Magnetic Performance Evaluation of Permanent Magnet Materials: Key Parameters, Testing Methods, and

Magnetic performance evaluation verifies the remanence, coercivity, and maximum energy product of permanent magnet materials such as NdFeB, SmCo, ferrite, and Alnico, ensuring magnets meet design specifications before they enter motors, sensors, and energy systems. This technical guide explains the key magnetic parameters, sample requirements, standardized test methods, and typical applications of permanent magnet characterization. All measurements described can be performed by accredited third-party laboratories in accordance with international standards such as IEC 60404-5 and ASTM A977.

Technical Overview

Permanent magnets are the functional core of modern electric traction motors, industrial servo systems, wind generators, sensors, and countless consumer devices. Unlike soft magnetic materials, permanent magnet materials are engineered to retain a strong remanent magnetization after the magnetizing field is removed, and their commercial value is determined almost entirely by a small set of magnetic properties measured along the demagnetization curve in the second quadrant of the hysteresis loop.

Magnetic performance evaluation is the systematic measurement of these properties under controlled, standardized conditions. The principal parameters include remanence (Br), normal coercivity (Hcb), intrinsic coercivity (Hcj), and the maximum energy product ((BH)max), together with the full demagnetization curve and its temperature dependence. Because magnet performance degrades with temperature and with demagnetizing loads, accurate characterization at both room temperature and operating temperature is essential for motor design, quality control, incoming inspection, and failure analysis.

International standards such as IEC 60404-5, IEC 60404-8-1, ASTM A977/A977M, GB/T 3217, and the corresponding JIS specifications define the accepted test methods and material grade classifications, enabling consistent comparison between suppliers and production lots.

Testing Samples

Sintered NdFeB Magnets

Sintered neodymium-iron-boron is the highest-performance commercial magnet family, with remanence values reaching approximately 1.4-1.5 T and intrinsic coercivity grades ranging from roughly 800 kA/m to more than 3000 kA/m. Evaluation typically covers grade verification of Br, Hcb, Hcj, and (BH)max against declared specifications, lot-to-lot consistency, and high-temperature stability for electric vehicle and wind power applications.

Samarium Cobalt Magnets

SmCo magnets, in both SmCo5 (1:5) and Sm2Co17 (2:17) compositions, offer low temperature coefficients and stable operation up to 250-350 °C, making them the standard choice in aerospace, defense, and high-temperature industrial equipment. Testing confirms room-temperature properties as well as elevated-temperature demagnetization curves and irreversible loss behavior.

Bonded and Injection-Molded Magnets

Bonded NdFeB, SmFeN, and ferrite-based composite magnets, produced by compression bonding or injection molding with polymer binders, exhibit lower remanence but excellent dimensional control and crack resistance. Evaluation focuses on Br, Hcj, and (BH)max of molded specimens, rings, and finished arc segments used in micromotors.

Hard Ferrite Magnets

Sintered strontium and barium ferrites, with remanence of approximately 200-450 mT, remain the most widely used magnets by volume. Testing verifies Br, Hcb, and Hcj at room temperature and at low temperature, where the temperature characteristic of coercivity must be confirmed to guarantee demagnetization resistance in cold environments.

Alnico and Other Magnet Families

Alnico alloys combine high remanence of up to about 1.3 T with relatively low coercivity, requiring careful handling in open-circuit conditions, while iron-chromium-cobalt alloys are ductile and machinable. These materials are evaluated using the same second-quadrant parameters, with particular attention to the shape-sensitive operating point.

Sample Geometry and Preparation

For closed-circuit hysteresisgraph testing, specimens must be prepared so that they form a low-reluctance joint with the electromagnet pole faces. Typical requirements include:

• Cylindrical specimens of approximately 10 mm diameter and 5-10 mm height, or equivalent rectangular blocks
• End faces ground flat, parallel, and perpendicular to the magnetization direction
• Sufficient volume to represent bulk properties and avoid surface oxidation and grain-boundary effects
• Delivery in a demagnetized state; full saturation magnetization is applied within the instrument before measurement

Testing Items

The scope of evaluation depends on the application, but the following parameters constitute the core deliverables of a permanent magnet test report.

Second-Quadrant Demagnetization Curve

• Remanence Br (mT or kG): the residual flux density at zero applied field
• Normal coercivity Hcb (kA/m or Oe): the reverse field at which flux density B falls to zero
• Intrinsic coercivity Hcj (kA/m or kOe): the reverse field at which intrinsic magnetization J falls to zero
• Maximum energy product (BH)max (kJ/m³ or MGOe): the peak of the energy product curve, indicating the energy available per unit volume
• Knee field Hk: the reverse field at which J drops to 90% of Br, marking the onset of irreversible demagnetization
• Recoil permeability: the slope of the recoil line, required for accurate magnetic circuit simulation

Temperature-Dependent Properties

• Temperature coefficient of Br, typically about -0.11 to -0.13 %/°C for NdFeB grades
• Temperature coefficient of Hcj, for example about -0.6 to -0.8 %/°C for standard NdFeB grades
• Elevated-temperature and cryogenic demagnetization curves, commonly from -40 °C to +200 °C and extended ranges for SmCo
• Reversible and irreversible losses after thermal cycling and long-term aging
• Curie temperature Tc

Open-Circuit and Assembly-Level Measurements

• Total magnetic flux or dipole moment by Helmholtz coil and fluxmeter
• Surface field mapping with calibrated Hall probes
• Air-gap flux density of magnetized rotors, stators, and magnetic assemblies
• Angular flux distribution and magnetization uniformity checks

Testing Methods

Methods are selected according to material grade, coercivity level, sample geometry, and whether the specimen is loose or already built into an assembly. All primary methods are traceable to IEC, ASTM, EN, GB, and JIS standards.

Closed-Circuit Hysteresisgraph Measurement

The reference method of IEC 60404-5 and ASTM A977/A977M places the specimen between the poles of a high-field electromagnet, forming a closed magnetic circuit. The sample is first saturated, then a symmetrical magnetization cycle is applied while search coils measure B and J directly. Typical expanded uncertainty is 1-2% for Br and 2-3% for Hcj. Conventional electromagnets generate pole fields of roughly 2-2.5 T, sufficient for most SmCo, ferrite, Alnico, and standard NdFeB grades.

Pulsed-Field Magnetometry for High-Coercivity Magnets

For very high-coercivity NdFeB grades where an electromagnet cannot fully saturate and reverse the specimen, a capacitor bank discharges into a pulsed solenoid producing fields of several tesla. The induced voltage is integrated to reconstruct the J-H loop, with eddy-current corrections applied for electrically conductive samples. This is the practical industrial method for ultra-high-Hcj EV-grade magnets.

Vibrating Sample Magnetometry

VSM measures the magnetic moment of a small specimen vibrating in a uniform applied field, offering high sensitivity across wide temperature ranges. Because the measurement is open-circuit, demagnetization-factor corrections are required to derive true intrinsic properties. VSM is widely used for material development, Curie temperature determination, and research-grade characterization.

Helmholtz Coil and Fluxmeter Measurement

A magnetized component is withdrawn from a calibrated Helmholtz coil while a fluxmeter integrates the induced voltage, yielding total flux or magnetic dipole moment. The technique is fast and non-destructive, making it ideal for 100% inspection of production lots and finished components, consistent with the withdrawal method described in IEC 60404-14.

Hall Probe Surface Field Testing

Calibrated Hall-effect gaussmeters measure flux density at defined points on a magnet or assembly surface, verifying magnetization direction, peak surface field, and uniformity for acceptance checks and failure investigations.

Variable-Temperature Magnetic Testing

Furnace- or cryostat-equipped hysteresigraphs record demagnetization curves at controlled set points, from which temperature coefficients, reversible and irreversible losses, and maximum operating temperature ratings are derived for thermal design of motors and generators.

Applications

Magnetic performance evaluation supports product development, supplier qualification, production quality control, and failure analysis across virtually every industry that relies on permanent magnets.

• New energy vehicles: traction motors, electric power steering motors, and auxiliary drive motors
• Industrial automation: servo motors, linear motors, magnetic couplings, and clamping systems
• Wind power and hydroelectric generators
• Consumer electronics: loudspeakers, earbuds, vibration motors, and magnetic fasteners
• Medical equipment: sensors, pumps, and diagnostic devices
• Aerospace and defense: actuators, gyroscope sensors, and microwave devices
• Magnetic separation, lifting, and holding systems
• Incoming material inspection and lot qualification for magnet users and motor manufacturers

FAQ

What is the difference between Hcb and Hcj?

Hcb is the reverse field at which the external flux density B of the magnet falls to zero, while Hcj is the reverse field at which the intrinsic magnetization J itself falls to zero. Hcj is always greater than or equal to Hcb and is the true measure of resistance to irreversible demagnetization.

Why can a magnet pass a room-temperature test but fail at operating temperature?

Both Br and Hcj decline as temperature rises; for NdFeB, Hcj can drop significantly at 150-180 °C. If the operating point crosses the knee of the demagnetization curve, irreversible loss occurs even though the same magnet met specification at room temperature. This is why elevated-temperature curves are essential for thermal design.

What sample size is required for a full B-H test?

A cylindrical specimen of about 10 mm diameter and 5-10 mm height, or an equivalent block with ground parallel end faces perpendicular to the magnetization axis, is preferred. Other geometries can often be accommodated with appropriate fixturing and coil winding.

When is pulsed-field testing necessary?

When the intrinsic coercivity exceeds the level that a standard electromagnet circuit can fully saturate and demagnetize, typically above approximately 2000 kA/m for ultra-high-coercivity EV-grade NdFeB, pulsed-field magnetometry provides the higher fields needed for an accurate measurement loop.

Can magnetized assemblies or rotors be tested without disassembly?

Yes. Helmholtz coil flux measurement, Hall probe surface mapping, and air-gap flux density checks provide non-destructive verification of finished, magnetized components and complete rotor assemblies.

Which standards govern permanent magnet magnetic testing?

Primary references include IEC 60404-5 for measurement methods, IEC 60404-8-1 for material specifications, ASTM A977/A977M for hysteresigraph testing of high-coercivity magnets, GB/T 3217, and related JIS national standards.

What measurement uncertainty can be expected?

In accredited third-party laboratories, typical expanded uncertainties (coverage factor k = 2) are approximately 1-2% for Br and Hcb and 2-3% for Hcj and (BH)max, depending on the material grade, geometry, and test temperature.