Comprehensive evaluation of the magnetic properties of permanent magnet materials delivers accurate, standards-based characterization of remanence, coercivity, and maximum energy product for NdFeB, SmCo, ferrite, and AlNiCo magnets. By applying internationally recognized methods such as IEC 60404-5, ASTM A977, and GB/T 3217, accredited third-party laboratories help manufacturers verify material grade, optimize motor and sensor designs, and ensure consistent quality across the supply chain.
Technical Overview
Permanent magnet materials are functional materials that retain a substantial remanent magnetization after the external magnetizing field is removed, providing a persistent magnetic field without continuous power input. They form the core magnetic circuit elements in modern electric motors, generators, sensors, actuators, and numerous consumer and industrial devices. Because device performance, efficiency, and reliability depend directly on the magnetic quality of these materials, a comprehensive and quantitative evaluation of their magnetic properties is essential at every stage — from research and development and incoming material inspection to finished product qualification.
The principal permanent magnet families covered by a comprehensive evaluation program include:
- Sintered neodymium-iron-boron (NdFeB) magnets, the highest-performance commercial magnets, widely used in traction motors, wind generators, and industrial servos.
- Bonded NdFeB magnets, produced from rapidly quenched powders blended with polymer binders, offering complex shapes and moderate energy density.
- Samarium-cobalt (SmCo) magnets, including SmCo5 (1:5) and Sm2Co17 (2:17) types, valued for excellent thermal stability and corrosion resistance.
- Hard ferrite (ceramic) magnets based on strontium or barium hexaferrite, providing low-cost, stable performance for high-volume applications.
- Aluminum-nickel-cobalt (AlNiCo) and iron-chromium-cobalt (FeCrCo) alloys, used where high temperature capability and low temperature coefficients are required.
- Magnetic powders and granular feedstock used in the production of bonded magnets and magnet assemblies.
A complete characterization typically combines DC hysteresis loop measurement in the second quadrant, magnetic moment determination, temperature-dependent measurements, and verification against grade specifications defined in standards such as IEC 60404-8-1, GB/T 13560, GB/T 4180, and JIS C 2502. The result is a full magnetic fingerprint of the material that supports grade confirmation, design margin assessment, and failure analysis.
Testing Samples
Magnetized and Demagnetized Solid Samples
Cylindrical, rectangular, ring, and arc-segment magnets in both the as-magnetized and thermally demagnetized states can be evaluated. For closed-circuit B-H measurement, samples are machined or prepared with parallel, ground end faces to ensure intimate contact with the pole pieces of the permeameter electromagnet, minimizing air gaps that would distort the measured demagnetization curve.
Small and Irregularly Shaped Specimens
Small magnets, bonded magnet parts, and irregular components are measured using open-circuit techniques such as Helmholtz coil flux integration for magnetic moment and vibrating sample magnetometry (VSM) for intrinsic hysteresis properties, in accordance with IEC 60404-14 and ASTM A882/A894 practices.
Magnetic Powders
Rare-earth and ferrite magnetic powders are evaluated after compaction into dense specimens or by using VSM measurement of loose or immobilized powder samples, allowing assessment of intrinsic coercivity and remanence of the raw material prior to magnet production.
Magnet Assemblies and Finished Products
Complete magnet assemblies — such as rotor stacks, magnetic couplings, and speaker magnet systems — can be assessed through surface field mapping, magnetic flux measurement, and magnetic moment testing to verify assembly-level magnetic performance against design targets.
Testing Items
A comprehensive magnetic property evaluation typically includes the following measurement items:
- Residual magnetic flux density (remanence, Br) — the magnetic induction remaining in the material when the magnetizing force returns to zero, expressed in millitesla (mT) or tesla (T).
- Magnetic polarization or intrinsic remanence (Jr or Jr/B relationship) determined from the intrinsic demagnetization curve J(H).
- Coercive field strength (normal coercivity, Hcb or BHc) — the demagnetizing field at which magnetic induction B falls to zero.
- Intrinsic coercive field strength (intrinsic coercivity, Hcj or JHc) — the field at which intrinsic magnetization vanishes, a critical indicator of resistance to demagnetization.
- Maximum energy product ((BH)max) — the peak of the B–H product along the demagnetization curve, defining the maximum useful magnetic energy per unit volume, typically in kJ/m³ or MGOe.
- Full second-quadrant demagnetization curve B(H) and intrinsic curve J(H), including the position of the knee point.
- Squareness factor or knee field (Hk) — the field at which intrinsic polarization drops to a defined fraction (commonly 90% or 80%) of Br, indicating curve squareness and demagnetization margin.
- Recoil permeability (μrec) — the effective slope of the recoil line, essential for magnetic circuit design and load line analysis.
- Temperature coefficients of remanence (α, %/°C) and intrinsic coercivity (β, %/°C) measured across the operating temperature range.
- Curie temperature (Tc) — the temperature at which ferromagnetic order is lost.
- Maximum operating temperature and irreversible flux loss at elevated temperatures.
- Total magnetic moment (m, in A·m² or mWb·turn equivalents) for open-circuit sample characterization.
- Surface magnetic field distribution (peak and mapped field strength, in mT) for finished magnets and assemblies.
- Optional supporting items: density verification, dimensional checks, and magnetic property uniformity screening across production lots.
Testing Methods
Closed-Circuit Hysteresisgraph Measurement (B–H Tracer)
The reference method for hard magnetic materials is defined in IEC 60404-5, ASTM A977/A977M, and GB/T 3217. The specimen is placed in a closed magnetic circuit formed by an electromagnet with adjustable pole shoes and a transducer coil system. The sample is first magnetized to saturation, then the demagnetization curve is traced by sweeping the field through the second quadrant. Search coils (B-coil and H-coil or Hall sensor) record induction and field strength simultaneously, yielding Br, Hcb, Hcj, (BH)max, the full B(H) and J(H) curves, and the knee point with typical uncertainties of about 1–3% for standard specimens.
Pulsed Field Magnetometry
For ultra-high-coercivity grades — such as NdFeB materials with Hcj above roughly 2000 kA/m — conventional electromagnets may not reach saturation. Pulsed field magnetometers apply capacitor-driven field pulses of 4–10 T or higher, and the induced signals are analyzed to reconstruct the hysteresis loop, in accordance with the principles of IEC TR 62575 and related pulsed-field practices.
Vibrating Sample Magnetometry (VSM)
VSM measurement provides high-sensitivity determination of hysteresis properties for small specimens, thin sections, and powders. The sample vibrates within detection coils in an applied field up to 2–3 T, and the induced voltage is calibrated against a nickel reference standard traceable to national metrology institutes. VSM is particularly suitable for R&D screening, coating studies, and grain-boundary diffusion development.
Helmholtz Coil and Fluxmeter Method
Per IEC 60404-14, the magnetic moment of a magnetized sample is measured by withdrawing or rotating the sample within a Helmholtz coil pair connected to an integrating fluxmeter. Combined with dimensional data, this yields the average magnetization and is the standard method for lot-acceptance testing of production magnets.
Temperature-Dependent Measurements
Thermal chambers integrated with the hysteresisgraph or VSM enable measurement of Br(T) and Hcj(T) from cryogenic conditions up to 200–300 °C (and beyond for SmCo and AlNiCo). From these data, the temperature coefficients α and β, reversible and irreversible loss components, and maximum continuous operating temperature are derived, often referenced to IEC 60404-8 series grade classification.
Surface Field and Flux Mapping
Calibrated Hall-effect teslameters and digital fluxmeters with search coils or Helmholtz fixtures are used for rapid verification of surface field strength, pole geometry, total flux, and angular orientation of magnetized assemblies, supporting 100% inline inspection and outgoing quality control.
Quality Assurance of Results
All instruments are calibrated with reference magnets traceable to national standards, measurement uncertainty budgets are documented, and test reports state the applied standard, sample condition (magnetized/demagnetized), temperature, and orientation of measurement relative to the magnetization direction.
Applications
Comprehensive magnetic property evaluation directly supports design, procurement, and quality decisions across a wide range of industries:
- Electric vehicle traction motors and auxiliary motors, where verifying Hcj at elevated temperature guarantees demagnetization withstand under fault and overload conditions.
- Wind turbine direct-drive and semi-direct generators, requiring long-term flux stability and uniformity across thousands of magnet blocks.
- Industrial servo motors, spindle motors, and PM stepper motors, where (BH)max and curve squareness determine torque density and efficiency.
- Loudspeakers, headphones, and acoustic transducers, benefiting from consistent Br and recoil permeability for stable sensitivity.
- Magnetic sensors, encoders, and Hall-effect position devices requiring tightly controlled magnet strength and pole definition.
- Magnetic separation equipment, magnetic couplings, holding systems, and magnetic chucks.
- Magnetic resonance imaging (MRI) and other scientific instrumentation using high-uniformity magnet assemblies.
- Aerospace and defense systems demanding full traceability of magnetic performance over extreme temperatures.
- Raw material qualification for magnet producers, including powder coercivity screening and grain-boundary-diffusion process validation.
- Failure analysis of demagnetized or degraded magnets recovered from service, supporting root-cause investigation.
FAQ
What is the difference between Hcb and Hcj?
Hcb (normal coercivity) is the field strength at which the external induction B of the magnet becomes zero, while Hcj (intrinsic coercivity) is the field at which the intrinsic magnetization J itself vanishes. Hcj is always greater than or equal to Hcb and is the key parameter for resistance to demagnetization by external fields, temperature, and adverse operating points.
Should the sample be magnetized or demagnetized before B–H curve testing?
Closed-circuit hysteresisgraph measurement begins with a thermally or AC demagnetized specimen, which is then saturated within the instrument, ensuring a well-defined virgin magnetization state. Helmholtz coil magnetic moment and surface field measurements, by contrast, require fully magnetized samples.
How is (BH)max determined?
The B–H product is computed point-by-point along the measured second-quadrant demagnetization curve, and (BH)max is the maximum value of this product. Together with Br and Hcj, it is used to confirm the material grade, for example against N35–N52 or temperature-stable grade specifications.
Why are temperature-dependent measurements important?
Both Br and Hcj decrease with temperature, and the reduction in Hcj is nonlinear for NdFeB materials. Measuring α, β, and irreversible loss at the actual operating temperature is the only reliable way to confirm that a motor magnet will not suffer permanent demagnetization in service.
What sample dimensions are suitable for testing?
Typical closed-circuit specimens are cylinders or rectangular blocks with a length-to-diameter or length-to-width aspect ratio of approximately 0.5 to 1.5 and diameters or widths of about 6 to 50 mm, with ground parallel end faces. Smaller, thinner, or complex parts are better suited to VSM or coil-flux methods.
What measurement uncertainty can be expected?
For well-prepared specimens measured on a calibrated hysteresisgraph, expanded uncertainties of approximately 1–3% on Br, (BH)max, Hcb, and Hcj are routinely achieved. Reports include the applicable standard, measurement direction, temperature, and stated uncertainty.
How long does a comprehensive evaluation take?
Standard room-temperature B–H curve measurement with report issuance is typically completed within a few working days. Programs including temperature coefficient determination across multiple temperatures, Curie temperature measurement, or large lot screening require additional time depending on sample count and test matrix complexity.