Every magnet grade has a kind of report card: the B-H curve, also called the demagnetization curve. Br, Hcj, Hcb and (BH)max are all read from it. When a supplier sends a certified test report, this is the measurement behind it.
What is the B-H curve?
Imagine a machine that magnetizes a sample to full strength, then applies an opposing field to demagnetize it. Plot the magnet's remaining strength against that opposing field, and you get a loop. The part where it resists being demagnetized is the demagnetization curve — the second quadrant.
From that curve you read: Br (starting strength), Hcj (resistance to demagnetization), and (BH)max (stored energy density).
How is it measured?
The sample sits between the poles of an electromagnet inside a closed magnetic circuit — no air gap. It is magnetized at several times its intrinsic coercivity, then the field is slowly swept while sensors record the curve. This is the standard method of GB/T 3217, aligned with IEC 60404-5.
Why must the circuit be closed? In open air, the magnet's own poles create a "self-demagnetizing" field that distorts the reading. Closing the circuit removes that effect, so the result is a true material property — the same for any shape of the same grade.
Why every manufacturer tests it
- It is the certificate — a grade name like N42 only states minimums; the curve shows what a batch actually delivered.
- It catches weak batches — under-magnetized or poorly sintered material shows up as a low, soft curve.
- It predicts real-world behavior — the shape of the curve tells you how a motor magnet will behave at high temperature.
A full curve needs a big machine and a full-size sample, so factories certify each batch with this method, then use faster checks — surface field, flux, moment — for day-to-day production control.
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