Permanent magnet material magnetic testing measures the intrinsic and technical magnetic properties of NdFeB, SmCo, AlNiCo, and hard ferrite magnets, including remanence (Br), coercivity (Hcb/Hcj), and maximum energy product ((BH)max). Testing is performed by accredited third-party laboratories in accordance with international standards such as IEC 60404-5, IEC 60404-14, ASTM A977/A977M, GB/T 3217, and JIS C 2502. Accurate characterization of the demagnetization curve is essential for motor design, lot acceptance, quality control, and failure analysis across the electric vehicle, wind power, industrial automation, and consumer electronics industries.
Technical Overview
Permanent magnet materials, including sintered and bonded neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo), aluminum-nickel-cobalt (AlNiCo), and hard ferrites, are the functional core of modern electric motors, generators, sensors, actuators, and magnetic assemblies. Their performance is defined by the second-quadrant demagnetization curve, which quantifies how much magnetic flux a magnet can deliver and how strongly it resists demagnetization under opposing fields and elevated temperatures.
Magnetic property testing of permanent magnet materials determines key parameters such as residual flux density (Br), normal coercivity (Hcb), intrinsic coercivity (Hcj), maximum energy product ((BH)max), knee field (Hk), and recoil permeability. Because measured values depend strongly on the measurement circuit and specimen geometry, testing must follow standardized methods, typically closed-circuit hysteresisgraph measurement in accordance with IEC 60404-5 and ASTM A977/A977M, or open-circuit magnetic moment measurement in accordance with IEC 60404-14. Accredited third-party laboratories provide traceable, reproducible results for incoming inspection, lot release, design verification, and dispute resolution.
Testing Samples
Sintered NdFeB Magnets
Sintered NdFeB offers the highest commercially available energy product and is widely used in traction motors and high-efficiency machines. Testing is typically performed on machined cylinders or rectangular blocks with a length-to-diameter ratio near unity, machined with parallel faces and clearly marked magnetization direction. High-Hcj grades (Hcj above 2000 kA/m) may require pulsed-field magnetization to reach saturation before measurement.
Bonded Permanent Magnets
Bonded NdFeB, bonded ferrite, and hybrid bonded magnets, produced by injection molding, compression bonding, or calendaring with polymer binders, exhibit lower but highly uniform properties. Isotropic bonded specimens can be tested without prior magnetization, and ring-shaped parts are commonly evaluated by flux or moment measurement using search coils or Helmholtz coils.
Samarium-Cobalt Magnets
Sintered SmCo magnets, in both the 1:5 (SmCo5) and 2:17 (Sm2Co17) families, provide excellent temperature stability and corrosion resistance, making them the preferred choice in aerospace and high-temperature applications. Testing often includes high-temperature demagnetization curves up to 250 °C or higher to verify performance margins.
AlNiCo Magnets
Cast and sintered AlNiCo alloys feature high remanence with relatively low coercivity, so their demagnetization behavior is strongly geometry-dependent. Testing emphasizes the full hysteresis loop and recoil characteristics, and specimens must be measured in a magnetically closed circuit to avoid errors caused by self-demagnetization.
Hard Ferrite Magnets
Sintered and bonded hard ferrite (strontium or barium hexaferrite) materials are low-cost, high-coercivity magnets used in motors, loudspeakers, and separators. Standard specimens include cylinders, blocks, and rings, and testing frequency spans production lot control to design qualification.
Magnetic Assemblies and Powders
Magnetized rotor segments, arc segments, and complete assemblies are evaluated by open-circuit flux and moment methods. Raw magnetic powders and small or irregular specimens are characterized using vibrating sample magnetometry (VSM) or SQUID magnetometry, with demagnetization factor corrections applied where appropriate.
Testing Items
Typical measurands and reported parameters for permanent magnet materials include:
- Residual flux density / remanence (Br), expressed in mT or kG
- Normal coercive field strength (Hcb), in kA/m or kOe
- Intrinsic coercive field strength (Hcj), in kA/m or kOe
- Maximum energy product ((BH)max), in kJ/m³ or MGOe
- Knee field (Hk) and squareness ratio (Hk/Hcj)
- Recoil permeability (μrec) and recoil line slope
- Complete normal (B–H) and intrinsic (J–H) demagnetization curves
- Open-circuit magnetic flux (total flux in mWb) and magnetic dipole moment (in Wb·m or A·m²)
- Temperature coefficients of Br (α) and Hcj (β), in %/K
- Irreversible flux losses after thermal exposure or thermal cycling
- Curie temperature and magnetization-temperature behavior
- Surface field distribution and flux uniformity mapping of assemblies
Testing Methods
Closed-Circuit Hysteresisgraph Testing
The reference method for magnetically hard materials is defined in IEC 60404-5, ASTM A977/A977M, GB/T 3217, and JIS C 2502. The specimen is clamped between the pole pieces of a high-permeability electromagnet to form a closed magnetic circuit, and wound B- and J-coils measure the flux density and magnetic polarization while the applied field H is cycled from saturation in one direction to saturation in the other. The method delivers the full hysteresis loop with typical uncertainties of 1–3 % and requires specimens with precise geometry, typically cylinders of 10 mm diameter and 10 mm height or equivalent blocks. The magnetizing field must be sufficient to saturate the material, generally several times the intrinsic coercivity, which limits conventional electromagnets to moderate-Hcj grades.
Pulsed Field Magnetometry (PFM)
For high-coercivity NdFeB and SmCo grades that cannot be saturated by dc electromagnets, capacitor-discharge pulsed field magnetometers apply fields of several tesla within milliseconds. The induced voltage in pickup coils is integrated to reconstruct the dynamic hysteresis loop, with eddy-current corrections applied for conductive specimens. PFM enables rapid measurement of very high-Hcj materials and small samples.
Open-Circuit Flux and Magnetic Moment Measurement
In accordance with IEC 60404-14, the magnetic dipole moment of a magnetized specimen is measured by withdrawing or rotating it within a Helmholtz coil or search coil connected to an electronic fluxmeter. This non-destructive method is fast and well suited to 100 % production inspection, lot uniformity assessment, and verification of magnetized assemblies, and results can be correlated with closed-circuit Br values through calibration specimens.
VSM and SQUID Magnetometry
Vibrating sample magnetometers measure the magnetic moment of small specimens, powders, flakes, and thin sections in open-circuit conditions over a wide temperature range. SQUID magnetometers provide the highest sensitivity for research-grade characterization. Both methods require careful correction for the specimen demagnetization factor when converting moment to material property values.
Temperature-Dependent Testing
Environmental chambers and cryostats extend hysteresisgraph, PFM, and VSM measurements from cryogenic temperatures (down to −196 °C using liquid nitrogen) to elevated temperatures (typically up to 200–300 °C, and beyond for SmCo). Repeated measurements yield the temperature coefficients of Br and Hcj, while hold and thermal-cycling tests quantify irreversible flux loss, which is critical for verifying demagnetization margins in traction motors and other thermally stressed applications.
Applications
Magnetic property testing of permanent magnet materials supports product development and quality assurance across many industries:
- Electric vehicle and hybrid traction motors, e-axles, and auxiliary motors, where thermal demagnetization resistance must be verified
- Direct-drive wind turbine generators and other renewable energy machines
- Industrial servo motors, brushless dc motors, linear motors, and magnetic couplings
- Consumer electronics including loudspeakers, vibration motors, wireless charging modules, and sensors
- Aerospace actuators, avionics, and satellite systems relying on temperature-stable SmCo magnets
- Medical devices, including MRI and implantable or wearable equipment
- Magnetic separators, holding systems, magnetic bearings, and research institutions
- Incoming material inspection, supplier lot release certification, and failure or demagnetization analysis of returned parts
FAQ
What is the difference between Hcb and Hcj?
Hcb is the normal coercivity, the opposing field at which the external flux density B in the magnet falls to zero, while Hcj is the intrinsic coercivity, the field at which the intrinsic magnetization J itself vanishes. Hcj is the true indicator of resistance to irreversible demagnetization, which is why motor designers specify a minimum Hcj, especially for high-temperature NdFeB grades.
Why can very high-coercivity magnets not be tested on a standard electromagnet?
A conventional hysteresisgraph electromagnet cannot generate fields high enough to fully saturate grades with Hcj above roughly 1600–2000 kA/m, because the iron yoke and pole pieces approach magnetic saturation. In such cases, specimens are pulse-magnetized before measurement on the hysteresisgraph, or the complete loop is measured directly on a pulsed field magnetometer.
What specimen dimensions are required for hysteresisgraph testing?
Standard practice follows IEC 60404-5 and ASTM A977/A977M, which recommend cylindrical specimens of approximately 10 mm diameter and 10 mm height, or blocks of similar volume with a length-to-diameter ratio near 1, machined with flat and parallel end faces and a marked orientation axis. Other geometries can be tested after machining or by agreement, with geometry-related corrections noted in the report.
How are the temperature coefficients of Br and Hcj determined?
The demagnetization curve is measured at room temperature and at one or more elevated temperatures, for example 80 °C, 100 °C, or 150 °C. The relative change of Br and Hcj per kelvin is then calculated, typically as a percentage per K over the specified range, and irreversible losses are assessed by re-measuring the specimen after it returns to room temperature.
Which standards govern permanent magnet magnetic testing?
The principal standards are IEC 60404-5 (measurement methods for magnetically hard materials), IEC 60404-14 (measurement of magnetic dipole moment by the withdrawal and rotation method), IEC 60404-8-1 (specifications for magnetically hard materials), ASTM A977/A977M (hysteresisgraph testing of high-coercivity permanent magnets), GB/T 3217 (Chinese national standard for permanent magnet test methods), and JIS C 2502 (Japanese standard for permanent magnet test methods).
Can magnetized production parts and assemblies be tested?
Yes. Magnetized parts, arc segments, and complete assemblies are routinely verified using Helmholtz coil and search-coil fluxmeter methods in accordance with IEC 60404-14, supplemented by Hall-probe surface field mapping. These open-circuit methods are fast, non-destructive, and suitable for sampling plans or full-lot screening.
How long does magnetic property testing take?
Standard room-temperature hysteresisgraph or flux measurements at an accredited third-party laboratory are typically completed within a few working days of sample receipt. Temperature coefficient testing, long-term aging, and thermal cycling programs require longer durations, typically from several days to several weeks depending on the test plan.