Ferromagnetism is the property of some materials, called ferromagnetic materials, to magnetize very intensely under the action of an external magnetic field and to remain magnetized for a long time when the field is canceled, thus becoming magnets. This property is maintained only below a certain temperature, called Curie temperature, above which the material behaves like a paramagnetic material. For iron, for example, this temperature is around 770 °C.
The magnetization of a ferromagnetic material can occur naturally or artificially, subjecting the material to a magnetic field. Natural ferromagnetic materials are, for example, magnetite, iron, cobalt, nickel and some transition metals. In ferromagnetic materials, the relative magnetic permeability of the material is not constant with the variation of the field, as instead occurs in diamagnetic materials and paramagnetic materials: the relationship between the magnetic induction field and the magnetic field is therefore not linear, nor even unique.
Ferromagnetic properties have a quantum origin and depend on the electronic structure of the materials and their crystalline structure. Ferromagnetism arises due to two effects from quantum mechanics: spin and the Pauli exclusion principle. Only atoms with partially filled shells (i.e., unpaired spins) can have a net magnetic moment, so ferromagnetism occurs only in materials with partially filled shells. Because of Hund’s rules, the first few electrons in a shell tend to have the same spin, thereby increasing the total dipole moment. These unpaired dipoles (often called simply “spins” even though they also generally include orbital angular momentum) tend to align in parallel to an external magnetic field, an effect called paramagnetism. Ferromagnetism involves an additional phenomenon, however: in a few substances, the dipoles tend to align spontaneously, giving rise to a spontaneous magnetization, even when there is no applied field.