coercivity

Coercivity is a measure of a ferromagnetic material's resistance to demagnetization. It is defined as the external magnetic field required to reduce the material’s magnetization to zero after the material has been saturated in a given direction. In practice, coercivity is read from a magnetization versus magnetic field (M–H) hysteresis loop and is denoted Hc.

Two related quantities are intrinsic coercivity (Hci) and practical or observed coercivity (Hc). Hci reflects the material’s microstructure and magnetic anisotropy and is largely independent of sample shape or demagnetizing effects. Observed Hc depends on geometry, defects, texture, and processing.

Measurement and interpretation: On a major hysteresis loop, Hc is the field value at which the magnetization crosses zero during the reversal from saturation. The corresponding field where magnetic flux density B becomes zero is related but distinct, as B can differ from M due to demagnetizing factors.

Units and scales: In SI units, Hc is expressed in amperes per meter (A/m). Older literature uses kiloamperes per meter (kA/m); cgs units use Oersted. Materials are often described as having low coercivity (soft magnets) or high coercivity (hard magnets). Soft magnetic materials have relatively small Hc, enabling easy magnetization and demagnetization, while hard magnetic materials, such as permanent magnets, exhibit high Hc and retain magnetization against demagnetizing forces.

Influencing factors include chemical composition, crystal structure, magnetic anisotropy, grain size, defects, texture, temperature, and mechanical stress. Applications span permanent magnets, electric motors, transformers, magnetic sensing, and data storage.

See also: magnetic hysteresis, remanence, saturation.