Demagnetising

Demagnetising, or demagnetization, is the process of reducing or eliminating the residual magnetization in a material or device. It is used to minimize unwanted magnetic fields that can interfere with nearby components, skew measurements, or affect performance. The concept applies to ferromagnetic materials, magnetic storage media, and permanent magnets.

Several methods are commonly employed. Thermal demagnetization involves heating the material above its Curie temperature (or near it) and then letting it cool in zero external field, allowing magnetic domains to randomize. Alternate-field demagnetization, or degaussing, passes the object through a coil carrying an alternating magnetic field whose amplitude is gradually reduced to zero, driving the net magnetization toward randomness. Reverse-field demagnetization applies a strong magnetic field in the opposite direction to the current magnetization to reduce the net magnetic moment.

Materials differ in their ease of demagnetization. Soft magnetic materials with low coercivity are typically easy to demagnetize, whereas hard permanent magnets require stronger fields or thermal treatment. Degaussing is widely used to erase data on magnetic storage media and to remove stray fields in instruments and assemblies. Thermal methods may affect mechanical properties or coatings, so care is needed with temperature-sensitive components.

Measurement of remaining magnetization is done with instruments such as gauss meters or magnetometers, which quantify residual flux density. Complete demagnetization is not always possible for very strong permanent magnets, but the goal is to reduce stray magnetism to acceptable levels for a given application.