In materials science, the coercivity, also called the coercive field, 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. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC.
When the coercive field of a ferromagnet is large, the material is said to be a hard or permanent magnet. Permanent magnets find application in electric motors, magnetic recording media (e.g. hard drives, floppy disks, or magnetic tape) and magnetic separation. A ferromagnet with a low coercive field is said to be soft and may be used in microwave devices, magnetic shielding, transformers or recording heads.
Coercivity can be measured using a B-H Analyzer.
Typically the coercivity of a magnetic material is determined by measurement of the hysteresis loop or 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 (called a magnetization curve) crosses zero is the coercivity. If an antiferromagnetic solid is present in the sample, the coercivities measured in increasing and decreasing fields may be unequal as a result of the exchange bias effect.
|Soft Wrought Iron||2|
|Ni1-xZnxFeO3, a microwave material||15-200|
|Alnico, a common refrigerator magnet||1500-2000|
|disk drive recording media||1700|
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 creep to zero. Creep occurs when reversal of magnetization by domain wall motion is thermally activated and is dominated by magnetic viscosity 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.
The squareness (M(H=0)/Ms) 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 boride magnets with high energy products but undesirably low Curie temperatures. Since the 1990s new exchange spring hard magnets with high coercivities have been developed.
WIPO PUBLISHES PATENT OF TOYOTA JIDOSHA FOR "COERCIVITY PERFORMANCE DETERMINATION DEVICE FOR COERCIVITY DISTRIBUTION MAGNET" (JAPANESE INVENTORS)
Feb 22, 2012; GENEVA, Feb. 20 -- Publication No. WO/2012/020479 was published on Feb. 16. Title of the invention: "Coercivity...
Wipo Publishes Patent of Nidec Sankyo, Yamamoto Hisashi for "Method for Determining Coercivity of Magnetic Recording Medium" (Japanese Inventor)
Dec 06, 2012; GENEVA, Dec. 6 -- Publication No. WO/2012/160871 was published on Nov. 29. Title of the invention: "METHOD FOR DETERMINING...