Intrinsic Demagnetization Curve Measurement: Principles, Methods, and Standards for Permanent Magnet

Intrinsic demagnetization curve measurement is the definitive method for characterizing the magnetic stability of permanent magnet materials such as NdFeB, SmCo, hard ferrites, and Alnico. By plotting magnetic polarization J against the internal demagnetizing field H, this test determines critical parameters including intrinsic coercivity (HcJ), knee-point field (Hk), and curve squareness that govern how a magnet behaves under demagnetizing stress. This article explains the underlying principles, sample requirements, measurement techniques, applicable international standards, and typical industrial applications of intrinsic demagnetization curve testing.

Technical Overview

The intrinsic demagnetization curve is the portion of the magnetic hysteresis loop, located in the second quadrant, that describes the relationship between the intrinsic magnetic polarization J and the internal magnetic field strength H of a permanently magnetized material. Unlike the normal demagnetization curve, which plots the magnetic flux density B against H, the intrinsic curve isolates the contribution of the material's own magnetization, since the two quantities are related by the fundamental expression J = B − μ0H (in SI units).

Physically, the intrinsic curve reveals how strongly the internal magnetization of a material resists being reduced or reversed by an opposing field. While the normal curve B(H) bends continuously toward zero and defines parameters such as remanence Br and normal coercivity HcB, the intrinsic curve J(H) remains nearly flat until the demagnetizing field approaches a critical value, after which magnetization collapses rapidly. The field at which J falls to zero is defined as the intrinsic coercivity HcJ (also written jHc or Hci), and for all practical permanent magnets HcJ is greater than or equal to HcB.

Intrinsic demagnetization curve measurement is essential because the shape of this curve, rather than any single parameter alone, determines whether a magnet will remain stable in service. Designers of electric machines pay particular attention to the knee point Hk, the field at which polarization has dropped to a specified fraction of Br (commonly 90%), and to the squareness ratio Hk/HcJ. A magnet operating beyond the knee of its intrinsic curve undergoes irreversible demagnetization, which can cause permanent loss of torque, sensor malfunction, or complete device failure. Consequently, accurate determination of the full intrinsic curve—often at elevated temperatures—is a core requirement in magnet qualification and quality control.

Testing Samples

Sintered NdFeB Magnets

Neodymium-iron-boron magnets represent the largest testing volume because of their widespread use in traction motors, servomotors, and actuators. Their relatively strong temperature sensitivity of HcJ makes full-temperature intrinsic curve characterization especially important, and grades are frequently differentiated by HcJ values ranging from roughly 12 kOe to over 35 kOe.

Sintered SmCo Magnets

Samarium-cobalt magnets, both SmCo5 and Sm2Co17 types, exhibit very high intrinsic coercivity and excellent thermal stability. Because their HcJ can exceed the capability of conventional electromagnets, these materials often require high-field or pulsed-field measurement systems to achieve genuine magnetic saturation before the demagnetization branch is recorded.

Hard Ferrite Magnets

Sintered and bonded barium/strontium ferrites have low remanence but high electrical resistivity and good chemical stability. Their intrinsic curves are routinely measured for applications such as motors for household appliances, loudspeakers, and magnetic assemblies, often following the same closed-circuit methodology used for rare-earth magnets.

Alnico Magnets

Alnico alloys possess high remanence but low coercivity and a strongly curved intrinsic characteristic, meaning their operating point must be carefully controlled by the magnetic circuit geometry. Intrinsic curve measurement helps define the permeance coefficient limits within which Alnico components can safely operate.

Bonded and Polymer-Bonded Magnets

Compression-bonded and injection-molded magnets, including NdFeB and SmFeN based composites, can be measured directly as molded test cylinders. Because their magnetic properties depend on the volume fraction of magnetic powder, intrinsic curve testing is frequently used to verify both powder quality and compounding consistency.

Sample Geometry and Preparation

For closed-circuit measurement, samples are typically machined as cylinders or rectangular blocks with two end faces that are parallel, flat, and perpendicular to the magnetization direction, ensuring intimate contact with the pole pieces of the electromagnet. Common nominal dimensions are on the order of 10 mm diameter and 5 to 10 mm in height, with an aspect ratio chosen to minimize demagnetizing effects. Open-circuit methods such as vibrating sample magnetometry accept much smaller specimens, including spheres, chips, and powders, but require correction for the self-demagnetizing field.

Testing Items

A complete intrinsic demagnetization curve measurement report typically includes the following measurands:

• Remanence flux density Br: the flux density remaining at H = 0 after saturation magnetization.
• Normal coercivity HcB: the reverse field at which the flux density B falls to zero.
• Intrinsic coercivity HcJ: the reverse field at which the magnetic polarization J falls to zero, the primary indicator of resistance to irreversible demagnetization.
• Maximum energy product (BH)max: derived from the normal curve, representing the maximum energy density available in the air gap.
• Knee-point field Hk: the demagnetizing field at which J has decreased to 90% (or another agreed fraction) of Br.
• Squareness ratio Hk/HcJ: a dimensionless figure of merit describing how rectangular the intrinsic curve is; values approaching 1.0 indicate high stability.
• Recoil permeability μrec: the effective slope of the demagnetization curve near Br, important for circuit modeling.
• Full tabulated and graphical J(H) and B(H) curves over the measured field range.
• Temperature coefficients α(Br) and α(HcJ): percentage changes per kelvin, obtained by repeating measurements at multiple temperatures.
• Curie temperature and elevated-temperature curve families (for example, at 80 °C, 120 °C, 150 °C, or 180 °C) when specified by the customer or product standard.

Testing Methods

Closed-Circuit Hysteresisgraph Method (Permeameter)

The dominant industrial technique is the closed-circuit hysteresisgraph, performed in accordance with IEC 60404-5, ASTM A977/A977M, and GB/T 3217. The specimen is clamped between the pole shoes of a high-power electromagnet, forming a closed magnetic circuit. A tightly fitting sensing coil (J-coil) around the specimen measures the polarization, while a calibrated Hall probe positioned in a cavity of the pole piece or adjacent to the sample surface measures the internal field H. The sample is first saturated with a strong pulsed or DC field—typically several times HcJ—after which the field is swept in the reverse direction to trace the complete demagnetization branch. This method provides the highest accuracy for bulk production magnets and is directly traceable to the relevant documentary standards.

Vibrating Sample Magnetometry (VSM)

VSM is a laboratory-grade open-circuit technique in which a small specimen vibrates within pickup coils and the induced signal is proportional to magnetic moment. It offers exceptional sensitivity for research quantities of material, thin sections, and powders, and modern systems reach fields of 2 to 3 T using superconducting or high-field resistive magnets. Because the sample is measured in open circuit, the applied field must be corrected for the self-demagnetizing field using the demagnetization factor of the specimen geometry; spherical samples are preferred to make this correction exact.

Pulsed Field Magnetometry (PFM)

Pulsed field magnetometers generate transient fields of 10 T or more by discharging a capacitor bank through a coil. This permits genuine saturation of ultra-high-coercivity grades, such as highly dysprosium-doped NdFeB or Sm2Co17, that cannot be fully magnetized by DC electromagnets. Careful control of eddy-current effects—through small sample cross-sections or conductive shielding corrections—is required, and the technique is standardized for high-coercivity materials in recent editions of IEC 60404-5 series guidance.

SQUID Magnetometry

Superconducting quantum interference device magnetometers provide the highest field strengths and sensitivities available and are used for reference measurements, thin films, and materials with extremely high coercivity where absolute accuracy at low magnetization levels is required.

Elevated and Sub-Zero Temperature Testing

Temperature-controlled chambers integrated around the measurement pole gap allow the entire intrinsic curve family to be recorded at set points from approximately −40 °C up to 200 °C or beyond, depending on the material. Specimens are allowed to thermally stabilize, re-saturated at temperature, and then measured, yielding the temperature-dependent HcJ(T) and Br(T) data necessary for motor thermal design and irreversible loss prediction.

Quality Assurance of Results

Accredited third-party laboratories ensure metrological traceability by calibrating field probes and coil constants against reference standards, verifying pole-face parallelism and sample contact, confirming complete saturation before measurement, and repeating measurements in accordance with the repeatability provisions of IEC 60404-5. Uncertainty budgets covering field strength, polarization, and derived quantities such as Hk are documented for each test configuration.

Applications

Intrinsic demagnetization curve data directly support engineering decisions across virtually every permanent magnet industry:

• Electric vehicle traction motors: verifying that HcJ at maximum winding temperature keeps the magnet operating point above the knee of the intrinsic curve under worst-case quadrature-axis demagnetizing current.
• Industrial servo and spindle motors: selecting magnet grades whose squareness ratio guarantees stable torque output across the duty cycle.
• Wind turbine generators: qualifying large volumes of magnet material for long-term stability in variable thermal and load conditions.
• Aerospace and defense components: demonstrating demagnetization resilience under shock, vibration, and extreme-temperature environments.
• Sensors, encoders, and Hall-effect devices: ensuring stable magnetic bias over the product lifetime.
• Loudspeakers and consumer audio: balancing remanence, coercivity, and cost for ferrite and NdFeB assemblies.
• Magnetic assemblies and chucks: computing load lines and confirming adequate permeance coefficients to avoid irreversible losses.
• Magnet manufacturing quality control: lot-by-lot verification of Br, HcB, HcJ, (BH)max, and Hk against grade specifications in standards such as IEC 60404-8-1 and the corresponding national material specification documents.
• Research and development: evaluating new compositions, grain-boundary diffusion processing, and aging behavior of next-generation magnetic materials.

FAQ

What is the difference between the normal and intrinsic demagnetization curves?

The normal curve plots flux density B versus internal field H, while the intrinsic curve plots magnetic polarization J = B − μ0H versus H. The normal curve is used to compute the energy product and the operating point in a magnetic circuit, whereas the intrinsic curve reveals the true stability of the material's magnetization and defines the intrinsic coercivity HcJ and knee point that determine irreversible demagnetization limits.

Why is HcJ so important if the motor designer works with the B(H) curve?

Because HcJ defines the field at which the material's magnetization itself collapses. As long as the operating point stays above the knee of the intrinsic J(H) curve, any demagnetization is reversible and the magnet recovers when the field is removed. Beyond the knee, the loss becomes permanent. HcJ therefore sets the true safety margin, especially at elevated temperature where it can decrease substantially.

What field strength is required to saturate the sample before measurement?

As a rule of thumb, the magnetizing field should be at least two to three times HcJ, and sufficient to drive the material well beyond its saturation knee. For high-coercivity NdFeB and SmCo grades this may demand 4 T or more, which is why pulsed-field magnetometers are sometimes necessary for full characterization.

At which temperatures should the intrinsic curve be measured?

Ambient measurement (typically 20–25 °C) is the baseline for grade identification. For any thermally loaded application, curves should additionally be recorded at the maximum continuous and maximum transient operating temperatures of the device so that the temperature coefficients and the position of the knee can be evaluated under realistic conditions.

Can bonded, plated, or assembled magnets be tested?

Yes. Bonded magnets can be measured as molded cylinders using the same closed-circuit method. Plated or coated magnets are tested routinely, provided the coating does not prevent flat, parallel contact with the pole faces. Fully assembled magnet systems with steel flux paths generally cannot be measured directly on a hysteresisgraph and require dedicated fixture-based or open-circuit assessment.

How are the results reported and against which standards?

Reports present Br, HcB, HcJ, (BH)max, Hk, squareness, and the full graphical J(H) and B(H) curves, with measurement uncertainty and reference to IEC 60404-5, ASTM A977/A977M, or GB/T 3217 as applicable. Material classification is then checked against the relevant part of the IEC 60404-8 series or the corresponding national permanent magnet material specifications.