Permanent Magnet Demagnetization Curve Testing: Standards, Methods, and Key Magnetic Parameters

Permanent magnet demagnetization curve testing measures the second-quadrant magnetic properties of hard magnetic materials such as sintered NdFeB, SmCo, hard ferrite, and Alnico, determining remanence (Br), coercivity (Hcb, Hcj), and maximum energy product ((BH)max). Conducted by accredited third-party laboratories in accordance with IEC 60404-5, ASTM A977/A977M, and GB/T 3217, this testing is essential for material grading, motor and sensor design verification, and incoming quality control. This article explains the measurement principles, sample requirements, test items, methods, and typical applications of demagnetization curve testing.

Technical Overview

The demagnetization curve is the portion of the hysteresis loop located in the second quadrant, describing how a permanent magnet behaves when subjected to an increasing opposing magnetic field. Two related curves are recorded during the test: the normal curve B(H), which shows the magnetic flux density B as a function of the applied field H, and the intrinsic curve J(H), which describes the magnetic polarization J of the material itself, where J = B − μ0H. Together, these curves provide the most complete quantitative description of a permanent magnet's usable magnetic performance.

For design engineers, the demagnetization curve is the primary input for circuit calculations. The operating point of a magnet in an application is defined by the intersection of the normal curve with the circuit load line, while the intrinsic curve indicates how close the material is to irreversible demagnetization under adverse fields, temperatures, or fault conditions. Key standard references include IEC 60404-5 for the hysteresisgraph measurement method, ASTM A977/A977M for high-coercivity materials, GB/T 3217 for Chinese-market testing, and the material specification standards IEC 60404-8-1 and GB/T 13560 for sintered NdFeB grading.

• The normal curve B(H) determines the external flux a magnet can deliver in a magnetic circuit.
• The intrinsic curve J(H) reveals resistance to irreversible demagnetization and magnet stability.
• The full curve enables derivation of operating points, load-line margins, and temperature derating factors.
• Results are reported in SI units (T, kA/m, kJ/m³) or CGS units (G, Oe, MGOe), where 1 T = 10 kG and 1 MGOe ≈ 7.958 kJ/m³.

Testing Samples

For closed-circuit hysteresisgraph testing to IEC 60404-5 and GB/T 3217, specimens are typically fine-ground cylindrical samples of approximately 10 mm diameter and 5 mm to 10 mm height. The two end faces must be flat, parallel, and perpendicular to the cylinder axis within tight tolerances so that no air gap forms between the sample and the pole pieces of the electromagnet, since even a small gap causes systematic underestimation of remanence. Prior to measurement, samples must be fully magnetized to saturation, generally requiring a magnetizing field of at least three times the intrinsic coercivity.

Sintered NdFeB (Neodymium-Iron-Boron) Magnets

Sintered NdFeB is the most frequently tested material, with remanence typically between 1.0 T and 1.5 T and maximum energy products reaching approximately 400 kJ/m³. Intrinsic coercivity ranges from about 950 kA/m for high-energy grades to above 3100 kA/m for heavy-dysprosium or grain-boundary-diffusion grades designed for elevated-temperature traction motor applications.

Samarium-Cobalt (SmCo) Magnets

SmCo magnets, including SmCo5 and Sm2Co17 types, combine moderate remanence of roughly 0.85 T to 1.15 T with excellent thermal stability, allowing continuous operating temperatures from 250 °C up to 350 °C for selected grades. Testing at elevated temperature is often required because these magnets are specified mainly for high-temperature or aerospace environments.

Hard Ferrite (Ceramic) Magnets

Sintered hard ferrite (strontium or barium hexaferrite) exhibits low remanence of about 0.2 T to 0.45 T and low cost. Because ferrites have a positive temperature coefficient of intrinsic coercivity, their worst-case demagnetization occurs at low temperature, so cryogenic or sub-zero curve measurement may be specified for outdoor and automotive applications.

Alnico and Bonded Magnets

Alnico magnets offer high remanence up to about 1.3 T but very low coercivity, so they must be tested and used in closed magnetic circuits or magnetized in place. Bonded NdFeB and bonded ferrite magnets, which mix magnetic powder with polymer binders, are tested to verify isotropic or anisotropic properties and to check the effect of the binder fraction on energy product.

Testing Items

A demagnetization curve test reports the complete second-quadrant characteristic together with the derived scalar parameters that define a permanent magnet grade. Typical test items include:

• Remanence Br: the residual flux density at zero applied field, the starting point of the demagnetization curve.
• Normal coercivity Hcb: the reverse field strength at which the flux density B falls to zero.
• Intrinsic coercivity Hcj: the reverse field at which the intrinsic polarization J falls to zero, marking irreversible loss of magnetization.
• Maximum energy product (BH)max: the peak of the B·H product along the curve, representing the maximum usable energy density.
• Knee point (Hd, Bd) and squareness Hk/Hcj: the field at which J has dropped to 90% of Br, used to assess curve shape and demagnetization margin.
• Recoil permeability μrec: the effective slope of the curve near Br, typically 1.02 to 1.10 for sintered NdFeB.
• Complete B–H and J–H curves: full digitized data supplied for finite-element simulation and design input.
• Temperature coefficients α(Br) and β(Hcj) and irreversible loss: measured by repeating the test across a temperature range.

Testing Methods

Closed-Circuit Hysteresisgraph Method

The reference method under IEC 60404-5, ASTM A977/A977M, and GB/T 3217 uses an electromagnet with parallel pole pieces between which the specimen is clamped, forming a closed magnetic circuit. A search coil (B-coil) wound around the middle of the sample measures flux density through a fluxmeter, while the field strength H is measured by a Hall probe positioned in a small hole close to the sample surface or by a tangential H-coil. The specimen is first saturated, then the field is swept through zero into the second quadrant to trace the complete loop. This method offers the highest accuracy for bulk magnet grades, with typical uncertainties of 1% to 3% on Br and Hcj, and measurements are performed at a controlled reference temperature.

Pulsed Field Magnetometer (PFM) Method

Ultra-high-coercivity NdFeB grades can exceed the saturation field capability of conventional electromagnets. A pulsed field magnetometer discharges a capacitor bank through a coil to generate transient fields of 4 T to 10 T, allowing full saturation and complete curve acquisition in milliseconds. Because the rapidly changing field induces eddy currents in electrically conductive rare-earth magnets, apparent Hcj values are corrected using field-dependent eddy-current models or dual-pulse-width techniques. PFM is widely used for production control of high-Hcj traction motor grades and for temperature-dependent characterization.

Vibrating Sample Magnetometer (VSM) Method

The VSM measures the magnetic moment of a small specimen vibrating in a pickup coil system under a DC or superconducting magnet field. It requires only a few milligrams to tens of milligrams of material, making it suitable for research, thin films, powders, and failure analysis where a full-size sample is unavailable. Because the measurement is performed in open circuit, results must be corrected for the demagnetization factor of the specimen geometry, which limits accuracy on bulk-grade verification compared with the closed-circuit method.

Temperature-Dependent Measurement

To characterize thermal stability, the demagnetization curve is measured inside a cryostat or furnace at fixed points, for example from −40 °C to +180 °C for NdFeB automotive grades or up to 350 °C for SmCo. Fitting Br and Hcj versus temperature yields the coefficients α(Br) and β(Hcj), which for sintered NdFeB are typically about −0.10 %/K to −0.13 %/K and −0.4 %/K to −0.8 %/K respectively. Repeated cycling also quantifies irreversible flux loss after exposure to the maximum operating temperature.

Applications

Demagnetization curve testing underpins material qualification and design assurance across every industry that relies on permanent magnets. Accredited third-party laboratories provide test reports used for grade certification to IEC 60404-8-1 and GB/T 13560, supplier qualification, incoming inspection, and failure investigation.

• Electric vehicle traction motors, generators, and EPS motors: verifying Hcj and knee point against short-circuit and overload demagnetization events.
• Wind turbine generators: confirming thermal stability of large NdFeB rotor assemblies.
• Industrial servos, linear motors, and magnetic couplings: supplying accurate B–H data for electromagnetic simulation.
• Sensors, actuators, and consumer electronics: validating Br and (BH)max for miniature magnet assemblies.
• Aerospace and defense: qualifying SmCo magnets for high-temperature reliability and establishing derating data.
• Quality control and dispute resolution: providing ISO/IEC 17025-traceable measurement evidence for supplier audits and batch acceptance.

FAQ

What is the difference between Hcb and Hcj?

Hcb (normal coercivity) is the reverse field at which the external flux density B of the magnet becomes zero, while Hcj (intrinsic coercivity) is the reverse field at which the material's intrinsic polarization J becomes zero. A magnet can still recover flux if the applied field stays below Hcj; beyond Hcj, permanent magnetization is lost. High-temperature grades are therefore specified mainly by minimum Hcj.

Which standards govern demagnetization curve testing?

The principal standards are IEC 60404-5 and GB/T 3217 for the closed-circuit hysteresisgraph method, ASTM A977/A977M for high-coercivity permanent magnet materials, and IEC 60404-8-1 and GB/T 13560 for material grade specifications. Laboratory competence is typically demonstrated under ISO/IEC 17025 accreditation.

What sample dimensions are required for testing?

The standard specimen for hysteresisgraph testing is a ground cylinder of about 10 mm diameter and 5 mm to 10 mm height with flat, parallel end faces. Non-standard geometries can often be tested, but machining a representative specimen is recommended because edge damage, coating thickness, and air gaps between faces and pole pieces directly bias the results.

How are very high coercivity magnets tested?

When Hcj approaches or exceeds the field limit of an electromagnet, a pulsed field magnetometer generating 4 T to 10 T is used, with eddy-current corrections applied for conductive NdFeB samples. Alternatively, elevated-temperature testing can be performed at a temperature where Hcj falls within electromagnet capability, and room-temperature values are derived through calibrated temperature coefficients.

Why must samples be fully saturated before measurement?

If the magnetizing field is insufficient, the measured Br, Hcj, and (BH)max will be artificially low because the specimen never reaches its saturated magnetic state. A magnetizing field of at least two to three times the intrinsic coercivity is generally applied before tracing the demagnetization curve to guarantee reproducible, grade-representative results.