Roy Lewallen wrote:

Al is usually the value for low flux density. That is, it's the value
you'll have when the flux level is low. Permeability will drop from
there at high flux levels.


Not to nit-pick but the permeability of nearly all powdered iron
formulations actually rises with increasing flux levels (AC) and then
falls off. For #26 material (u=75), the effect is very much exagerated
with the permeability increasing nearly 300% at ~5000 Gauss and then
falling very quickly. However the permeability does drop for any value
of DC bias current and larger DC bias currents produce greater
reductions in permeability.

73, Larry Benko, W0QE