Editing Ferromagnetism (section)

===Exchange interaction===
{{Main|Exchange interaction}}
When two nearby atoms have unpaired electrons, whether the electron spins are parallel or antiparallel affects whether the electrons can share the same orbit as a result of the quantum mechanical effect called the [[exchange interaction]]. This in turn affects the electron location and the [[Coulomb force|Coulomb (electrostatic) interaction]] and thus the energy difference between these states.

The exchange interaction is related to the Pauli exclusion principle, which says that two electrons with the same spin cannot also be in the same spatial state (orbital). This is a consequence of the [[spin–statistics theorem]] and that electrons are [[fermions]]. Therefore, under certain conditions, when the [[atomic orbital|orbitals]] of the unpaired outer [[valence electron]]s from adjacent atoms overlap, the distributions of their [[electric charge]] in space are farther apart when the electrons have parallel spins than when they have opposite spins. This reduces the [[electrostatic energy]] of the electrons when their spins are parallel compared to their energy when the spins are antiparallel, so the parallel-spin state is more stable. This difference in energy is called the [[exchange energy]]. In simple terms, the outer electrons of adjacent atoms, which repel each other, can move further apart by aligning their spins in parallel, so the spins of these electrons tend to line up.

This energy difference can be orders of magnitude larger than the energy differences associated with the [[magnetic dipole–dipole interaction]] due to dipole orientation,<ref name=Chikazumi2>{{cite book |last=Chikazumi |first=Sōshin |title=Physics of ferromagnetism |year=2009 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-956481-1 |edition=2nd |others=English edition prepared with the assistance of C.&nbsp;D. Graham, Jr. |pages=129–130}}</ref> which tends to align the dipoles antiparallel. In certain doped semiconductor oxides, [[RKKY interaction]]s have been shown to bring about periodic longer-range magnetic interactions, a phenomenon of significance in the study of [[Spintronics|spintronic materials]].<ref>{{cite journal |last1=Assadi |first1=M.&nbsp;H.&nbsp;N. |last2=Hanaor |first2=D.&nbsp;A.&nbsp;H. |title=Theoretical study on copper's energetics and magnetism in TiO<sub>2</sub> polymorphs |journal= Journal of Applied Physics |year=2013 |volume=113 |issue=23 |pages=233913-1–233913-5 |doi=10.1063/1.4811539 |arxiv=1304.1854 |bibcode=2013JAP...113w3913A |s2cid=94599250}}</ref>

The materials in which the exchange interaction is much stronger than the competing dipole–dipole interaction are frequently called ''magnetic materials''. For instance, in iron (Fe) the exchange force is about 1,000 times stronger than the dipole interaction. Therefore, below the Curie temperature, virtually all of the dipoles in a ferromagnetic material will be aligned. In addition to ferromagnetism, the exchange interaction is also responsible for the other types of spontaneous ordering of atomic magnetic moments occurring in magnetic solids: antiferromagnetism and ferrimagnetism. There are different exchange interaction mechanisms which create the magnetism in different ferromagnetic,<ref>{{Cite journal |last1=García |first1=R. Martínez |last2=Bilovol |first2=V. |last3=Ferrari |first3=S. |last4=de la Presa |first4=P. |last5=Marín |first5=P. |last6=Pagnola |first6=M. |date=2022-04-01 |title=Structural and magnetic properties of a BaFe12O19/NiFe2O4 nanostructured composite depending on different particle size ratios |url=https://www.sciencedirect.com/science/article/pii/S030488532101132X |journal=Journal of Magnetism and Magnetic Materials |volume=547 |pages=168934 |doi=10.1016/j.jmmm.2021.168934 |s2cid=245150523 |issn=0304-8853}}</ref> ferrimagnetic, and antiferromagnetic substances—these mechanisms include [[Exchange interaction#Direct exchange interactions in solids|direct exchange]], [[RKKY interaction|RKKY exchange]], [[double exchange]], and [[superexchange]].