Coercivity is the single most critical parameter defining a permanent magnet's ability to resist demagnetization under thermal stress, external fields, and operational load. Permanent magnet material coercivity testing measures normal coercivity (HcB) and intrinsic coercivity (Hcj) in accordance with international standards such as IEC 60404-5, ASTM A977/A977M, and GB/T 3217. This article explains the testing principles, sample requirements, measurement methods, and typical applications for sintered NdFeB, SmCo, ferrite, Alnico, and bonded magnet materials.
Technical Overview
Coercivity, denoted Hc, is the magnetic field strength required to reduce the magnetization of a previously saturated permanent magnet to zero. It is the key indicator of a magnet's demagnetization resistance and long-term stability. Two distinct values are reported for permanent magnet materials: normal coercivity (HcB), the reverse field at which the magnetic flux density B in the material reaches zero, and intrinsic coercivity (Hcj), the reverse field at which the intrinsic magnetization J itself falls to zero. HcB is always smaller than Hcj and is limited by the saturation flux density, whereas Hcj reflects the fundamental resistance of the material's microstructure to domain wall motion and magnetization reversal.
Coercivity is determined by tracing the complete hysteresis loop or the second-quadrant demagnetization curve after the specimen has been saturated to a reference state. The result is expressed in amperes per meter (A/m) or kiloamperes per meter (kA/m), and may also be reported in oersteds (Oe), where 1 Oe equals approximately 79.577 A/m. Typical intrinsic coercivity ranges are approximately 900 to over 3000 kA/m for sintered NdFeB, 600 to 2500 kA/m for SmCo, 200 to 400 kA/m for hard ferrites, and only 40 to 130 kA/m for Alnico. Accredited third-party laboratories perform these measurements under the framework of IEC 60404-5, ASTM A977/A977M, and the Chinese national standard GB/T 3217, ensuring traceable and comparable results for procurement, quality control, and design verification.
Testing Samples
Sintered Neodymium-Iron-Boron (NdFeB)
Sintered NdFeB offers the highest energy density but exhibits a strong negative temperature coefficient of coercivity, typically between -0.4 and -0.6 percent per kelvin for Hcj. High-temperature grades (SH, UH, EH, and above) with Hcj exceeding 2000 kA/m require pulsed-field or superconducting measurement techniques because conventional electromagnets cannot generate sufficient saturating fields. Specimens are usually machined as cylinders or rectangular blocks with a favorable length-to-diameter ratio to ensure uniform magnetization in a closed magnetic circuit.
Samarium-Cobalt (SmCo)
SmCo magnets, including both SmCo5 and Sm2Co17 types, combine high coercivity with excellent thermal stability, making coercivity verification at elevated temperature especially important for aerospace and downhole applications. Measurement procedures are largely identical to those for NdFeB, with Hcj values commonly between 800 and 2500 kA/m.
Hard Ferrites and Alnico
Barium and strontium ferrites have moderate coercivity and very low conductivity, which simplifies pulsed or closed-circuit testing. Alnico materials possess high remanence but low coercivity, so they must be measured in a magnetically closed circuit or in conjunction with their magnetic circuit geometry, since open-circuit handling alone can partially demagnetize the sample.
Bonded, Semi-Hard, and Powder Materials
Bonded magnets produced with polymer binders and isotropic NdFeB powders, as well as semi-hard materials for relays and sensors, are tested on compacted or injection-molded specimens. Because eddy currents are negligible in these composites, pulsed-field magnetometry can be applied with minimal dynamic correction.
Testing Items
Coercivity testing is normally delivered as part of a complete demagnetization-curve characterization, which includes:
- Intrinsic coercivity (Hcj) - resistance of the material's magnetization to reversal, the primary acceptance parameter for high-temperature grades
- Normal coercivity (HcB) - reverse field at which flux density in the material reaches zero
- Remanence (Br) - residual flux density at zero applied field after saturation
- Maximum energy product ((BH)max) - figure of merit for magnet performance
- Demagnetization curve knee point and recoil permeability - indicators of operating-point margin in motor design
- Temperature coefficients of Hcj and Br, and irreversible loss coefficients measured at reference temperatures such as 80, 100, 120, 150, 180, and 200 degrees Celsius
Testing Methods
Closed-Circuit Hysteresisgraph Method
Following IEC 60404-5 and ASTM A977/A977M, the specimen is clamped between the poles of an electromagnet or a dedicated permeameter conforming to IEC 60404-15, forming a closed magnetic circuit that minimizes demagnetizing effects. The sample is first saturated, then driven through a full magnetization cycle while search coils or a Hall-effect sensor record flux density B and a compensated field sensor records the internal field strength H. This method provides the highest accuracy, typically within one to two percent, and is the reference technique for measuring Br, HcB, Hcj, and (BH)max on materials whose coercivity does not exceed the achievable saturation field, generally up to about 800 kA/m for Hcj when using water-cooled laboratory electromagnets.
Pulsed-Field Magnetometer Method
For high-coercivity sintered NdFeB and SmCo, a capacitor-discharge pulse coil generates fields of 5 to 10 tesla, sufficient to saturate materials with Hcj above 3000 kA/m. Encircling detection coils measure the time derivative of magnetization, and integrated signals reconstruct the intrinsic hysteresis loop. Because fast field pulses induce eddy currents in electrically conductive sintered materials, dynamic corrections based on field-rate analysis or comparison with static reference measurements are applied in accordance with ASTM A977/A977M. Modern pulsed-field magnetometers achieve repeatability better than two percent and are the standard industrial tool for grading high-coercivity production lots.
Vibrating Sample Magnetometer Method
Vibration sample magnetometry, standardized in IEC 60404-15 for calibrated measurements, is used for small specimens, thin films, powders, and research-scale samples. The open-circuit geometry introduces a demagnetizing field that must be corrected using the sample's shape-dependent demagnetization factor, which is the principal source of uncertainty in Hcj determination with this method.
Elevated-Temperature Coercivity Measurement
Thermally controlled furnaces and heated pole pieces allow the demagnetization curve to be recorded at fixed operating temperatures. The measured Hcj values at multiple temperatures yield the temperature coefficient of intrinsic coercivity, which designers use to verify that a selected magnet grade retains sufficient demagnetization margin at maximum service temperature, in line with the class designation requirements of IEC 60404-8-1.
Applications
Coercivity test data directly support material qualification and design assurance across the permanent magnet industry. Typical applications include:
- Electric vehicle traction motors, where high Hcj grades prevent irreversible demagnetization under fault-current and high-temperature conditions
- Wind turbine generators, verifying long-term stability of large sintered NdFeB rotor assemblies
- Industrial servo and spindle motors, confirming knee-point margins for reliable operation deep in the second quadrant
- Aerospace actuation and downhole tooling, relying on elevated-temperature Hcj data for SmCo and high-temperature NdFeB
- Sensors, relays, magnetic couplings, and separators, where batch-level coercivity screening ensures consistent magnetic performance
- Incoming inspection and supplier qualification, where accredited third-party laboratories provide ISO/IEC 17025 traceable certificates of analysis
FAQ
What is the difference between HcB and Hcj?
HcB is the reverse field at which the external flux density of the magnet becomes zero, meaning the magnet temporarily produces no net output but is not fully demagnetized. Hcj is the reverse field at which the intrinsic magnetization of the material itself is reduced to zero. Hcj must always exceed HcB, and a large Hcj-to-HcB separation indicates a demagnetization curve with a well-defined knee and greater stability margin.
Which standards govern coercivity testing?
The principal standards are IEC 60404-5 for the measurement of permanent magnet properties by closed-circuit methods, ASTM A977/A977M for high-coercivity materials using hysteresigraphs including pulsed-field systems, IEC 60404-15 for permeameters and calibrated magnetic measurement instruments, and GB/T 3217 as the corresponding Chinese national method. Material classification limits are defined in IEC 60404-8-1.
What sample preparation is required?
Specimens should have clean, parallel end faces and a geometry that ensures uniform magnetization, commonly cylinders of approximately 10 mm diameter and 10 mm height for NdFeB and SmCo. Samples must be fully demagnetized before testing, then saturated in the measuring direction, and their dimensions measured accurately because flux-density calculations depend directly on cross-sectional area.
Why cannot all magnets be tested on an electromagnet hysteresisgraph?
Water-cooled electromagnets with iron pole pieces saturate at fields of roughly 2 to 3 tesla and cannot reliably saturate sintered NdFeB grades with Hcj above about 800 to 1000 kA/m. Incomplete saturation leads to understated Hcj values, so high-coercivity grades are instead measured with pulsed-field magnetometers or superconducting magnet systems.
How is temperature-related coercivity loss assessed?
The demagnetization curve is measured at several stabilized temperatures, and the intrinsic coercivity values are fitted to derive the temperature coefficient, expressed as percent change per kelvin. This coefficient, together with irreversible-loss testing at the maximum operating temperature, determines whether a magnet grade is suitable for a given thermal environment.