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| :''Not to be confused with [[Ferrimagnetism]]; for an overview see [[Magnetism]]''
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| [[Image:MagnetEZ.jpg|thumb|A magnet made of [[alnico]], an iron alloy. Ferromagnetism is the physical theory which explains how materials become magnets.]]
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| '''Ferromagnetism''' is the basic mechanism by which certain materials (such as [[iron]]) form [[permanent magnet]]s, or are attracted to [[magnet]]s. In [[physics]], several different types of [[magnetism]] are distinguished. Ferromagnetism (including [[ferrimagnetism]])<ref>{{harvnb|Chikazumi|2009|p=118}}</ref> is the strongest type; it is the only type that creates forces strong enough to be felt, and is responsible for the common phenomena of magnetism [[Magnet#Common uses of magnets|encountered in everyday life]]. Other substances respond weakly to magnetic fields with two other types of magnetism, [[paramagnetism]] and [[diamagnetism]], but the forces are so weak that they can only be detected by sensitive instruments in a laboratory. An everyday example of ferromagnetism is a [[refrigerator magnet]] used to hold notes on a refrigerator door. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today".<ref name="bozorth">Richard M. Bozorth, ''Ferromagnetism'', first published 1951, reprinted 1993 by [[IEEE]] Press, New York as a "Classic Reissue." ISBN 0-7803-1032-2.</ref> | | * Only registered users will be able to execute this rendering mode. |
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| 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 other materials that are noticeably attracted to them. Only a few substances are ferromagnetic. The common ones are [[iron]], [[nickel]], [[cobalt]] and most of their alloys, some compounds of [[Rare earth magnet|rare earth metals]], and a few naturally-occurring minerals such as [[lodestone]].
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| Ferromagnetism is very important in industry and modern technology, and is the basis for many electrical and electromechanical devices such as [[electromagnet]]s, [[electric motor]]s, [[Electric generator|generators]], [[transformer]]s, and [[magnetic storage]] such as [[tape recorder]]s, and [[hard disk]]s.
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| ==History and distinction from ferrimagnetism== | | <!--'''PNG''' (currently default in production) |
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| Historically, the term ''ferromagnet'' was used for any material that could exhibit spontaneous magnetization: a net magnetic moment in the absence of an external magnetic field. This general definition is still in common use. More recently, however, different classes of spontaneous magnetization have been identified when there is more than one magnetic ion per [[primitive cell]] of the material, leading to a stricter definition of "ferromagnetism" that is often used to distinguish it from ferrimagnetism. In particular, a material is "ferromagnetic" in this narrower sense only if ''all'' of its magnetic ions add a positive contribution to the net magnetization. If some of the magnetic ions ''subtract'' from the net magnetization (if they are partially ''anti''-aligned), then the material is "ferrimagnetic".<ref>{{cite journal|last=Herrera|first=J. M.|coauthors=Bachschmidt, A, Villain, F, Bleuzen, A, Marvaud, V, Wernsdorfer, W, Verdaguer, M|title=Mixed valency and magnetism in cyanometallates and Prussian blue analogues|journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|date=13 January 2008|volume=366|issue=1862|pages=127–138|doi=10.1098/rsta.2007.2145}}</ref> If the moments of the aligned and anti-aligned ions balance completely so as to have zero net magnetization, despite the magnetic [[Order (crystal lattice)|ordering]], then it is an [[antiferromagnet]]. These alignment effects only occur at [[temperature]]s below a certain critical temperature, called the [[Curie temperature]] (for ferromagnets and ferrimagnets) or the [[Néel temperature]] (for antiferromagnets).
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| Among the first investigations of ferromagnetism are the pioneering works of [[Aleksandr Stoletov]] on measurement of the [[magnetic permeability]] of ferromagnetics, known as the [[Stoletov curve]].
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| ==Ferromagnetic materials==<!-- [[Ferromagnetic materials]] redirects here --> | | ==Demos== |
| {{See also|Category:Ferromagnetic materials}}
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| {| class="wikitable" style="float:right;margin:0 0 1em 1em;"
| | Here are some [https://commons.wikimedia.org/w/index.php?title=Special:ListFiles/Frederic.wang demos]: |
| |+ style="font-size: 80%"|Curie temperatures for some crystalline ferromagnetic (* = 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>
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| ! Material
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| ! Curie <br/>temp. (K)
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| | [[Cobalt|Co]]
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| | 1388
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| | [[Iron|Fe]]
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| | 1043
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| | [[Hematite|Fe<sub>2</sub>O<sub>3</sub>]]<sup>*</sup>
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| | 948
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| | [[Magnetite|FeOFe<sub>2</sub>O<sub>3</sub>]]<sup>*</sup>
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| | 858
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| | [[Ferrite (magnet)|NiOFe<sub>2</sub>O<sub>3</sub>]]<sup>*</sup>
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| | 858
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| | CuOFe<sub>2</sub>O<sub>3</sub><sup>*</sup>
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| | 728
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| | MgOFe<sub>2</sub>O<sub>3</sub><sup>*</sup>
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| | 713
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| | [[Manganese|Mn]][[Bismuth|Bi]]
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| | 630
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| | [[Nickel|Ni]]
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| | 627
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| | Mn[[Antimony|Sb]]
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| | 587
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| | MnOFe<sub>2</sub>O<sub>3</sub><sup>*</sup>
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| | 573
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| | [[Yttrium iron garnet|Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>]]<sup>*</sup>
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| | 560
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| | [[Chromium(IV) oxide|CrO<sub>2</sub>]]
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| | 386
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| | Mn[[Arsenic|As]]
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| | 318
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| | [[Gadolinium|Gd]]
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| | 292
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| | [[Dysprosium|Dy]]
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| | 88
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| | [[Europium|Eu]]O
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| | 69
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| <!-- 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. -->
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| The table on the right lists a selection of ferromagnetic and ferrimagnetic compounds, along with the temperature above which they cease to exhibit spontaneous magnetization (see [[Ferromagnetism#Curie temperature|Curie temperature]]).
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| Ferromagnetism is a property not just of the chemical make-up of a material, but of its crystalline structure and microscopic organization. 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.
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| One can also make amorphous (non-crystalline) ferromagnetic metallic alloys by very rapid [[quenching]] (cooling) of a liquid 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.
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| <!--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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| 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-orbitals.
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| ===Actinide ferromagnets===
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| A number of [[actinide]] compounds are ferromagnets at room temperature or become ferromagnets below the Curie temperature (T<sub>C</sub>). [[Plutonium|Pu]][[Phosphorus|P]] is one actinide [[Nitrogen family|pnictide]] that is a paramagnet and has [[Cubic crystal system|cubic symmetry]] at room temperature, but upon cooling undergoes a lattice distortion to [[Tetragonal crystal system|tetragonal]] when cooled to below its T<sub>c</sub> = 125 K. PuP has an [[easy axis]] of <100>,<ref name=Lander>{{cite journal |author=Lander GH, Lam DJ |title=Neutron diffraction study of PuP: The electronic ground state |journal=Phys Rev B. |year=1976 |volume=14 |issue=9 |pages=4064–7 |doi=10.1103/PhysRevB.14.4064|bibcode = 1976PhRvB..14.4064L }}</ref> so that
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| at 5 K.<ref name=Mueller>{{cite journal |author=Mueller MH, Lander GH, Hoff HA, Knott HW, Reddy JF |title=Lattice distortions measured in actinide ferromagnets PuP, NpFe<sub>2</sub>, and NpNi<sub>2</sub> |journal= J Phys Colloque C4, supplement |month=Apr |year=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}}</ref> The lattice distortion is presumably a consequence of strain induced by the magnetoelastic interactions as the [[magnetic moment]]s aligned parallel within [[magnetic domain]]s.
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| In [[Neptunium|Np]]Fe<sub>2</sub> the easy axis is <111>.<ref name=Aldred>{{cite journal |author=Aldred AT, Dunlap BD, Lam DJ, Lander GH, Mueller MH, 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=530–44 |doi=10.1103/PhysRevB.11.530|bibcode = 1975PhRvB..11..530A }}</ref> Above 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 c along the unique trigonal axis (after the distortion has begun) and a as the distance in the plane perpendicular to c. In the cubic phase this reduces to <math>\scriptstyle\frac{c}{a}</math> = 1.00. Below the Curie temperature
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| :<math>\frac{c}{a} - 1 = -(120 \pm 5) \times 10^{-4}</math>
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| which is the largest strain in any actinide compound.<ref name=Mueller/> NpNi<sub>2</sub> undergoes a similar lattice distortion below 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.
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| ===Lithium gas===
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| In 2009, a team of MIT physicists demonstrated that a lithium gas cooled to less than one Kelvin can exhibit ferromagnetism.<ref>{{cite journal |author=G-B Jo, Y-R Lee, J-H Choi, C. A. Christensen, T. H. Kim, J. H. Thywissen, D. E. Pritchard, and 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 }}</ref> The team cooled [[fermion]]ic lithium-6 to less than 150 billionths of one Kelvin above absolute zero using infrared [[laser cooling]]. This demonstration is the first time that ferromagnetism has been demonstrated in a gas.
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| ==Explanation==
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| The [[Bohr–van Leeuwen theorem]] shows that magnetism cannot occur in purely classical solids. Without [[quantum mechanics]], there would be no [[diamagnetism]], paramagnetism or ferromagnetism. The property of ferromagnetism is due to the direct influence of two effects from quantum mechanics: [[spin (physics)|spin]] and the [[Pauli exclusion principle]].<ref>{{cite book
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| |last = Feynman
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| |first = Richard P.
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| |title = The Feynman Lectures on Physics, Vol.2
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| |publisher = Addison-Wesley
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| |year = 1963
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| |location = USA
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| |pages = Ch. 37|isbn = 0-201-02011-4H}}</ref>
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| ===Origin of magnetism===
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| One of the fundamental properties of an [[electron]] (besides that it carries charge) is that it has a [[Electron magnetic dipole moment|dipole moment]], i.e. it behaves itself as a tiny magnet. This dipole moment comes from the more fundamental property of the electron that it has quantum mechanical [[Spin (physics)|spin]]. The quantum mechanical nature of this spin causes the electron to only be able to be in two states, with the magnetic field either pointing "up" or "down" (for any choice of up and down). The spin of the electrons in atoms is the main source of ferromagnetism, although there is also a contribution from the [[planetary orbit|orbital]] [[angular momentum]] of the electron about the [[nucleus (atomic structure)|nucleus]]. When these tiny magnetic dipoles are aligned in the same direction, their individual magnetic fields add together to create a measurable macroscopic field.
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| However in materials with a filled [[electron shell]], the total dipole moment of the electrons is zero because the spins are in up/down pairs. Only atoms with partially filled shells (i.e., unpaired spins) can have a net magnetic moment, so ferromagnetism only occurs in materials with partially filled shells. Because of [[Hund's rules]], the first few electrons in a shell tend to have the same spin, thereby increasing the total dipole moment.
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| These unpaired dipoles (often called simply "spins" even though they also generally include angular momentum) tend to align in parallel to an external magnetic field, an effect called paramagnetism. Ferromagnetism involves an additional phenomenon, however: the dipoles tend to align spontaneously, giving rise to a [[spontaneous magnetization]], even when there is no applied field.
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| ===Exchange interaction===
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| {{Main|Exchange interaction}}
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| According to classical [[electromagnetism]], two nearby magnetic dipoles will tend to align in ''opposite'' directions, so their magnetic fields will oppose one another and cancel out. However, this effect is very weak, because the magnetic fields generated by individual spins are small and the resulting alignment is easily destroyed by [[thermal fluctuations]]. In a few materials, a much stronger interaction between spins arises because the change in the direction of the spin leads to a change in [[electrostatic]] repulsion between neighboring electrons, due to a particular [[Quantum mechanics|quantum mechanical]] effect called the [[exchange interaction]]. At short distances, the exchange interaction is much stronger than the dipole-dipole magnetic interaction. As a result, in a few materials, the ferromagnetic ones, nearby spins tend to align in the same direction.
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| The exchange interaction is related to the [[Pauli exclusion principle]], which says that two electrons with the same spin cannot also have the same "position". 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 further 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 anti-parallel, so the parallel-spin state is more stable. In simple terms, the electrons, which repel one another, can move "further apart" by aligning their spins, so the spins of these electrons tend to line up. This difference in energy is called the [[exchange energy]].
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| 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 1000 times stronger than the dipole interaction. Therefore below the Curie temperature virtually all of the dipoles in a ferromagnetic material will be aligned.
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| The [[exchange interaction]] is also responsible for the other types of spontaneous ordering of atomic magnetic moments occurring in magnetic solids, [[antiferromagnetism]] and ferrimagnetism.
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| There are different exchange interaction mechanisms which create the magnetism in different ferromagnetic, ferrimagnetic, and antiferromagnetic substances. These mechanisms include [[Exchange_interaction#Direct_exchange_interactions_in_solids|direct exchange]], [[RKKY interaction|RKKY exchange]], [[double exchange]], and [[superexchange]].
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| ===Magnetic anisotropy===
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| {{Main|Magnetic anisotropy}}
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| Although the exchange interaction keeps spins aligned, it does not align them in a particular direction. Without [[magnetic anisotropy]], the spins in a magnet randomly change direction in response to [[thermal fluctuations]] and the magnet is [[superparamagnetic]]. There are several kinds of magnetic anisotropy, the most common of which is [[magnetocrystalline anisotropy]]. This is a dependence of the energy on the direction of magnetization relative to the [[crystallographic lattice]]. Another common source of anisotropy, [[inverse magnetostriction]], is induced by internal [[deformation (mechanics)|strains]]. [[Single-domain (magnetic)|Single-domain magnets]] also can have a ''shape anisotropy'' due to the magnetostatic effects of the particle shape. As the temperature of a magnet increases, the anisotropy tends to decrease, and there is often a [[superparamagnetism#Blocking temperature|blocking temperature]] at which a transition to superparamagnetism occurs.<ref name=Aharoni>{{cite book|last = Aharoni|first = Amikam|author-link=Amikam Aharoni|title=Introduction to the Theory of Ferromagnetism|publisher=[[Clarendon Press]]|year = 1996|isbn=0-19-851791-2|url=http://www.oup.com/us/catalog/general/subject/Physics/ElectricityMagnetism/?view=usa&ci=9780198508090}}</ref>
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| ===Magnetic domains===
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| {{Main|Magnetic domain}}
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| The above would seem to suggest that every piece of ferromagnetic material should have a strong magnetic field, since all the spins are aligned, yet iron and other ferromagnets are often found in an "unmagnetized" state. [[Image:Weiss-Bezirke1.png|thumb|Weiss domains microstructure]] The reason for this is that a bulk piece of ferromagnetic material is divided into tiny ''[[magnetic domains]]''<ref name="Feynman">{{cite book
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| | title = The Feynman Lectures on Physics, Vol. I
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| | publisher = California Inst. of Technology
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| | date = 1963
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| | location = USA
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| | pages = 37.5-37.6
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| | url = http://books.google.com/books?id=bDF-uoUmttUC&pg=SA4-PA4&dq=%22inclined+plane%22++%22conservation+of+energy%22&hl=en&sa=X&ei=gQtdT6iLCanSiAK22tCsCw&ved=0CGwQ6AEwBg#v=onepage&q=%22inclined%20plane%22%20%20%22conservation%20of%20energy%22&f=false
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| | isbn = 0-201-02117-XP}}</ref> (also known as ''Weiss domains''). Within each domain, the spins are aligned, but (if the bulk material is in its lowest energy configuration, i.e. ''unmagnetized''), the spins of separate domains point in different directions and their magnetic fields cancel out, so the object has no net large scale magnetic field.
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| Ferromagnetic materials spontaneously divide into magnetic domains because the ''[[exchange interaction]]'' is a short-range force, so over long distances of many atoms the tendency of the magnetic dipoles to reduce their energy by orienting in opposite directions wins out. If all the dipoles in a piece of ferromagnetic material are aligned parallel, it creates a large magnetic field extending into the space around it. This contains a lot of [[magnetostatics|magnetostatic]] energy. The material can reduce this energy by splitting into many domains pointing in different directions, so the magnetic field is confined to small local fields in the material, reducing the volume of the field. The domains are separated by thin [[domain wall]]s a number of molecules thick, in which the direction of magnetization of the dipoles rotates smoothly from one domain's direction to the other.
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| Thus, a piece of iron in its lowest energy state ("unmagnetized") generally has little or no net magnetic field. However, if it is placed in a strong enough external magnetic field, the domain walls will move, reorienting the domains so more of the dipoles are aligned with the external field. The domains will remain aligned when the external field is removed, creating a magnetic field of their own extending into the space around the material, thus creating a "permanent" magnet. The domains do not go back to their original minimum energy configuration when the field is removed because the domain walls tend to become 'pinned' or 'snagged' on defects in the crystal lattice, preserving their parallel orientation. This is shown by the [[Barkhausen effect]]: as the magnetizing field is changed, the magnetization changes in thousands of tiny discontinuous jumps as the domain walls suddenly "snap" past defects.
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| This magnetization as a function of the external field is described by a [[Hysteresis loop|hysteresis curve]]. Although this state of aligned domains found in a piece of magnetized ferromagnetic material is not a minimal-energy configuration, it is [[metastable]], and can persist for long periods, as shown by samples of [[magnetite]] from the sea floor which have maintained their magnetization for millions of years.
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| Alloys used for the strongest permanent magnets are "hard" alloys made with many defects in their crystal structure where the domain walls "catch" and stabilize. The net magnetization can be destroyed by heating and then cooling ([[Annealing (metallurgy)|annealing]]) the material without an external field, however. The thermal motion allows the domain boundaries to move, releasing them from any defects, to return to their low-energy unaligned state.
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| ===Curie temperature===
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| {{Main|Curie temperature}}
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| As the temperature 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, although it still responds paramagnetically to an external field. Below that temperature, there is a [[spontaneous symmetry breaking]] and random domains form (in the absence of an external field). 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.
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| The study of ferromagnetic phase transitions, especially via the simplified [[Ising model|Ising]] spin model, had an important impact on the development of statistical physics. There, it was first clearly shown that [[mean field theory]] approaches failed to predict the correct behavior at the critical point (which was found to fall under a ''universality class'' that includes many other systems, such as liquid-gas transitions), and had to be replaced by [[renormalization group]] theory.
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| ==See also==
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| *[[Ferromagnetic material properties]]
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| *[[Thermo-magnetic motor]]
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| ==References==
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| {{Reflist|2}}
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| ==Bibliography==
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| {{Refbegin}}
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| *{{cite book|last=Ashcroft|first=Neil W.|first2=N. David |last2=Mermin |title=Solid state physics|year=1977|publisher=Holt, Rinehart and Winston|location=New York|isbn=978-0-03-083993-1|edition=27. repr.}}
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| *{{cite book|last=Chikazumi|first=Sōshin|title=Physics of ferromagnetism|year=2009|publisher=Oxford University Press|location=Oxford|isbn=9780199564811|edition=2nd |others= English edition prepared with the assistance of C.D. Graham, Jr |ref=harv}}
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| *{{cite book|last=Jackson|first=John David|title=Classical electrodynamics|year=1998|publisher=Wiley|location=New York|isbn=978-0-471-30932-1|edition=3rd}}
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| *E. P. Wohlfarth, ed., ''Ferromagnetic Materials'' (North-Holland, 1980).
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| *"Heusler alloy," ''Encyclopædia Britannica Online'', retrieved Jan. 23, 2005.
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| *F. Heusler, W. Stark, and E. Haupt, ''Verh. der Phys. Ges.'' '''5''', 219 (1903).
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| *[[Sergei Vonsovsky|S. Vonsovsky]] ''Magnetism of elementary particles'' (Mir Publishers, Moscow, 1975).
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| *[[Sergei Tyablikov|Tyablikov S. V.]] (1995): ''Methods in the Quantum Theory of Magnetism.'' Springer; 1st edition. ISBN 0-306-30263-2.
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| {{Refend}}
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| ==External links==
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| *[http://www.lightandmatter.com/html_books/0sn/ch11/ch11.html Electromagnetism] - a chapter from an online textbook
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| *{{cite web
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| |last = Sandeman
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| |first = Karl
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| |title = Ferromagnetic Materials
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| |work = DoITPoMS
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| |publisher = Dept. of Materials Sci. and Metallurgy, Univ. of Cambridge
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| |date = January 2008
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| |url = http://www.msm.cam.ac.uk/doitpoms/tlplib/ferromagnetic/printall.php
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| |accessdate = 2008-08-27}} Detailed nonmathematical description of ferromagnetic materials with animated illustrations
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| {{magnetic states}}
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| [[Category:Quantum phases]]
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| [[Category:Concepts in physics]]
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| [[Category:Magnetic alloys]]
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| [[Category:Magnetic ordering]]
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| [[Category:Phase transitions]]
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