Coercivity, also called themagnetic coercivity,coercive field orcoercive force, is a measure of the ability of aferromagnetic material to withstand an externalmagnetic field without becomingdemagnetized. Coercivity is usually measured inoersted orampere/meter units and is denotedHC.
An analogous property inelectrical engineering andmaterials science,electric coercivity, is the ability of aferroelectric material to withstand an externalelectric field without becomingdepolarized.
Ferromagnetic materials with high coercivity are called magneticallyhard, and are used to makepermanent magnets. Materials with low coercivity are said to be magneticallysoft. The latter are used intransformer andinductorcores,recording heads,microwave devices, andmagnetic shielding.
Coercivity in aferromagnetic material is the intensity of the appliedmagnetic field (H field) required to demagnetize that material, after the magnetization of the sample has been driven tosaturation 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 strongestrare-earth magnets lose almost none of the magnetization atHCn.
| Material | Coercivity (kA/m) |
|---|---|
| Supermalloy (16Fe:79Ni:5Mo) | 0.0002[2]: 131, 133 |
| Permalloy (Fe:4Ni) | 0.0008–0.08[3] |
| Iron filings (0.9995wt) | 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 themagnetic hysteresis loop, also called themagnetization curve, as illustrated in the figure above. The apparatus used to acquire the data is typically avibrating-sample or alternating-gradientmagnetometer. The applied field where the data line crosses zero is the coercivity. If anantiferromagnet is present in the sample, the coercivities measured in increasing and decreasing fields may be unequal as a result of theexchange 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, slowlyrelax to zero. Relaxation occurs when reversal of magnetization by domain wall motion isthermally activated and is dominated bymagnetic viscosity.[22] The increasing value of coercivity at high frequencies is a serious obstacle to the increase ofdata rates in high-bandwidth magnetic recording, compounded by the fact that increased storage density typically requires a higher coercivity in the media.[citation needed]
At the coercive field, thevector component of the magnetization of a ferromagnet measured along the applied field direction is zero. There are two primary modes ofmagnetization reversal:single-domain rotation anddomain 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 andmagnetocrystalline 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 likegrain boundaries andimpurities serve asnucleation sites for reversed-magnetization domains. The role of domain walls in determining coercivity is complicated since defects maypin domain walls in addition to nucleating them. The dynamics of domain walls in ferromagnets is similar to that of grain boundaries andplasticity inmetallurgy since both domain walls and grain boundaries are planar defects.[citation needed]
As with anyhysteretic process, the area inside the magnetization curve during one cycle represents thework 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 includemagnetostriction 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, althoughmaximum energy product is also commonly quoted. The 1980s saw the development ofrare-earth magnets with high energy products but undesirably lowCurie temperatures. Since the 1990s newexchange spring hard magnets with high coercivities have been developed.[24]
