effect is not caused by domains flipping, but by domain walls getting "snagged" on crystal defects.

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