Editing Magnetocaloric effect (section)

== Working materials ==

The MCE is an intrinsic property of a magnetic solid. This thermal response of a solid to the application or removal of magnetic fields is maximized when the solid is near its magnetic ordering temperature. Thus, the materials considered for magnetic refrigeration devices should be magnetic materials with a magnetic phase transition temperature near the temperature region of interest.<ref name="doi10.1002/aenm.201200167">{{Cite journal | doi = 10.1002/aenm.201200167| title = Materials Challenges for High Performance Magnetocaloric Refrigeration Devices| journal = Advanced Energy Materials| volume = 2| issue = 11| pages = 1288| year = 2012| last1 = Smith | first1 = A. | last2 = Bahl | first2 = C. R. H. | last3 = Bjørk | first3 = R. | last4 = Engelbrecht | first4 = K. | last5 = Nielsen | first5 = K. K. | last6 = Pryds | first6 = N. | bibcode = 2012AdEnM...2.1288S| s2cid = 98040294}}</ref> For refrigerators that could be used in the home, this temperature is room temperature. The temperature change can be further increased when the [[Phase transition#Order parameters|order-parameter]] of the phase transition changes strongly within the temperature range of interest.<ref name="doi10.1088/0022-327/38/23/R01"/>

The magnitudes of the magnetic entropy and the adiabatic temperature changes are strongly dependent upon the magnetic ordering process. The magnitude is generally small in [[antiferromagnet]]s, [[ferrimagnet]]s and [[spin glass]] systems but can be much larger for ferromagnets that undergo a magnetic phase transition. First order phase transitions are characterized by a discontinuity in the magnetization changes with temperature, resulting in a latent heat.<ref name="doi10.1002/aenm.201200167"/> Second order phase transitions do not have this latent heat associated with the phase transition.<ref name="doi10.1002/aenm.201200167"/>

In the late 1990s Pecharksy and Gschneidner reported a magnetic entropy change in {{chem|Gd|5|(Si|2|Ge|2|)}} that was about 50% larger than that reported for Gd metal, which had the largest known magnetic entropy change at the time.<ref name="auto2">{{Cite journal | doi = 10.1103/PhysRevLett.78.4494| title = Giant Magnetocaloric Effect in Gd_{5}(Si_{2}Ge_{2})| journal = Physical Review Letters| volume = 78| issue = 23| pages = 4494| year = 1997| last1 = Pecharsky | first1 = V. K.| last2 = Gschneidner, Jr. | first2 = K. A.|bibcode = 1997PhRvL..78.4494P }}</ref> This giant magnetocaloric effect (GMCE) occurred at 270 K, which is lower than that of Gd (294 K).<ref name="auto1"/> Since the MCE occurs below room temperature these materials would not be suitable for refrigerators operating at room temperature.<ref name="doi10.1038/NMAT3951">{{Cite journal | doi = 10.1038/NMAT3951| title = Caloric materials near ferroic phase transitions| journal = Nature Materials| volume = 13| issue = 5| pages = 439–50| year = 2014| last1 = Moya | first1 = X.| last2 = Kar-Narayan | first2 = S.| last3 = Mathur | first3 = N. D.|bibcode = 2014NatMa..13..439M | pmid=24751772| url = https://www.repository.cam.ac.uk/bitstream/1810/267195/1/Caloric%20review_open%20access%20version.pdf}}</ref> Since then other alloys have also demonstrated the giant magnetocaloric effect. These include {{chem|Gd|5|(Si|''x''|Ge|1−''x''|)|4}}, {{chem|La(Fe|''x''|Si|1−''x''|)|13|H|''x''}} and {{chem|MnFeP|1−''x''|As|''x''}} alloys.<ref name="doi10.1002/aenm.201200167"/><ref name="doi10.1038/NMAT3951"/> Gadolinium and its alloys undergo second-order phase transitions that have no magnetic or thermal [[hysteresis]].<ref>{{Cite journal | doi = 10.1038/srep02291| title = Integrating giant microwave absorption with magnetic refrigeration in one multifunctional intermetallic compound of LaFe11.6Si1.4C0.2H1.7| journal = Scientific Reports| volume = 3| pages = 2291| year = 2013| last1 = Song | first1 = N. N. | last2 = Ke | first2 = Y. J. | last3 = Yang | first3 = H. T. | last4 = Zhang | first4 = H. | last5 = Zhang | first5 = X. Q. | last6 = Shen | first6 = B. G. | last7 = Cheng | first7 = Z. H. |bibcode = 2013NatSR...3.2291S | pmid=23887357 | pmc=3724178}}</ref> However, the use of rare earth elements makes these materials very expensive.

{{chem|Ni|2|Mn-X}} (X = Ga, Co, In, Al, Sb) Heusler alloys are also promising candidates for magnetic cooling applications because they have Curie temperatures near room temperature and, depending on composition, can have martensitic phase transformations near room temperature.<ref name="auto"/> These materials exhibit the [[magnetic shape memory]] effect and can also be used as actuators, energy harvesting devices, and sensors.<ref>{{Cite journal | doi = 10.1002/adma.201002753| pmid = 20957766| title = Size Effects on Magnetic Actuation in Ni-Mn-Ga Shape-Memory Alloys| journal = Advanced Materials| volume = 23| issue = 2| pages = 216–32| year = 2011| last1 = Dunand | first1 = D. C. | last2 = Müllner | first2 = P. | bibcode = 2011AdM....23..216D| s2cid = 4646639}}</ref> When the martensitic transformation temperature and the Curie temperature are the same (based on composition) the magnitude of the magnetic entropy change is the largest.<ref name="doi10.1088/0022-327/38/23/R01"/> In February 2014, GE announced the development of a functional Ni-Mn-based magnetic refrigerator.<ref>{{cite web|url= http://www.ge.com/research/live/magnetic_refrigeration/|title= GE Global Research Live|access-date= 2015-02-18|archive-date= 2015-02-18|archive-url= https://web.archive.org/web/20150218202913/http://www.ge.com/research/live/magnetic_refrigeration/|url-status= dead}}</ref><ref>{{cite web|url=http://www.gizmag.com/ge-magnetocaloric-refrigerator/30835/|title=Your next fridge could keep cold more efficiently using magnets|work=gizmag.com|date=2014-02-14}}</ref>

The development of this technology is very material-dependent and will likely not replace vapor-compression refrigeration without significantly improved materials that are cheap, abundant, and exhibit much larger magnetocaloric effects over a larger range of temperatures. Such materials need to show significant temperature changes under a field of two tesla or less, so that permanent magnets can be used for the production of the magnetic field.<ref name="ReferenceA">{{Cite journal | doi = 10.1088/0034-4885/68/6/R04| title = Recent developments in magnetocaloric materials| journal = Reports on Progress in Physics| volume = 68| issue = 6| pages = 1479| year = 2005| last1 = Gschneidnerjr | first1 = K. A. | last2 = Pecharsky | first2 = V. K. | last3 = Tsokol | first3 = A. O. |bibcode = 2005RPPh...68.1479G | s2cid = 56381721| url = https://zenodo.org/record/1235742}}</ref><ref>{{Cite journal | doi = 10.1016/S0304-8853(99)00397-2| title = Magnetocaloric effect and magnetic refrigeration| journal = Journal of Magnetism and Magnetic Materials| volume = 200| issue = 1–3| pages = 44–56| year = 1999| last1 = Pecharsky | first1 = V. K. | last2 = Gschneidner Jr | first2 = K. A. |bibcode = 1999JMMM..200...44P }}</ref>

=== Paramagnetic salts ===

The original proposed refrigerant was a [[paramagnetism|paramagnetic]] [[salt (chemistry)|salt]], such as [[cerium]] [[magnesium]] [[nitrate]].  The active magnetic dipoles in this case are those of the [[electron shell]]s of the paramagnetic atoms.

In a paramagnetic salt ADR, the heat sink is usually provided by a pumped {{chem|4|He}} (about 1.2&nbsp;K) or {{chem|3|He}} (about 0.3&nbsp;K) [[cryostat]].  An easily attainable 1&nbsp;T magnetic field is generally required for initial magnetization. The minimum temperature attainable is determined by the self-magnetization tendencies of the refrigerant salt, but temperatures from 1 to 100 mK are accessible. [[Dilution refrigerator]]s had for many years supplanted paramagnetic salt ADRs, but interest in space-based and simple to use lab-ADRs has remained, due to the complexity and unreliability of the dilution refrigerator.

At a low enough temperature, paramagnetic salts become either [[diamagnetism|diamagnetic]] or ferromagnetic, limiting the lowest temperature that can be reached using this method.{{Citation needed|reason=What is this transition temperature called?|date=June 2023}}

=== Nuclear demagnetization ===

One variant of adiabatic demagnetization that continues to find substantial research application is nuclear demagnetization refrigeration (NDR).  NDR follows the same principles, but in this case the cooling power arises from the [[Spin (physics)#Magnetic moments|magnetic dipoles of the nuclei]] of the refrigerant atoms, rather than their electron configurations.  Since these dipoles are of much smaller magnitude, they are less prone to self-alignment and have lower intrinsic minimum fields.  This allows NDR to cool the nuclear spin system to very low temperatures, often 1 μK or below. Unfortunately, the small magnitudes of nuclear magnetic dipoles also makes them less inclined to align to external fields.  Magnetic fields of 3 teslas or greater are often needed for the initial magnetization step of NDR.

In NDR systems, the initial heat sink must sit at very low temperatures (10–100 mK).  This precooling is often provided by the mixing chamber of a dilution refrigerator<ref>{{Cite journal | doi = 10.1016/j.cryogenics.2021.103390| issn=0011-2275| title = Development of Dilution refrigerators – A review | journal = Cryogenics| volume = 121| year = 2022| last1 = Zu | first1 = H.| last2 = Dai | first2 = W.| last3 = de Waele | first3 = A.T.A.M.| s2cid=244005391}}</ref> or a paramagnetic salt.