What Does Permanent Magnet material testing Measure?
A permanent magnet material is characterised by its intrinsic magnetic properties — the values that define how strong the magnet is, how hard it is to demagnetise, and how much energy it can store per unit volume. These are not measured by a single number but by a demagnetisation curve (the second-quadrant B-H loop), from which four parameters are extracted: remanence (Br), the normal coercivity (Hcb) and intrinsic coercivity (Hcj), and the maximum energy product (BH)max. Permanent magnet material testing is the standardised measurement of these properties, governed by GB/T 3217-2013 in China, IEC 60404-5 internationally and ASTM A977 in the US. It also extends to dimensional, coating/corrosion, microstructural and temperature-stability tests, because the same block of NdFeB can meet its magnetic spec on paper yet fail in service through dimensional drift, coating breakdown, or irreversible thermal demagnetisation. This is distinct from magnetic-particle testing (MT), which is an NDT method for finding cracks in steel parts — not a test of magnet materials at all.
Why Do Br, Hcb, Hcj and (BH)max All Matter?
Each parameter answers a different design question, and a complete report reports all four:
- Br (remanence, in T or G) — the residual flux density left in the magnet after the magnetising field is removed. Higher Br means more flux for a given magnet volume, which directly drives motor torque density.
- Hcb (normal coercivity, in kA/m or Oe) — the reverse field needed to bring the flux density B back to zero on the demagnetisation curve. It is a measure of the magnet's resistance to demagnetisation in the external circuit.
- Hcj (intrinsic coercivity, in kA/m or Oe) — the reverse field needed to bring the magnetisation M to zero. Hcj is always ≥ Hcb, and it is the parameter that governs demagnetisation resistance at elevated temperature or under reverse fields — the critical number for EV motors and high-temperature service.
- (BH)max (maximum energy product, in kJ/m³ or MGOe) — the largest B×H rectangle that fits under the demagnetisation curve; the single best figure of merit for "how strong is this magnet per unit volume."
How Do the Four Magnet Families Compare on These Parameters?
Magnet materials trade these parameters differently, which is why the test report must be read against the declared grade of the specific material. Typical ranges (per published material data and GB/T 3217 method):
| Family | Br (T) | Hcb (kA/m) | Hcj (kA/m) | (BH)max (kJ/m³) | Tc (°C) |
|---|---|---|---|---|---|
| NdFeB (sintered) | 1.10–1.48 | 800–1000 | 955–2785 | 230–440 | 310–400 |
| SmCo (2:17) | 1.00–1.15 | 700–870 | 1432–2388 | 190–265 | 700–850 |
| Alnico | 0.60–1.35 | 40–140 | 40–160 | 10–88 | 760–890 |
| Ferrite (ceramic) | 0.36–0.44 | 230–340 | 250–400 | 22–38 | 450–460 |
The trade-offs are stark. NdFeB dominates on (BH)max (hence EV/wind-turbine use) but has the lowest Curie temperature and needs Hcj-rich grades to survive heat. SmCo sacrifices some Br but keeps its properties to 700–850 °C. Alnico has high Br and Tc but such low Hcj that it demagnetises easily under any reverse field. Ferrite is cheap and corrosion-proof but stores a tenth of NdFeB's energy. The grade designation encodes the expected (BH)max: an N52 NdFeB grade denotes (BH)max ≈ 52 MGOe (≈ 410–420 kJ/m³), an N35 ≈ 35 MGOe — the "N" number is essentially the energy-product spec the magnet must hit.
How Is the Demagnetisation Curve Actually Measured?
The fundamental test is a closed-circuit hysteresisgraph (permeameter) that traces the full B-H loop, but the choice of magnetising source depends on the material's Hcj — and this is where GB/T 3217-2013 diverges from older practice:
- For Hcj ≤ 600 kA/m (Alnico, Ferrite) — an electromagnet closed magnetic circuit is sufficient. The machined sample (typically a cube) is wound with a search coil and clamped between high-permeability pole pieces; a DC magnetiser drives the sample from origin → saturation → demagnetisation → reverse saturation → back to saturation, and the fluxmeter records B and H continuously to build the loop.
- For Hcj > 600 kA/m (high-grade NdFeB, SmCo) — a conventional electromagnet cannot supply enough field to saturate the sample, so the standard requires a pulsed-field (or superconducting) magnet to reach full saturation. Where GB/T 3217 cannot obtain a complete J(H) curve, the test is performed per GB/T 29628 (high-intrinsic-coercivity permanent magnet measurement) instead.
Because the hysteresisgraph test requires machining the sample to precise dimensions and winding a coil around it, it is expensive and slow — so in production, manufacturers supply one B-H curve per lot, not per part. For the routine acceptance testing of individual magnets, two faster methods are used instead.
What Are the Production Test Methods — and When Do You Use Which?
For day-to-day quality control of finished magnets, the closed-circuit loop is impractical. Three faster methods cover the production floor:
| Method | What it measures | Sample need | Best use |
|---|---|---|---|
| Gaussmeter + Hall probe | Local surface flux density at one point (G or T) | None — contact the surface | Quick polarity / gross-defect check; not a material-spec value |
| Helmholtz coil + fluxmeter | Total magnetic flux / magnetic moment (Wb, mV·s) | Magnet fits inside coil (coil Ø ≥ 3× magnet) | Batch acceptance — fast, repeatable, gives an effective Br |
| Hysteresisgraph (permeameter) | Full demagnetisation curve → Br, Hcb, Hcj, (BH)max | Machined cube with search coil | Lot-level material characterisation, one per lot |
Two operational details determine whether the fast methods give trustworthy numbers. With a Gaussmeter, the reading is extremely position-sensitive — the probe must sit in the identical spot each time, so brass fixtures locate it repeatably; and the surface reading in Gauss is not the same as the Br in the datasheet. With a Helmholtz coil, the coil diameter must be at least three times the largest magnet dimension, the magnet must be withdrawn perpendicular to the coil plane, the coil must be kept away from metal surfaces and stray fields, and the fluxmeter must be de-drifted (10–30 min) before measurement. Skip these steps and the batch "passes" a non-comparable number.
What Are the Physical and Environmental Tests?
Magnetic performance alone does not qualify a magnet for service. A complete material test program adds:
- Dimensional inspection — digital calipers, CMM or laser scanning to verify length, perpendicularity, parallelism, R/C angles; even small shifts change the magnetic circuit and fit in motor assemblies.
- Density test — Archimedes method; verifies sintering quality and material homogeneity (under-density means under-sintered, weaker magnet).
- Coating / corrosion resistance — most NdFeB is Ni-Cu-Ni or epoxy coated because bare NdFeB rusts rapidly. Salt spray (ASTM B117) and HAST / autoclave tests (e.g. 130 °C / 100 % RH / 96 h weight-loss, the "BCT test") quantify coating integrity. Adhesion and thickness are checked separately. See our coating testing capability.
- Microstructure and composition — SEM/EDS for grain structure, ICP for composition, infrared C/S and O/N/H analysers for impurity elements (oxygen, carbon, sulphur) that degrade magnetic performance.
- Mechanical strength — Instron tensile / compression / bend; NdFeB is brittle and cracks under impact, a real failure mode in assembly.
How Does Temperature Affect Magnetic Properties — Reversible vs Irreversible?
Temperature is the single biggest in-service variable for permanent magnets, and the mechanism splits into two regimes that the test program must distinguish:
- Reversible losses — within the operating range, Br and Hcj change linearly with temperature and recover on cooling. Quantified by the reversible temperature coefficients α (for Br) and β (for Hcj), expressed in %/°C and measured per GB/T 24270 (and JJF 1239 for rare-earth magnets). NdFeB's α ≈ −0.12 %/°C for Br, meaning Br drops ~12 % over a 100 °C rise — a large effect that motor designers must budget for.
- Irreversible losses — once the operating point crosses the knee of the demagnetisation curve (driven by high temperature plus the demagnetising field in the circuit), the magnet does not recover when cooled. Push past the Curie temperature (Tc) and the magnetism is lost completely. This is why the demagnetisation curve is often re-measured at the maximum operating temperature (per the relevant temperature-grade spec) rather than only at room temperature — a room-temperature B-H curve does not reveal high-temperature failure.
For the test-program context on thermal conditioning shared with these measurements, see our high-temperature test capability.
FAQ
What is the difference between Hcb and Hcj, and which matters more?
Hcb is the reverse field where flux density B = 0 on the demagnetisation curve; Hcj is the reverse field where magnetisation M = 0. Hcj ≥ Hcb always, and Hcj governs demagnetisation resistance under temperature and reverse fields. For EV-motor, aerospace and high-temperature service, Hcj is the parameter to specify and test.
Why do high-coercivity NdFeB grades need a pulsed-field test?
A conventional electromagnet closed circuit cannot generate enough field to saturate a magnet with Hcj > 600 kA/m — without saturation the measured loop is incomplete and the reported Br/Hcj are wrong. GB/T 3217-2013 therefore routes such materials to the pulsed-field method of GB/T 29628 for a valid measurement.
Is a Gaussmeter surface reading the same as the Br on the datasheet?
No. The Gaussmeter reports local surface flux density at one point, which depends heavily on probe position and magnet geometry. Br is an intrinsic material property from the closed-loop B-H curve. A Gaussmeter is fine for quick polarity or gross-defect checks, but it cannot verify a material specification.
What does an "N52" NdFeB grade mean?
The grade number encodes the nominal (BH)max in MGOe — N52 ≈ 52 MGOe (≈ 410–420 kJ/m³), N35 ≈ 35 MGOe. Letters after the number (SH, UH, EH) denote the maximum operating-temperature class (e.g. SH ≤ 150 °C, UH ≤ 180 °C, EH ≤ 200 °C) via higher Hcj. A test report should verify that the measured (BH)max and Hcj meet the declared grade.
Why test at the operating temperature, not just room temperature?
A magnet that meets spec at 25 °C can drop out of spec at 150 °C through both the reversible coefficient (recoverable) and, more dangerously, irreversible demagnetisation if the operating point crosses the curve's knee. The worst-case thermal condition is what the motor actually sees, so the demagnetisation curve must be measured at that temperature.
Our Testing Capabilities
As an ISO/IEC 17025-accredited third-party laboratory, Beijing ZKGX Research provides permanent magnet material testing aligned to GB/T 3217-2013, GB/T 29628, GB/T 24270, IEC 60404-5 and ASTM A977:
- Magnetic properties — full demagnetisation curve (Br, Hcb, Hcj, (BH)max, recoil permeability) by closed-circuit hysteresisgraph for Hcj ≤ 600 kA/m materials, and pulsed-field methods for high-Hcj NdFeB/SmCo per GB/T 29628.
- Batch acceptance testing — Helmholtz-coil + fluxmeter total-flux/magnetic-moment measurement (coil Ø ≥ 3× magnet, de-drifted fluxmeter, perpendicular withdrawal), and Gaussmeter surface-field checks with brass-located Hall probes.
- Temperature-stability characterisation — reversible temperature coefficients α(Br)/β(Hcj) per GB/T 24270 / JJF 1239, and demagnetisation curves measured at operating temperature (high-temperature grades SH/UH/EH/AH).
- Physical and environmental — dimensional (CMM/laser), density (Archimedes), coating integrity and corrosion (salt spray ASTM B117, HAST/autoclave weight-loss), microstructure (SEM/EDS), composition (ICP, C/S, O/N/H), and mechanical strength.
- Materials covered — sintered and bonded NdFeB, SmCo (1:5 and 2:17), Alnico, ferrite, and permanent-magnet assemblies.
Sample types include machined test cubes for hysteresisgraph characterisation, finished magnets for batch acceptance, and magnet assemblies. For related metallic-material work see our Magnesium alloy testing. If you have a specific material family, declared grade (e.g. N48SH), target application or compliance standard, contact the laboratory to confirm the exact test set and reporting format.