Editing Ferromagnetism (section)

==Materials==<!-- [[Ferromagnetic materials]] redirects here -->
{{See also|Category:Ferromagnetic materials}}

{| class="wikitable" style="float:right;margin:0 0 1em 1em;"
|+ Curie temperatures for some crystalline ferromagnetic and ferrimagnetic materials<ref>{{cite book|last=Kittel|first=Charles|author-link=Charles Kittel|title=[[Introduction to Solid State Physics]]|edition=sixth|publisher=[[John Wiley and Sons]]|year=1986|isbn=0-471-87474-4}}</ref><ref>{{cite journal|journal = IRM Quarterly|year = 2000|volume = 10|issue = 3|page = 6|author = Jackson, Mike|publisher = Institute for Rock Magnetism|url = http://www.irm.umn.edu/quarterly/irmq10-3.pdf|title = Wherefore Gadolinium? Magnetism of the Rare Earths|access-date = 2016-08-08|archive-url = https://web.archive.org/web/20170712151422/http://www.irm.umn.edu/quarterly/irmq10-3.pdf|archive-date = 2017-07-12}}</ref>
|-
! Material
! Curie <br />{{abbr|temp.|temperature}} (K)
|-
| [[Cobalt|Co]]
| 1388
|-
| [[Iron|Fe]]
| 1043
|-
| [[Iron(III) oxide|Fe<sub>2</sub>O<sub>3</sub>]]{{efn|name=fi|Ferrimagnetic material}}
| 948
|-
| [[Trevorite|NiOFe<sub>2</sub>O<sub>3</sub>]]{{efn|name=fi}}
| 858
|-
| [[Cuprospinel|CuOFe<sub>2</sub>O<sub>3</sub>]]{{efn|name=fi}}
| 728
|-
| [[Magnesioferrite|MgOFe<sub>2</sub>O<sub>3</sub>]]{{efn|name=fi}}
| 713
|-
| [[Bismanol|MnBi]]
| 630
|-
| [[Nickel|Ni]]
| 627
|-
| [[neodymium magnet|Nd<sub>2</sub>Fe<sub>14 </sub>B]] 
| 593
|-
| Mn[[Antimony|Sb]]
| 587
|-
| [[Jacobsite|MnOFe<sub>2</sub>O<sub>3</sub>]]{{efn|name=fi}}
| 573
|-
| [[Yttrium iron garnet|Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>]]{{efn|name=fi}}
| 560
|-
| [[Chromium(IV) oxide|CrO<sub>2</sub>]]
| 386
|-
| [[Manganese arsenide|MnAs]]
| 318
|-
| [[Gadolinium|Gd]]
| 292
|-
| [[Terbium|Tb]]
| 219
|-
| [[Dysprosium|Dy]]
| 88
|-
| [[Europium(II) oxide|EuO]]
| 69
<!-- The numbers in this table currently come from Kittel, as referenced in the text. Please do not add new numbers without adding the corresponding reference. -->
|-
| colspan=2 | {{notelist}}
|}

Ferromagnetism is an unusual property that occurs in only a few substances. The common ones are the [[transition metal]]s [[iron]], [[nickel]], and [[cobalt]], as well as their [[alloy]]s and alloys of [[rare-earth metal]]s. It is a property not just of the chemical make-up of a material, but of its crystalline structure and microstructure. Ferromagnetism results from these materials having many unpaired electrons in their d-[[Block (periodic table)|block]] (in the case of iron and its relatives) or f-block (in the case of the rare-earth metals), a result of [[Hund's rule of maximum multiplicity]]. There are ferromagnetic metal alloys whose constituents are not themselves ferromagnetic, called [[Heusler alloy]]s, named after [[Fritz Heusler]]. Conversely, there are non-magnetic alloys, such as types of [[stainless steel]], composed almost exclusively of ferromagnetic metals.

Amorphous (non-crystalline) ferromagnetic metallic alloys can be made by very rapid [[quenching]] (cooling) of an alloy. These have the advantage that their properties are nearly isotropic (not aligned along a crystal axis); this results in low [[coercivity]], low [[hysteresis]] loss, high permeability, and high electrical resistivity. One such typical material is a transition metal-[[metalloid]] alloy, made from about 80% transition metal (usually Fe, Co, or Ni) and a metalloid component ([[Boron|B]], [[Carbon|C]], [[Silicon|Si]], [[Phosphorus|P]], or [[Aluminium|Al]]) that lowers the [[melting point]].
<!--changes here are not correct; commenting out until sorted out on talk: One example of such an amorphous alloy is Fe<sub>80</sub>B<sub>20</sub> (Metglas 2605) which has a Curie temperature of 647&nbsp;K and a room-temperature (300&nbsp;K) saturation magnetization of 1.58&nbsp;[[tesla (unit)|teslas]] (1,257&nbsp;[[gauss]]), compared with 1,043&nbsp;K and 2.15&nbsp;T (1,707&nbsp;gauss) for pure iron from above. The melting point, or more precisely the glass transition temperature, is only 714&nbsp;K for the alloy versus a melting point of 1,811&nbsp;K for pure iron.-->

A relatively new class of exceptionally strong ferromagnetic materials are the [[rare-earth magnet]]s. They contain [[lanthanide]] elements that are known for their ability to carry large magnetic moments in well-localized [[F-orbital|f-orbitals]].

The table lists a selection of ferromagnetic and ferrimagnetic compounds, along with their [[Curie temperature]] (''T''<sub>C</sub>), above which they cease to exhibit spontaneous magnetization.

=== Unusual materials ===
Most ferromagnetic materials are metals, since the conducting electrons are often responsible for mediating the ferromagnetic interactions. It is therefore a challenge to develop ferromagnetic insulators, especially [[Multiferroics|multiferroic]] materials, which are both ferromagnetic and [[ferroelectric]].<ref>{{Cite journal |last=Hill |first=Nicola A. |date=2000-07-01 |title=Why Are There so Few Magnetic Ferroelectrics? |journal=The Journal of Physical Chemistry B |volume=104 |issue=29 |pages=6694–6709 |doi=10.1021/jp000114x |issn=1520-6106}}</ref>

A number of [[actinide]] compounds are ferromagnets at room temperature or exhibit ferromagnetism upon cooling. [[Plutonium|Pu]][[Phosphorus|P]] is a paramagnet with [[Cubic crystal system|cubic symmetry]] at [[room temperature]], but which undergoes a structural transition into a [[Tetragonal crystal system|tetragonal]] state with ferromagnetic order when cooled below its {{Nowrap|1=''T''<sub>C</sub> = 125 K}}. In its ferromagnetic state, PuP's [[easy axis]] is in the ⟨100⟩ direction.<ref name=Lander>{{cite journal |author=Lander G. H. |author2=Lam D. J.  |title=Neutron diffraction study of PuP: The electronic ground state |journal=Phys. Rev. B |year=1976 |volume=14 |issue=9 |pages=4064–4067 |doi=10.1103/PhysRevB.14.4064 |bibcode=1976PhRvB..14.4064L }}</ref>

In [[Neptunium|Np]]Fe<sub>2</sub> the easy axis is ⟨111⟩.<ref name=Aldred>{{cite journal |author=Aldred A. T. |author2=Dunlap B. D. |author3=Lam D. J. |author4=Lander G. H. |author5=Mueller M. H. |author6=Nowik I. |title=Magnetic properties of neptunium Laves phases: NpMn<sub>2</sub>, NpFe<sub>2</sub>, NpCo<sub>2</sub>, and NpNi<sub>2</sub> |journal=Phys. Rev. B |year=1975 |volume=11 |issue=1 |pages=530–544 |doi=10.1103/PhysRevB.11.530 |bibcode=1975PhRvB..11..530A }}</ref> Above {{nowrap|''T''<sub>C</sub> ≈ 500&nbsp;K}}, NpFe<sub>2</sub> is also paramagnetic and cubic. Cooling below the Curie temperature produces a [[rhombohedral]] distortion wherein the rhombohedral angle changes from 60° (cubic phase) to 60.53°. An alternate description of this distortion is to consider the length {{Mvar|c}} along the unique trigonal axis (after the distortion has begun) and {{Mvar|a}} as the distance in the plane perpendicular to {{Mvar|c}}. In the cubic phase this reduces to {{nowrap|{{Mvar|c}}/{{Mvar|a}} {{=}} 1.00}}. Below the Curie temperature, the lattice acquires a distortion

: <math>\frac{c}{a} - 1 = -(120 \pm 5) \times 10^{-4},</math>

which is the largest strain in any [[actinide]] compound.<ref name=Mueller>{{cite journal |author=Mueller M. H. |author2=Lander G. H. |author3=Hoff H. A. |author4=Knott H. W. |author5=Reddy J. F. |title=Lattice distortions measured in actinide ferromagnets PuP, NpFe<sub>2</sub>, and NpNi<sub>2</sub> |journal=J. Phys. Colloque C4, Supplement |date=Apr 1979 |volume=40 |issue=4 |pages=C4-68–C4-69 |url=http://hal.archives-ouvertes.fr/docs/00/21/88/17/PDF/ajp-jphyscol197940C421.pdf |archive-url=https://web.archive.org/web/20110509221218/http://hal.archives-ouvertes.fr/docs/00/21/88/17/PDF/ajp-jphyscol197940C421.pdf |archive-date=2011-05-09 |url-status=live}}</ref> NpNi<sub>2</sub> undergoes a similar lattice distortion below {{nowrap|''T''<sub>C</sub> {{=}} 32 K}}, with a strain of (43&nbsp;±&nbsp;5)&nbsp;×&nbsp;10<sup>−4</sup>.<ref name=Mueller/> NpCo<sub>2</sub> is a ferrimagnet below 15&nbsp;K.

In 2009, a team of [[MIT]] physicists demonstrated that a [[lithium]] gas cooled to less than one&nbsp;[[kelvin]] can exhibit ferromagnetism.<ref>{{cite journal |author1=G.-B. Jo |author2=Y.-R. Lee |author3=J.-H. Choi |author4=C. A. Christensen |author5=T. H. Kim |author6=J. H. Thywissen |author7=D. E. Pritchard |author8=W. Ketterle |title=Itinerant Ferromagnetism in a Fermi Gas of Ultracold Atoms |journal= Science |year=2009 |volume=325  |pages=1521–1524 |doi=10.1126/science.1177112 |pmid=19762638 |issue=5947 |bibcode=2009Sci...325.1521J |arxiv=0907.2888 |s2cid=13205213 }}</ref> The team cooled [[fermion]]ic [[lithium-6]] to less than {{nowrap|150 nK}} (150&nbsp;billionths of one&nbsp;kelvin) using infrared [[laser cooling]]. This demonstration is the first time that ferromagnetism has been demonstrated in a gas.

In rare circumstances, ferromagnetism can be observed in compounds consisting of only s-[[Block (periodic table)|block]] and p-block elements, such as [[rubidium sesquioxide]].<ref>{{cite journal | last1=Attema | first1=Jisk J. | last2=de Wijs | first2=Gilles A. | last3=Blake | first3=Graeme R. | last4=de Groot | first4=Robert A. | title=Anionogenic Ferromagnets | journal=Journal of the American Chemical Society | publisher=American Chemical Society (ACS) | volume=127 | issue=46 | year=2005 | issn=0002-7863 | doi=10.1021/ja0550834 | pages=16325–16328| pmid=16287327 | bibcode=2005JAChS.12716325A | url=https://pure.rug.nl/ws/files/10178653/2005JAmChemSocAttema.pdf }}</ref>

In 2018, a team of [[University of Minnesota]] physicists demonstrated that body-centered tetragonal [[ruthenium]] exhibits ferromagnetism at room temperature.<ref>{{cite journal |author1=Quarterman, P. |author2=Sun, Congli |author3=Garcia-Barriocanal, Javier |author4=D. C., Mahendra |author5=Lv, Yang |author6=Manipatruni, Sasikanth |author7=Nikonov, Dmitri E. |author8=Young, Ian A. |author9=Voyles, Paul M. |author10=Wang, Jian-Ping |title=Demonstration of Ru as the 4th ferromagnetic element at room temperature |journal=Nature Communications |year=2018 |volume=9 |issue=1 |page=2058 |doi=10.1038/s41467-018-04512-1 |pmid=29802304 |bibcode=2018NatCo...9.2058Q |pmc=5970227 }}</ref>

===Electrically induced ferromagnetism===
Recent research has shown evidence that ferromagnetism can be induced in some materials by an [[electric current]] or voltage. Antiferromagnetic LaMnO<sub>3</sub> and SrCoO have been switched to be ferromagnetic by a current. In July 2020, scientists reported inducing ferromagnetism in the abundant [[diamagnetic]] material [[iron pyrite]] ("fool's gold") by an applied voltage.<ref name="Phys">{{cite news |title='Fool's gold' may be valuable after all |url=https://phys.org/news/2020-07-gold-valuable.html |access-date=17 August 2020 |work=phys.org |language=en}}</ref><ref name="Voigt">{{cite journal |last1=Walter |first1=Jeff |last2=Voigt |first2=Bryan |last3=Day-Roberts |first3=Ezra |last4=Heltemes |first4=Kei |last5=Fernandes |first5=Rafael M. |last6=Birol |first6=Turan |last7=Leighton |first7=Chris |title=Voltage-induced ferromagnetism in a diamagnet |journal=Science Advances |date=1 July 2020 |volume=6 |issue=31 |pages=eabb7721 |doi=10.1126/sciadv.abb7721 |pmid=32832693 |pmc=7439324 |bibcode= 2020SciA....6.7721W|language=en |issn=2375-2548 |doi-access=free }}</ref> In these experiments, the ferromagnetism was limited to a thin surface layer.