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===Origin of magnetism===
One of the fundamental properties of an [[electron]] (besides that it carries charge) is that it has a [[Electron magnetic dipole moment|dipole moment]], i.e., it behaves itself as a tiny magnet. This dipole moment comes from the more fundamental property of the electron that it has quantum mechanical [[Spin (physics)|spin]]. The quantum mechanical nature of this spin causes the electron to only be able to be in two states, with the magnetic field either pointing "up" or "down" (for any choice of up and down). The spin of the electrons in atoms is the main source of ferromagnetism, although there is also a contribution from the [[planetary orbit|orbital]] [[angular momentum]] of the electron about the [[nucleus (atomic structure)|nucleus]]. When these tiny magnetic dipoles are aligned in the same direction, their individual magnetic fields add together to create a measurable macroscopic field.
However, in materials with a filled [[electron shell]], the total dipole moment of the electrons is zero because the spins are in up/down pairs. Only atoms with partially filled shells (i.e., unpaired spins) can have a net magnetic moment, so ferromagnetism only occurs 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.
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