Magnetic property guide
How to Read a Permanent-Magnet Demagnetization Curve
Direct answer: A permanent-magnet demagnetization curve shows how magnetic flux density or polarization changes against an opposing field, usually in the second quadrant and at a stated temperature. Read it by confirming the curve convention, locating Br and coercivity intercepts, identifying the knee, placing the load line, and checking the resulting operating point under normal and adverse conditions.
What a demagnetization curve is
A permanent-magnet demagnetization curve is the part of a magnetic material characteristic used to review how the material responds as an opposing field is applied after magnetization. It is commonly shown in the second quadrant. The exact meaning depends on whether the graph presents a normal B-H curve, an intrinsic J-H curve, or another stated convention.
The curve is material-level evidence measured under defined conditions. It does not by itself state the field at a product surface, pull force, torque, waveform, useful air-gap flux, safe application temperature, or the survival of a finished assembly.
Start by confirming the curve identity
- Curve convention: normal flux density B, intrinsic polarization J, or another defined quantity.
- Axes and units: do not compare graphs until both axes and unit systems are clear.
- Material state: grade or compound, production route, orientation, and magnetization state.
- Temperature: the same material can have a different curve at another temperature.
- Direction: confirm that the data represents the relevant material orientation.
Engineering Evidence · EM-MAG-04 · Revision A
Permanent-magnet curve-reading card
Read the curve as a controlled sequence: identify the convention and temperature, locate the intercepts and knee, place the circuit load line, read the operating point, stress the worst credible condition, and release the design only with representative finished-part or assembly evidence.
| Step | Question to control | Evidence to retain |
|---|---|---|
| Identify | Which curve, units, temperature, orientation, and material state? | Approved source and exact data convention |
| Locate | Where are Br, Hcb, Hcj where applicable, BHmax, and the knee? | Values and labels from that approved curve |
| Place | How do geometry, air gap, return path, leakage, and steel define the load line? | Drawing, model assumptions, tolerances, and assembly state |
| Read | Where is the operating point and how much margin remains? | Calculated or modeled working point and boundary |
| Stress | What changes at peak temperature, reverse field, minimum section, or larger air gap? | Adverse-condition cases and irreversible-loss criterion |
| Release | Does the finished product meet the useful magnetic target? | Representative measurement and acceptance record |
The main points on the curve
Remanence Br
Remanence Br is the residual magnetic flux density under the stated curve condition when the applied field is zero. It is one material anchor, not a surface-field or force result.
Normal and intrinsic coercivity
Hcb and Hcj describe different zero-crossing conditions. They should not be exchanged or treated as one universal resistance number. Use the curve and definition attached to the exact proposed material state.
The knee
The knee is the region where the curve bends more sharply. Its location and engineering significance depend on the curve convention, material, and temperature. An operating point moving toward or beyond a critical knee region can indicate reduced margin and possible irreversible change, but the acceptance boundary remains project-specific.
Maximum energy product
BHmax is taken from the relevant second-quadrant curve and supports controlled material energy-density comparison. It does not locate the final working point or prove useful product output.
What the load line adds
The magnet does not operate on the material curve in isolation. Magnet dimensions, magnetization direction, air gap, steel return path, leakage, neighboring fields, and assembly interfaces define the magnetic circuit. A load line represents that circuit relationship, and its intersection with the curve gives the operating point for the stated case.
Changing magnet length, cross-section, air gap, surrounding steel, coating build, assembly position, or tolerance can move the operating point even when the material label stays unchanged.
Why temperature and adverse fields must be checked
A room-temperature curve is not a complete operating-envelope review. Compare the approved curves or data for continuous, peak, assembly, storage, and fault conditions. Also consider external opposing fields, armature reaction, magnetizing or demagnetizing process exposure, and changes in air gap or steel condition.
If a part appears weaker after heat exposure, follow the separate heat-demagnetization diagnostic sequence before assigning irreversible material loss.
Common curve-reading mistakes
- Comparing values from different curve conventions, units, temperatures, or orientations.
- Using Br, Hcj, or BHmax as a direct prediction of force, torque, or surface field.
- Selecting a grade before defining geometry, air gap, return path, and adverse fields.
- Ignoring minimum dimensions, coating build, assembly tolerances, or peak temperature.
- Treating a generic curve diagram as the approved material data for production.
- Releasing a design without representative magnetic and application-level validation.
Inputs for a controlled engineering review
- Target field, force, torque, flux, waveform, or sensing result with position and temperature.
- 2D drawing and 3D model with magnetization direction, air gap, and surrounding magnetic circuit.
- Material or grade intent plus approved normal and intrinsic curve data where applicable.
- Continuous, peak, assembly, storage, and fault temperatures and dwell times.
- External opposing fields and allowable irreversible magnetic change.
- Prototype quantity, annual demand, inspection method, and release evidence.
Review the sintered NdFeB product route, the custom engineering and assembly process, and the quality and inspection approach. Use the RFQ path when the actual drawing and operating conditions are ready for review.
Boundary: exact curve values, grade recommendations, temperature margins, feasibility, price, MOQ, lead time, and acceptance criteria remain specific to the proposed material, circuit, drawing, quantity, and validation plan.
Frequently asked questions
What does a permanent-magnet demagnetization curve show?
It shows how magnetic flux density or polarization changes as an opposing field is applied under a stated curve convention, material state, orientation, and temperature.
What is the difference between a B-H curve and a J-H curve?
A normal B-H curve tracks magnetic flux density B, while an intrinsic J-H curve tracks polarization or intrinsic induction. Their zero crossings describe different coercivity conditions and must not be interchanged.
What is the operating point of a permanent magnet?
It is the intersection between the relevant material curve and the load line representing the magnetic circuit for a stated geometry, air gap, return path, assembly, and condition.
Why does the knee of the curve matter?
Moving the operating point toward a critical knee region can reduce margin and increase irreversible-loss risk under heat or opposing fields. The exact boundary depends on the approved material curve and project criterion.
What information is needed for a demagnetization review?
Provide the approved curves, material or grade, drawing, magnetization, magnetic circuit, air gap, temperature history, adverse fields, assembly state, tolerances, required function, and representative validation method.