Exchange interactions in magnetism and hyperfine fields

Abstract

So far we have discussed the effects of ferromagnetic and antiferromagnetic order in solids in terms of molecular fields, without attempting to explain the origins of the molecular fields. Dipole-dipole magnetic interactions between local magnetic moments are far too weak to explain the effects observed. Curie temperatures in strong ferromagnets can be more than 1000 K, so that a thermal energy of kT _C ≃10^-7 J per atom is necessary to destroy the magnetically ordered state. The dipole ineraction energy is of the order of µ ²_B /a³, about 10^-9 J per atom since the interatomic separation a is about 0.1 nm (1Å). The dipole interactions can therefore be at the most only a correction to the primary effect.