There are at least three different mechanisms which can give rise to ferromagnetic order in iron.
- First is due to the band electrons called band magnetism or itinerant magnetism which is an exchange interaction between conduction electrons.
Second is indirect exchange i.e. exchange between unpaired d electrons and conduction electrons.
The third is the direct exchange between localized magnetic moments of two neighbouring Fe ions as described by the Heisenberg model.
Which one of them is responsible for ferromagnetism in iron (and also cobalt and nickel) and why? I expect that the third effect would be least because d orbitals are inner orbitals and do not have much overlap.
I read this, this, this and the question titled "What is the difference between a localized and itinerant magnetism?". None seem to address my concern.