Temperatures that Permanent Magnets operate
All permanent magnets experience variations in field strength (flux density) as the ambient temperature increases or decreases. How these variations in operating temperature influence magnetic strength depend on the type of magnetic material and the degree above or below ambient temperature in which they are expected to operate. The shape of the magnet is also a factor in how heat affects magnetic strength, and generally speaking, the thicker the magnetised axis of the magnet, the more resistance to power reduction from heat it displays.
As the ambient temperature increases or decreases, changes occur at a molecular level in the magnetic material and this can disrupt the alignment of magnetic particles. Increased heat causes molecules to move faster and in a less uniform manner and is generally disruptive of the field density, reducing magnetic alignment and power. However, up to a point in some materials, lower temperatures (even sub-zero) than ambient can actually increase magnetic power, as the molecules become more aligned and condensed.
The Maximum Operating temperature for each type of magnetic material is different. Above this temperature the magnet will experience an incremental decline in power to a point where the loss of magnetic field is either Permanent, Reversible or Irreversible. Reversible power losses are recovered as the magnet’s temperature returns to ambient. Irreversible power losses will not recover to their original level as the magnet cools and will permanently remain lower. Permanent power losses occur when Curie temperatures are reached and the structure of the magnetic material is permanently damaged and is no longer able to be magnetised.
Alnico material is highly resistant to temperature fluctuations and can retain their magnetic power up to 500℃ (Alnico8 grade: 525℃).
Samarium Cobalt materials have good resistance to heat and are not permanently damaged up to around 300-350°C depending on the grade.
Standard grade (N grade) Neodymium magnets have the highest magnetic energy in temperatures up to 80℃, but above this temperature they rapidly demagnetise and are usurped in performance by other materials. Higher than standard grades of Neodymium are available to overcome this problem and are identified with a suffix code (including M, SH, UH, EH, AH). Unlike other materials, in below zero temperatures, Neodymium material shows outstanding resistance to demagnetisation and can actually increase in energy output.
Ferrite material, although intrinsically low in magnetic energy, has good resistance to heat and can remain operational at temperatures up to 250℃ before serious demagnetisation begins. This tolerance range makes them highly suitable for DC motors and electric generators.