Loss of Permanent Magnet Magnetization Caused by Heat Exposure
Loss of Magnetization Caused by Heat Exposure – Exposing a magnet to high temperatures can decrease its strength or permanently damage its structure. Usually this happens at very high temperatures, but for certain neodymium magnets this can happen as low as 60°C.
Magnets are characterized by two temperature thresholds: the working temperature and the Curie temperature. These are important parameters to evaluate magnets performance and resistance when exposed to heat.
What is the working temperature of a magnet?
The working temperature is the maximum temperature a magnet can withstand for prolonged periods of time without incurring in a loss of performance, either temporary or permanent.
What is the maximum working temperature for a neodymium magnet?
The working temperature of standard Neodymium magnets is 80°C. Special neodymium magnets can be manufactured to have a higher temperature tolerance. This is indicated by a letter written after the grade of magnetization. A neodymium magnet with an “M” (like N35 M) can be heated up to 100°C. An “H” magnet up to 120°C, “SH” up to 150°C, “UH” up to 180°C, “EH” up to 200°C, and a “TH” up to 220°C. Our standard neodymium magnets (no letter shown) have a working temperature of up to 80°C; ask our team if you need neodymium magnets with higher heat tolerance.
What is the maximum working temperature for ferrite and Alnico magnets?
Ferrite and especially Alnico magnets have a very high maximum working temperature. Ferrite magnets can operate at a temperature of up to 250°C, while Alnico magnets up to 650°C.
What is the Curie temperature of a magnet?
The Curie temperature is the temperature above which a magnet loses all of its magnetization permanently. In this case, the structure is compromised beyond repair and remagnetization is no longer possible.
Which is the Curie temperature for a neodymium magnet?
Standard neodymium magnets have a Curie temperature of 310 °C (340-350°C for special M, H, SH, UH, EH, TH magnets).
Which is the Curie temperature of ferrite and alnico magnets?
The Curie temperature of Ferrite magnets is 450°C, while for Alnico Magnets is 700–850°C.
What happens if you heat a magnet above its maximum working temperature?
Heating a magnet above its maximum working temperature will cause a temporary or permanent loss of magnetization (adhesive force). The end result will change based on how much the heat exceeds the working or (eventually) the Curie temperature of the magnet. Loss of Magnetization Heat Exposure
Here are three different scenarios:
Temporary loss of performance
If the heat reaches values just above the maximum working temperature, the magnet will lose some magnetic strength while it remains hot.
Neodymium magnets lose about 0.11% of their strength for each 1°C in excess above the working threshold. This loss can be recovered by cooling the magnet.
The end result is not affected by either duration of the exposure nor the amount of times a magnet is heated and cooled. Be careful nevertheless, because repeated and rapid heating and cooling cycles can cause the magnet to break or crack. Magnets that have to be used in critical environments should be designed accordingly (shape, size and choice of materials). Ask our technical team for assistance.
Permanent loss of performance
If the heat reaches values significantly higher than the maximum working temperature, the magnet is weakened permanently. Cooling it down may recover some magnetization, but some of the loss will be permanent. Loss of Magnetization Heat Exposure
However, provided that the Curie temperature was not reached, this loss is recoverable by exposing the magnet to a sufficiently strong external magnetic field. This process of remagnetization (similar to what happens during manufacturing) can remagnetize a weakened magnet and restore its original strength.
Irreversible loss of magnetization
If the heat surpasses the Curie temperature, the structure of the magnet is altered permanently and the loss of magnetization cannot be recovered. Remagnetization is no longer possible.
The shape and size of a magnet, together with the direction of magnetization (axial or diametrical, for example) can substantially change its heat tolerance.
Moreover, the time of exposure can make a difference for larger magnets. The outer part can reach critical temperatures while the core can be less impacted, resulting in an irregular magnetization. Anyway, even if just a portion of the magnet exceeds the working temperature or Curie temperature thresholds, the damage happens regardless, and prolonged exposure does not make much of a difference. Loss of Magnetization Heat Exposure
Contact our technical team for further specifications about our magnets and an evaluation of your use case.