Coercivity
360835
224071466
2008-07-07T04:44:24Z
Megan Reyes
7433561
/* Experimental determination */
{{dablink|This article is about a property of magnetic fields. For other meanings, see [[Coercion (disambiguation)]].}}
[[Image:B-H loop.png|thumb|A family of [[hysteresis]] loops for grain-oriented electrical steel (B<sub>R</sub> denotes ''[[remanence]]'' and H<sub>C</sub> is the ''coercivity'').]]
In [[materials science]], the '''coercivity''', also called the '''coercive field''', of a [[ferromagnet|ferromagnetic material]] is the intensity of the applied [[magnetic field]] required to reduce the [[magnetization]] of that material to zero ''after'' the magnetization of the sample has been driven to [[saturation (magnetic)|saturation]]. Coercivity is usually measured in [[oersted]] or [[ampere]]/meter units and is denoted H<sub>C</sub>.
When the coercive field of a ferromagnet is large, the material is said to be a '''hard''' or '''permanent''' magnet. Permanent magnets find application in [[electric motor]]s, magnetic recording media (e.g. [[Hard disk|hard drives]], [[floppy disk]]s, or [[magnetic tape]]) and [[Mineral processing#Magnetic Separation|magnetic separation]]. A ferromagnet with a low coercive field is said to be '''soft''' and may be used in [[microwave]] devices, [[magnetic shielding]], [[transformer]]s or [[recording head]]s.
Coercivity can be measured using a [[B-H Analyzer]].
== Experimental determination ==
[[Image:Coercivity.png|thumb|The magnitude of the coercive field of a ferromagnet can be determined via simple graphical analysis of the hysteresis loop.]]
Typically the [[coercivity]] of a magnetic material is determined by measurement of the [[hysteresis]] loop or magnetization curve as illustrated in the figure. The apparatus used to acquire the data is typically a vibrating-sample or alternating-gradient [[magnetometer]]. The applied field where the data (called a '''magnetization curve''') crosses zero is the coercivity. If an [[antiferromagnet|antiferromagnetic]] solid is present in the sample, the coercivities measured in increasing and decreasing fields may be unequal as a result of the [[exchange bias]] effect.
{| class="wikitable"
! Material
! Coercivity (Oersteds)
|-
| align="center" | [[Supermalloy]] {{Iron|15.7}}{{Nickel|79}}{{Molybdenum|5}}{{Manganese|0.3}}
| align="center" | [http://mysite.du.edu/~jcalvert/phys/iron.htm#Magn 0.002]
<!-- |-
| align="center" | {{Iron}}
| align="center" | [http://mysite.du.edu/~jcalvert/phys/iron.htm#Magn 0.05] -->
|-
| align="center" | [[Permalloy]], {{Nickel|81}}{{Iron|19}}
| align="center" | [http://dx.doi.org/10.1063/1.365100 0.05-1]
|-
| align="center" | [[Silicon Iron]]
| align="center" | [http://cartech.ides.com/datasheet.aspx?E=193~192~191~190~189&CK=1967748 0.4-0.9]
|-
| align="center" | [[Wrought Iron|Soft Wrought Iron]]
| align="center" | [http://books.google.com/books?id=G0cOAAAAYAAJ&pg=PA133&lpg=PA133 2]
|-
| align="center" | {{Cobalt}}
| align="center" | [http://dx.doi.org/10.1063/1.348151 20]
|-
| align="center" | {{Nickel}}
| align="center" | [http://dx.doi.org/10.1063/1.355560 150]
|-
| align="center" | [[Ferrite (magnet)|Ni<sub>1-x</sub>Zn<sub>x</sub>FeO<sub>3</sub>]], a microwave material
| align="center" | [http://dx.doi.org/10.1109/20.619559 15-200]
|-
| align="center" | [[Alnico]], a common refrigerator magnet
| align="center" | [http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=1066731 1500-2000]
|-
| align="center" | {{Cobalt}}{{Platinum}}{{Chromium}} disk drive recording media
| align="center" | [http://dx.doi.org/10.1109/20.278737 1700]
|-
| align="center" | [[NdFeB]]
| align="center" | [http://dx.doi.org/10.1063/1.353563 10,000]
|-
| align="center" | {{Iron|48}}{{Platinum|52}}
| align="center" | 12,300+
|-
| align="center" | [[Samarium-cobalt magnet|SmCo]]<sub>5</sub>
| align="center" | [http://dx.doi.org/10.1063/1.368075 40,000]
|+ Coercivities of representative soft and hard magnets
|}
The coercivity of a material depends on the time scale over which a magnetization curve is measured. The magnetization of a material measured at an applied reversed field which is nominally smaller than the coercivity may, over a long time scale, slowly [[Creep (deformation)|creep]] to zero. Creep occurs when reversal of magnetization by domain wall motion is [[Arrhenius equation|thermally activated]] and is dominated by [http://dx.doi.org/10.1063/1.336671 magnetic viscosity]. The increasing value of coercivity at high frequencies is a serious obstacle to the increase of [[Data transfer rate|data rates]] in high-[[Bandwidth (computing)|bandwidth]] magnetic recording, compounded by the fact that increased storage density typically requires a higher coercivity in the media.
== Theory ==
At the coercive field, the [[Vector (spatial)|vector component]] of the magnetization of a ferromagnet measured along the applied field direction is zero. There are two primary modes of magnetization reversal: rotation and [[domain wall]] motion. When the magnetization of a material reverses by rotation, the magnetization component along the applied field is zero because the vector points in a direction orthogonal to the applied field. When the magnetization reverses by domain wall motion, the net magnetization is small in every vector direction because the moments of all the individual domains sum to zero. Magnetization curves dominated by rotation and [[magnetocrystalline anisotropy]] are found in relatively perfect magnetic materials used in fundamental research[http://www3.interscience.wiley.com/cgi-bin/fulltext/109858911/PDFSTART]. Domain wall motion is a more important reversal mechanism in real engineering materials since defects like [[grain boundary|grain boundaries]] and [[impurity|impurities]] serve as [[nucleation]] sites for reversed-magnetization domains. The role of domain walls in determining coercivity is complex since defects may '''pin''' domain walls in addition to nucleating them. The dynamics of domain walls in ferromagnets is similar to that of grain boundaries and [[Plasticity (physics)|plasticity]] in [[metallurgy]] since both domain walls and grain boundaries are planar defects.
== Significance ==
As with any [[hysteresis|hysteretic]] process, the area inside the magnetization curve during one cycle is [[Work (thermodynamics)|work]] that is performed on the magnet. Common dissipative processes in magnetic materials include [[magnetostriction]] and domain wall motion. The coercivity is a measure of the degree of magnetic hysteresis and therefore characterizes the lossiness of soft magnetic materials for their common applications.
The '''squareness''' (M(H=0)/M<sub>s</sub>){{Fact|date=December 2007}} and coercivity are figures of merit for hard magnets although '''energy product''' (saturation magnetization times coercivity) is most commonly quoted. The 1980s saw the development of rare earth boride magnets with high energy products but undesirably low [[Curie temperature]]s. Since the 1990s new [http://dx.doi.org/10.1109/20.102931 exchange spring] hard magnets with high coercivities have been developed.
==See also==
==References==
{{reflist}}
*J.D. Livingston, [http://dx.doi.org/10.1063/1.328996 "A review of coercivity mechanisms,"] J. Appl. Phys. 52, 2541 (1981).
*R. V. Lapshin, [http://www.nanoworld.org/homepages/lapshin/publications.htm#analytical1995 “Analytical model for the approximation of hysteresis loop and its application to the scanning tunneling microscope”], Review of Scientific Instruments, volume 66, number 9, pages 4718-4730, 1995.
*Min Chen and David E. Nikles, "Synthesis, Self-Assembly, and Magnetic Properties of Fe<sub>x</sub>Co<sub>y</sub>Pt<sub>100-x-y</sub>Nanoparticles," Nano Lett. '''2''', 211 -214 (2002).
== External links ==
* [http://www.bama.ua.edu/~tmewes/Java/Reversal/reversal.shtml Magnetization reversal applet (coherent rotation)]
* For a table of coercivities of various magnetic recording media, see "[http://www.fujifilmusa.com/JSP/fuji/epartners/bin/Degauss_Data_Tape.pdf Degaussing Data Storage Tape Magnetic Media]" ([[PDF]]), at fujifilmusa.com.
[[Category:Physical quantity]]
[[Category:Condensed matter physics]]
[[Category:Electric and magnetic fields in matter]]
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