Magnetic property testing is the systematic characterization of how advanced materials respond to magnetic fields, measuring parameters such as permeability, coercivity, remanence, core loss and Curie temperature. As new materials such as nanocrystalline alloys, soft magnetic composites, rare-earth permanent magnets and magnetic thin films are deployed in electric vehicles, renewable energy and power electronics, accurate magnetic testing to IEC, ASTM, EN, GB/T and JIS standards has become essential for material qualification, design input and quality control. This article explains the key test samples, testing items, measurement methods, applications and frequently asked questions related to magnetic property testing of new materials.
Technical Overview
Magnetic property testing is a fundamental part of the development, qualification and quality control of advanced magnetic materials. It provides quantitative data on how a material is magnetized, how it retains magnetization, and how it behaves under alternating fields and varying temperatures. For new materials such as amorphous and nanocrystalline alloys, soft magnetic composites, rare-earth permanent magnets, ferrites, magnetic thin films and magnetic nanomaterials, reliable magnetic data is the basis for material selection, electromagnetic simulation and lifetime prediction.
The test framework for magnetic materials is well standardized. The IEC 60404 series and its EN adoption define methods for electrical steel, soft magnetic materials and permanent magnets, while ASTM standards such as ASTM A343, ASTM A596, ASTM A772, ASTM A977 and ASTM A342 cover alternating-current and direct-current measurement procedures. Chinese national standards GB/T 3655, GB/T 13789 and GB/T 3217 correspond to the Epstein frame, single sheet tester and permanent magnet methods respectively, and the JIS C 2550 series covers equivalent procedures for electrical steel. Accredited third-party laboratories operating under ISO/IEC 17025 provide traceable measurements with documented uncertainty for R&D, incoming inspection, supplier qualification and failure analysis.
Testing Samples
Soft Magnetic Metals and Alloys
This category includes grain-oriented and non-oriented electrical steel, amorphous and nanocrystalline ribbons, nickel-iron (permalloy) and cobalt-iron alloys, and iron-silicon, iron-silicon-aluminum and iron-phosphorus based materials. Typical specimen forms are Epstein strips, rectangular sheet pieces for the single sheet tester, and ring or toroidal cores wound with primary and secondary windings.
Permanent Magnet Materials
Sintered and bonded NdFeB, SmCo, AlNiCo and hard ferrite magnets are tested as machined blocks, discs or cylinders with parallel pole faces. Specimens may be supplied magnetized or unmagnetized, and anisotropic materials must be oriented so that the easy axis is aligned with the magnetizing direction of the test apparatus.
Ferrites and Soft Magnetic Composites
MnZn and NiZn soft ferrites, pressed powder cores and soft magnetic composites (SMC) are usually tested as toroidal cores. The ring specimen method is preferred because it forms a closed magnetic circuit and minimizes demagnetization effects caused by specimen geometry.
Thin Films, Nanomaterials and Functional Magnets
Magnetic thin films and multilayers for sensors and spintronic devices, magnetic nanoparticles, ferrofluids, magnetocaloric materials and superconducting tapes require high-sensitivity instruments such as vibrating sample magnetometers or SQUID magnetometers, and the specimen mass or film area must be accurately determined to obtain meaningful magnetization values.
Weakly Magnetic and Paramagnetic Materials
Austenitic stainless steels, aluminum and titanium alloys, ceramics, polymers and composites are tested for relative permeability close to unity. This verification is frequently required for components used in or near MRI and NMR systems, electron microscopes and precision instruments where residual magnetism can disturb fields or attract contaminants.
Testing Items
Magnetic property testing covers direct-current (DC), alternating-current (AC) and temperature-dependent parameters. The scope of a test program is selected according to the material class and the end application.
DC Magnetic Properties
- Hysteresis loop and initial magnetization curve
- Saturation magnetization and saturation flux density
- Remanence Br and coercivity Hcb
- Intrinsic coercivity Hcj
- Maximum energy product (BH)max and demagnetization curve
- Initial permeability, maximum permeability and recoil permeability
AC Magnetic Properties
- AC hysteresis loop and dynamic B-H characteristics
- Specific total loss (core loss) at specified frequency and flux density, for example P1.5/50
- Amplitude permeability versus flux density and frequency
- Complex permeability (real part and imaginary part) over a wide frequency range
- Inductance, Q factor and loss factor for cores and components
Temperature-Dependent Properties
- Curie temperature and magnetization versus temperature (M-T) curves
- Temperature coefficients of remanence and intrinsic coercivity
Hysteresis loops measured at elevated or cryogenic temperatures to evaluate demagnetization resistance in traction motors, generators and cryogenic equipment.
Other Measured Parameters
- Magnetic moment and total flux of permanent magnets
- Surface flux density distribution and peak surface field
- Magnetic susceptibility of weakly magnetic materials
- Magnetostriction coefficient
- Magnetic anisotropy and AC susceptibility
- Long-term and temperature-cycled irreversible flux loss of magnets
Testing Methods
Epstein Frame and Single Sheet Tester
The Epstein frame method in IEC 60404-2, ASTM A343 and GB/T 3655 measures specific total loss and AC permeability of electrical steel strips at power frequencies. The single sheet tester of IEC 60404-3 and GB/T 13789 allows larger sheets to be evaluated and is widely used for production control because it requires less specimen preparation.
Toroidal (Ring) Specimen Method
Ring specimens wound with magnetizing and sensing windings provide a closed magnetic circuit suitable for both DC measurements per IEC 60404-4 and ASTM A596 and AC measurements in the range from power frequency up to hundreds of kilohertz per IEC 60404-6 and ASTM A772. The method is the reference technique for ferrites, powder cores, amorphous and nanocrystalline ribbons wound into cores.
Closed-Circuit Hysteresisgraph and Permeameter
Permanent magnets are characterized in an electromagnetic yoke or permeameter that closes the magnetic circuit, in accordance with IEC 60404-5, ASTM A977 and GB/T 3217. The instrument traces the complete demagnetization curve, from which Br, Hcb, Hcj and (BH)max are derived. Specimens are first saturated with a strong pulsed or DC field, and temperature-controlled measurements generate the temperature coefficients required for motor design.
Helmholtz Coil and Fluxmeter Method
The magnetic moment of a complete magnet or assembly is measured by withdrawing or rotating it inside a Helmholtz coil connected to a fluxmeter. This open-circuit method is fast and non-destructive, and is commonly used for production screening and calibration of permanent magnet components.
Vibrating Sample Magnetometry and SQUID Magnetometry
Vibrating sample magnetometers (VSM) and SQUID magnetometers measure magnetization as a function of applied field and temperature with very high sensitivity, typically from liquid helium temperatures up to high temperatures. They are the preferred tools for research on thin films, nanoparticles, magnetocaloric compounds and any material available only in small quantities. Coercivity of soft magnetic materials in open circuits can also be determined with a coercimeter per IEC 60404-7.
Supplementary Techniques
- AC susceptibility measurement using mutual-inductance coils for phase transitions and superconductivity studies
- Gaussmeter and calibrated Hall probes for surface field mapping of magnet assemblies
- Relative permeability measurement of weakly magnetic materials per ASTM A342
- Magnetostriction measurement by strain gauges or optical interferometry
- Thermal demagnetization and AC degaussing prior to reference measurements
Applications
Magnetic property testing supports the entire life cycle of new magnetic materials, from laboratory research through industrial qualification to in-service failure analysis. Key application areas include:
- Electric vehicle traction motors, generators and drive units requiring verified demagnetization resistance
- Power transformers, reactors, inductors and wireless charging systems where core loss determines efficiency
- Wind turbines and renewable energy converters using large permanent magnet generators
- Consumer electronics including speakers, sensors, actuators and magnetic fasteners
- Data storage, MRAM and spintronic devices based on magnetic thin films
- MRI-compatible components and biomedical magnetic nanomaterials
- Aerospace, defense and nuclear facilities with strict requirements on permeability and long-term magnetic stability
- University research and national programs on advanced and functional magnetic materials
FAQ
How should samples be prepared before magnetic testing?
Ring cores should have a uniform cross-section and a ratio of outer to inner diameter close to the values recommended by the relevant standard so that the magnetizing field stays uniform. Epstein strips and sheet specimens must be flat, deburred and free of residual stress. Permanent magnet specimens need parallel pole faces and defined orientation. Soft magnetic specimens are usually thermally demagnetized or AC degaussed before measurement, and the specimen mass or dimensions must be supplied so that results can be normalized.
What is the difference between Hcb and Hcj?
Hcb is the normal coercivity at which the flux density of the magnet becomes zero, while Hcj is the intrinsic coercivity at which the intrinsic magnetization is fully overcome. Hcj indicates resistance to irreversible demagnetization under opposing fields and elevated temperature, and for NdFeB and SmCo magnets it is usually far larger than Hcb. Both values, together with Br and (BH)max, are reported on the demagnetization curve.
Which method should be chosen: ring test, hysteresisgraph or VSM?
Ring tests are best for soft magnetic materials and cores because the closed circuit reproduces actual working conditions up to high frequencies. A hysteresisgraph with a closed yoke is the reference method for bulk permanent magnets. VSM or SQUID magnetometry is chosen when the specimen is very small, a thin film, a powder or a nanomaterial, or when measurements over wide temperature ranges are needed. Accredited third-party laboratories can advise on the most appropriate method for a given material and data requirement.
Can magnetic properties be measured at extreme temperatures?
Yes. Temperature-controlled hysteresisgraph and VSM measurements are routinely performed from cryogenic temperatures to several hundred degrees Celsius. These data yield Curie temperature, temperature coefficients of Br and Hcj, and demagnetization behavior under realistic operating conditions, which are essential inputs for motor and generator thermal design.
How are core losses reported for soft magnetic materials?
Specific total loss is reported in watts per kilogram or watts per cubic meter at defined combinations of flux density and frequency, for example P1.5/50 meaning the loss at 1.5 T and 50 Hz. Curves of loss versus frequency and flux density, together with complex permeability spectra, allow designers to interpolate performance at arbitrary operating points.
Why do nearly non-magnetic parts still need magnetic testing?
Materials with relative permeability only slightly above unity can still distort highly uniform fields in MRI, NMR and precision analytical instruments. Measurement of relative permeability per ASTM A342 verifies compliance with specifications that commonly limit permeability to values such as 1.005 or below, ensuring safety, image quality and equipment performance.
