Magnets may lose their magnetism at high temperatures, which is mainly related to the material characteristics of the magnets and the level of temperature.
1. The concept of Curie temperature
Each magnetic material has a specific Curie temperature. The Curie temperature refers to the temperature at which the magnetism of the material changes abruptly. When the temperature rises to the Curie temperature, the magnetism of the magnetic material will drop sharply, changing from ferromagnetic or ferrimagnetic to paramagnetic. For example, Nd-Fe-B magnets, which are a common type of high-performance magnets, generally have a Curie temperature in the range of around 310 - 350 °C. When the temperature approaches or exceeds this range, their magnetism will weaken rapidly.
2. The principle of how high temperature affects magnetism
Under normal temperatures, the magnetic domains inside magnetic materials are regularly arranged. The orderly arrangement of these magnetic domains gives the materials their magnetism. Magnetic domains can be regarded as small magnetic regions. When the temperature rises, the thermal motion of atoms intensifies. As the temperature continues to increase, the energy of the thermal motion of atoms is sufficient to disrupt the orderly arrangement of magnetic domains. Once the orderly arrangement of magnetic domains is disrupted, the magnetism of the magnet will weaken.
3. Differences in the performance of different materials
• Nd-Fe-B magnets: As mentioned earlier, the magnetism of Nd-Fe-B magnets decreases significantly at high temperatures. However, through some special processes, such as adding trace elements or using coatings, its high-temperature resistance can be improved to some extent.
• SmCo magnets: These magnets have a relatively high Curie temperature and can work at temperatures as high as around 300 °C. They also have good corrosion resistance and oxidation resistance. Therefore, in a high-temperature environment, SmCo magnets can maintain their magnetism relatively better.
• Ferrite magnets: Their magnetism is relatively weak, but within a certain temperature range, the change in their magnetism with temperature is relatively gentle. However, when the temperature is too high, they will also lose their magnetism. It's just that the temperature range they can withstand is slightly higher than that of Nd-Fe-B magnets.
4. The recovery of magnetism after temperature reduction
For some magnets, their magnetism may be partially recovered when the temperature drops. This is because as the temperature decreases, the thermal motion of atoms weakens and magnetic domains have the opportunity to rearrange. However, if the temperature is too high and leads to irreversible changes in the magnetic domain structure, then even if the temperature drops, the magnetism cannot be fully restored to its original state. For example, after Nd-Fe-B magnets experience excessive temperatures, the degree of their magnetism recovery may be greatly affected, while SmCo magnets are more likely to recover to a better magnetic state when the temperature returns to normal.
