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Asked in GUJCET 2007 · Dia, para and ferromagnetism
Idea: a domain is a small region in which a very large number of atomic dipoles are locked parallel to one another, so the region is magnetised to saturation even with no field applied.
The locking is done by the quantum exchange interaction between neighbouring atoms, and only ferromagnetic materials - iron, cobalt, nickel, gadolinium - have it strongly enough.
In a paramagnetic material the atomic dipoles are permanent but uncoupled, so they stay randomly oriented until a field partly aligns them.
In a diamagnetic material the atoms carry no net moment at all; the field merely induces a small opposing one.
So domain formation is a necessary feature of ferromagnetism.
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