Rémanence

Rémanence, in magnetism, refers to the residual magnetization that remains in a ferromagnetic material after the external magnetic field has been removed. It is commonly described in terms of the remanent flux density, Br, which is the magnetization left by the material once the applied field is reduced to zero. The phenomenon is a consequence of the magnetic domain structure: when a material is exposed to a magnetic field, domains align, and after the field is removed, some degree of this alignment persists, creating a permanent internal magnetic field.

The size of the remanence depends on the material and its history. Factors such as composition, microstructure, temperature, and mechanical stress influence Br and the overall magnetic hysteresis. In the context of hysteresis, remanence is the point on the B-H curve that remains when the external field is reduced to zero after saturation. Remanence is always less than or equal to the saturation magnetization (Ms). The coercive field, Hc, describes the reverse field needed to demagnetize the material, and it distinguishes soft magnetic materials from hard, high-retentivity magnets.

Rémanence has practical significance in several areas. Permanent magnets rely on high remanence to retain magnetic performance without a continual external field. In data storage and electronic devices, remanent magnetization helps determine stability and readout characteristics. In geology and paleomagnetism, rocks can acquire remanent magnetization—thermoremanent, chemical, or detrital—that records the direction and intensity of Earth's magnetic field in the past, aiding studies of plate tectonics and ancient environments.

See also: magnetization, hysteresis, ferromagnetism, permanent magnets, paleomagnetism.