Editing Ferromagnetism (section)

=== 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>