Thermal Showdown: Can HRE‑Free NdFeB Hold at 120°C?
A practical guide to reversible/irreversible losses and grade selection for HRE‑free N/M/H/SH magnets at extreme temperatures.
In today’s push for electrification and decarbonization, neodymium–iron–boron (NdFeB) permanent magnets are the core of motors and sensors. Historically, achieving high-temperature capability required adding expensive and supply-volatile "heavy rare earth" (HRE) elements such as dysprosium (Dy) and terbium (Tb) [1, 3]. HRE‑free technology is now changing the playbook: even under elevated temperatures, magnets can still deliver robust performance.
This article explains how much magnetic performance drops for HRE‑free grades such as N, M, H, and SH at their respective maximum operating temperatures.
Why do magnets fear heat? Two components of thermal demagnetization
Performance loss under heat generally consists of two parts:
- Reversible loss: like a spring—flux decreases as temperature rises, and fully recovers after cooling [5, 9].
- Irreversible loss: the critical risk. If the design limit (knee point) is exceeded, flux will not recover after cooling and re-magnetization is required [5, 10].
Performance at maximum temperatures (vs. HRE-containing grades)
Based on test results and material datasheets, typical thermal coefficients at the corresponding limit temperatures are:
| Grade | Limit test temperature | Br temp. coeff. α (%/°C) | Hcj coeff. β (%/°C) |
|---|---|---|---|
| N | 80 °C | -0.12 | -0.60 |
| M | 100 °C | -0.12 | -0.58 |
| H | 120 °C | -0.11 | -0.58 |
| SH | 150 °C | -0.10 ~ -0.12 | -0.55 |
Conclusion: how large is the total thermal loss (%)?
Assuming ambient temperature increases from 20 °C to the test temperature, the total flux loss can be estimated as:
- N (80 °C): total loss ~ 12.2%
Calculation: reversible 7.2% (60 °C × 0.12) + allowable irreversible < 5%. - M (100 °C): total loss ~ 14.6%
Calculation: reversible 9.6% (80 °C × 0.12) + allowable irreversible < 5%. - H (120 °C): total loss ~ 16.0%
Calculation: reversible 11.0% (100 °C × 0.11) + allowable irreversible < 5%. - SH (150 °C): total loss ~ 20.6%
Calculation: reversible 15.6% (130 °C × 0.12) + allowable irreversible < 5%.
Expert note: At 150 °C, reversible drift typically exceeds 15%, and demagnetization resistance may drop to roughly 30% of its room-temperature margin. Under such extreme conditions, consider a high aspect ratio (L/D > 0.7) geometry to keep irreversible loss within a safe range. If your application operates steadily at 150 °C, HRE‑free SH grade can be highly cost-effective—but requires more rigorous magnetic-circuit calculation.