Every number on a magnet data sheet — Br, Hcb, Hcj and (BH)max — comes from one picture: the hysteresis loop. You do not need to be an engineer to read it. Here is what the curve is and why it matters.
What the curve is
Magnets have a memory. Magnetize one, remove the field, and it stays magnetized. That memory is called hysteresis. If you sweep the magnetic field up, down and back again, the magnet traces a closed loop — the hysteresis loop.
Permanent magnets spend their working life in the second quadrant of that loop, redrawn on its own as the demagnetization curve. It shows how the magnet behaves when something tries to demagnetize it — an opposing field from a coil, another magnet, or heat.
Why it matters
The curve is where all four key parameters come from:
- Br — where the curve crosses the vertical axis: the strength that stays after magnetizing.
- Hcb — where the curve crosses the horizontal axis: the opposing field that cancels the output.
- Hcj — where the intrinsic curve crosses: the opposing field that destroys the magnetization for good.
- (BH)max — the largest rectangle under the curve: the useful energy per volume.
| Parameter | Where to find it on the curve |
|---|---|
| Br | Curve meets the vertical axis |
| Hcb | Curve meets the horizontal axis |
| Hcj | Intrinsic curve meets the horizontal axis |
| (BH)max | Largest rectangle under the curve |
Heat reshapes the curve
As temperature rises, the curve shrinks and its "knee" moves toward the origin. A magnet that looks safe at room temperature can pass its knee at 80 °C and lose strength permanently. That is why suppliers publish whole families of curves at 20, 60, 100 and 120 °C, and why hot applications need high-Hcj grades.
Read on: the four key parameters explained → · surface field vs. remanence vs. flux →