Editing Ferromagnetism (section)

===Origin of atomic magnetism===
One of the fundamental properties of an [[electron]] (besides that it carries charge) is that it has a [[Electron magnetic moment|magnetic dipole moment]], i.e., it behaves like a tiny magnet, producing a [[magnetic field]]. This dipole moment comes from a more fundamental property of the electron: its quantum mechanical spin. Due to its quantum nature, the spin of the electron can be in one of only two states, with the magnetic field either pointing "up" or "down" (for any choice of up and down). Electron spin in atoms is the main source of ferromagnetism, although there is also a contribution from the [[Atomic orbital|orbital]] [[angular momentum]] of the electron about the [[Atomic nucleus|nucleus]]. When these magnetic dipoles in a piece of matter are aligned (point in the same direction), their individually tiny magnetic fields add together to create a much larger macroscopic field.

However, materials made of atoms with filled [[electron shell]]s have a total dipole moment of zero: because the electrons all exist in pairs with opposite spin, every electron's magnetic moment is cancelled by the opposite moment of the second electron in the pair. Only atoms with partially filled shells (i.e., [[Unpaired electron|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 an otherwise unoccupied shell tend to have the same spin, thereby increasing the total dipole moment.

These [[unpaired electron|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{{snd}} leading to a macroscopic effect called [[paramagnetism]]. In ferromagnetism, however, the magnetic interaction between neighboring atoms' magnetic dipoles is strong enough that they align with ''each other'' regardless of any applied field, resulting in the [[spontaneous magnetization]] of so-called [[#Magnetic domains|domains]]. This results in the large observed [[magnetic permeability]] of ferromagnetics, and the ability of magnetically hard materials to form [[permanent magnets]].