Do Permanent Magnets ever Lose Their Magnetism or Get Weaker?
Actually permanent magnets are not permanent. A Permanent Magnet is a material which has ability to resist demagnetization, including filed demagnetization and thermal demagnetization. The ability is characterized by a physical parameter called coercivity.
In regard to field demagnetization, if a demagnetizing field or reverse field is smaller than a permanent magnet’s coercivity, the permanent magnet will keep the same magnetic flux (the demagnetizing process is linear and reversible when the demagnetizing field is lower than a threshold value) or will lose some flux (the demagnetizing process is nonlinear and irreversible when the demagnetizing field is higher than a threshold value) when the demagnetizing field removed. If a demagnetizing field excesses a permanent magnet’s coercivity, the permanent magnet will be demagnetized and remagnetized reversely.
In regard to thermal demagnetization, for a same material type, a permanent magnet with higher coercivity and/or higher Curie temperature usually has higher ability to resist magnetic flux loss when the working temperature increases. If the temperature excess a permanent magnet’s maximum working temperature, the magnetic flux loss will excess 5% and the decrease becomes faster subsequently. If the temperature is higher than a permanent magnet’s Curie temperature, the magnet will be totally demagnetized.
As results, to choose a permanent magnet for a motor or a device/machine, it is necessary to take the motor’s demagnetizing/reverse field and maximum working temperature into consideration so as to determine the permanent magnet’s coercivity.
Finally, it should also be mentioned that some permanent magnets, especially sintered NdFeB magnets, are vulnerable to corrosion and/or oxidation. If they suffer from corrosion/oxidation over time, their microchemistry and microstructure will change so that they will lose some magnetism even become invalid. In order to prevent from corrosion/oxidation, these permanent magnets will choose suitable coatings according to their working environments.