Temperature Effects on Magnets
Temperature Effects on Neodymium Magnets
Neodymium Magnets are very susceptible to losing their effective magnetic field at elevated temperatures. The Neodymium magnet grade should be carefully selected to account for the operational temperature of the application and the magnet’s geometry. All magnet alloy will gain or lose effective magnetic field as the temperature flucuates. Neo magnets will lose effective field as the temperature increases and are said to have a Negative Temperature Coefficient. When compared to other magnet alloy options, neo magnets lose magnetic field at a faster rate. Neo magnets effectively have a higher Temperature Coefficient (TC) when compared to other commercial alloys.
This high rate of change relative to temperature exposure results in Neo magnets being very susceptible to demagnetizing from elevated operating environments. The magnet may be exposed to a temperature which does not fully demagnetize, but reduces its’ magnetic performance to a state that it no longer is able to provide sufficient magnet field to support the application.
The loss that has been discussed thus far is considered reversible. This means that the magnet will recover the loss as it cools back down. What is typically published by suppliers of magnets is the Reversible Temperature Coefficient (RTC) for each particular grade of Neo for both the Residual Induction (Br) and the Intrinsic coercive Force (Hci). The RTC as well as the recommended Maximum Operating Temperature must be considered when selecting a Neo magnet grade for an application above ambient room temperature.
Temperature Effects on Samarium Cobalt Magnets
Sintered Samarium Cobalt rare earth magnets are extremely resistant to demagnetization and can operate at temperatures up to 500F (260C). There are many Samarium Cobalt grades which can withstand higher temperatures, but several factors will dictate the overall performance of the Samarium Cobalt rare earth magnet. One of the most pertinent variables is the geometry of the magnet or magnetic circuit. Samarium Cobalt magnets, which are relatively thin compared to their pole cross-section (Magnetic Length / Pole Area), will demagnetize easier than Samarium Cobalt magnets which are thick.
Magnetic geometries utilizing backing plates, yokes, or return path structures will respond better to increased temperatures. The maximum recommended operating temperatures listed on the Samarium Cobalt magnetic characteristics page do not take into account all geometry conditions. Please contact a Dura team member for Samarium Cobalt rare earth magnet design assistance when elevated temperatures are involved in your application.
Temperature Effects on Alnico Magnets
Alnico magnets offer the best temperature characteristics of any standard production magnet material available. Alnico magnets can be used for continuous duty applications where temperature extremes up to 930°F (500°C) can be expected. Temperatures above 1000°F will result in permanent metallurgical changes which can only be recovered by reheat treating. Excursions to lower temperatures pose less of a problem for most Alnico applications; however, each individual circuit should be examined carefully to determine the effects which may occur as a result of operating at those extremes. A Dura team member can provide specific guidelines concerning the temperature characteristics for a given Alnico grade magnet and make recommendations for their proper use.
Temperature Effects on Ceramic Magnets
Ceramic (Ferrite) magnets are susceptible to demagnetization when exposed to temperature extremes. There are grades which have better resistance to high and low temperatures, but several factors will dictate the performance of the Ceramic magnet. One of the most pertinent variables is the geometry of the magnet or magnetic circuit. Magnets which are thin relative to their pole cross-section (Magnetic Length / Pole Area) will demagnetize easier than magnets which are thick. Magnetic geometries utilizing backing plates, yokes, or return path structures will respond better to temperature changes. The maximum recommended operating temperatures listed on the Ceramic magnetic characteristics page does not take into account all geometry conditions.
Caution when using Ceramic magnet in the cold:
Unlike Neodymium, Samarium Cobalt, and Alnico, Ceramic magnets have a Positive Temperature Coefficient for the Intrinsic Coercive Force (Hci) (β). This means that as the temperature increases the magnet may exhibit an increase in net field. This is up to a certain point and the degree of increase is dependent upon the geometry of the magnet. The converse is also true and this is where some designs may have issues. As a Ceramic Magnet experiences a temperature decrease, the net field decreases. This is unlike all other commercial magnet alloys which experience a net field increase when the temperature decreases. Applications where failures may occur could be sensor trigger when the field is not sufficient to trigger a sensor in colder climates. Refer to the Available Ceramic Magnet Grades section of our website for specific thermal performance.