Coercivity

This article is about the property of magnetic fields. For other uses, see Coercion (disambiguation).
A family of hysteresis loops for grain-oriented electrical steel. BR denotes retentivity and HC is the coercivity. The wider the outside loop is, the higher the coercivity. Movement on the loops is counterclockwise.

In electrical engineering and materials science, the 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. An analogous property, electric coercivity, is the ability of a ferroelectric material to withstand an external electric field without becoming depolarized.

For ferromagnetic material the coercivity 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. Thus coercivity measures the resistance of a ferromagnetic material to becoming demagnetized. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC. It can be measured using a B-H analyzer or magnetometer.

Ferromagnetic materials with high coercivity are called magnetically hard materials, and are used to make permanent magnets. Materials with low coercivity are said to be magnetically soft. The latter are used in transformer and inductor cores, recording heads, microwave devices, and magnetic shielding.

Experimental determination

Coercivities of some magnetic materials:
Material Coercivity
[Oe (kA/m)]
Supermalloy
16Fe:79Ni:5Mo		 0.0025 (0.0002)[1]:131,133
Permalloy
Fe:4Ni		 0.01–1 (0.00080–0.07958)[2]
Iron filings, 0.9995 Fe 0.05–470 (0.0040–37.4014)[3][4]
Electrical steel (11Fe:Si) 0.4–0.9 (0.032–0.072)[5]
Raw iron (1896) 2 (0.16)[6]
Nickel, 0.99 pure 0.7–290 (0.056–23.077)[4][7]
Ferrite magnet
ZnxFeNi1-xO3		 15–200 (1.2–15.9)[8]
2Fe:Co,[9] Iron pole 240 (19)[4]
Cobalt, 0.99 pure
 10–900 (0.80–71.62)[10]
Alnico
 380–6,030 (30–480)[11]:146–148
Disk drive recording medium
Cr:Co:Pt		 1,700 (140)[12]
Neodymium magnet
NdFeB		 10,000–12,000 (800–950)[13][14]
12Fe:13Pt, Fe48Pt52 >= 12,300 (980)[15]
?(Dy,Nb,Ga,Co):2Nd:14Fe:B 25,600–26,300 (2,040–2,090)[16][17]
Samarium-cobalt magnet
2Sm:17Fe:3N (10 K)		 < 500–35,000 (40–2,785)[18][19]
Samarium-cobalt magnet 40,000 (3,200)[20]

Typically the coercivity of a magnetic material is determined by measurement of the magnetic hysteresis loop, also called the 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 line crosses zero is the coercivity. If an 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.

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 relax to zero. Relaxation occurs when reversal of magnetization by domain wall motion is thermally activated and is dominated by magnetic viscosity.[21] 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.

Theory

The coercivity is a horizontal intercept of the hysteresis loop.

At the coercive field, the vector component of the magnetization of a ferromagnet measured along the applied field direction is zero. There are two primary modes of magnetization reversal: single-domain 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.[22] Domain wall motion is a more important reversal mechanism in real engineering materials since defects like grain boundaries and 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.

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 squareness (saturation remanence divided by saturation magnetization) 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 magnets with high energy products but undesirably low Curie temperatures. Since the 1990s new exchange spring hard magnets with high coercivities have been developed.[23]

See also

References

  1. ↑ Tumanski, S. (2011). Handbook of magnetic measurements. Boca Raton, FL: CRC Press. ISBN 9781439829523.
  2. ↑ M. A. Akhter-D. J. Mapps-Y. Q. Ma Tan-Amanda Petford-Long-R. Doole; Mapps; Ma Tan; Petford-Long; Doole (1997). "Thickness and grain-size dependence of the coercivity in permalloy thin films". Journal of Applied Physics. 81 (8): 4122. Bibcode:1997JAP....81.4122A. doi:10.1063/1.365100.
  3. ↑ Archived February 4, 2008, at the Wayback Machine.
  4. 1 2 3 "Magnetic Properties of Solids". Hyperphysics.phy-astr.gsu.edu. Retrieved 22 November 2014.
  5. ↑ "timeout". Cartech.ides.com. Retrieved 22 November 2014.
  6. ↑ Dynamo-electric machinery. Books.google.com. Retrieved 22 November 2014.
  7. ↑ M. S. Miller-F. E. Stageberg-Y. M. Chow-K. Rook-L. A. Heuer; Stageberg; Chow; Rook; Heuer (1994). "Influence of rf magnetron sputtering conditions on the magnetic, crystalline, and electrical properties of thin nickel films". Journal of Applied Physics. 75 (10): 5779. Bibcode:1994JAP....75.5779M. doi:10.1063/1.355560. Retrieved 22 November 2014.
  8. ↑ Zhenghong Qian; Geng Wang; Sivertsen, J.M.; Judy, J.H. (1997). "IEEE Xplore - Sign In". IEEE Transactions on Magnetics. 33 (5): 3748–3750. Bibcode:1997ITM....33.3748Q. doi:10.1109/20.619559. Retrieved 22 November 2014.
  9. ↑ Handbook of Charged Particle Optics, Second Edition. Books.google.com. Retrieved 22 November 2014.
  10. ↑ Luo, Hongmei; Wang, Donghai; He, Jibao; Lu, Yunfeng (2005). "Magnetic Cobalt Nanowire Thin Films". The Journal of Physical Chemistry B. Pubs.acs.org. 109 (5): 1919–22. doi:10.1021/jp045554t. PMID 16851175. Retrieved 22 November 2014.
  11. ↑ Tumanski, S. (2011). Handbook of magnetic measurements. Boca Raton, FL: CRC Press. ISBN 9781439829523.
  12. ↑ Yang, M.M.; Lambert, S.E.; Howard, J.K.; Hwang, C. (1991). "IEEE Xplore - Sign In". IEEE Transactions on Magnetics. 27 (6): 5052–5054. Bibcode:1991ITM....27.5052Y. doi:10.1109/20.278737. Retrieved 22 November 2014.
  13. ↑ C. D. Fuerst-E. G. Brewer; Brewer (1993). "High‐remanence rapidly solidified Nd‐Fe‐B: Die‐upset magnets (invited)". Journal of Applied Physics. 73 (10): 5751. Bibcode:1993JAP....73.5751F. doi:10.1063/1.353563. Retrieved 22 November 2014.
  14. ↑ "WONDERMAGNET.COM - NdFeB Magnets, Magnet Wire, Books, Weird Science, Needful Things". Wondermagnet.com. Retrieved 22 November 2014.
  15. ↑ Chen & Nikles 2002
  16. ↑
  17. ↑
  18. ↑ Nakamura, H.; Kurihara, K.; Tatsuki, T.; Sugimoto, S.; Okada, M.; Homma, M. (1992). "IEEE Xplore - Sign In". IEEE Translation Journal on Magnetics in Japan. 7 (10): 798–804. doi:10.1109/TJMJ.1992.4565502. Retrieved 22 November 2014.
  19. ↑ "High coercivity Sm2Fe17Nx and related phases in sputtered film samples". Cat.inist.fr. Retrieved 22 November 2014.
  20. ↑ M. F. de Campos-F. J. G. Landgraf-N. H. Saito-S. A. Romero-A. C. Neiva-F. P. Missell-E. de Morais-S. Gama-E. V. Obrucheva-B. V. Jalnin; Landgraf; Saito; Romero; Neiva; Missell; De Morais; Gama; Obrucheva; Jalnin (1998). "Chemical composition and coercivity of SmCo5 magnets". Journal of Applied Physics. 84: 368. Bibcode:1998JAP....84..368D. doi:10.1063/1.368075. Retrieved 22 November 2014.
  21. ↑ Gaunt 1986
  22. ↑ Genish et al. 2004
  23. ↑ Kneller & Hawig 1991
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