Residual magnetization curve testing measures the remanence (Br/Mr), coercivity, and demagnetization behavior of permanent and soft magnetic materials, providing the essential data required for magnet design, qualification, and quality control. Performed by accredited third-party laboratories in accordance with IEC 60404, ASTM, and GB/T standards, this testing verifies that magnetic materials meet performance specifications for motors, sensors, transformers, and advanced electronic applications.
Technical Overview
The residual magnetization curve describes the magnetization retained by a ferromagnetic or ferrimagnetic material after an external magnetizing field is removed, and how that retained magnetization evolves as a reverse field is applied. On the complete hysteresis loop, the remanence point is located where the applied field H returns to zero after saturation, while the second-quadrant branch between remanence and coercivity constitutes the demagnetization curve that dominates permanent magnet engineering.
Two complementary curve systems are recorded during testing. The normal curve relates magnetic flux density B to the applied field H, while the intrinsic curve relates magnetic polarization J (where J = B − μ₀H, proportional to magnetization M) to H. From these curves the key figures of merit are extracted: remanence Br (or residual magnetization Mr), normal coercivity HcB, intrinsic coercivity HcJ, maximum energy product (BH)max, knee field Hk, and squareness ratio.
Measurements are performed following international and national standards, including IEC 60404-5 for magnetically hard materials, IEC 60404-4 and IEC 60404-6 for DC magnetic properties of soft materials, ASTM A977/A977M for high-coercivity permanent magnets measured with hysteresisgraphs, ASTM A34/A34M for general test practices, and GB/T 3217 for permanent magnet measurement. Accredited third-party laboratories apply these methods with calibrated instruments and certified reference materials to ensure traceable, repeatable results.
Testing Samples
Permanent Magnet Materials
Sintered and bonded neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo₅, Sm₂Co₁₇), aluminum-nickel-cobalt (AlNiCo), hard ferrites (barium and strontium hexaferrites), and iron-chromium-cobalt magnets. Typical specimens are cylinders or rectangular blocks sized to the permeameter pole diameter—commonly 10 mm diameter by 10 mm height for NdFeB per IEC 60404-5—with ground parallel faces to minimize air gaps in closed-circuit fixtures.
Soft Magnetic Materials
Electrical steel (grain-oriented and non-oriented), pure iron and low-carbon steels, nickel-iron alloys (permalloy), cobalt-iron alloys, amorphous and nanocrystalline ribbons, soft ferrites, and iron powder composites. Specimens are prepared as ring (toroidal) cores, strips, or block samples depending on material form and the applicable standard.
Special and Research Specimens
Magnetic powders and bonded granules, thin films and multilayers on substrates, magnetic nanoparticles dispersed in carriers, geological samples, and finished magnet assemblies. Small or irregular specimens are typically characterized on open-circuit magnetometers with appropriate demagnetization-factor corrections.
Testing Items
Core parameters reported from the residual magnetization curve include:
- Residual magnetization Mr and remanence Br of the material after saturation
- Saturation magnetization Ms and saturation polarization Js
- Normal coercivity HcB (the reverse field at which B returns to zero)
- Intrinsic coercivity HcJ (the reverse field at which J returns to zero)
- Maximum energy product (BH)max and the corresponding optimal operating point
- Knee field Hk (the field at 90% or 80% of Br) and demagnetization-curve squareness
- Full major hysteresis loops, initial magnetization curves, and minor (recoil) loops
- Recoil permeability μrec for dynamic operating conditions
- Hysteresis loss per cycle and remanence-to-saturation squareness ratio Mr/Ms
- Temperature coefficients of Br and HcJ, and irreversible losses at elevated or cryogenic temperatures
Testing Methods
Closed-Circuit Hysteresisgraph (Permeameter) Method
The specimen is clamped between the pole pieces of a high-field electromagnet, forming a closed magnetic circuit. A wound B-coil (with J-compensation winding) and an H-coil or Hall probe record flux density and field strength while the magnetizing current sweeps between positive and negative saturation. Fields of approximately 2 T or more are achievable, sufficient for most NdFeB and SmCo grades, and results are directly traceable to IEC 60404-5 and ASTM A977 requirements. Magnetizing fields of at least three to five times HcJ are applied before recording the demagnetization branch to guarantee full saturation.
Ring Specimen (Toroid) Method
Preferred for soft magnetic materials, the toroidal specimen carries a primary magnetizing winding and a secondary measuring winding. Field strength is calculated from the magnetizing current via Ampere's law, and flux density is obtained from the integrated secondary voltage via Faraday's law. Because the magnetic path is completely closed within the specimen, no demagnetization correction is required, yielding highly accurate quasistatic and DC hysteresis loops in line with IEC 60404-4 and IEC 60404-6.
Vibrating Sample Magnetometry (VSM)
In open-circuit VSM, the specimen vibrates within a uniform field between electromagnet poles (up to about 2-3 T) or inside a superconducting magnet (up to 7-9 T), while stationary pickup coils measure a signal proportional to the magnetic moment. VSM accommodates tiny, irregular, powdered, and thin-film samples with sensitivity in the 10⁻⁶ emu range, making it the workhorse instrument for research and development. An effective demagnetization factor is applied to correct the internal field of the specimen.
Pulsed Field Magnetometry and Related Techniques
Capacitor-bank discharge through a solenoid produces transient fields of 5-10 T within milliseconds, enabling saturation and full-loop measurement of very high-coercivity magnets, with eddy-current corrections applied during analysis. SQUID magnetometers complement these methods where ultra-high sensitivity is required for weakly magnetic or nanoscale specimens.
All instruments are calibrated against certified reference materials, such as high-purity nickel spheres supplied by national metrology institutes, with typical expanded uncertainties of 1-2% for Br and 1-3% for HcJ at a 95% confidence level.
Applications
- Quality control of permanent magnets for electric vehicle traction motors, industrial servo motors, and wind turbine generators
- Verification of magnet grades and supplier conformity against datasheet values for Br, HcJ, and (BH)max
- Design data for sensors, actuators, loudspeakers, magnetic couplings, bearings, separators, and MRI assemblies
- Characterization of soft magnetic cores for transformers, inductors, and chokes, including loss and squareness evaluation
- Residual magnetism checks after machining, welding, heat treatment, or handling to prevent particle attraction and sensor interference
- Aerospace and defense component qualification, degaussing verification, and magnetic cleanliness programs
- Research on magnetic thin films, spintronic materials, recording media, and novel permanent magnet compositions
FAQ
What is the difference between remanence Br and residual magnetization Mr?
Residual magnetization Mr is the magnetization (in A/m) remaining at zero applied field, whereas remanence Br is the corresponding flux density (in T or mT) read from the normal curve. The two are related through Br = μ₀Mr and describe the same physical point of the hysteresis loop.
Which standards govern the testing?
IEC 60404-5 covers permanent magnet measurement with hysteresisgraphs, IEC 60404-4 and IEC 60404-6 cover DC properties of soft magnetic materials, ASTM A977/A977M covers high-coercivity permanent magnets, ASTM A34/A34M defines general test practices, and GB/T 3217 specifies the equivalent Chinese method for permanent magnet materials.
What sample size and geometry are required?
For closed-circuit permeameter testing, cylindrical or block specimens with ground parallel faces—commonly 10 mm diameter by 10 mm height for NdFeB—are used, and the sample face should not exceed roughly 70% of the pole cap diameter. VSM testing accepts specimens from sub-millimeter films to pieces of several millimeters, while ring specimens are wound to specification for soft material testing.
Why do open-circuit and closed-circuit results differ?
In open-circuit measurement, the sample's own poles create a demagnetizing field that reduces the internal field, shifting the apparent loop unless a demagnetization factor N is corrected for. Closed-circuit fixtures such as permeameters and toroids minimize this self-demagnetization and provide values consistent with material standards.
Can very high-coercivity magnets be fully saturated?
Grades with HcJ above roughly 2000 kA/m may exceed conventional electromagnet capability. In these cases, pulsed field magnetometers or superconducting-magnet VSM systems supply fields of 5 T and above to reach saturation and record the complete demagnetization curve.
What accuracy and turnaround time can be expected?
Typical expanded uncertainties are 1-2% for remanence and 1-3% for coercivity at a 95% confidence level. Standard measurements are usually completed within three to five working days, while variable-temperature studies and calibration-critical programs may require additional time.