Intrinsic Coercivity (Hcj) Testing for Permanent Magnet Materials

Intrinsic coercivity (Hcj) testing measures a permanent magnet's ability to resist demagnetization and is one of the most decisive quality parameters for NdFeB, SmCo, ferrite, and bonded magnet materials. This technical guide explains the measurement principle, applicable standards such as IEC 60404-5 and ASTM A977, typical sample requirements, testing procedures, and the industrial applications of Hcj characterization in accredited third-party laboratories.

Technical Overview

Intrinsic coercivity, designated Hcj (also written as jHc or Hci), is the demagnetizing field strength at which the intrinsic magnetic polarization J of a fully saturated permanent magnet falls to zero. It is read directly from the second-quadrant intrinsic demagnetization (J–H) curve and is expressed in amperes per meter (A/m) in SI units or oersteds (Oe) in CGS units, where 1 kOe is equivalent to approximately 79.58 kA/m.

Hcj must be clearly distinguished from the normal coercivity Hcb (bHc), which is the field strength at which the magnetic flux density B of the magnet in a closed magnetic circuit reaches zero. Because B = μ0H + J, the intrinsic curve always lies above the normal curve in the second quadrant, and Hcj is always greater than or equal to Hcb. A large separation between the two values indicates that the magnet retains substantial intrinsic polarization even when its external flux is fully suppressed, which is exactly the behavior required for stable operation under strong demagnetizing loads.

From an engineering perspective, Hcj quantifies the material's resistance to demagnetization caused by external reverse fields, armature reaction in electric machines, and elevated operating temperatures. Physically, it is governed by the magnetocrystalline anisotropy of the main phase and by microstructural features such as grain boundary phases. This is why heavy rare earth elements such as dysprosium and terbium are deliberately added to high-grade sintered NdFeB magnets to increase Hcj.

Hcj is also strongly temperature dependent. For sintered NdFeB, the temperature coefficient β(Hcj) typically ranges from −0.4 %/°C to −0.7 %/°C depending on grade. Consequently, the room-temperature value quoted on a datasheet must always be verified against the requirement at the maximum operating temperature of the end application.

Testing Samples

Sintered NdFeB Magnets

Sintered neodymium-iron-boron is the most frequently tested material because it combines very high remanence with moderate to very high coercivity. Commercial grades span from N-series (Hcj ≥ 955 kA/m, or 12 kOe) through SH, UH, EH, and AH grades with Hcj values up to approximately 2785 kA/m (35 kOe) or higher.

Samarium Cobalt Magnets

Both SmCo5 (1:5) and Sm2Co17 (2:17) types are tested. Sintered SmCo typically exhibits Hcj in the range of roughly 1600 kA/m to 2600 kA/m and offers superior thermal stability, making it suitable for high-temperature aerospace and automotive applications.

Hard Ferrite Magnets

Anisotropic and isotropic strontium and barium ferrites show comparatively low Hcj, generally between about 250 kA/m and 400 kA/m, but remain widely used in cost-sensitive, high-volume applications.

Bonded and Other Magnet Materials

Compression-bonded and injection-molded NdFeB, bonded SmCo, alnico, and other magnetically hard materials can also be characterized. Alnico deserves special attention because its low Hcj (often below 150 kA/m) makes it highly susceptible to demagnetization during handling and measurement.

Sample Geometry and Preparation

For closed-circuit hysteresisgraph measurements, specimens are typically machined as cylinders (for example, 10 mm in diameter by 7 to 10 mm in height) or rectangular blocks with ground, parallel end faces that ensure full contact with the pole shoes of the electromagnet. The magnetization direction must be clearly marked so the sample is oriented with its easy axis along the field direction. Finished parts are usually represented by a machined test coupon taken from the same production lot.

Testing Items

A complete intrinsic coercivity test campaign typically reports the following magnetic parameters derived from the measured hysteresis loops:

• Intrinsic coercive force Hcj (jHc, Hci) — the core deliverable of the test
• Normal coercive force Hcb (bHc) — the field at which flux density B reaches zero
• Remanence Br — the residual flux density at zero applied field
• Maximum energy product (BH)max — the peak of the B–H product in the second quadrant
• Full second-quadrant demagnetization curves, both J–H and B–H
• Knee field Hk and squareness ratio Hk/Hcj — indicators of loop rectangularity and design margin
• Recoil permeability μrec — relevant for circuit modeling after partial demagnetization
• Temperature coefficients α(Br) and β(Hcj) — obtained from measurements at multiple set temperatures

Testing Methods

Closed-Circuit Hysteresisgraph Method

The reference method follows IEC 60404-5 and ASTM A977/A977M. The specimen is clamped between the pole shoes of a high-performance electromagnet to form a closed magnetic circuit with minimal air gap. A J-coil (wound directly around the specimen) measures intrinsic polarization, while a calibrated Hall probe or measuring coil adjacent to the sample measures the applied field H. The sample is first saturated with a strong magnetizing pulse or field, after which the demagnetization branch is swept and the complete loop is recorded. This method delivers the highest accuracy and is the standard approach for production release testing and certification.

Vibrating Sample Magnetometer (VSM)

The VSM is an open-circuit technique suited to small specimens, coatings, thin sections, and research quantities of material. The sample vibrates within pickup coils while a controlled field is swept, and the magnetic moment is measured with high sensitivity. Because the measurement is open circuit, corrections for the demagnetization factor of the specimen geometry are required when deriving the intrinsic J–H loop, and the achievable maximum field limits testing of the highest-coercivity grades.

Pulsed Field Magnetometer Method

For very high-coercivity materials, where conventional electromagnets cannot deliver sufficient saturating fields, a capacitor-discharge pulsed magnet generating fields of several tesla is used. Inductive pickup coils record the transient response, and dedicated software corrects for eddy-current and dynamic effects to reconstruct the hysteresis loop.

Temperature-Dependent Measurements

By integrating environmental chambers, cryostats, or furnaces with the measurement system, Hcj can be characterized across a defined temperature range, commonly from −60 °C up to +250 °C. Measuring at several set points allows the temperature coefficient β(Hcj) to be calculated and provides the data needed to confirm safe operation at the application's maximum temperature.

Applicable Standards

• IEC 60404-5 — permanent magnet (magnetically hard) materials, methods of measurement of magnetic properties
• IEC 60404-8-1 — specification for individual magnetically hard materials
• ASTM A977/A977M — magnetic properties of high-coercivity permanent magnet materials using hysteresisgraphs
• GB/T 3217 — Chinese national standard for the measurement of magnetic properties of permanent magnet materials

Applications

Intrinsic coercivity testing is applied throughout the magnet supply chain, from powder and material development to incoming inspection and failure analysis:

• Grade verification and lot release of sintered NdFeB, SmCo, and ferrite magnets for electric vehicle traction motors, servomotors, and generators
• Qualification of magnets for direct-drive wind turbine generators and aerospace actuators exposed to high demagnetizing loads
• Validation of magnet stability in sensors, loudspeakers, magnetic couplings, holding systems, and magnetic separation equipment
• Support for motor design and simulation by supplying accurate demagnetization curves, knee fields, and temperature coefficients
• Investigation of thermal or field-induced demagnetization failures, including assessment of machined, coated, or thermally aged parts
• Verification of heavy rare earth content effects, grain boundary diffusion processes, and new high-coercivity alloy formulations

FAQ

What is the difference between Hcj and Hcb?

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 field at which the intrinsic polarization J reaches zero. Hcj is always greater than or equal to Hcb and is the parameter that truly reflects the material's intrinsic resistance to demagnetization.

What magnetizing field is required before measurement?

The specimen must be fully saturated prior to tracing the demagnetization curve. In practice, a field of at least 2.5 to 3 times the expected Hcj is applied, and saturation is confirmed when a further increase in the magnetizing field no longer changes the measured loop.

Can a finished magnet be tested directly?

Closed-circuit hysteresisgraph testing requires machined specimens with flat, parallel faces and dimensions compatible with the pole gap. Finished parts with complex geometries are normally tested by machining a representative coupon from the same production batch, while small or irregular samples may be evaluated on a VSM with appropriate geometry corrections.

Why is the measured Hcj sometimes lower than the datasheet value?

Possible causes include measurement uncertainty, insufficient saturation field, poor contact between sample and pole shoes, thermal aging or corrosion of the magnet, grade substitution in the supply chain, or surface damage introduced during machining. Comparative testing in an accredited third-party laboratory is the recommended way to resolve such discrepancies.

How are units converted between SI and CGS?

The conversion factor is 1 Oe = 79.577 A/m, so 1 kOe equals 79.577 kA/m. For example, a minimum Hcj of 20 kOe corresponds to approximately 1592 kA/m, the typical threshold for SH-grade sintered NdFeB.

How should results be reported?

A professional test report should state the applied standard, sample geometry and orientation, saturation field, measurement temperature, the measured Hcj (with Hcb, Br, and (BH)max as applicable), the full demagnetization curves, calibration traceability, and the measurement uncertainty.