Neodymium magnets are the strongest in the world - at room temperature. Heat is their weakness. Warm one up and it gets weaker; cool it down and most of the strength returns. Heat it too much, and the loss becomes permanent. Here is what happens and how to pick a magnet that survives.
Two kinds of loss
Reversible loss: heat makes the tiny magnetic regions inside the magnet jitter, so its output drops. When it cools down, the regions settle back and the strength mostly returns.
Irreversible loss: past a certain point, part of the magnet stops contributing for good. Cooling it down does not bring the lost strength back. That is the failure mode that kills magnets inside hot motors and engines.
The numbers
As a rough guide, an NdFeB magnet loses about 0.12% of its strength per °C of heating. Warm it 100 °C and it loses roughly 12% - noticeable, but recoverable within limits.
Working temperature by grade
Each grade series carries a nominal maximum working temperature:
| Series | Max working temp (approx.) |
|---|---|
| N | 80 °C |
| M | 100 °C |
| H | 120 °C |
| SH | 150 °C |
| UH | 180 °C |
| EH | 200 °C |
| TH | 230 °C |
Reference values per GB/T 13560-2017; the real limit also depends on shape.
One more catch: shape matters
The working temperature is not a fixed number. A thin, flat magnet resists demagnetization poorly, so it fails at a much lower temperature than a thick magnet of the same grade. A 1 mm thick N50H disc can already lose strength above 60 °C.
What to do
For hot applications - EV motors, engines, industrial machinery - choose a higher series (SH, UH, EH). It costs a little strength and money, but that is the standard price of heat resistance. Remember: hot job → higher letter.
Read on: how grade names work → · why shape matters →