The origin of magnetic ordering is the quantum mechanical exchange effect. The physics behinds the exchange effect is the coulombic interactions between the two spins. It depends on whether the spins from a singlet (or) a triplet.
The singlet state has symmetric wave function in real space, where as triplet has anti-symmetric wave function is real space. Due to this electrons stay out of each other way when they are in triplet state. as a result of which the coulombic interaction between two electrons is less when they in triplet state, then they are in singlet state.
So the interaction energy (or) exchange interaction is depends on the relative orientation of the spins of the electrons. This interactions, the spins of the electrons is known as exchange interaction which leads to the molecular field.
Ferro magnetic materials exhibits spontaneous magnetization even in the absence of externally applied field (Hext = 0) it means that atomic magnetic moments gets aligned even in the absence of applied magnetic field (Hext = 0). It implies that there is a strong internal field exists within the magnetic material. Which makes the atomic magnetic moments aligned with one another.