Editing Magnetism (section)

== Types ==
[[File:Magnetism.svg|thumb|center|upright=3|Hierarchy of types of magnetism.<ref name=Meyers1>{{cite book |title=Introductory solid state physics |author=HP Meyers |url=https://books.google.com/books?id=Uc1pCo5TrYUC&pg=PA322 |page=362; Figure 11.1 |isbn= 9781420075021 |year=1997 |publisher=CRC Press |edition=2}}</ref>]]

=== Diamagnetism ===
{{main|Diamagnetism}}
Diamagnetism appears in all materials and is the tendency of a material to oppose an applied magnetic field, and therefore, to be repelled by a magnetic field. However, in a material with paramagnetic properties (that is, with a tendency to enhance an external magnetic field), the paramagnetic behavior dominates.<ref name=Westbrook>

{{cite book |title=MRI (Magnetic Resonance Imaging) in practice |author1=Catherine Westbrook |author2=Carolyn Kaut |author3=Carolyn Kaut-Roth |isbn=978-0-632-04205-0 |url=https://books.google.com/books?id=Qq1SHDtS2G8C&pg=PA217 |page=217 |edition=2|publisher=Wiley-Blackwell |year=1998}}

</ref> Thus, despite its universal occurrence, diamagnetic behavior is observed only in a purely diamagnetic material. In a diamagnetic material, there are no unpaired electrons, so the intrinsic electron magnetic moments cannot produce any bulk effect. In these cases, the magnetization arises from the electrons' orbital motions, which can be understood [[classical physics|classically]] as follows:

{{Blockquote|When a material is put in a magnetic field, the electrons circling the nucleus will experience, in addition to their [[Coulomb's law|Coulomb]] attraction to the nucleus, a [[Lorentz force]] from the magnetic field. Depending on which direction the electron is orbiting, this force may increase the [[centripetal force]] on the electrons, pulling them in towards the nucleus, or it may decrease the force, pulling them away from the nucleus. This effect systematically increases the orbital magnetic moments that were aligned opposite the field and decreases the ones aligned parallel to the field (in accordance with [[Lenz's law]]). This results in a small bulk magnetic moment, with an opposite direction to the applied field.}}

This description is meant only as a [[heuristic]]; the [[Bohr–Van Leeuwen theorem]] shows that diamagnetism is impossible according to classical physics, and that a proper understanding requires a [[quantum mechanics|quantum-mechanical]] description.

All materials undergo this orbital response. However, in paramagnetic and ferromagnetic substances, the diamagnetic effect is overwhelmed by the much stronger effects caused by the unpaired electrons.

=== Paramagnetism ===
{{main|Paramagnetism}}
In a paramagnetic material there are unpaired electrons; i.e., [[atomic orbital|atomic]] or [[molecular orbital]]s with exactly one electron in them. While paired electrons are required by the [[Pauli exclusion principle]] to have their intrinsic ('spin') magnetic moments pointing in opposite directions, causing their magnetic fields to cancel out, an unpaired electron is free to align its magnetic moment in any direction. When an external magnetic field is applied, these magnetic moments will tend to align themselves in the same direction as the applied field, thus reinforcing it.

=== Ferromagnetism ===
{{main|Ferromagnetism}}
A ferromagnet, like a paramagnetic substance, has unpaired electrons. However, in addition to the electrons' intrinsic magnetic moment's tendency to be parallel to an applied field, there is also in these materials a tendency for these magnetic moments to orient parallel to each other to maintain a lowered-energy state. Thus, even in the absence of an applied field, the magnetic moments of the electrons in the material spontaneously line up parallel to one another.

Every ferromagnetic substance has its own individual temperature, called the [[Curie temperature]], or Curie point, above which it loses its ferromagnetic properties. This is because the thermal tendency to disorder overwhelms the energy-lowering due to ferromagnetic order.

Ferromagnetism only occurs in a few substances; common ones are [[iron]], [[nickel]], [[cobalt]], their [[alloy]]s, and some alloys of [[rare-earth]] metals.

==== Magnetic domains ====

{{main|Magnetic domains}}

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The magnetic moments of atoms in a [[Ferromagnetism|ferromagnetic]] material cause them to behave something like tiny permanent magnets. They stick together and align themselves into small regions of more or less uniform alignment called [[magnetic domains]] or [[Weiss domains]]. Magnetic domains can be observed with a [[magnetic force microscope]] to reveal magnetic domain boundaries that resemble white lines in the sketch. There are many scientific experiments that can physically show magnetic fields.

When a domain contains too many molecules, it becomes unstable and divides into two domains aligned in opposite directions so that they stick together more stably.

When exposed to a magnetic field, the domain boundaries move, so that the domains aligned with the magnetic field grow and dominate the structure (dotted yellow area), as shown at the left. When the magnetizing field is removed, the domains may not return to an unmagnetized state. This results in the ferromagnetic material's being magnetized, forming a permanent magnet.

When magnetized strongly enough that the prevailing domain overruns all others to result in only one single domain, the material is [[Saturation (magnetic)|magnetically saturated]]. When a magnetized ferromagnetic material is heated to the [[Curie point]] temperature, the molecules are agitated to the point that the magnetic domains lose the organization, and the magnetic properties they cause cease. When the material is cooled, this domain alignment structure spontaneously returns, in a manner roughly analogous to how a liquid can [[freezing|freeze]] into a crystalline solid.

=== Antiferromagnetism ===
[[File:Antiferromagnetic ordering.svg|thumb|Antiferromagnetic ordering]]
{{main|Antiferromagnetism}}
In an [[antiferromagnet]], unlike a ferromagnet, there is a tendency for the intrinsic magnetic moments of neighboring valence electrons to point in ''opposite'' directions. When all atoms are arranged in a substance so that each neighbor is anti-parallel, the substance is '''antiferromagnetic'''. Antiferromagnets have a zero net magnetic moment because adjacent opposite moment cancels out, meaning that no field is produced by them. Antiferromagnets are less common compared to the other types of behaviors and are mostly observed at low temperatures. In varying temperatures, antiferromagnets can be seen to exhibit diamagnetic and ferromagnetic properties.

In some materials, neighboring electrons prefer to point in opposite directions, but there is no geometrical arrangement in which ''each'' pair of neighbors is anti-aligned. This is called a [[spin canting|canted antiferromagnet]] or [[spin ice]] and is an example of [[geometrical frustration]].

=== Ferrimagnetism ===
[[File:Ferrimagnetic ordering.svg|thumb|[[Ferrimagnetic]] ordering]]
{{main|Ferrimagnetism}}
Like ferromagnetism, '''ferrimagnets''' retain their magnetization in the absence of a field. However, like antiferromagnets, neighboring pairs of electron spins tend to point in opposite directions. These two properties are not contradictory, because in the optimal geometrical arrangement, there is more magnetic moment from the sublattice of electrons that point in one direction, than from the sublattice that points in the opposite direction.

Most [[Ferrite (magnet)|ferrites]] are ferrimagnetic. The first discovered magnetic substance, [[magnetite]], is a ferrite and was originally believed to be a ferromagnet; [[Louis Néel]] disproved this, however, after discovering ferrimagnetism.

=== Superparamagnetism ===
[[File:Magnetic orders.webm|thumb|Magnetic orders: comparison between ferro, antiferro and ferrimagnetism]]
{{Main|Superparamagnetism}}
When a ferromagnet or ferrimagnet is sufficiently small, it acts like a single magnetic spin that is subject to [[Brownian motion]]. Its response to a magnetic field is qualitatively similar to the response of a paramagnet, but much larger.

=== Nagaoka magnetism ===
Japanese physicist Yosuke Nagaoka conceived of a type of magnetism in a square, two-dimensional lattice where every lattice node had one electron. If one electron was removed under specific conditions, the lattice's energy would be minimal only when all electrons' spins were parallel.

A variation on this was achieved experimentally by arranging the atoms in a triangular [[Moiré pattern|moiré]] lattice of [[molybdenum diselenide]] and [[tungsten disulfide]] monolayers. Applying a weak magnetic field and a voltage led to ferromagnetic behavior when 100–150% more electrons than lattice nodes were present. The extra electrons delocalized and paired with lattice electrons to form doublons. Delocalization was prevented unless the lattice electrons had aligned spins. The doublons thus created localized ferromagnetic regions. The phenomenon took place at 140 millikelvins.<ref>{{Cite magazine |last=Greshko |first=Michael |date=January 20, 2024 |title=Scientists Just Discovered a New Type of Magnetism |url=https://www.wired.com/story/scientists-discovered-new-type-magnetism-physics-electrons/ |access-date=2024-02-08 |magazine=Wired |language=en-US |issn=1059-1028}}</ref>

=== Other types of magnetism ===
* [[Metamagnetism]]
* [[Molecule-based magnets]]
* [[Single-molecule magnet]]
* [[Amorphous magnet]]