Certain materials posses permanent magnetic moments in
the absence of an external magnetic field. This is known as
ferromagnetism.
Permanent magnetic moments in ferromagnetic materials
arise due to uncancelled electron spins by virtue of their
electron structure.
The coupling interactions of electron spins of adjacent atoms
cause alignment of moments with one another.
The origin of this coupling is attributed to the electron
structure. Ferromagnetic materials like Fe (26 – [Ar] 4s23d6)
have incompletely filled d orbitals and hence unpaired electron
spins.