What is coercivity (Hc)?
Short answer: coercivity (Hc) is a magnet's resistance to demagnetisation: how strong an opposing field must be to remove the magnetisation. It is given in kA/m or kOe (1 kOe ≈ 79.6 kA/m). Standard neodymium has an intrinsic coercivity (Hcj) of at least 955 kA/m (12 kOe). Grades for high temperatures have more: SH at least 1,590 kA/m (20 kOe), EH 2,390 kA/m (30 kOe).
Hcb and Hcj: two kinds of coercivity
- Hcb (normal coercivity): the opposing field at which the flux density B goes to zero. Important for designing a magnetic circuit.
- Hcj or Hci (intrinsic coercivity): the opposing field at which the magnetisation of the material itself goes to zero. This determines how well a magnet resists heat and opposing fields, and therefore its temperature class.
Coercivity per temperature class (neodymium)
| Class | Hcj minimum | Max. operating temperature (approx.) |
|---|---|---|
| N | 955 kA/m (12 kOe); N50/N52: 876 kA/m (11 kOe) | 80 °C (N50/N52: 60 – 70 °C) |
| M | 1,114 kA/m (14 kOe) | 100 °C |
| H | 1,353 kA/m (17 kOe) | 120 °C |
| SH | 1,592 kA/m (20 kOe) | 150 °C |
| UH | 1,990 kA/m (25 kOe) | 180 °C |
| EH | 2,388 kA/m (30 kOe) | 200 °C |
| AH | 2,786 kA/m (35 kOe) | 220 – 230 °C |
Higher coercivity is achieved by adding heavy rare earth metals, mainly dysprosium (Dy) and terbium (Tb). These are expensive, which is why SH and UH magnets cost more than an ordinary N grade.
Why is coercivity important?
Coercivity drops as the magnet gets warmer: for neodymium approx. 0.5 – 0.6% per °C. A magnet with too little coercivity for its temperature permanently loses strength. Shape also matters: a thin, flat magnet has a stronger internal opposing field and therefore needs more coercivity than a thick one. More in what happens to a magnet at high temperatures?
Coercivity compared with other magnets
| Material | Hcj (approx.) |
|---|---|
| Neodymium | 955 – 2,800 kA/m |
| Samarium cobalt | 1,200 – 2,400 kA/m |
| Ferrite | 200 – 400 kA/m |
| AlNiCo | 40 – 160 kA/m |
Because of its low coercivity, AlNiCo can be partially demagnetised with a strong magnet; with neodymium this hardly works at room temperature.
Frequently asked questions
What is the difference between remanence and coercivity?
Remanence (Br) says how strong the magnet is; coercivity (Hc) says how well it stays strong. See what is remanence (Br)?
Does an N52 have a higher coercivity than an N35?
No, if anything lower (11 kOe versus 12 kOe). That is why an N52 is more sensitive to heat.
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