Physical Performance Testing of Permanent Magnetic Materials

Permanent magnetic materials are the core functional components of electric motors, generators, sensors, and electronic assemblies, and their physical performance directly determines the efficiency, reliability, and lifespan of the end product. This article provides a comprehensive guide to the magnetic, mechanical, thermal, and electrical testing of permanent magnet materials such as NdFeB, SmCo, ferrite, and AlNiCo, covering internationally recognized standards including IEC 60404-5, ASTM A977, and GB/T 3217. Whether you are a magnet manufacturer, motor designer, or quality engineer, understanding these testing methods helps ensure material consistency, specification compliance, and dependable long-term performance.

Technical Overview

Permanent magnetic materials, also known as magnetically hard materials, retain a substantial remanent magnetization after the removal of an external magnetizing field. The principal families include sintered neodymium-iron-boron (NdFeB), sintered samarium-cobalt (SmCo5 and Sm2Co17), bonded magnets (compression-bonded or injection-molded), sintered hard ferrites (BaFe12O19 / SrFe12O19), and cast or sintered aluminum-nickel-cobalt (AlNiCo) alloys. Because these materials are produced through powder metallurgy or casting processes, their properties are highly sensitive to composition, microstructure, oxygen content, and process control, which makes systematic physical performance testing indispensable.

Physical performance testing of permanent magnets covers four major domains: magnetic properties (remanence, coercivity, energy product, and demagnetization behavior), mechanical properties (density, hardness, and strength), thermal properties (temperature coefficients, Curie temperature, and thermal expansion), and electrical and dimensional characteristics. These tests are governed by international and national standards such as IEC 60404-5, IEC 60404-8-1, ASTM A977/A977M, ASTM B962, ISO 3369, ISO 3325, ISO 6507-1, GB/T 3217, and GB/T 13560, and are routinely performed by accredited third-party laboratories as well as in-house quality departments.

• Incoming material inspection and lot release for magnet manufacturers
• Design validation and grade verification for motor and generator developers
• Failure analysis of demagnetized or degraded magnetic assemblies
• Regulatory compliance and customer specification verification

Testing Samples

Sintered NdFeB Magnets

Sintered NdFeB is the highest-performance commercial magnet family, with maximum energy products typically ranging from 200 to 400 kJ/m³ (25 to 50 MGOe). Testing focuses on second-quadrant demagnetization curves, intrinsic coercivity (particularly for dysprosium- or terbium-enriched high-temperature grades), and consistency of magnetic output across production lots.

Sintered SmCo Magnets

Samarium-cobalt magnets offer excellent thermal stability and corrosion resistance, with usable operating temperatures up to 250-350 °C. Testing emphasizes temperature coefficients, high-temperature demagnetization behavior, and long-term flux stability for aerospace and industrial applications.

Bonded and Injection-Molded Magnets

Bonded magnets combine magnetic powder with polymer binders, offering complex geometries and tight tolerances at moderate energy products. Testing addresses magnetic properties at defined packing densities, mechanical integrity, and dimensional accuracy.

Hard Ferrite and AlNiCo Magnets

Sintered ferrite magnets are low-cost, high-resistivity materials widely used in motors, loudspeakers, and separators, while AlNiCo magnets provide high remanence with low coercivity and exceptional temperature stability. Testing is tailored to their comparatively low coercive force and specific application profiles.

Magnetic Powders, Assemblies, and Finished Components

Accelerator-quality testing also covers raw magnetic powders, magnetized sub-assemblies, rotors, and magnetized finished parts, for which non-destructive flux and moment measurement methods are applied without sectioning the component.

Testing Items

Magnetic Properties

• Residual flux density (remanence), Br
• Magnetic polarization coercive force (intrinsic coercivity), Hcj
• Magnetic flux density coercive force, Hcb
• Maximum magnetic energy product, (BH)max
• Full second-quadrant demagnetization curves (B-H and J-H curves)
• Knee field Hk and squareness ratio Hk/Hcj
• Recoil permeability, μrec
• Total magnetic flux and magnetic moment of finished magnets

Mechanical and Physical Properties

• Density (Archimedes method)
• Vickers hardness, HV
• Compressive strength
• Transverse rupture (flexural) strength
• Tensile strength (for bonded magnet grades, where applicable)

Thermal Properties

• Temperature coefficients of Br and Hcj, typically between -40 °C and +200 °C
• Irreversible flux loss after thermal aging and thermal cycling
• Curie temperature, Tc
• Coefficient of thermal expansion (CTE), parallel and perpendicular to the orientation direction
• Maximum continuous and maximum operating temperatures

Electrical and Dimensional Characteristics

• Electrical resistivity (four-terminal method)
• Dimensional tolerances and geometric accuracy (coordinate measurement)
• Coating thickness and surface integrity for plated or coated magnets

Testing Methods

Hysteresisgraph (B-H Tracer) Method

The hysteresisgraph, operated in a closed magnetic circuit with a high-permeability electromagnet yoke and pole shoes, is the reference method for measuring the complete demagnetization curve of permanent magnets, in accordance with IEC 60404-5, ASTM A977/A977M, and GB/T 3217. Specimens, typically cylindrical blocks of approximately 10 mm in diameter and height with ground parallel faces, are first pulse-magnetized to saturation and then demagnetized in a controlled manner while B and H are recorded. The method delivers Br, Hcb, Hcj, (BH)max, and Hk with typical measurement uncertainties of 1-3%.

Pulse Field Magnetometry (PFM)

For modern high-coercivity NdFeB grades whose Hcj exceeds the saturation limit of conventional electromagnets (generally above 1600-2400 kA/m), pulse field magnetometers apply pulsed fields of up to 4000-8000 kA/m. The induced signals are analyzed with eddy-current corrections to yield accurate intrinsic demagnetization curves for Dy- and Tb-containing grades.

Vibrating Sample Magnetometer (VSM)

VSM measurements are used for small specimens, research and development, magnetic powders, and temperature-dependent studies between cryogenic and elevated temperatures. Although open-circuit corrections must be applied, the technique offers high sensitivity and supports Curie temperature determination through thermomagnetic analysis.

Helmholtz Coil and Fluxmeter Method

For quality control of finished, magnetized components, a Helmholtz coil paired with an electronic fluxmeter provides a rapid, non-destructive measurement of the total magnetic moment or flux. This method is ideal for 100% inspection of production lots and for verifying assembly-level magnetic output.

Mechanical Testing Methods

Density is determined by the Archimedes immersion principle in accordance with ISO 3369 and ASTM B962. Vickers hardness is measured according to ISO 6507-1 and ASTM E384. Compressive strength is evaluated by uniaxial compression testing consistent with ASTM E9, and transverse rupture strength is measured by three-point bending in accordance with ISO 3325 and ASTM B528, all using specimens oriented relative to the magnetization direction.

Thermal Characterization Methods

Temperature coefficients α(Br) and α(Hcj) are obtained by measuring demagnetization curves at a series of stabilized temperatures (for example 20, 80, 100, 150, and 200 °C) and calculating the relative change per kelvin. Irreversible losses are assessed through aging tests at elevated temperature followed by remagnetization-capable flux measurement. Curie temperature is determined by thermomagnetic analysis, and thermal expansion is measured with a push-rod dilatometer per ASTM E228.

Electrical and Dimensional Testing

Electrical resistivity is measured with a four-terminal (four-point probe) technique, which is particularly relevant for distinguishing conductive rare-earth magnets from insulating ferrites in eddy-current-sensitive designs. Dimensional verification employs coordinate measuring machines and optical projection systems, while coating thickness on nickel-, zinc-, or epoxy-coated magnets is confirmed by X-ray fluorescence or cross-sectional metallography.

Applications

Physical performance testing of permanent magnet materials supports quality assurance and product development across a wide spectrum of industries, wherever reliable magnetic output over the service life is critical.

• Electric and hybrid vehicle traction motors, EPS motors, and actuators
• Wind turbine direct-drive and semi-direct generators
• Industrial servo motors, robotics, and automation systems
• Consumer electronics including loudspeakers, earphones, vibration motors, and hard disk drives
• Medical devices such as MRI assemblies, implantable pumps, and diagnostic sensors
• Aerospace and defense components including traveling wave tubes, magnetrons, gyroscopes, and radar systems
• Magnetic separators, holding systems, and magnetic couplings
• Home appliances, power tools, and HVAC blower motors

FAQ

What is the difference between Hcb and Hcj?

Hcb is the normal coercivity, the reverse field at which the magnetic flux density B in the magnet falls to zero, whereas Hcj is the intrinsic coercivity, the field at which the intrinsic magnetization J is fully overcome. Hcj is the decisive parameter for a magnet's resistance to demagnetization under adverse fields and elevated temperatures.

Which standards govern the magnetic testing of permanent magnets?

The primary standards are IEC 60404-5 and ASTM A977/A977M for hysteresisgraph measurement, IEC 60404-8-1 and GB/T 13560 for material specifications and grade classification, and GB/T 3217 as the Chinese national method for permanent magnet magnetic property measurement. Japanese JIS C 2501 and JIS C 2502 address equivalent material and test requirements.

What sample shape is required for B-H curve tracing?

Standard practice, per IEC 60404-5, uses cylindrical or rectangular block specimens with uniform cross-section and ground parallel end faces, commonly around 10 mm in size, with the magnetization axis aligned to the measurement direction. Specimens must be fully saturated before measurement and, for high-coercivity grades, may require pulse magnetization.

Why are temperature coefficients measured?

Remanence and coercivity of all permanent magnets decrease with rising temperature. The coefficients α(Br) and α(Hcj), typically expressed in %/K, allow designers to predict magnetic output at operating temperature and to select grades, such as high-Hcj NdFeB or SmCo, that will not suffer irreversible demagnetization in service.

Can very high-coercivity NdFeB grades be measured accurately?

Yes. When the intrinsic coercivity exceeds the field capability of an electromagnet hysteresisgraph, pulse field magnetometry is applied. Modern PFM systems with eddy-current correction deliver results that agree closely with conventional methods for grades with Hcj well above 2400 kA/m (30 kOe).

How are finished magnetized parts tested without destruction?

The Helmholtz coil and fluxmeter method measures the total magnetic moment of an assembled or finished magnet non-destructively, making it the standard approach for lot inspection, assembly verification, and periodic stability monitoring of components that cannot be sectioned.