Coercivity analysis is a fundamental magnetic characterization technique used to determine the magnetic field strength required to demagnetize a material after it has been saturated. This comprehensive guide covers the technical principles of coercivity measurement, applicable sample types, standardized testing methods, and practical applications in quality control and materials research. Accurate coercivity data is essential for verifying the performance of permanent magnets, soft magnetic components, and magnetic recording media across a wide range of industries.
Technical Overview
Coercivity, denoted as Hc, is one of the most important intrinsic magnetic properties of ferromagnetic and ferrimagnetic materials. It represents the intensity of the external magnetic field that must be applied in the opposite direction to reduce the magnetization of a previously saturated sample to zero. For permanent magnet materials, two distinct parameters are measured: the normal coercivity (HcB), which is determined from the flux density versus field strength (B–H) curve, and the intrinsic coercivity (Hcj or Hci), which is determined from the intrinsic magnetization versus field strength (J–H) curve. The intrinsic coercivity reflects the material's resistance to demagnetization and is a critical specification parameter in magnet design and procurement.
Coercivity is highly sensitive to microstructural features such as grain size, phase distribution, impurity content, residual stress, and heat treatment condition. As a result, coercivity analysis serves not only as a performance verification tool but also as an indirect indicator of manufacturing process consistency and material integrity. Measurements are typically performed at room temperature, but temperature-dependent coercivity characterization is increasingly required for applications involving elevated operating temperatures, such as electric vehicle traction motors and aerospace components.
Accredited third-party laboratories conduct coercivity analysis in accordance with internationally recognized standards, including the IEC 60404 series, ASTM A977, ASTM A341/A341M, and corresponding GB standards, ensuring traceable and comparable results for both research and commercial purposes.
Testing Samples
Permanent Magnet Materials
Sintered and bonded neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo), aluminum-nickel-cobalt (AlNiCo), and hard ferrite (ceramic) magnets constitute the majority of coercivity analysis requests. These samples are typically submitted as finished magnet components, block specimens, or cylinder samples, and their intrinsic coercivity values may range from several hundred kA/m to more than 3000 kA/m for high-grade NdFeB grades.
Soft Magnetic Materials
Silicon steel sheets, pure iron, low-carbon steel, permalloy, amorphous and nanocrystalline ribbons, soft ferrites, and iron powder composites are analyzed to determine their coercive force, which should be as low as possible to minimize hysteresis losses. Ring-shaped (toroidal) specimens and strip samples are commonly tested using closed magnetic circuit methods.
Magnetic Powders and Recording Media
Magnetic powders intended for bonded magnet production, as well as thin films, magnetic tapes, and recording heads, can be characterized using open-circuit techniques such as vibrating sample magnetometry. Powder samples are typically immobilized in a non-magnetic matrix or measured in dedicated holders to ensure reproducible packing and orientation.
Magnetic Assemblies and Components
Finished rotor assemblies, sensor magnets, magnetic couplings, and magnetized devices may be evaluated using coercimeters or hysteresisgraph systems with appropriate fixtures, provided that geometry-dependent demagnetizing effects are properly accounted for.
Testing Items
- Intrinsic coercivity (Hcj / Hci) of permanent magnet materials
- Normal coercivity (HcB) derived from the B–H hysteresis loop
- Coercive force (Hc) of soft magnetic materials
- Remanent coercivity (Hcr) for open-circuit and rock-magnetism type analyses
- Full hysteresis loop parameters: remanence (Br), maximum energy product ((BH)max), saturation magnetization, and initial magnetization curve
- Temperature-dependent coercivity and temperature coefficients (α of Hcj) over specified temperature ranges
- Demagnetization curve analysis at elevated temperatures for motor-grade magnets
- Aging and stability studies of coercivity after thermal, humidity, or corrosive exposure
Testing Methods
Closed-Circuit Hysteresisgraph Method (IEC 60404-5)
The sample is placed in a closed magnetic circuit formed by an electromagnet with pole pieces and a flux measuring coil (B-coil) plus a field sensing coil (H-coil). The material is first magnetized to saturation using a pulsed or direct field, after which a symmetrical hysteresis loop is recorded. This method provides the highest accuracy for permanent magnets with rectangular geometries and yields both HcB and Hcj directly. Specimen faces must be ground parallel and in intimate contact with the pole pieces to minimize air gaps.
Vibrating Sample Magnetometer Method (ASTM A977)
In VSM testing, the sample vibrates within a uniform magnetic field while pickup coils detect the induced voltage proportional to the magnetic moment. The instrument sweeps the field to record a complete hysteresis loop from which coercivity is extracted. VSM offers high sensitivity, small sample requirements, and suitability for irregular shapes, films, and powders, although corrections for the demagnetizing factor must be applied for open-circuit measurements.
Permeameter and Ring-Core Method (IEC 60404-4, IEC 60404-6, ASTM A596)
Soft magnetic materials are tested using ring specimens wound with primary magnetizing and secondary measuring windings, or using permeameters that form a closed yoke circuit around strip or bar samples. The coercive force is determined from the slowly traced quasi-static hysteresis loop, with careful degaussing of the yoke and controlled field sweep rates to avoid rate-dependent errors.
Coercimeter Method (IEC 60404-7)
An open-circuit technique in which the specimen is first magnetized to saturation in a solenoid and then exposed to an increasing opposing field. The field strength at which the net magnetization reaches zero, corrected for self-demagnetization, gives the remanent coercivity Hcr. This method is well suited to quality inspection of mass-produced small magnets and is widely used in industrial sorting applications.
Pulse Magnetometer Method
High-coercivity grades of NdFeB and SmCo that exceed the saturation capability of conventional electromagnets are saturated using capacitor-driven pulsed fields, after which the demagnetization curve is measured in a hysteresisgraph. This combined approach ensures full saturation, which is a prerequisite for valid Hcj determination.
Applications
Coercivity analysis supports a broad spectrum of engineering and commercial activities, including incoming material inspection, supplier qualification, and lot release testing of permanent magnets for electric motors, generators, wind turbine direct-drive systems, acoustic transducers, and magnetic separators. In the soft magnetic sector, coercive force data are used to evaluate hysteresis losses in transformer cores, motor laminations, relay armatures, and shielding materials, and to verify the effectiveness of annealing treatments.
In research and development, coercivity measurements enable correlation between composition, processing parameters, and microstructure, supporting the development of high-anisotropy magnet alloys, grain-boundary diffusion processes, and thin-film recording media. Failure analysis teams also rely on coercivity comparisons between service-exposed and reference samples to identify thermal degradation, corrosion-induced demagnetization, or mechanical damage in returned components.
FAQ
What is the difference between HcB and Hcj? HcB is the field at which the magnetic flux density B in the material reaches zero, while Hcj is the field at which the intrinsic magnetization J reaches zero. Hcj is always greater than HcB, and it is the more meaningful indicator of a magnet's resistance to demagnetization under external fields and thermal stress.
Why must the sample be fully saturated before measurement? If the magnetization field does not reach saturation, the measured coercivity will be underestimated and the hysteresis loop will be minor rather than the major loop. For high-coercivity rare-earth magnets, pulsed fields of several thousand kA/m are often required to guarantee full saturation.
How does temperature affect coercivity results? Coercivity generally decreases as temperature rises. Permanent magnet specifications therefore include temperature coefficients, typically expressed as a percentage change in Hcj per degree Celsius, and elevated-temperature demagnetization curves are essential for motor and automotive applications.
What sample preparation is required? Closed-circuit measurements require two parallel, ground, and clean contact faces. Open-circuit VSM testing requires knowledge of sample dimensions and shape to correct for the demagnetizing factor. Powders should be uniformly packed and immobilized, and all samples must be demagnetized or consistently magnetized prior to the measurement sequence as specified by the applicable standard.
Which standards govern coercivity testing? Key references include IEC 60404-5 for permanent magnet hysteresisgraph testing, IEC 60404-7 for coercimeter measurements, IEC 60404-4 and IEC 60404-6 for soft magnetic materials, ASTM A977 for VSM determination of intrinsic coercivity, ASTM A341/A341M for direct-current magnetic properties, and the corresponding GB standards for the Chinese national framework.