Permanent Magnet Demagnetization Curve Testing: Principles, Standards, and Methods

Permanent magnet demagnetization curve testing is the core procedure for characterizing the second-quadrant magnetic properties of hard magnetic materials such as NdFeB, SmCo, hard ferrite, and Alnico. By measuring key parameters including remanence (Br), coercivity (HcB and HcJ), and maximum energy product ((BH)max), accredited third-party laboratories verify whether magnetic materials conform to IEC 60404-8-1 and equivalent national standards. This article explains the measurement principles, sample requirements, test items, methods, and typical applications of demagnetization curve testing for permanent magnet materials.

Technical Overview

The demagnetization curve is the portion of the hysteresis loop located in the second quadrant of the B–H plane, describing how the magnetic flux density (B) and magnetic polarization (J) of a permanent magnet decrease as an applied demagnetizing field (H) increases in the negative direction. Two curves are measured simultaneously: the normal curve B(H), from which remanence Br and normal coercivity HcB are read, and the intrinsic curve J(H), from which intrinsic coercivity HcJ is determined. The relationship between the two curves is defined by the equation B = J + μ0H in SI units.

The demagnetization curve is the single most important data set for permanent magnet design and qualification, because it defines the operating point of a magnet in any magnetic circuit and determines its resistance to demagnetization by external fields and by temperature. In addition to the curve itself, the maximum energy product (BH)max, the knee field Hk, and the recoil permeability μrec are extracted from the measured data. Testing is generally performed by accredited third-party laboratories or in-house metrology laboratories in accordance with internationally recognized standards, including:

• IEC 60404-5: Measurement of magnetic properties of permanent magnet (magnetically hard) materials by the closed-circuit hysteresisgraph method
• IEC 60404-8-1: Specifications for individual materials, Part 8-1, magnetically hard materials, which defines the grade designation system for NdFeB, SmCo, ferrite, and Alnico magnets
• ASTM A977/A977M: Standard test method for magnetic properties of high-coercivity permanent magnet materials using hysteresigraphs
• GB/T 3217: Chinese national standard for the measurement of the demagnetization curve of permanent magnet materials
• JIS C 2501: Japanese standard covering test methods for permanent magnet materials

Because magnet performance depends strongly on temperature, demagnetization curve testing is frequently extended to elevated and sub-ambient temperatures in order to determine the temperature coefficients of Br and HcJ and to verify that the knee point does not enter the operating region of the magnetic circuit at maximum service temperature.

Testing Samples

Sintered NdFeB (Neodymium-Iron-Boron)

Sintered NdFeB is the most frequently tested material due to its dominant role in electric machines. Typical test specimens are precision-machined cylinders, commonly 10 mm in diameter and 10 mm in height, or rectangular blocks with ground, parallel end faces. The easy-axis (magnetization) direction must be along the cylinder axis, and the specimen must be fully demagnetized before measurement.

Samarium-Cobalt (SmCo5 and Sm2Co17)

SmCo magnets combine moderate energy product with excellent temperature stability and are therefore tested not only at room temperature but also at elevated temperatures up to 300–400 °C, depending on the grade. Specimen geometry and preparation requirements are similar to those for NdFeB.

Hard Ferrites (Barium and Strontium Hexaferrite)

Ferrite magnets exhibit low remanence and relatively low cost, and are usually supplied as larger blocks or rings. Because their intrinsic coercivity is high and their conductivity is negligible, they are well suited to both closed-circuit hysteresisgraph and pulsed-field measurement, with specimen dimensions adjusted to the pole diameter of the measuring electromagnet.

Alnico Alloys

Alnico possesses high remanence but very low coercivity and a strongly curved demagnetization characteristic. Closed-circuit measurement with carefully matched pole pieces is essential, because open-circuit self-demagnetization would otherwise distort the measured curve. Length-to-diameter ratio requirements are therefore more critical for Alnico than for rare-earth materials.

Bonded and Injection-Molded Magnets

Bonded magnets, produced from NdFeB or ferrite powders in polymer binders, exhibit lower remanence and reduced coercivity compared with their sintered counterparts. They can be tested as machined specimens in a hysteresisgraph or as small samples in a vibrating sample magnetometer (VSM), and anisotropic grades require specimen orientation along the easy axis.

Regardless of material class, all specimens must have end faces parallel to within a tight tolerance, a documented magnetization direction, and accurately measured cross-sectional dimensions, since the cross-sectional area enters directly into the calculation of B and J from the measured flux. Specimens are AC-demagnetized prior to testing so that each measurement starts from a well-defined, reproducible initial state.

Testing Items

A standard demagnetization curve test report covers the following measured and derived quantities:

• Residual flux density (remanence) Br, expressed in mT or kG
• Normal coercivity HcB, the demagnetizing field at which B reaches zero, in kA/m or Oe
• Intrinsic coercivity HcJ, the field at which magnetic polarization J reaches zero, in kA/m or kOe
• Maximum energy product (BH)max, in kJ/m³ or MGOe, together with the associated operating point
• Complete second-quadrant B–H and J–H curves, and third-quadrant extension when full demagnetization behavior is required
• Knee field Hk, commonly defined as the field at which J falls to 90% or 95% of Br, indicating the onset of irreversible demagnetization
• Recoil permeability μrec, the effective slope of the recoil line used in magnetic circuit design
• Temperature coefficients α(Br) and β(HcJ), in %/°C, determined over specified temperature intervals
• Irreversible loss of flux or moment after thermal exposure or demagnetizing field exposure
• Magnetic moment and open-circuit flux of complete magnet assemblies, measured with Helmholtz coils and fluxmeters
• Optional items such as Curie temperature determination and recoil-loop measurements

Testing Methods

Closed-Circuit Hysteresisgraph Method

The reference method of IEC 60404-5, GB/T 3217, and ASTM A977/A977M clamps the specimen between the pole shoes of a high-permeability electromagnet so that the magnetic circuit is closed and the internal demagnetizing field is minimized. A B-sensing coil wound around the specimen measures flux density, a compensated J-coil measures magnetic polarization, and a Hall probe adjacent to the specimen measures the applied field H. The specimen is first impulse- or capacitor-discharge-magnetized to saturation, typically with a field several times HcJ, after which the field is swept into the second quadrant and the full demagnetization curve is recorded. This method delivers the lowest measurement uncertainty and is the basis of grade certification.

Vibrating Sample Magnetometer (VSM) Method

VSM testing is an open-circuit technique in which a small specimen vibrates within pickup coils under a swept DC field. It requires only milligram- to gram-scale samples, making it ideal for material development, coating studies, and cases where machining a standard specimen is impractical. Because the specimen is in an open-circuit state, corrections for self-demagnetization must be applied, and the method is generally used for comparative and research purposes rather than grade certification.

Pulsed-Field Magnetometer Method

Modern high-HcJ NdFeB grades, particularly those enriched with dysprosium or terbium, can exceed the saturation capability of conventional electromagnets. Capacitor-driven pulsed-field magnetometers generate fields of 5–10 T within a coil, enabling full magnetization and demagnetization of these grades. Eddy-current effects in electrically conductive sintered materials are corrected analytically or suppressed through specimen geometry and pulse-shape control, and the method is validated against closed-circuit results.

Temperature-Dependent Testing

To determine temperature behavior, the specimen is stabilized inside a temperature-controlled chamber fitted between the electromagnet poles, and complete demagnetization curves are recorded at set points such as 20 °C, 80 °C, 100 °C, 120 °C, 150 °C, 180 °C, and 200 °C, or at sub-ambient points down to approximately −80 °C where required. Linear regression of Br and HcJ versus temperature over the specified interval yields the coefficients α and β, while comparison of room-temperature curves before and after thermal cycling quantifies irreversible losses.

Traceability and Measurement Uncertainty

Calibration is performed using certified reference magnets traceable to national metrology institutes, and laboratories report expanded uncertainties typically within ±1% to ±2% for Br and ±1% to ±3% for HcJ, depending on material and method. Repeat measurements on multiple specimens from the same batch are used to assess homogeneity and statistical spread, which is particularly important for production lot release.

Applications

Demagnetization curve testing underpins quality control, design verification, and failure analysis across every industry that relies on permanent magnets:

• Traction and auxiliary motors for electric and hybrid vehicles, where grade conformance and high-temperature knee behavior directly determine motor safety margins
• Direct-drive wind turbine generators and industrial PM synchronous machines requiring long-term stability under load and thermal cycling
• Servo motors, brushless DC motors, and stepper motors for robotics and automation
• Magnetic couplings, separators, holding systems, and magnetic chucks in industrial equipment
• Position and speed sensors, encoders, and instrumentation where flux stability governs accuracy
• Consumer electronics, loudspeakers, and magnetic assemblies where miniaturization demands verified energy product
• Aerospace and medical device components, including actuators and magnet assemblies subject to strict qualification and lot-acceptance testing
• Material verification for import and export inspection, supplier audits, and dispute resolution through accredited third-party laboratories

In each of these areas, the measured demagnetization curve provides the objective basis for grade designation in accordance with IEC 60404-8-1 and for validating that the magnet operating point remains safely above the knee of the curve under worst-case combinations of reverse field and temperature.

FAQ

What is the difference between HcB and HcJ?

HcB is the normal coercivity, the demagnetizing field at which the external flux density B falls to zero, meaning the magnet no longer produces useful external flux. HcJ is the intrinsic coercivity, the field at which the internal magnetic polarization J falls to zero and the magnetization itself is destroyed. HcJ is always greater than HcB and is the primary indicator of a magnet resistance to irreversible demagnetization by external fields and temperature.

Why must the specimen be saturated before measurement?

The demagnetization curve is a material property only when the specimen starts from a fully saturated magnetic state. Incomplete saturation produces a minor loop with reduced Br and distorted knee behavior, leading to non-reproducible and non-comparable results. Standards therefore require magnetization with a field several times the material HcJ prior to recording the curve.

What specimen dimensions are required for closed-circuit testing?

The most common geometry is a cylinder of 10 mm diameter and 10 mm height, or an equivalent block with ground parallel faces, sized so the pole shoes fully cover the end faces. Specimen dimensions must remain within the uniform-field region of the electromagnet, and very small, thin, or complex-shaped samples are better characterized by VSM or by flux-moment measurement.

Can a finished magnet be tested without cutting a specimen?

The magnetic moment and open-circuit flux of a complete magnet can be measured directly using a Helmholtz coil and fluxmeter, which is standard practice for production screening. However, a full demagnetization curve requires a machined test specimen, which is normally taken from the same production batch and heat-treatment lot as the finished parts.

How are temperature coefficients determined?

Complete demagnetization curves are measured at a reference temperature and at several elevated temperatures after thermal stabilization. Linear regression of Br and HcJ against temperature over the specified interval yields α(Br) and β(HcJ) in %/°C, and the shift of the knee point with temperature establishes the practical maximum operating temperature for the application.

How is very high-coercivity NdFeB tested when electromagnet fields are insufficient?

Grades with HcJ above roughly 2000 kA/m may require pulsed-field magnetometers or high-field closed-circuit systems to guarantee saturation. Where the available field is marginal, laboratories verify saturation by confirming that repeated magnetization at increasing peak fields produces no further increase in Br before accepting the measured curve.

What accuracy can be expected from a professional test?

Using calibrated reference magnets and controlled procedures, accredited third-party laboratories typically achieve expanded uncertainties of about ±1% to ±2% on Br, (BH)max, and HcB, and about ±1% to ±3% on HcJ, with full uncertainty budgets reported alongside the measured curves.