Coercivity


Coercivity
A family of hysteresis loops for grain-oriented electrical steel (BR denotes remanence and HC is the coercivity).

In materials science, the coercivity, also called the coercive field or coercive force, of a 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. 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. Permanent magnets find application in electric motors, magnetic recording media (e.g. hard drives, floppy disks, or magnetic tape) and magnetic separation.

Materials with low coercivity are said to be magnetically soft. They are used in transformer and inductor cores, recording heads. microwave devices, and magnetic shielding.

Contents

Experimental determination

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,[1] 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.

Coercivities of soft and hard magnets:
hardness grows as crystal (domain) size and strew,
shrinks as smoothness or glassiness
Material Coercivity
[Oe]
[.1Mn:]6Fe:27Ni:Mo, Supermalloy 0.002
Fe:4Ni, Permalloy 0.011
.9995 iron–filings 0.05470
11Fe:Si, silicon iron 0.4–0.9
Raw iron 2 (1896)
.99 Nickel 0.7290
ZnxFeNi1-xO3,
ferrite for magnetron
15–200
2Fe:Co, Iron pole 240
>.99 cobalt 10900
6Al:18Fe:8Co:Cu:6Ni–
3Ti:8Al:20Fe:20Co:2Cu:8Ni
,
alnico 5–9, fridge magnet and stronger
6402000
Cr:Co:Pt,
disk drive recording media
1700
2Nd:14Fe:B, neodymium-iron-boron 10,00012,000
12Fe:13Pt, Fe48Pt52 12,300+[2]
 ?(Dy,Nb,Ga,Co):2Nd:14Fe:B 25,60026,300
2Sm:17Fe:3N, samarium-iron-nitrogen <50035,000 (10 K)
Sm:5Co, samarium-cobalt 40,000

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.[3] 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.[4] 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 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 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,[1] 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.[5]

See also

References

External links


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Look at other dictionaries:

  • coercivity — [kō΄ər siv′ə tē] n. the magnetic intensity, usually measured in oersteds, needed to completely demagnetize a substance that has been fully magnetized …   English World dictionary

  • coercivity —   adj.    the measure (usually expressed in Oersteds) of a magnetic material s resistance to being recorded or erased. Magnetic stripe cards are either low coercivity or high coercivity …   Locksmith dictionary

  • coercivity — noun Date: 1898 the property of a material determined by the value of the coercive force when the material has been magnetized to saturation …   New Collegiate Dictionary

  • coercivity — /koh euhr siv i tee/, n. Elect. the magnetic intensity needed to reduce to zero the magnetic flux density of a fully magnetized magnetic specimen or to demagnetize a magnet. Also called coercive force. [1895 1900; COERCIVE + ITY] * * * …   Universalium

  • coercivity — noun a) The quality of being coercive b) The intensity the magnetic field which must be applied in order to reduce the magnetization of a ferromagnetic material to zero after the magnetization of the sample has been driven to saturation Syn:… …   Wiktionary

  • coercivity — [ˌkəʊə sɪvɪti] noun Physics the resistance of a magnetic material to changes in magnetization, equivalent to the field intensity necessary to demagnetize the fully magnetized material …   English new terms dictionary

  • coercivity — co·er·civ·i·ty …   English syllables

  • coercivity — co•er•civ•i•ty [[t]ˌkoʊ ərˈsɪv ɪ ti[/t]] n. elm the magnetic intensity needed to reduce to zero the magnetic flux density of a fully magnetized magnetic specimen or to demagnetize a magnet • Etymology: 1895–1900 …   From formal English to slang

  • coercivity — ˌkōˌərˈsivəd.ē noun ( es) Etymology: coercive + ity : the property of a material determined by the value of the coercive force when the material has been magnetized to saturation …   Useful english dictionary

  • high coercivity —   adj.    a high energy form of magnetic encoding measuring in the range of 2400 to 4000 Oe which is resistant to accidental erasure …   Locksmith dictionary