Ferromagnetism: Difference between revisions
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imported>Chetvorno →top: Refrigerator magnets are not usually ferromagnetic; they are most often made of ferrimagnetic ferrites |
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Magnetic permeability describes the induced magnetization of a material due to the presence of an external magnetic field. For example, this temporary magnetization inside a steel plate accounts for the plate's attraction to a magnet. Whether or not that steel plate then acquires permanent magnetization depends on both the strength of the applied field and on the [[coercivity]] of that particular piece of steel (which varies with the steel's chemical composition and any heat treatment it may have undergone). | Magnetic permeability describes the induced magnetization of a material due to the presence of an external magnetic field. For example, this temporary magnetization inside a steel plate accounts for the plate's attraction to a magnet. Whether or not that steel plate then acquires permanent magnetization depends on both the strength of the applied field and on the [[coercivity]] of that particular piece of steel (which varies with the steel's chemical composition and any heat treatment it may have undergone). | ||
In [[physics]], multiple types of material [[magnetism]] have been distinguished. Ferromagnetism (along with the similar effect [[ferrimagnetism]]) is the strongest type and is responsible for the common phenomenon of everyday magnetism.<ref name="Chikazumi">{{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. D. Graham, Jr. |page=118}}</ref> A common example of a permanent magnet is a [[refrigerator magnet]].<ref name="bozorth">Bozorth, Richard M. ''Ferromagnetism'', first published 1951, reprinted 1993 by [[IEEE]] Press, New York as a "Classic Reissue". {{ISBN|0-7803-1032-2}}.</ref> Substances respond weakly to magnetic fields by three other types of magnetism—[[paramagnetism]], [[diamagnetism]], and [[antiferromagnetism]]—but the forces are usually so weak that they can be detected | In [[physics]], multiple types of material [[magnetism]] have been distinguished. Ferromagnetism (along with the similar effect [[ferrimagnetism]]) is the strongest type and is responsible for the common phenomenon of everyday magnetism.<ref name="Chikazumi">{{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. D. Graham, Jr. |page=118}}</ref> A common example of a permanent magnet is a [[refrigerator magnet]].<ref name="bozorth">Bozorth, Richard M. ''Ferromagnetism'', first published 1951, reprinted 1993 by [[IEEE]] Press, New York as a "Classic Reissue". {{ISBN|0-7803-1032-2}}.</ref> Substances respond weakly to magnetic fields by three other types of magnetism—[[paramagnetism]], [[diamagnetism]], and [[antiferromagnetism]]—but the forces are usually so weak that they can only be detected by lab instruments. | ||
Permanent magnets (materials that can be [[Magnetization|magnetized]] by an external [[magnetic field]] and remain magnetized after the external field is removed) are either ferromagnetic or ferrimagnetic, as are the materials that are strongly attracted to them. Relatively few materials are ferromagnetic; the common ones are the metals [[iron]], [[cobalt]], [[nickel]] and most of their [[alloy]]s, and certain [[Rare-earth magnet|rare-earth metals]]. | Permanent magnets (materials that can be [[Magnetization|magnetized]] by an external [[magnetic field]] and remain magnetized after the external field is removed) are either ferromagnetic or ferrimagnetic, as are the materials that are strongly attracted to them. Relatively few materials are ferromagnetic; the common ones are the metals [[iron]], [[cobalt]], [[nickel]] and most of their [[alloy]]s, and certain [[Rare-earth magnet|rare-earth metals]]. | ||
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Historically, the term ''ferromagnetism'' was used for any material that could exhibit [[spontaneous magnetization]]: a net magnetic moment in the absence of an external magnetic field; that is, any material that could become a [[magnet]]. This definition is still in common use.<ref>{{cite book |editor-last1=Somasundaran |editor-first1=P. |title=Encyclopedia of surface and colloid science |date=2006 |publisher=Taylor & Francis |location=New York |isbn=978-0-8493-9608-3 |page=3471 |edition=2nd}}</ref> | Historically, the term ''ferromagnetism'' was used for any material that could exhibit [[spontaneous magnetization]]: a net magnetic moment in the absence of an external magnetic field; that is, any material that could become a [[magnet]]. This definition is still in common use.<ref>{{cite book |editor-last1=Somasundaran |editor-first1=P. |title=Encyclopedia of surface and colloid science |date=2006 |publisher=Taylor & Francis |location=New York |isbn=978-0-8493-9608-3 |page=3471 |edition=2nd}}</ref> | ||
In a landmark paper in 1948, [[Louis Néel]] showed that two | In a landmark paper in 1948, [[Louis Néel]] showed that spontaneous magnetization can arise from two distinct types of magnetic ordering. In ferromagnetism in the strict sense, magnetic moments align parallel to one another. In ''[[ferrimagnetism]]'', magnetic moments are arranged in two antiparallel sublattices with different magnitudes, resulting in a net magnetization.<ref>{{cite book |last1=Cullity |first1=B. D. |last2=Graham |first2=C. D. |chapter=6. Ferrimagnetism |title=Introduction to Magnetic Materials |date=2011 |publisher=John Wiley & Sons |isbn=978-1-118-21149-6}}</ref><ref>{{cite book |last1=Aharoni |first1=Amikam |title=Introduction to the theory of ferromagnetism |date=2000 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-850809-0 |edition=2nd}}</ref>{{rp|28–29}} When the opposing moments are equal in magnitude and cancel completely, the alignment is known as ''[[antiferromagnetism]]''; such materials do not exhibit spontaneous magnetization.{{clear}} | ||
{{clear}} | |||
==Materials==<!-- [[Ferromagnetic materials]] redirects here --> | ==Materials==<!-- [[Ferromagnetic materials]] redirects here --> | ||
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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. | 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]]. | Amorphous (non-crystalline) ferromagnetic metallic alloys are sometimes called [[Asperomagnetism|asperomagnets.]] These 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 K and a room-temperature (300 K) saturation magnetization of 1.58 [[tesla (unit)|teslas]] (1,257 [[gauss]]), compared with 1,043 K and 2.15 T (1,707 gauss) for pure iron from above. The melting point, or more precisely the glass transition temperature, is only 714 K for the alloy versus a melting point of 1,811 K for pure iron.--> | <!--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 K and a room-temperature (300 K) saturation magnetization of 1.58 [[tesla (unit)|teslas]] (1,257 [[gauss]]), compared with 1,043 K and 2.15 T (1,707 gauss) for pure iron from above. The melting point, or more precisely the glass transition temperature, is only 714 K for the alloy versus a melting point of 1,811 K for pure iron.--> | ||
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=== Unusual materials === | === 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> | 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 |bibcode=2000JPCB..104.6694H |issn=1520-6106}}</ref> | ||
A number of [[actinide]] compounds are ferromagnets at room temperature or exhibit ferromagnetism upon cooling. | A number of [[actinide]] compounds are ferromagnets at room temperature or exhibit ferromagnetism upon cooling. PuP 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>{{Relevance inline|discuss=Way too specific, except the first sentence|date=January 2026}} | ||
In | In NpFe<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 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> | : <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 ± 5) × 10<sup>−4</sup>.<ref name=Mueller/> NpCo<sub>2</sub> is a ferrimagnet below 15 K. | 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 ± 5) × 10<sup>−4</sup>.<ref name=Mueller/> NpCo<sub>2</sub> is a ferrimagnet below 15 K.{{Relevance inline|discuss=Does this add anything to the topic of ferromagnetism in general?|date=January 2026}} | ||
In 2009, a team of [[MIT]] physicists demonstrated that a [[lithium]] gas cooled to less than one [[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 billionths of one kelvin) using infrared [[laser cooling]]. This demonstration is the first time that ferromagnetism has been demonstrated in a gas. | In 2009, a team of [[MIT]] physicists demonstrated that a [[lithium]] gas cooled to less than one [[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 billionths of one 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 | In rare circumstances, ferromagnetism can be observed in compounds consisting of only [[Block (periodic table)|s-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 | hdl=11370/b216ba1e-93ed-418d-831d-cb32130d4db5 | 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> | 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>{{Relevance inline|discuss=Is this relevant for ferromagnetism?|date=January 2026}} | ||
===Electrically induced ferromagnetism=== | ===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 | | 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 |article-number=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. | ||
==Explanation== | ==Explanation== | ||
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===Exchange interaction=== | ===Exchange interaction=== | ||
{{Main|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. | 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 | 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 the same direction. | ||
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. 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. H. N. |last2=Hanaor |first2=D. A. 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> | 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. 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. H. N. |last2=Hanaor |first2=D. A. 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 | | 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 |article-number=168934 |doi=10.1016/j.jmmm.2021.168934 |s2cid=245150523 |issn=0304-8853|url-access=subscription }}</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]]. | ||
===Magnetic anisotropy=== | ===Magnetic anisotropy=== | ||
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===Curie temperature=== | ===Curie temperature=== | ||
{{Main|Curie temperature}} | {{Main|Curie temperature}} | ||
As the temperature of a material increases, thermal motion, or [[entropy]], competes with the ferromagnetic tendency for dipoles to align. When the temperature rises beyond a certain point, called the [[Curie temperature]], there is a second-order [[phase transition]] and the system can no longer maintain a spontaneous magnetization, so its ability to be magnetized or attracted to a magnet disappears, although it still responds [[Paramagnetism|paramagnetically]] to an external field. Below that temperature, there is a [[spontaneous symmetry breaking]] and magnetic moments become aligned with their neighbors. The Curie temperature itself is a [[critical point (thermodynamics)|critical point]], where the [[magnetic susceptibility]] is theoretically infinite and, although there is no net magnetization, domain-like spin correlations fluctuate at all length scales. | As the temperature of a material increases, thermal motion, or [[entropy]], competes with the ferromagnetic tendency for dipoles to align. When the temperature rises beyond a certain point, called the [[Curie temperature]], there is a second-order [[phase transition]] and the system can no longer maintain a spontaneous magnetization, so its ability to be magnetized or attracted to a magnet disappears, although it still responds [[Paramagnetism|paramagnetically]] to an external field. Below that temperature, there is a [[spontaneous symmetry breaking]] and magnetic moments become aligned with their neighbors. The Curie temperature itself is a [[critical point (thermodynamics)|critical point]], where the [[magnetic susceptibility]] is theoretically infinite and, although there is no net magnetization, domain-like spin correlations fluctuate at all length scales. | ||
==See also== | ==See also== | ||
* {{annotated link|Ferromagnetic material properties}} | * {{annotated link|Ferromagnetic material properties}} | ||
* {{annotated link|Hysteresis}} | * {{annotated link|Hysteresis}} | ||
* {{annotated link|Neodymium magnet}} | |||
* {{annotated link|Orbital magnetization}} | * {{annotated link|Orbital magnetization}} | ||
* {{annotated link|Stoner criterion}} | * {{annotated link|Stoner criterion}} | ||
* {{annotated link|Thermo-magnetic motor}} | * {{annotated link|Thermo-magnetic motor}} | ||
==References== | ==References== | ||
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{{Authority control}} | {{Authority control}} | ||
[[Category:Ferromagnetism| ]] | |||
[[Category:Quantum phases]] | [[Category:Quantum phases]] | ||
[[Category:Magnetic hysteresis]] | [[Category:Magnetic hysteresis]] | ||
[[Category:Physical phenomena]] | [[Category:Physical phenomena]] | ||