Determination of the magnetic properties of materials quantifies parameters such as permeability, saturation induction, remanence, coercivity, core loss, and Curie temperature, which govern the performance of soft and hard magnetic materials in electrical and electronic engineering. This technical guide explains the measurable magnetic parameters, applicable sample types, standard test methods based on IEC 60404, ASTM, and GB/T standards, and the main industrial and research applications of magnetic property testing in accredited third-party laboratories.
Technical Overview
Magnetism originates from the spin and orbital motion of electrons, and the response of a material to an applied magnetic field defines its technical magnetic behavior. Depending on this response, materials are classified as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, ferrimagnetic, or superparamagnetic. Determination of magnetic properties quantifies this response through parameters such as permeability, saturation magnetic induction, remanence, coercivity, and energy losses, which together determine whether a material is suited for efficient flux conduction (soft magnetic materials) or for storing magnetic energy over long periods (hard or permanent magnetic materials).
Measurements are performed under controlled DC or AC magnetization conditions, using either closed magnetic circuits (ring specimens, permeameters, pole-cap fixtures) or open-circuit configurations (vibrating sample magnetometers, extraction and balance methods). The applied field strength H, the resulting magnetic induction B or magnetization J (M), and the temperature are controlled or monitored, and the characteristic B–H or J–H curves are recorded. Results are reported with calibration traceability to certified reference materials maintained by national metrology institutes, and the test procedures follow internationally recognized standards such as the IEC 60404 series, ASTM A341, ASTM A596, ASTM A977, and the corresponding GB/T and JIS C 2550 series of methods.
Testing Samples
Soft Magnetic Materials
Soft magnetic materials are characterized by high permeability and low coercivity and are tested mainly to evaluate how easily they can be magnetized, demagnetized, and remagnetized. Typical samples include:
- Grain-oriented and non-oriented electrical steel sheets and strips
- Nickel–iron soft magnetic alloys (permalloy-type) and cobalt–iron alloys
- Soft ferrites, including MnZn and NiZn grades
- Amorphous and nanocrystalline ribbons and toroidal cores
- Pure iron, iron powder cores, and soft magnetic composites
Hard (Permanent) Magnetic Materials
Permanent magnet materials are characterized by high coercivity and high remanence and are tested to verify their ability to maintain a stable magnetic field. Typical samples include:
- Sintered and bonded neodymium–iron–boron (NdFeB) magnets
- Samarium–cobalt (SmCo) magnets, including 1:5 and 2:17 types
- Aluminum–nickel–cobalt (AlNiCo) alloys
- Hard ferrite (barium and strontium hexaferrite) magnets
Weakly Magnetic and Non-Magnetic Materials
Many structural and functional materials are required to be nominally non-magnetic, and their residual susceptibility must be verified. Typical samples include austenitic stainless steels, titanium alloys, copper and aluminum alloys, ceramics, polymers, paramagnetic and diamagnetic chemicals, and geological or biomedical powders.
Magnetic Thin Films, Powders, and Components
Advanced applications require testing of magnetic thin films and multilayers, magnetic recording media, magnetic nanoparticles, magnetic powders, and finished components such as transformer cores, inductor cores, motor stators and rotors, relay armatures, sensor elements, and magnet assemblies.
Testing Items
The scope of magnetic property determination covers the full set of DC and AC parameters used in material specification sheets and design calculations:
- Saturation magnetic induction (Bs) and saturation magnetization (Ms)
- Remanence or residual flux density (Br)
- Coercive field strength (HcB) and intrinsic coercivity (Hcj)
- Maximum energy product (BH)max
- Initial permeability (μi), maximum permeability (μm), and amplitude permeability
- Complex permeability (μ′, μ″) as a function of frequency
- Specific total loss (core loss, W/kg) at specified induction and frequency
- Full hysteresis loop and normal magnetization curve
- Second-quadrant demagnetization curves at room, elevated, or cryogenic temperatures
- Curie temperature (Tc) and temperature coefficients of Br and Hcj
- Magnetic susceptibility (χ) and effective magnetic moment (μeff)
- Anisotropy field (Ha) and magnetostriction coefficient (λs)
- Magnetic moment of powders, fluids, and irregularly shaped specimens
Testing Methods
DC Hysteresis Measurement: Ring Method and Permeameter
In the ring method, a toroidal specimen is wound with a primary (magnetizing) and secondary (measuring) winding. The magnetizing field H is calculated from the ampere-turns per meter of the mean magnetic path length, while the induction B is obtained by integrating the voltage induced in the secondary winding with a calibrated fluxmeter. This closed-circuit geometry eliminates demagnetizing field corrections and yields the intrinsic DC hysteresis loop, from which μi, μm, Br, Hc, and Bs are derived. The method is specified in IEC 60404-4 and ASTM A596. For bars, rods, and wires that cannot be made into rings, a permeameter (closed yoke fixture) is used in accordance with IEC 60404-4 and ASTM A341, with the specimen clamped in a magnetic circuit so that a uniform field is applied along its axis.
Hysteresisgraph Method for Permanent Magnets
Permanent magnets are measured in a closed magnetic circuit formed by an electromagnet with shaped pole caps matching the specimen cross-section. Surrounding search coils measure B and J (the intrinsic induction), and the second-quadrant demagnetization curve is recorded after full saturation with a pulsed or steady strong field. The curve yields Br, HcB, Hcj, and the (BH)max energy product. Measurements can be performed inside temperature chambers, typically from about −80 °C to +250 °C, to determine temperature coefficients and reversible losses. Relevant procedures are given in IEC 60404-5, ASTM A977, and GB/T 3217.
Vibrating Sample Magnetometry and SQUID Magnetometry
The vibrating sample magnetometer (VSM) is an open-circuit method in which a small specimen vibrates in a uniform field between pole pieces and induces a signal in pick-up coils proportional to its magnetic moment. The instrument is calibrated with a certified nickel sphere or equivalent reference, allowing absolute measurement of the moment in emu or A·m². The VSM is ideal for powders, thin films, small irregular specimens, and temperature-dependent studies up to the Curie point. SQUID magnetometers extend the sensitivity by several orders of magnitude and are used for ultra-weak paramagnetic and diamagnetic signals and for research-grade characterization. Both methods are widely applied for thermomagnetic analysis to determine Curie temperatures and phase transformations.
AC Magnetic Testing: Epstein Frame, Single Sheet Tester, and B–H Analyzer
AC properties are essential for materials operating in alternating fields, such as electrical steel in transformers and motors. In the Epstein frame method, specified in IEC 60404-2, ASTM A804, and GB/T 3655, a standard set of strips is inserted into four solenoid arms to form a closed magnetic circuit, and the specific total loss, AC permeability, and dynamic B–H loop are measured at controlled sinusoidal induction, typically 1.0–1.8 T at 50 or 60 Hz. The single sheet tester (IEC 60404-3, GB/T 13789) allows direct testing of full production sheets without cutting Epstein strips. For ferrites, amorphous ribbons, and nanocrystalline cores, an AC B–H analyzer or impedance/LCR system measures the complex permeability and core losses of wound ring specimens over frequencies from power-line to the megahertz range, following IEC 60404-6 and related practices.
Susceptibility and Magnetic Balance Methods
Weakly magnetic materials are characterized by their volume or mass susceptibility. Gouy and Faraday balance methods, as well as modern Evans-type magnetic balances, measure the force exerted on a specimen in a field gradient and yield the susceptibility directly, from which the effective magnetic moment is calculated using the Curie law. AC susceptibility systems based on mutual-inductance or mutual-inductance bridge detection provide both the in-phase (χ′) and out-of-phase (χ″) components, useful for superconductors, spin glasses, and nanoparticle research. Verification of nominally non-magnetic engineering alloys is commonly performed by permeability measurement at low field in accordance with ASTM A342.
Curie Temperature and Specialized Measurements
Curie temperatures are determined by thermomagnetic analysis: the magnetization of a saturated specimen is monitored while heating in a small applied field, and the temperature at which the magnetization vanishes is reported. Alternatives include the peak in initial permeability versus temperature and thermomagnetometry with a mass-change balance fitted with a magnet. Specialized techniques complement the standard methods, including magnetostriction measurement with strain gauges or optical dilatometry, magneto-optical Kerr effect (MOKE) for thin-film hysteresis loops, and ferromagnetic resonance for anisotropy fields.
Applications
Magnetic property determination supports material qualification, product design, process control, and failure analysis across many industries:
- Qualification of electrical steel and soft magnetic cores for transformers, motors, and generators
- Incoming inspection and batch release testing of NdFeB and SmCo magnets for electric vehicle traction motors, wind turbine generators, and industrial drives
- Design data for ferrite and powder cores used in switched-mode power supplies, inductors, and wireless charging systems
- Aerospace and defense applications, including relays, actuators, magnetrons, and microwave components
- Verification of nominally non-magnetic parts for MRI environments, cryogenic systems, and precision instrumentation
- Research and development of magnetic recording media, spintronic devices, and soft magnetic nanocrystalline alloys
- Biomedical engineering, including magnetic nanoparticles for separation, hyperthermia, and targeted delivery
- Failure analysis, such as detecting oxidation and aging of sintered magnets or crystallization of amorphous ribbons through loss and coercivity drift
FAQ
What is the difference between HcB and Hcj?
HcB is the normal coercivity at which the magnetic flux density B in the magnet becomes zero, while Hcj is the intrinsic coercivity at which the intrinsic magnetization J (or M) becomes zero. Because Hcj measures resistance to irreversible demagnetization of the material itself, it is always larger than HcB, and it is the critical parameter for magnets exposed to strong opposing fields or elevated temperatures.
What is the difference between closed-circuit and open-circuit measurements?
In closed-circuit methods, such as the ring method or the hysteresisgraph with pole caps, the magnetic flux is confined within a complete magnetic path, so the internal field equals the applied field and the intrinsic material properties are obtained directly. In open-circuit methods, such as VSM measurements of a free-standing specimen, the specimen's own demagnetizing field opposes the applied field and must be corrected using the demagnetization factor determined from the specimen geometry.
What specimen forms and dimensions are required?
Ring specimens are typically machined or wound to a defined ratio of outer to inner diameter to keep the field uniform. Epstein testing requires strips of standard width and length sheared parallel and perpendicular to the rolling direction of electrical steel. Permanent magnets are tested as rectangular or cylindrical blocks with parallel, ground faces. VSM testing accepts powders, films, and small irregular pieces, typically with masses from a few milligrams upward. In all cases, specimens must be demagnetized to a reproducible initial state before measurement.
How is core loss measured and reported?
Core loss is measured with an Epstein frame, single sheet tester, or wound ring specimen by controlling a sinusoidal flux waveform and measuring the product of induced voltage and magnetizing current, or by digital waveform acquisition and integration. The result is reported as specific total loss in W/kg at a stated peak induction and frequency, for example 1.5 T and 50 Hz, in accordance with IEC 60404-2, IEC 60404-3, or IEC 60404-6.
Can magnetic properties be measured at extreme temperatures?
Yes. Hysteresisgraph and VSM systems can be fitted with cryostats and furnaces, allowing demagnetization curves and magnetization to be recorded from cryogenic temperatures up to several hundred degrees Celsius. These data are used to derive temperature coefficients of Br and Hcj, reversible and irreversible losses, and Curie temperatures for magnet design and derating.
How are results assured to be accurate and comparable?
Accredited third-party laboratories calibrate magnetometers, fluxmeters, and field-measuring coils against certified reference materials traceable to national metrology institutes, control specimen preparation and demagnetized initial states, apply standardized sweep rates, and report measurement uncertainties. Following the applicable IEC 60404, ASTM, JIS C 2550, or GB/T procedures ensures that results are comparable between laboratories and compliant with material specifications.