Ι Low Temperature Vs. Permanent Magnets
While it’s well-known that the performance of permanent magnets generally decreases as temperature rises, their behavior at low temperatures is less familiar. As the temperature decreases, two significant changes occur in many magnets. First, the remanence (Br) increases slightly, which is typical for most ferromagnetic materials. This small increase can notably improve the energy product (BH)max, sometimes doubling it as temperatures approach absolute zero.
Second, intrinsic coercivity (Hcj) rises, except in hard ferrites. For many magnets, intrinsic coercivity can increase substantially—doubling or even tripling between room temperature and extremely low temperatures. This rise in coercivity is linked to a corresponding increase in anisotropy at lower temperatures. Understanding these effects is crucial when selecting permanent magnets for use in cold environments.
Low Temperature Behavior of Different Magnet Types
In this section, we will explore the behavior of different permanent magnets at low temperatures. Understanding how these materials react to cold is essential for assessing their performance in low-temperature environments.
- Neodymium Magnets: Neodymium magnets increase in flux output and intrinsic coercivity as the temperature decreases. However, as the temperature falls, they undergo a spin reorientation. Most studies indicate this transition occurs at around -138°C, where the magnetization direction shifts to a 30° cone from the easy axis. This reorientation leads to a decrease in remanence as the temperature drops. While this spin reorientation is generally not a major concern, it’s important for anyone using Neodymium magnets at cryogenic temperatures to be aware of it and design accordingly. To avoid the spin reorientation, Praseodymium has been substituted for part or all of the Neodymium in Neodymium magnets.
This results in a material with lower flux output at room temperature but avoids the spin reorientation effect. With 80% Praseodymium substitution, these magnets perform successfully down to at least 10 K. Such formulations are used in applications like rocket propulsion sensors and low-temperature free-electron lasers.
- Samarium Cobalt Magnets: Both SmCo5 and Sm2Co17 types Samarium Cobalt magnets perform well at cryogenic temperatures. At low temperatures, their intrinsic coercivity increases significantly, while remanence sees a modest rise. Many studies report the successful use of Samarium Cobalt magnets at temperatures as low as 2 K.
- Alnico Magnets: AlNiCo magnets show minimal sensitivity to temperature when it comes to their demagnetization characteristics. However, As noted in Permanent Magnets and Their Applications by Parkers and Studders, an irreversible loss in magnetization of up to 10% can occur after exposure to temperatures as low as -190°C. This loss is also influenced by the self-demagnetizing stress, which depends on the magnet’s geometry and the magnetic circuit. Without a detailed evaluation, -75°C is typically considered a reasonable lower limit for AlNiCo magnets.
- Ferrite Magnets: Ferrite magnets have a unique characteristic in that their intrinsic coercivity decreases as the temperature drops. By the time a ferrite magnet reaches -60°C, it has already lost about one-third of its room temperature intrinsic coercivity. Additionally, Parker and Studders report mild low-temperature demagnetization after exposure to -60°C. For these reasons, ferrite magnets are generally not recommended for use below -60°C.
