1. Cooling treatment (for the case of reversible demagnetization)
When a magnet loses its magnetism due to a temperature increase, but the temperature has not exceeded the Curie temperature of the material by too much and the magnetic domain structure has not undergone irreversible changes, the magnetism can be restored by cooling. For example, for some ordinary ferrite magnets, if they lose their magnetism in an environment slightly above the normal working temperature, when they are placed in an environment at room temperature or a lower temperature, as the temperature drops, the thermal motion of atoms weakens and magnetic domains have the opportunity to rearrange, and the magnetism may gradually recover. This is because in a low-temperature environment, the orderly arrangement of magnetic domains is easier to achieve, just like under normal temperature conditions, and the regular arrangement of magnetic domains enables the magnet to exhibit its magnetism again.
2. Remagnetization (applicable to various situations)
• Using an external magnetic field for magnetization: If a magnet loses its magnetism at high temperatures and the magnetism does not recover or the recovery is not satisfactory after cooling, it can be remagnetized using an external magnetic field. For magnets made of different materials, the required strength of the external magnetic field and the magnetization methods may vary. For example, for Nd-Fe-B magnets, a magnetizer can be used to generate a strong magnetic field to remagnetize them. The demagnetized Nd-Fe-B magnet is placed in the magnetic field of the magnetizer, and under the action of the external strong magnetic field, the magnetic domains are rearranged, thus restoring its magnetism. However, the magnetism after remagnetization may be somewhat different from the original magnetism because the high temperature may have already had a certain impact on the internal structure of the magnet.
• Using the interaction between magnetic materials for magnetization: The magnetism can also be restored by bringing the demagnetized magnet into contact with a stronger magnetic material. For example, a small demagnetized magnet is closely attached to a large, strongly magnetic magnet for a period of time. Under the influence of the magnetic field of the large magnet, the internal magnetic domains of the small magnet will gradually be rearranged, thus restoring part of its magnetism. This method is relatively simple, but the degree of recovered magnetism may be limited, and for some magnets with severely damaged internal structures due to high temperatures, this method may not work well.
It should be noted that if a magnet exceeds the Curie temperature of the material by a large margin at high temperatures, resulting in irreversible changes to the magnetic domain structure, it will be very difficult or even impossible to restore the magnetism. For example, when Nd-Fe-B magnets are kept in an environment far above their Curie temperature (310 - 350 °C) for a long time and the internal magnetic domain structure is severely damaged, it is very difficult to restore them to their original magnetic state even by using the above methods.
