Coercivity

Last updated
A family of hysteresis loops for grain-oriented electrical steel, a soft magnetic material. BR denotes retentivity and HC is the coercivity. The wider the outside loop is, the higher the coercivity. Movement on the loops is counterclockwise. B-H loop.png
A family of hysteresis loops for grain-oriented electrical steel, a soft magnetic material. BR denotes 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.

Contents

An analogous property in electrical engineering and materials science, electric coercivity, is the ability of a 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 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.

Definitions

Graphical definition of different coercivities in flux-vs-field hysteresis curve (B-H curve), for a hypothetical hard magnetic material. Coercivities in B-H curve.svg
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. Coercivities in M-H curve.svg
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
MaterialCoercivity
(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 pole19 [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 magnetic hysteresis loop, also called the magnetization curve, as illustrated in the figure above. 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.[ 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 relax to zero. Relaxation occurs when reversal of magnetization by domain wall motion is thermally activated and is dominated by 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 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. [23] 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.[ 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

Related Research Articles

<span class="mw-page-title-main">Ferromagnetism</span> Mechanism by which materials form into and are attracted to magnets

Ferromagnetism is a property of certain materials that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagnetic materials are familiar metals that are noticeably attracted to a magnet, a consequence of their substantial magnetic permeability. Magnetic permeability describes the induced magnetization of a material due to the presence of an external magnetic field. This temporarily induced magnetization, for example, inside a steel plate, accounts for its attraction to the permanent magnet. Whether or not that steel plate acquires a permanent magnetization itself depends not only on the strength of the applied field but on the so-called coercivity of the ferromagnetic material, which can vary greatly.

<span class="mw-page-title-main">Magnet</span> Object that has a magnetic field

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, cobalt, etc. and attracts or repels other magnets.

<span class="mw-page-title-main">Hysteresis</span> Dependence of the state of a system on its history

Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of the moment often form a loop or hysteresis curve, where there are different values of one variable depending on the direction of change of another variable. This history dependence is the basis of memory in a hard disk drive and the remanence that retains a record of the Earth's magnetic field magnitude in the past. Hysteresis occurs in ferromagnetic and ferroelectric materials, as well as in the deformation of rubber bands and shape-memory alloys and many other natural phenomena. In natural systems it is often associated with irreversible thermodynamic change such as phase transitions and with internal friction; and dissipation is a common side effect.

Magnetostriction is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of iron.

<span class="mw-page-title-main">Magnetic refrigeration</span> Phenomenon in which a suitable material can be cooled by a changing magnetic field

Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators.

Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. 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".

<span class="mw-page-title-main">Magnetic hysteresis</span> Application of an external magnetic field to a ferromagnet

Magnetic hysteresis occurs when an external magnetic field is applied to a ferromagnet such as iron and the atomic dipoles align themselves with it. Even when the field is removed, part of the alignment will be retained: the material has become magnetized. Once magnetized, the magnet will stay magnetized indefinitely. To demagnetize it requires heat or a magnetic field in the opposite direction. This is the effect that provides the element of memory in a hard disk drive.

<span class="mw-page-title-main">Rock magnetism</span> The study of magnetism in rocks

Rock magnetism is the study of the magnetic properties of rocks, sediments and soils. The field arose out of the need in paleomagnetism to understand how rocks record the Earth's magnetic field. This remanence is carried by minerals, particularly certain strongly magnetic minerals like magnetite. An understanding of remanence helps paleomagnetists to develop methods for measuring the ancient magnetic field and correct for effects like sediment compaction and metamorphism. Rock magnetic methods are used to get a more detailed picture of the source of the distinctive striped pattern in marine magnetic anomalies that provides important information on plate tectonics. They are also used to interpret terrestrial magnetic anomalies in magnetic surveys as well as the strong crustal magnetism on Mars.

A samarium–cobalt (SmCo) magnet, a type of rare-earth magnet, is a strong permanent magnet made of two basic elements: samarium and cobalt.

<span class="mw-page-title-main">Barkhausen effect</span>

The Barkhausen effect is a name given to the noise in the magnetic output of a ferromagnet when the magnetizing force applied to it is changed. Discovered by German physicist Heinrich Barkhausen in 1919, it is caused by rapid changes of size of magnetic domains.

<span class="mw-page-title-main">Ferrite (magnet)</span> Ferrimagnetic ceramic material composed of rust and a metallic element

A ferrite is a ceramic material made by mixing and firing iron(III) oxide with one or more additional metallic elements, such as strontium, barium, manganese, nickel, and zinc. They are ferrimagnetic, meaning they are attracted by magnetic fields and can be magnetized to become permanent magnets. Unlike other ferromagnetic materials, most ferrites are not electrically conductive, making them useful in applications like magnetic cores for transformers to suppress eddy currents. Ferrites can be divided into two families based on their resistance to being demagnetized.

Exchange bias or exchange anisotropy occurs in bilayers of magnetic materials where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the soft magnetization curve of a ferromagnetic film. The exchange bias phenomenon is of tremendous utility in magnetic recording, where it is used to pin the state of the readback heads of hard disk drives at exactly their point of maximum sensitivity; hence the term "bias."

Gallium manganese arsenide, chemical formula (Ga,Mn)As is a magnetic semiconductor. It is based on the world's second most commonly used semiconductor, gallium arsenide,, and readily compatible with existing semiconductor technologies. Differently from other dilute magnetic semiconductors, such as the majority of those based on II-VI semiconductors, it is not paramagnetic but ferromagnetic, and hence exhibits hysteretic magnetization behavior. This memory effect is of importance for the creation of persistent devices. In (Ga,Mn)As, the manganese atoms provide a magnetic moment, and each also acts as an acceptor, making it a p-type material. The presence of carriers allows the material to be used for spin-polarized currents. In contrast, many other ferromagnetic magnetic semiconductors are strongly insulating and so do not possess free carriers. (Ga,Mn)As is therefore a candidate as a spintronic material.

In magnetism, a nanomagnet is a nanoscopic scale system that presents spontaneous magnetic order (magnetization) at zero applied magnetic field (remanence).

The article Ferromagnetic material properties is intended to contain a glossary of terms used to describe ferromagnetic materials, and magnetic cores.

In magnetism, single domain refers to the state of a ferromagnet in which the magnetization does not vary across the magnet. A magnetic particle that stays in a single domain state for all magnetic fields is called a single domain particle. Such particles are very small. They are also very important in a lot of applications because they have a high coercivity. They are the main source of hardness in hard magnets, the carriers of magnetic storage in tape drives, and the best recorders of the ancient Earth's magnetic field.

In electromagnetism, the Stoner–Wohlfarth model is a widely used model for the magnetization of ferromagnets with a single-domain. It is a simple example of magnetic hysteresis and is useful for modeling small magnetic particles in magnetic storage, biomagnetism, rock magnetism and paleomagnetism.

<span class="mw-page-title-main">Demagnetizing field</span> Internal magnetic field generated by a magnet

The demagnetizing field, also called the stray field, is the magnetic field (H-field) generated by the magnetization in a magnet. The total magnetic field in a region containing magnets is the sum of the demagnetizing fields of the magnets and the magnetic field due to any free currents or displacement currents. The term demagnetizing field reflects its tendency to act on the magnetization so as to reduce the total magnetic moment. It gives rise to shape anisotropy in ferromagnets with a single magnetic domain and to magnetic domains in larger ferromagnets.

A domain wall is a term used in physics which can have similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.

<span class="mw-page-title-main">Exchange spring magnet</span>

An exchange spring magnet is a magnetic material with high coercivity and high saturation properties derived from the exchange interaction between a hard magnetic material and a soft magnetic material, respectively.

References

  1. 1 2 Giorgio Bertotti (21 May 1998). Hysteresis in Magnetism: For Physicists, Materials Scientists, and Engineers. Elsevier Science. ISBN   978-0-08-053437-4.
  2. ↑ Tumanski, S. (2011). Handbook of magnetic measurements. Boca Raton, FL: CRC Press. ISBN   9781439829523.
  3. ↑ 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.
  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. 1 2 3 "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. ↑ 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.
  9. ↑ Zhenghong Qian; Geng Wang; Sivertsen, J.M.; Judy, J.H. (1997). "Ni Zn ferrite thin films prepared by Facing Target Sputtering". IEEE Transactions on Magnetics. 33 (5): 3748–3750. Bibcode:1997ITM....33.3748Q. doi:10.1109/20.619559.
  10. ↑ Orloff, Jon (2017-12-19). Handbook of Charged Particle Optics, Second Edition. CRC Press. ISBN   9781420045550 . Retrieved 22 November 2014.
  11. ↑ Luo, Hongmei; Wang, Donghai; He, Jibao; Lu, Yunfeng (2005). "Magnetic Cobalt Nanowire Thin Films". The Journal of Physical Chemistry B. 109 (5): 1919–22. doi:10.1021/jp045554t. PMID   16851175.
  12. ↑ "Cast ALNICO Permanent Magnets" (PDF). Arnold Magnetic Technologies. Retrieved 4 November 2023.
  13. ↑ Yang, M.M.; Lambert, S.E.; Howard, J.K.; Hwang, C. (1991). "Laminated CoPt Cr/Cr films for low noise longitudinal recording". IEEE Transactions on Magnetics. 27 (6): 5052–5054. Bibcode:1991ITM....27.5052Y. doi:10.1109/20.278737.
  14. ↑ 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.
  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. ↑ Bai, G.; Gao, R.W.; Sun, Y.; Han, G.B.; Wang, B. (January 2007). "Study of high-coercivity sintered NdFeB magnets". Journal of Magnetism and Magnetic Materials. 308 (1): 20–23. Bibcode:2007JMMM..308...20B. doi:10.1016/j.jmmm.2006.04.029.
  18. ↑ Jiang, H.; Evans, J.; O’Shea, M.J.; Du, Jianhua (2001). "Hard magnetic properties of rapidly annealed NdFeB thin films on Nb and V buffer layers". Journal of Magnetism and Magnetic Materials. 224 (3): 233–240. Bibcode:2001JMMM..224..233J. doi:10.1016/S0304-8853(01)00017-8.
  19. ↑ Nakamura, H.; Kurihara, K.; Tatsuki, T.; Sugimoto, S.; Okada, M.; Homma, M. (October 1992). "Phase Changes and Magnetic Properties of Sm 2 Fe 17 N x Alloys Heat-Treated in Hydrogen". IEEE Translation Journal on Magnetics in Japan. 7 (10): 798–804. doi:10.1109/TJMJ.1992.4565502.
  20. ↑ Rani, R.; Hegde, H.; Navarathna, A.; Cadieu, F. J. (15 May 1993). "High coercivity Sm 2 Fe 17 N x and related phases in sputtered film samples". Journal of Applied Physics. 73 (10): 6023–6025. Bibcode:1993JAP....73.6023R. doi:10.1063/1.353457. INIST   4841321.
  21. ↑ de Campos, M. F.; Landgraf, F. J. G.; Saito, N. H.; Romero, S. A.; Neiva, A. C.; Missell, F. P.; de Morais, E.; Gama, S.; Obrucheva, E. V.; Jalnin, B. V. (1998-07-01). "Chemical composition and coercivity of SmCo5 magnets". Journal of Applied Physics. 84 (1): 368–373. Bibcode:1998JAP....84..368D. doi:10.1063/1.368075. ISSN   0021-8979.
  22. ↑ Gaunt 1986
  23. ↑ Genish et al. 2004
  24. ↑ Kneller & Hawig 1991
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.