Editing Remanence

{{Short description|Magnetization left behind in a material}}
{{About|magnetic remanence|the data storage term|Data remanence}}

'''Remanence''' or '''remanent magnetization''' or '''residual magnetism''' is the [[magnetization]] left behind in a [[ferromagnetic]] material (such as [[iron]]) after an external [[magnetic field]] is removed.<ref name=Chikazumi>{{Harvnb|Chikazumi|1997}}</ref> Colloquially, when a magnet is "magnetized", it has remanence.<ref>Strictly speaking, it is still in the Earth's field, but that has little effect on the remanence of a [[hard magnet]].</ref>  The remanence of magnetic materials provides the magnetic memory in [[magnetic storage]] devices, and is used as a source of information on the past [[Earth's magnetic field]] in [[paleomagnetism]]. The word remanence is from remanent + -ence, meaning "that which remains".<ref>{{Cite web|url=https://www.etymonline.com/word/remanence|title=remanence {{!}} Origin and meaning of remanence by Online Etymology Dictionary|website=www.etymonline.com|language=en|access-date=2020-01-20}}</ref>

The equivalent term  '''residual magnetization''' is generally used in engineering applications. In [[transformer]]s, [[electric motors]] and [[Electric generator|generators]] a large residual magnetization is not desirable (see also [[electrical steel]]) as it is an unwanted contamination, for example, a magnetization remaining in an [[electromagnet]] after the current in the coil is turned off. Where it is unwanted, it can be removed by [[degaussing]].

Sometimes the term '''retentivity''' is used for remanence measured in units of [[magnetic flux density]].<ref>{{cite web|title=Magnetic Tape Storage and Handling|url=http://www.clir.org/pubs/reports/pub54/glossary.html}}</ref>

==Types==

===Saturation remanence===
[[Image:B-H loop.png|thumb|'''Fig. 1''' A family of AC [[hysteresis]] loops for grain-oriented electrical steel ('''''B'''''<sub>r</sub> denotes ''remanence'' and '''''H'''''<sub>c</sub> is the ''[[coercivity]]'').]]
The default definition of magnetic remanence is the magnetization remaining in zero field after a large magnetic field is applied (enough to achieve [[saturation (magnetic)|saturation]]).<ref name=Chikazumi/> The effect of a magnetic [[hysteresis loop]] is measured using instruments such as a vibrating sample [[magnetometer]]; and the zero-field intercept is a measure of the remanence. In [[physics]] this measure is converted to an average [[magnetization]] (the total [[magnetic moment]] divided by the volume of the sample) and denoted in equations as '''''M'''''<sub>r</sub>. If it must be distinguished from other kinds of remanence, then it is called the ''saturation remanence'' or ''saturation isothermal remanence (SIRM)'' and denoted by '''''M'''''<sub>rs</sub>.

In engineering applications the residual magnetization is often measured using a [[B-H analyzer]], which measures the response to an AC magnetic field  (as in Fig. 1). This is represented by a [[flux density]] '''''B'''''<sub>r</sub>. This value of remanence is one of the most important parameters characterizing [[permanent magnet]]s; it measures the strongest magnetic field they can produce. [[Neodymium magnets]], for example, have a remanence approximately equal to 1.3 [[tesla (unit)|Tesla]].

=== Isothermal remanence ===
Often a single measure of remanence does not provide adequate information on a magnet. For example, magnetic tapes contain a large number of small magnetic particles (see [[magnetic storage]]), and these particles are not identical.  Magnetic minerals in rocks may have a wide range of magnetic properties (see [[rock magnetism]]). One way to look inside these materials is to add or subtract small increments of remanence. One way of doing this is first [[Magnet#Magnetization and demagnetization|demagnetizing]] the magnet in an AC field, and then applying a field '''''H''''' and removing it.  This remanence, denoted by '''''M'''''<sub>r</sub>('''''H'''''), depends on the field.<ref name=Wohlfarth>{{Harvnb|Wohlfarth|1958}}</ref> It is called the ''initial remanence''<ref>{{Harvnb|McCurrie|Gaunt|1966}}</ref> or the ''isothermal remanent magnetization (IRM)''.<ref name="Néel">{{Harvnb|Néel|1955}}</ref>

Another kind of IRM can be obtained by first giving the magnet a saturation remanence in one direction and then applying and removing a magnetic field in the opposite direction.<ref name=Wohlfarth/>  This is called ''demagnetization remanence'' or ''DC demagnetization remanence'' and is denoted by symbols like '''''M'''''<sub>d</sub>('''''H'''''), where '''''H''''' is the ''magnitude'' of the field.<ref name=Pfeiffer>{{Harvnb|Pfeiffer|1990}}</ref>  Yet another kind of remanence can be obtained by demagnetizing the saturation remanence in an ac field.  This is called ''AC demagnetization remanence'' or ''alternating field demagnetization remanence'' and is denoted by symbols like '''''M'''''<sub>af</sub>('''''H''''').

If the particles are noninteracting single-domain particles with uniaxial [[anisotropy]], there are simple linear relations between the remanences.<ref name=Wohlfarth/>

=== Anhysteretic remanence ===
Another kind of laboratory remanence is ''anhysteretic remanence'' or ''anhysteretic remanent magnetization (ARM)''. This is induced by exposing a magnet to a large alternating field plus a small [[DC bias]] field. The amplitude of the alternating field is gradually reduced to zero to get an ''anhysteretic magnetization'', and then the bias field is removed to get the remanence. The anhysteretic magnetization curve is often close to an average of the two branches of the [[hysteresis loop]],<ref name=Bozorth>{{Harvnb|Bozorth|1993}}</ref> and is assumed in some models to represent the lowest-energy state for a given field.<ref>{{Harvnb|Jiles|Atherton|1986}}</ref> There are several ways for experimental measurement of the anhysteretic magnetization curve, based on fluxmeters and DC biased demagnetization.<ref>{{Harvnb|Nowicki|2018}}</ref> ARM has also been studied because of its similarity to the write process in some magnetic recording technology<ref>{{Harvnb|Jaep|1969}}</ref> and to the acquisition of [[natural remanent magnetization]] in rocks.<ref>{{Harvnb|Banerjee|Mellema|1974}}</ref>

==Examples==
{{expand section|date=September 2016}}
{| class="wikitable sortable"
|-
! Material !! Remanence !! References
|-
| [[Ferrite (magnet)]] || {{convert|0.35|T|abbr=on}} || <ref>{{cite web |url=http://www.hilltech.com/products/emc_components/Amorphous_Shielding.html |title=Amorphous Magnetic Cores |author=<!--Staff writer(s); no by-line.--> |year=2006 |publisher=Hill Technical Sales |access-date=18 January 2014}}</ref>
|-
| [[Samarium-cobalt magnet]] || {{convert|0.82|–|1.16|T|abbr=on}} || <ref name="John Wiley and Sons">{{cite book|url=https://books.google.com/books?id=_y3LSh1XTJYC&pg=PT232|page=232|title=Design of Rotating Electrical Machines|author1=Juha Pyrhönen |author2=Tapani Jokinen |author3=Valéria Hrabovcová |publisher=John Wiley and Sons|year=2009|isbn=978-0-470-69516-6}}</ref> 
|-
| [[AlNiCo]] 5 || {{convert|1.28|T|abbr=on}} ||
|-
| [[Neodymium magnet]] || {{convert|1|-|1.3|T|abbr=on}} || <ref name="John Wiley and Sons"/>
|-
| [[Steel]]s || {{convert|0.9|-|1.4|T|abbr=on}} || <ref>{{cite web|url=https://www.arnoldmagnetics.com/wp-content/uploads/2017/10/Cobalt-Essential-to-High-Performance-Magnetics-Baylis-and-Constantinides-Cobalt-Conference-2012.-psn-hi-res.pdf|title=COBALT: Essential to High Performance Magnetics|publisher=Arnold Magnetic Technologies|date=2012}}</ref><ref>{{cite book|url={{google books |plainurl=y |id=YBKk4kWSle0C}}|title=Electric Machinery|last1=Fitzgerald|first1=A.E.|last2=Kingsley|first2=Charles Jr.|last3=Umans|first3=Stephen D.|publisher=McGraw-Hill|year=2003|isbn=978-0-07-366009-7|edition=6th|pages=688 pages}}</ref>
|}

==See also==
*[[Coercivity]]
*[[Hysteresis]]
*[[Rock magnetism]]
*[[Thermoremanent magnetization]]
*[[Viscous remanent magnetization]]

==Notes==
{{Reflist|3}}

==References==
{{Refbegin|2}}
*{{cite journal
  |doi = 10.1016/0012-821X(74)90190-3
  |last1 = Banerjee
  |first1 = S. K.
  |last2 = Mellema
  |first2 = J. P.
  |title = A new method for the determination of paleointensity from the A.R.M. properties of rocks
  |journal = Earth Planet. Sci. Lett.
  |volume = 23
  |pages = 177–184
  |date = 1974
|bibcode = 1974E&PSL..23..177B
  |issue = 2 }}
*{{cite book
  |last = Bozorth
  |first = Richard M.
  |title = Ferromagnetism
  |publisher = [[Wiley-IEEE Press]]
  |year = 1993
  |orig-year = Reissue of 1951 publication
  |series = AN IEEE Press Classic Reissue
  |isbn = 0-7803-1032-2
}}
*{{cite book
  |last = Chikazumi
  |first = Sōshin
  |title = Physics of Ferromagnetism
  |publisher = [[Clarendon Press]]
  |date = 1997
  |isbn = 0-19-851776-9
}}
*{{cite journal
  |doi = 10.1063/1.1657638
  |last = Jaep
  |first = W. F.
  |title = Anhysteretic magnetization of an assembly of single-domain particles
  |journal = J. Appl. Phys.
  |volume = 40
  |pages = 1297–1298
  |date = 1969
|bibcode = 1969JAP....40.1297J
  |issue = 3 }}
*{{cite journal
  |doi = 10.1016/0304-8853(86)90066-1
  |last1 = Jiles
  |first1 = D. C.
  |last2 = Atherton
  |first2 = D. L.
  |title = Theory of ferromagnetic hysteresis
  |journal = J. Magn. Magn. Mater.
  |volume = 61
  |pages = 48–60
  |date = 1986
  |issue = 1–2
|bibcode = 1986JMMM...61...48J }}
*{{cite journal
|doi = 10.1080/14786436608212648
|last1 = McCurrie
|first1 = R. A.
|last2 = Gaunt
|first2 = P.
|title = The magnetic properties of platinum cobalt near the equiatomic composition part I. the experimental data
|journal = Phil. Mag.
|volume = 13
|pages = 567–577
|date = 1966
|bibcode = 1966PMag...13..567M
|issue = 123 }}
*{{cite journal
|doi = 10.1080/00018735500101204
|last = Néel
|first = Louis
|title = Some theoretical aspects of rock magnetism
|journal = Adv. Phys.
|volume = 4
|pages = 191–243
|date = 1955
|bibcode = 1955AdPhy...4..191N
|issue = 14 |url = https://hal.archives-ouvertes.fr/hal-02888358/file/N%C3%A9el%20-%201955%20-%20Some%20theoretical%20aspects%20of%20rock-magnetism.pdf
}}
*{{cite journal
|doi = 10.3390/ma11102021
|last = Nowicki
|first = M.
|title = Anhysteretic Magnetization Measurement Methods for Soft Magnetic Materials
|journal = Materials
|volume = 11
|pages = 2021
|date = 2018
|issue = 10
|pmid = 30340358
|pmc = 6213293
|bibcode = 2018Mate...11.2021N
|doi-access = free
}}
*{{cite journal
|doi = 10.1002/pssa.2211180133
|last = Pfeiffer
|first = H.
|title = Determination of anisotropy field distribution in particle assemblies taking into account thermal fluctuations
|journal = Physica Status Solidi
|volume = 118
|pages = 295–306
|year = 1990
|issue = 1
|bibcode = 1990PSSAR.118..295P }}
*{{cite journal
|doi = 10.1063/1.1723232
|last = Wohlfarth
|first = E. P.
|title = Relations between different modes of acquisition of the remanent magnetization of ferromagnetic particles
|journal = J. Appl. Phys.
|volume = 29
|pages = 595–596
|date = 1958
|bibcode = 1958JAP....29..595W
|issue = 3 }}
{{Refend}}

==External links==
*[http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magperm.html Coercivity and Remanence in Permanent Magnets]
*[http://www.coolmagnetman.com/ Magnet Man]

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[[Category:Computer engineering]]
[[Category:Rock magnetism]]
[[Category:Magnetic hysteresis]]

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