Coercivity

Source: Wikipedia, the free encyclopedia.
This article is about the property of magnetic fields. For other uses, see Coercion (disambiguation).
retentivity
and HC is the coercivity. The wider the outside loop is, the higher the coercivity. Movement on the loops is counterclockwise.

Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a

ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in oersted or ampere
/meter units and is denoted HC.

An analogous property in

ferroelectric material to withstand an external electric field without becoming depolarized
.

Ferromagnetic materials with high coercivity are called magnetically hard, and are used to make

magnetic shielding
.

Definitions

Graphical definition of different coercivities in flux-vs-field hysteresis curve (B-H curve), for a hypothetical hard magnetic material.
Equivalent definitions for coercivities in terms of the magnetization-vs-field (M-H) curve, for the same magnet.

Coercivity in a

ferromagnetic material is the intensity of the applied magnetic field (H field) required to demagnetize that material, after the magnetization of the sample has been driven to saturation
by a strong field. This demagnetizing field is applied opposite to the original saturating field. There are however different definitions of coercivity, depending on what counts as 'demagnetized', thus the bare term "coercivity" may be ambiguous:

The distinction between the normal and intrinsic coercivity is negligible in soft magnetic materials, however it can be significant in hard magnetic materials.[1] The strongest rare-earth magnets lose almost none of the magnetization at HCn.

Experimental determination

Coercivities of some magnetic materials
Material Coercivity
(kA/m)
Supermalloy
(16Fe:79Ni:5Mo) 		0.0002[2]: 131, 133 
Permalloy (Fe:4Ni) 0.0008–0.08[3]
Iron filings (0.9995 wt) 0.004–37.4[4][5]
Electrical steel (11Fe:Si) 0.032–0.072[6]
Raw iron (1896) 0.16[7]
Nickel (0.99 wt) 0.056–23[5][8]
Ferrite magnet
(ZnxFeNi1−xO3) 		1.2–16[9]
2Fe:Co,[10] iron pole 19[5]
Cobalt (0.99 wt) 0.8–72[11]
Alnico 30–150[12]
Disk drive recording medium
(Cr:Co:Pt) 		140[13]
Neodymium magnet (NdFeB) 800–950[14][15]
12Fe:13Pt (Fe48Pt52) ≥980[16]
?(Dy,Nb,Ga(Co):2Nd:14Fe:B) 2040–2090[17][18]
Samarium-cobalt magnet
(2Sm:17Fe:3N; 10 K) 		<40–2800[19][20]
Samarium-cobalt magnet 3200[21]

Typically the coercivity of a magnetic material is determined by measurement of the

antiferromagnet is present in the sample, the coercivities measured in increasing and decreasing fields may be unequal as a result of the exchange bias effect.[citation needed
]

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

magnetic viscosity.[22] The increasing value of coercivity at high frequencies is a serious obstacle to the increase of data rates in high-bandwidth magnetic recording, compounded by the fact that increased storage density typically requires a higher coercivity in the media.[citation needed
]

Theory

At the coercive field, the

impurities serve as nucleation sites for reversed-magnetization domains. The role of domain walls in determining coercivity is complicated 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 in metallurgy since both domain walls and grain boundaries are planar defects.[citation needed
]

Significance

As with any hysteretic process, the area inside the magnetization curve during one cycle represents the work that is performed on the material by the external field in reversing the magnetization, and is dissipated as heat. 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 saturation remanence and coercivity are figures of merit for hard magnets, although maximum energy product is also commonly quoted. The 1980s saw the development of rare-earth magnets with high energy products but undesirably low Curie temperatures. Since the 1990s new exchange spring hard magnets with high coercivities have been developed.[24]

See also

References

  1. ^
    ISBN 978-0-08-053437-4
    .
  2. ISBN 9781439829523
    .
  3. doi:10.1063/1.365100
    .
  4. ^ Calvert, J. B. (6 December 2003) [13 December 2002]. "Iron". mysite.du.edu. Archived from the original on 2007-09-15. Retrieved 2023-11-04.
  5. ^ a b c "Magnetic Properties of Solids". Hyperphysics.phy-astr.gsu.edu. Retrieved 22 November 2014.
  6. ^ "timeout". Cartech.ides.com. Retrieved 22 November 2014.[permanent dead link]
  7. ^ Thompson, Silvanus Phillips (1896). Dynamo-electric machinery. Retrieved 22 November 2014.
  8. doi:10.1063/1.355560
    .
  9. doi:10.1109/20.619559
    .
  10. ISBN 9781420045550
    . Retrieved 22 November 2014.
  11. PMID 16851175
    .
  12. ^ "Cast ALNICO Permanent Magnets" (PDF). Arnold Magnetic Technologies. Retrieved 4 November 2023.
  13. doi:10.1109/20.278737
    .
  14. doi:10.1063/1.353563
    .
  15. ^ "WONDERMAGNET.COM - NdFeB Magnets, Magnet Wire, Books, Weird Science, Needful Things". Wondermagnet.com. Archived from the original on 11 February 2015. Retrieved 22 November 2014.
  16. ^ Chen & Nikles 2002
  17. doi:10.1016/j.jmmm.2006.04.029
    .
  18. doi:10.1016/S0304-8853(01)00017-8
    .
  19. doi:10.1109/TJMJ.1992.4565502
    .
  20. doi:10.1063/1.353457. INIST 4841321
    .
  21. ISSN 0021-8979
    .
  22. ^ Gaunt 1986
  23. ^ Genish et al. 2004
  24. ^ Kneller & Hawig 1991

External links
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