Editing Curie temperature (section)

== Materials with magnetic moments that change properties at the Curie temperature ==

Ferromagnetic, paramagnetic, ferrimagnetic, and antiferromagnetic structures are made up of intrinsic magnetic moments. If all the electrons within the structure are paired, these moments cancel out due to their opposite spins and angular momenta. Thus, even with an applied magnetic field, these materials have different properties and no Curie temperature.<ref name=Ibach/><ref name=Levy>{{harvnb|Levy|1968|pp=236–39}}</ref>

=== Paramagnetic ===
{{main|Paramagnetism}}

A material is paramagnetic only above its Curie temperature. Paramagnetic materials are non-magnetic when a [[magnetic field]] is absent and magnetic when a magnetic field is applied. When a magnetic field is absent, the material has disordered magnetic moments; that is, the magnetic moments are asymmetrical and not aligned. When a magnetic field is present, the magnetic moments are temporarily realigned parallel to the applied field;<ref name=Dekker1>{{harvnb|Dekker|1958|pp=217–20}}</ref><ref name=Levy4>{{harvnb|Levy|1968|pp=}}</ref> the magnetic moments are symmetrical and aligned.<ref name=Fan>{{harvnb|Fan|1987|pp=164–65}}</ref> The magnetic moments being aligned in the same direction are what causes an induced magnetic field.<ref name=Fan/><ref name=Dekker>{{harvnb|Dekker|1958|pp=454–55}}</ref>

For paramagnetism, this response to an applied magnetic field is positive and is known as [[magnetic susceptibility]].<ref name=Ibach>{{harvnb|Ibach|Lüth|2009}}</ref> The magnetic susceptibility only applies above the Curie temperature for disordered states.<ref name=Mendelssohn3>{{harvnb|Mendelssohn|1977|p=162}}</ref>

Sources of paramagnetism (materials which have Curie temperatures) include:<ref name=Levy1>{{harvnb|Levy|1968|pp=198–202}}</ref>
* All atoms that have unpaired electrons;
* Atoms that have inner shells that are incomplete in electrons;
* [[Radical (chemistry)|Free radicals]];
* Metals.

Above the Curie temperature, the atoms are excited, and the spin orientations become randomized<ref name=Levy/> but can be realigned by an applied field, i.e., the material becomes paramagnetic. Below the Curie temperature, the intrinsic structure has undergone a [[phase transition]],<ref name=Cusack>{{harvnb|Cusack|1958|p=269}}</ref> the atoms are ordered, and the material is ferromagnetic.<ref name=Fan/> The paramagnetic materials' induced magnetic fields are very weak compared with ferromagnetic materials' magnetic fields.<ref name=Cusack/>

=== Ferromagnetic ===
{{main|Ferromagnetism}}

Materials are only ferromagnetic below their corresponding Curie temperatures. Ferromagnetic materials are magnetic in the absence of an applied magnetic field.

When a magnetic field is absent the material has [[spontaneous magnetization]] which is a result of the ordered magnetic moments; that is, for ferromagnetism, the atoms are symmetrical and aligned in the same direction creating a permanent magnetic field.

The magnetic interactions are held together by [[exchange interaction]]s; otherwise thermal disorder would overcome the weak interactions of magnetic moments. The exchange interaction has a zero probability of parallel electrons occupying the same point in time, implying a preferred parallel alignment in the material.<ref name=Hall1>{{harvnb|Hall|Hook|1994|pp=220–21}}</ref> The Boltzmann factor contributes heavily as it prefers interacting particles to be aligned in the same direction.<ref name=Palmer>{{harvnb|Palmer|2007|pp=}}</ref> This causes [[Ferromagnetism|ferromagnets]] to have strong magnetic fields and high Curie temperatures of around {{convert|1000|K|C}}.<ref name=Hall3>{{harvnb|Hall|Hook|1994|p=220}}</ref>

Below the Curie temperature, the atoms are aligned and parallel, causing spontaneous magnetism; the material is ferromagnetic. Above the Curie temperature the material is paramagnetic, as the atoms lose their ordered magnetic moments when the material undergoes a phase transition.<ref name=Cusack/>

=== Ferrimagnetic ===
{{main|Ferrimagnetism}}

Materials are only ferrimagnetic below their corresponding Curie temperature. Ferrimagnetic materials are magnetic in the absence of an applied magnetic field and are made up of two different [[ions]].<ref name=Jullien158/>

When a magnetic field is absent the material has a spontaneous magnetism which is the result of ordered magnetic moments; that is, for ferrimagnetism one ion's{{clarify|date=May 2022}} magnetic moments are aligned facing in one direction with certain magnitude and the other ion's magnetic moments are aligned facing in the opposite direction with a different magnitude. As the magnetic moments are of different magnitudes in opposite directions there is still a spontaneous magnetism and a magnetic field is present.<ref name=Jullien158>{{harvnb|Jullien|Guinier|1989|pp=158–59}}</ref>

Similar to ferromagnetic materials the magnetic interactions are held together by exchange interactions. The orientations of moments however are anti-parallel which results in a net momentum by subtracting their momentum from one another.<ref name=Jullien158/>

Below the Curie temperature the atoms of each ion are aligned anti-parallel with different momentums causing a spontaneous magnetism; the material is ferrimagnetic. Above the Curie temperature the material is paramagnetic as the atoms lose their ordered magnetic moments as the material undergoes a phase transition.<ref name=Jullien158/>

=== Antiferromagnetic and the Néel temperature ===
{{main|Antiferromagnetism}}

Materials are only antiferromagnetic below their corresponding '''Néel temperature''' or '''magnetic ordering temperature''', ''T''<sub>N</sub>. This is similar to the Curie temperature as above the Néel Temperature the material undergoes a [[phase transition]] and becomes paramagnetic. That is, the thermal energy becomes large enough to destroy the microscopic magnetic ordering within the material.<ref name="spaldin">{{cite book|last1=Spaldin|first1=Nicola A.|author-link=Nicola Spaldin |title=Magnetic materials : fundamentals and device applications|date=2006|publisher=Cambridge Univ. Press|location=Cambridge|isbn=9780521016582|pages=89–106|edition=Repr.}}</ref> It is named after [[Louis Néel]] (1904–2000), who received the 1970 [[Nobel Prize in Physics]] for his work in the area.

The material has equal magnetic moments aligned in opposite directions resulting in a zero magnetic moment and a net magnetism of zero at all temperatures below the Néel temperature. Antiferromagnetic materials are weakly magnetic in the absence or presence of an applied magnetic field.

Similar to ferromagnetic materials the magnetic interactions are held together by exchange interactions preventing thermal disorder from overcoming the weak interactions of magnetic moments.<ref name=Hall1/><ref name=Jullien10/> When disorder occurs it is at the Néel temperature.<ref name=Jullien10>{{harvnb|Jullien|Guinier|1989|pp=156–57}}</ref>

Listed below are the Néel temperatures of several materials:<ref>{{cite book |first=Charles |last=Kittel |title=[[Introduction to Solid State Physics]] |edition=8th |location=New York |publisher=John Wiley & Sons |year=2005 |isbn=978-0-471-41526-8 }}</ref>
{| class="wikitable sortable"
|-
! Substance
! Néel temperature ([[kelvin|K]])
|-
| [[Manganese(II) oxide|MnO]]
| 116
|-
| [[Manganese(II) sulfide|MnS]]
| 160
|-
| [[Manganese(II) telluride|MnTe]]
| 307
|-
| [[Manganese(II) fluoride|MnF<sub>2</sub>]]
| 67
|-
| [[Iron(II) fluoride|FeF<sub>2</sub>]]
| 79
|-
| [[Iron(II) chloride|FeCl<sub>2</sub>]]
| 24
|-
| [[Iron(II) iodide|FeI<sub>2</sub>]]
| 9
|-
| [[Iron(II) oxide|FeO]]
| 198
|-
| [[Iron oxychloride|FeOCl]]
| 80
|-
| [[Chromium(II) chloride|CrCl<sub>2</sub>]]
| 25
|-
| [[Chromium(II) iodide|CrI<sub>2</sub>]]
| 12
|-
| [[Cobalt(II) oxide|CoO]]
| 291
|-
| [[Nickel(II) chloride|NiCl<sub>2</sub>]]
| 50
|-
| [[Nickel(II) iodide|NiI<sub>2</sub>]]
| 75
|-
| [[Nickel(II) oxide|NiO]]
| 525
|-
| KFeO<sub>2</sub>
| 983<ref>{{cite journal |last1=Ichida |first1=Toshio |title=Mössbauer Study of the Thermal Decomposition Products of K2FeO4 |journal=Bulletin of the Chemical Society of Japan |date=1973 |volume=46 |issue=1 |pages=79–82 |doi=10.1246/bcsj.46.79|doi-access=free }}</ref>
|-
| [[Chromium|Cr]]
| 308
|-
| [[Chromium(III) oxide|Cr<sub>2</sub>O<sub>3</sub>]]
| 307
|-
| Nd<sub>5</sub>Ge<sub>3</sub>
|50
|}