How Temperature Affects Neodymium Magnets (N vs M vs H vs SH)
The prototype worked all winter; the field returns began in August. Where the temperature lines sit for N, M, H and SH — and why the paper rating is not a promise.
The most expensive magnet failures we see share a script: the prototype worked all winter, the product shipped in spring, and by August the field returns began. Nothing broke. Nothing looked different. The magnets had simply crossed a temperature line nobody had drawn. Here is where those lines sit and how to stay on the right side of them.
Two kinds of loss
Reversible loss happens every day: NdFeB output drops about 0.11 % per °C as it warms, and comes back completely on cooling. Design margin absorbs it. Irreversible loss is the killer: past a threshold, domains flip and the flux does not return when the part cools. Only re-magnetizing restores it — impossible inside an assembled product.
The temperature ladder
| Series | Rated max* | Typical territory |
|---|---|---|
| N (N35–N54) | 80 °C | Consumer devices, indoor fixtures, packaging |
| M (38M–52M) | 100 °C | Enclosed electronics, warm actuators |
| H | 120 °C | Small motors, automotive interior |
| SH | 150 °C | Motor rotors, under-hood sensors, power tools |
*Guideline, not guarantee — see below.
Why the rating on paper is not a promise
The rated temperature assumes a favourable working point. Three things pull the real limit lower:
- Shape. Thin, wide magnets operate low on the B–H curve and hit the knee earlier. A coin-thin N45 disc can start losing flux in the 60s °C.
- Opposing fields. Stall a motor and the winding pushes a demagnetizing field into a rotor that is already hot — the two stresses multiply.
- The real local temperature. Not ambient: the magnet’s own spot, next to windings or electronics, at full load, in summer sun. We routinely see 30–40 °C between “room” and reality.
Choosing with your eyes open
- Measure or honestly estimate the magnet’s local worst-case temperature.
- Add margin — 20 °C is a sane default where field data is thin.
- Pick the series that clears it, then the N number that meets force, per the grade guide.
- If you land above 150 °C, stop and talk to us — geometry changes, hybrid circuits or ferrite may beat exotic grades on cost.
Tell us the temperature story of your application — it is the single highest-value sentence in any RFQ we receive.
FAQ
My magnet got hot once. Is it ruined?
If it stayed above its knee only briefly, some loss likely occurred but may be small. Compare pull or flux against a spare kept cool; loss beyond a few percent means the event crossed the line.
Does cold hurt neodymium magnets?
Cold slightly strengthens NdFeB. Only extreme cryogenic use needs review; for normal winters, no concern.
Are there grades above SH?
Yes — UH, EH and AH classes extend ratings toward 220 °C at rising cost. Ask us before specifying them; a design change is often the cheaper path.