unmagnetization

Unmagnetization, commonly called demagnetization, is the reduction or elimination of a material's net magnetization. It is applied to ferromagnetic and ferrimagnetic materials, including permanent magnets, to minimize magnetic bias, interference, or retained magnetic memory. Complete removal of magnetization is often impractical for some materials, but the goal is to lower residual magnetization to acceptable levels for a given application.

The process relies on disturbing the alignment of magnetic domains. Heating above the Curie temperature (for a ferromagnet) or exposing the material to a rapidly varying magnetic field (AC demagnetization or degaussing) causes domains to randomize. After the field is removed and the material cools in zero field, the domains tend to settle in random orientations, reducing the net moment. Other methods include applying a decreasing alternating-field sequence that gradually reduces the remaining magnetization.

Common techniques include degaussing with a coil that applies an AC magnetic field whose amplitude decays over several cycles; thermal demagnetization by controlled heating; and reverse-field demagnetization using a magnetizing field opposite to the original magnetization. Some components are designed to withstand minor remanence and are demagnetized during maintenance.

Applications include preventing interference in sensors and measuring devices, ensuring safe handling of lifting magnets, removing tool magnetization to prevent sticking of metallic parts, and ensuring consistent behavior in magnetic storage or timing devices. Limitations include that not all materials demagnetize fully and some magnetization can reappear if stored near strong magnetic fields. See also demagnetization, degaussing, magnetization.