Koercivita

Koercivita, in magnetism, is the property of a ferromagnetic material that describes the intensity of the external magnetic field required to reduce the material's magnetization to zero after the material has been saturated. In SI units this coercive field is called Hc and is measured in amperes per meter (A/m); in older literature it is given in oersteds.

When a magnetic specimen is cycled through a hysteresis loop, the coercivity corresponds to the field value at which magnetization crosses zero during demagnetization. The remanent magnetization Br (or Mr) is the magnetization left at zero applied field after saturation; together with Hc this explains how a material behaves as a permanent magnet.

Materials are often described as soft or hard magnets based on their coercivity. Soft magnets have low coercivity, enabling easy magnetization and demagnetization; they are used in transformers and inductors. Hard magnets have high coercivity and retain magnetization, making them suitable for permanent magnets such as NdFeB or SmCo alloys.

Coercivity depends on intrinsic factors like magnetocrystalline anisotropy and extrinsic factors such as grain size, microstructure, impurities, porosity and temperature. Increasing anisotropy, optimizing grain boundaries and adding certain alloying elements can raise Hc, while temperature generally lowers coercivity.

Applications include electric motors, generators, magnetic storage media, and magnetic shielding. The concept is distinct from retentivity (remanence), which measures remaining magnetization at zero field.