Coercivity

Coercivity (coercivity), also known as coercivity or coercivity, is one of the characteristics of magnetic materials. It refers to the magnetic field strength required to reduce the magnetization to zero after the magnetic material has been magnetized to magnetic saturation. Coercive force represents the ability of a magnetic material to resist demagnetization. It will be represented by the symbol of HC, and the unit is A/m (international standard system) or Oe (gauss unit system). The coercivity can be measured with a magnetometer or B-H analyzer.

　　 If the coercivity of ferromagnetic materials (including ferrimagnetic materials) is large, it is called hard magnetism and can be used as the material of the *** magnet. ****** Magnets can be used in motors, magnetic storage media (such as hard drives, disks or tapes), and magnetic separators in ore processing.

Ferromagnetic materials with low coercivity are called soft magnetic materials, which can be used in the iron cores of transformers and inductors, recording heads of magnetic storage media, microwave equipment, and electromagnetic shielding (English: Electromagnetic shielding) In the device.

　　The coercivity of magnetic materials is generally obtained by measuring the hysteresis curve (also called the magnetization curve). The instrument that collects data generally uses a vibrating sample magnetometer or an alternating gradient magnetometer. When the measured magnetic flux density data is zero, the corresponding magnetic field strength is the coercive force. . If the product contains ferromagnetic materials, the measured coercivity will be different when the magnetic field increases and decreases. This is due to the effect of the exchange bias field (English: exchange bias).

　　The measured coercivity of the material is also related to the time it takes to measure the magnetization curve. The magnetization of a magnet material measured in a reverse magnetic field will be less than the coercive force, and if it is measured under the same conditions for a long time, its value will relax to zero. The relaxation phenomenon is due to the effect of heat generated by the magnetic domain under a reverse magnetic field, and is also affected by the magnetic viscosity [1]. The coercivity of some materials increases with frequency. This is a major obstacle for high-bandwidth (English: bandwidth (computing)) magnetic storage devices to increase data speed, because increasing storage density means more storage devices. High coercivity.

　　The coercivity of soft ferromagnetic materials and hard ferromagnetic materials

　　 The "hardness" of ferromagnetic materials increases as the crystal becomes larger

　　 decreases with the increase of smoothness and glass quality

Material

Coercivity
[Oe (A/m)]

[.1Mn:]6Fe:27Ni:Mo, super alloy

0.002(0.16)

Fe:4Ni, Permalloy

0.01–1(0.8-80)

.9995 Iron

0.05–470 (4-37,000)

11Fe:Si, silicon steel

0.4–0.9(32-72)

Wrought iron (1896)

2 (160)

.99 Nickel

0.7–290(56-23,000)

ZnxFeNi1-xO3,
Ferrite used in multi-cavity magnetron

15–200 (1200-16,000)

2Fe:Co[10], magnet rod

240(19,000)

>.99 Cobalt

10-900 (800-72,000)

6Al:18Fe:8Co:Cu:6Ni--
3Ti:8Al:20Fe:20Co:2Cu:8Ni, Al-Ni-Co
alnico 5-9, refrigerator magnet or stronger magnet

640–2000(51,000-1.6*105)

Cr:Co:Pt,
Hard disk storage medium

1700 (1.4*105)

2Nd:14Fe:B, neodymium iron boron magnet

10,000–12,000 ((8-9.5)*105)

12Fe:13Pt, Fe48Pt52

12,300 (9.8*105)

? (Dy,Nb,Ga,Co):2Nd:14Fe:B

25,600/–26,300 (2*106)

2Sm:17Fe:3N, samarium-iron-nitrogen (10 K)

<500–35,000 (40,000-2.8*106)

Sm:5Co, samarium cobalt magnet

40,000 (3.2*106)

　　 If a ferromagnetic material is placed in an external magnetic field with coercive force, the component of the magnetization along the direction of the external magnetic field is zero. There are two ways of magnetization reversal: single domain (magnetic) rotation and domain wall (magnetism) motion. When the magnetization of the material is reversed due to single domain rotation, the component of the magnetization extending the direction of the applied magnetic field is zero, and the magnetization will be perpendicular to the applied magnetic field. When the magnetization of the material is reversed due to the movement of the magnetic domain walls, the sum of the magnetization of all small magnetic domains is close to zero, and the total magnetization is very small. In some basic research, the magnetization of ideal magnetic materials is mainly affected by single domain rotation and magnetocrystalline anisotropy (English: magnetocrystalline anisotropy) [23]. In the magnetic materials used in practical engineering, impurities (English: Impurity) and grain boundaries (English: grain boundary) are the nucleation sources of the reverse magnetization magnetic domain. At this time, the magnetization reversal is mainly controlled by the movement of the magnetic domain wall. However, the influence of the magnetic domain wall motion on the coercivity is quite complicated, so crystal defects may be the source of nucleation, but they may also fix the magnetic domain wall. The role of magnetic domain walls in ferromagnetic materials is similar to the role of grain boundaries in plastic deformation, because both magnetic domain walls and grain boundaries are surface defects in crystal defects.

　　 If the material has hysteresis and has been magnetized, the area in the magnetization curve is the work required to apply a reverse applied magnetic field to the material to make the material have reverse magnetization. This energy will eventually be lost in the form of heat. Common dissipation processes in magnetic materials include magnetostriction and the movement of magnetic domain walls. Coercivity can be used to measure the degree of hysteresis, and can also be used as an indicator of loss in soft ferromagnetic materials in general applications.

The squareness of a magnetic material is the quotient of the remanence divided by the coercivity. The squareness and coercivity of hard ferromagnetic materials are two important performance indicators of hard ferromagnetic materials, but the product of the two magnetic energy Product is more often mentioned.