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Asked in GSEB Board March 2023 · Dia, para and ferromagnetism
Idea: in a ferromagnet the atomic dipoles are locked parallel inside domains, and a modest field can swing whole domains round.
That gives a very large magnetisation for a small H, so χ1 and μᵣ=1+χ reaches 10³ to 10⁵: the permeability is high.
When the field is removed the domains do not go back to their random arrangement, so a large magnetisation remains: the retentivity is high as well.
How easily that remaining magnetisation can be destroyed - the coercivity - depends on the material: soft iron gives it up easily, steel does not. Both are ferromagnetic.
So ferromagnetic materials have high permeability and high retentivity.
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