Demagnetised

Demagnetised, or demagnetized, describes a material whose magnetic moment has been reduced to near zero. In ferromagnetic materials, magnetic moments align in domains; the overall magnetization is the sum of these alignments. Demagnetisation removes or greatly reduces this net magnetization, either within a component or for an entire piece.

Common methods to achieve demagnetisation include heating the material above its Curie temperature, causing magnetic order to disappear, and then cooling in zero field so no preferred direction is established. A more practical method is AC demagnetisation or degaussing: the material is exposed to an alternating magnetic field whose amplitude is gradually reduced to zero, progressively randomizing domain orientations. Another method is to apply a strong magnetic field in the opposite direction and then remove the field, which can reverse and cancel existing magnetization. Mechanical shock can also disturb domain structures in some materials.

Applications and implications vary. Demagnetisation is used to erase data on magnetic storage media, to remove stray magnetic fields from tools, housings, or measuring instruments, and to prevent unwanted attraction or interference in sensitive equipment. It is also performed in manufacturing to return a material to a non-magnetic state after processing.

Notes: The term can be spelled demagnetised (British) or demagnetized (American). The effectiveness depends on material properties such as coercivity and magnetic history; some residual magnetization may remain after treatment, especially in high-coercivity hard magnets. Demagnetising a permanent magnet requires appropriate energy input and can risk damage if overheating occurs.