Coercivity testing is the fundamental evaluation used to determine the demagnetization resistance of permanent magnet materials such as sintered NdFeB, SmCo, and hard ferrites. Accredited third-party laboratories measure normal coercivity (Hcb) and intrinsic coercivity (Hcj) in accordance with IEC 60404-5, ASTM A977, and GB/T 3217 to verify magnetic grades, assess thermal stability, and support quality control across the electric vehicle, wind energy, and industrial motor industries. This article explains the testing principles, sample requirements, instrumentation, and typical applications of coercivity measurement for permanent magnet materials.
Technical Overview
Coercivity is one of the most critical magnetic properties of permanent magnet materials, defining their ability to resist demagnetization by external fields, elevated temperatures, or adverse operating conditions. Two distinct coercivity values are measured on the demagnetization curve: normal coercivity (Hcb), the reverse field at which magnetic flux density B falls to zero, and intrinsic coercivity (Hcj), the reverse field at which intrinsic magnetization J is fully cancelled. Intrinsic coercivity is generally regarded as the more meaningful indicator of thermal and field stability, because significant irreversible demagnetization begins well before Hcb is reached once the knee of the curve is passed.
Coercivity measurement is performed on the second-quadrant demagnetization curve, from which remanence (Br), maximum energy product ((BH)max), knee field (Hk), and recoil permeability are also derived. Test results are used for grade classification (for example, N, M, H, SH, UH, EH, and AH temperature classes of sintered NdFeB), incoming material inspection, and design verification. Principal international and national standards governing the measurement include:
- IEC 60404-5 — Methods of measurement of the magnetic properties of magnetically hard (permanent magnet) materials
- IEC 60404-8-1 — Specifications for individual materials: magnetically hard materials
- ASTM A977/A977M — Standard test method for magnetic properties of high-coercivity permanent magnet materials using hysteresigraphs
- ASTM A341/A341M — Standard test method for DC magnetic properties using permeameters and the DC permeameter method
- GB/T 3217 — Test methods for the magnetic properties of permanent magnet (magnetically hard) materials
- GB/T 13560 — Specification for sintered neodymium-iron-boron permanent magnet materials
- JIS C 2502 — Japanese standard covering test methods for permanent magnet materials
Testing Samples
Sintered NdFeB (Neodymium-Iron-Boron)
Sintered NdFeB exhibits the highest maximum energy product among commercial magnet materials, with intrinsic coercivity typically ranging from approximately 960 kA/m for standard grades to beyond 3100 kA/m for heavy-rare-earth-enriched EH and AH grades. Because these grades can exceed the field capability of conventional electromagnets, pulse-magnetization and high-field measurement techniques are often required. Samples are usually supplied as cylinders or rectangular blocks with parallel, ground end faces.
Sintered SmCo (Samarium-Cobalt)
SmCo magnets of the SmCo5 and Sm2Co17 families combine high coercivity with excellent temperature stability, supporting continuous operation up to 250-350 degrees Celsius. Coercivity verification, together with temperature coefficient testing, is essential for aerospace, defense, and downhole applications where irreversible flux loss cannot be tolerated.
Bonded and Injection-Molded Magnets
Compression-bonded NdFeB, injection-molded ferrite, and bonded SmFeN materials contain magnet powder embedded in a polymer matrix. Their isotropic or anisotropic coercivity values are verified to confirm powder quality, magnetization alignment, and process consistency. Because bonded materials are magnetically softer than sintered equivalents, careful saturation and repeatability checks are applied.
Hard Ferrites
Sintered strontium and barium ferrites (for example, SrFe12O19) remain widely used in automotive sensors, motors, and loudspeakers. Typical intrinsic coercivity values fall between roughly 240 and 400 kA/m, and testing confirms both room-temperature properties and stability across the operating temperature range.
Alnico and FeCrCo
Alnico and iron-chromium-cobalt alloys offer high remanence and exceptional temperature tolerance but relatively low coercivity. Their low Hcb values place them at risk of demagnetization in adverse field conditions, so accurate coercivity characterization and load-line analysis are particularly important for these material families.
Magnetic Assemblies and Finished Components
Finished rotors, magnet segments, and magnetized assemblies can be evaluated using open-circuit techniques such as Helmholtz-coil flux measurements combined with 3D field mapping, providing verification of the assembled magnetic state without destroying the component.
Testing Items
Coercivity testing programs typically report the following magnetic parameters, measured at reference temperature (usually 23 degrees Celsius) and, where required, at elevated or sub-ambient temperatures:
- Remanence / retentivity (Br), expressed in mT or kG
- Normal coercive field strength (Hcb), in kA/m or Oe
- Intrinsic coercive field strength (Hcj / Hci), in kA/m or Oe
- Maximum energy product ((BH)max), in kJ/m³ or MGOe
- Knee field (Hk) at 90% or 80% of Br, and squareness ratio Hk/Hcj
- Recoil permeability and full second-quadrant demagnetization curves
- Temperature coefficients of Br (alpha) and Hcj (beta)
- Irreversible flux losses after thermal cycling and long-term aging exposure
- Magnetic moment and surface flux density for assemblies (Helmholtz coil method)
Testing Methods
Closed-Circuit Hysteresisgraph Method
The reference method specified in IEC 60404-5, ASTM A977/A977M, and GB/T 3217 places the specimen in a closed magnetic circuit between the poles of a DC electromagnet. Search coils around the sample measure flux density, while tangential Hall probes at the pole surface measure field strength. The specimen is first pulse-magnetized to technical saturation, then the applied field is reversed to trace the complete hysteresis loop. This method delivers the lowest uncertainty, typically within 1-2% for Br and Hcj, and is suitable for materials with Hcj up to approximately 2000-2400 kA/m, limited by the saturation field of the electromagnet.
Vibrating Sample Magnetometer (VSM)
VSM testing measures the magnetic moment of a small specimen vibrating in a uniform applied field, with detection via pickup coils. It is well suited for material development, small or irregular samples, and high-coercivity grades when equipped with superconducting or high-field magnets. Because the measurement is open-circuit, results must be corrected for the self-demagnetizing field using the demagnetization factor of the specimen geometry, and sample shape therefore strongly influences accuracy.
Pulsed-Field Magnetometry
For ultra-high-coercivity grades, including grain-boundary-diffused NdFeB with Hcj above 2400 kA/m, pulsed-field magnetometers apply capacitor-discharge fields of 5-10 tesla to achieve full saturation. The induced signals in calibrated pickup coils reconstruct the magnetization curve. This method is indispensable for advanced EV traction motor grades whose coercivity exceeds conventional electromagnet capability.
Temperature-Dependent Coercivity Testing
Thermal chambers and cryostats integrated with hysteresigraph or VSM systems allow measurement from approximately -196 degrees Celsius (liquid nitrogen) up to 250 degrees Celsius or higher. These tests quantify the negative temperature coefficient of Hcj in NdFeB (typically around -0.5 to -0.7% per kelvin depending on grade) and validate performance at maximum operating temperature, including the position of the magnet working point relative to the curve knee.
Calibration and Traceability
Measurements are calibrated against certified reference materials traceable to national metrology institutes such as NIST, with periodic verification of coil constants, probe positioning, and electromagnet field uniformity. Full measurement uncertainty budgets and instrument calibration records are provided in accredited test reports.
Applications
Coercivity testing services support a broad range of industries and product categories, including:
- Electric vehicle traction motors, where high Hcj grades must withstand stator reaction fields and thermal excursions without irreversible loss
- Wind turbine generators and industrial servo motors requiring long-term magnetic stability
- Automotive sensors, actuators, and EPS/ADAS components with strict reliability requirements
- Aerospace, marine, and oil-and-gas downhole systems operating at extreme temperatures
- Consumer electronics, loudspeakers, magnetic couplings, and magnetic separation equipment
- Grade verification and incoming inspection for magnet manufacturers, distributors, and end users
- R&D programs for heavy-rare-earth reduction, grain boundary diffusion, and new magnet alloys
FAQ
What is the difference between Hcb and Hcj?
Hcb is the reverse field at which the external flux density B of the magnet reaches zero, while Hcj is the reverse field at which the intrinsic polarization J of the material itself is reduced to zero. Hcj is always greater than or equal to Hcb, and it provides the more reliable measure of resistance to irreversible demagnetization.
What sample size and geometry are required?
Closed-circuit hysteresigraph measurements typically use cylindrical samples of approximately 10 mm diameter and 10 mm height, or rectangular blocks with similar proportions, with ground parallel end faces to minimize air gaps. VSM testing accepts much smaller specimens, usually a few milligrams to tens of milligrams, provided the geometry is well defined for demagnetization-factor correction.
Why must the sample be saturated before measurement?
Coercivity can only be correctly determined from the major hysteresis loop, which requires the specimen to first reach technical saturation. Incomplete saturation produces minor loops that understate Br, Hcb, Hcj, and (BH)max, leading to false rejection or misclassification of the material grade.
How is testing performed for high-temperature grades?
Specimens are measured inside a controlled thermal enclosure at specified temperatures, and the measured demagnetization curves are compared with the magnet operating load line. Temperature coefficients and irreversible losses after thermal cycling are reported to confirm suitability for the intended application class.
What accuracy and turnaround can be expected?
With calibrated hysteresigraph systems, typical expanded measurement uncertainty is approximately 1-2% for Br and Hcj. Routine single-temperature tests are commonly completed within three to five working days, while temperature-dependent studies and failure-analysis programs may require longer schedules depending on sample quantity and test points.
Can magnetized assemblies be tested non-destructively?
Yes. Helmholtz-coil magnetic moment measurement, combined with flux density mapping and, where necessary, thermal exposure followed by remeasurement, allows the coercivity-related stability of finished assemblies to be assessed without disassembly or destruction.