How does Temperature Affect a Magnet?
Extreme temperatures can literally make or break a magnet, so it is very important to take this into consideration before selecting a magnet that will be used in either extremely hot or cold environments. Some of the earliest permanent magnets were, in fact, made by the application of extreme heat. If a fire tong was heated until it was red-hot, then left to cool oriented in the direction of the earth's magnetic field, it became a magnet. William Gilbert and other early researchers learned very quickly that this phenomenon also worked in reverse. Get a magnet red-hot, and when it cooled it was not longer a magnet unless it cooled while its long axis was oriented north - south.
Extreme heat will lessen the effectiveness of any magnet. Advanced designs, however, have done much to overcome many of the problems associated with high temperatures. Well-designed electromagnets have provisions for cooling built in, utilizing either air or oil baths to dissipate excess heat. Permanent magnets are also designed to minimize the effects of extreme temperatures. Erium 45 Alnico magnets, for example, lose only 10 of their magnetism at 800 °F, although the Erium 25 Ceramic magnet loses 81% magnetic potential at that temperature. The key again is to select the proper magnet for the job you have to do.