Retentivity

Retentivity, in magnetism, is the ability of a ferromagnetic material to retain magnetization after the external magnetic field has been removed. It is the residual magnetization, typically denoted Mr, i.e., the magnetization remaining at H = 0. The corresponding magnetic flux density left in the material is the remanent flux density Br. Retentivity is read from the hysteresis loop as the value of M when the applied field returns to zero; the remanence is the analogous quantity in B.

Measurement and relation: The magnetization curve M–H is obtained by saturating the sample, reducing the field to zero, and measuring M. At H = 0, M = Mr and B = Br. In many texts Br ≈ μ0 Mr in a specimen with negligible demagnetizing effects.

Factors and materials: Retentivity depends on magnetic anisotropy, crystallographic structure, grain size, impurities, defects, and temperature. Materials with high retentivity, known as hard magnetic materials, include NdFeB and SmCo permanent magnets, and certain iron alloys; these retain substantial magnetization and resist demagnetization. Soft magnetic materials, such as pure iron or permalloy, have low retentivity and high permeability, allowing rapid response but little residual magnetization.

Applications and behavior: High retentivity is desirable for permanent magnets and data storage, while low retentivity is advantageous in transformer cores and other devices requiring easy demagnetization. Thermal effects: Retentivity generally decreases with increasing temperature and falls toward zero near the Curie temperature.

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