Permanent Magnet Material Magnetic Performance Testing: Methods, Standards, and Applications

Permanent magnet material magnetic performance testing is a critical laboratory process for evaluating the intrinsic and technical magnetic properties of hard magnetic materials such as NdFeB, SmCo, AlNiCo, and hard ferrites. By measuring parameters including remanence, coercivity, and maximum energy product in accordance with IEC, ASTM, and GB standards, manufacturers can verify material grade, ensure product consistency, and validate performance under demanding operating conditions. This article provides a comprehensive technical overview of permanent magnet magnetic testing, covering sample requirements, measured parameters, instrumentation, testing methods, and typical industrial applications.

Technical Overview

Permanent magnet materials are functional materials that retain a persistent magnetic field after magnetization and are fundamental to modern electric motors, generators, sensors, actuators, and numerous electromagnetic devices. The magnetic performance of these materials directly determines the efficiency, power density, reliability, and service life of the end products in which they are installed. Magnetic performance experiments are therefore essential for material qualification, incoming inspection, production quality control, and research and development of new magnet formulations.

A complete magnetic characterization of a permanent magnet material is built around the measurement of its hysteresis loop, from which the second-quadrant demagnetization curve is derived. This curve describes how the material resists demagnetization when subjected to opposing fields and thermal stresses, and it yields the key figures of merit used universally in magnet specification: remanence flux density (Br), magnetic polarization coercivity (Hcj), flux density coercivity (Hcb), and maximum energy product, expressed as (BH)max. Because permanent magnets operate in an open or partially closed magnetic circuit in real applications, laboratory testing must carefully control the magnetic state of the sample, including its saturation magnetization history, geometry-dependent demagnetization factor, and temperature.

Testing is performed by accredited third-party laboratories and in-house quality facilities using standardized methods defined by international and national standards bodies. The principal standards include the IEC 60404 series (notably IEC 60404-5 for permanent magnet materials measurement methods), ASTM A977/A977M for hysteresis properties of high-coercivity materials using vibrating sample magnetometry, ASTM A976 for magnetic classification of materials, and Chinese national standards such as GB/T 3217 and the GB/T 13560 series for sintered NdFeB magnets. Together, these standards specify sample preparation, saturation procedures, measurement conditions, and uncertainty limits, ensuring that results are comparable between laboratories worldwide.

Testing Samples

Sintered NdFeB (Neodymium-Iron-Boron) Magnets

Sintered NdFeB is the highest-performance commercial permanent magnet material, offering remanence values up to approximately 1.5 T and energy products exceeding 400 kJ/m³. Testing focuses not only on room-temperature properties but also on intrinsic coercivity grades, temperature coefficients, and irreversible flux losses at elevated temperature, since the material's magnetic performance degrades above its maximum operating temperature. Samples are typically machined cylinders, blocks, or discs with precisely controlled dimensions, and surface coatings such as nickel or epoxy are evaluated for their influence on measurement accuracy.

Samarium-Cobalt (SmCo) Magnets

SmCo magnets, in both SmCo5 (1:5) and Sm2Co17 (2:17) compositions, provide excellent thermal stability and corrosion resistance with operating temperatures up to 350 °C or higher. Testing emphasizes high-temperature demagnetization curves, reversible and irreversible flux loss, and long-term aging behavior, which are critical for aerospace, defense, and downhole petroleum applications.

Aluminium-Nickel-Cobalt (AlNiCo) Magnets

AlNiCo alloys exhibit low coercivity but excellent temperature stability and are tested primarily for remanence, energy product, and stability across wide temperature ranges. Due to their low coercivity, AlNiCo samples must be magnetized in a closed magnetic circuit or handled carefully to avoid accidental demagnetization during testing and transport.

Hard Ferrite (Ceramic) Magnets

Sintered strontium and barium ferrites are low-cost, high-coercivity materials used extensively in motors, loudspeakers, and holding systems. Testing covers Br, Hcb, Hcj, and (BH)max at both room and elevated temperatures, following IEC 60404-8-1 and related material specification standards.

Bonded and Injection-Molded Magnets

Composite magnets made from magnetic powders (NdFeB or ferrite) mixed with polymer binders exhibit isotropic or anisotropic behavior depending on the manufacturing process. Testing determines the reduced Br typical of bonded materials, typically 0.5–0.7 T for bonded NdFeB, along with coercivity and thermal stability data needed for miniaturized motor and sensor designs.

Testing Items

Magnetic performance experiments on permanent magnet materials encompass a comprehensive set of measurable parameters:

• Remanence flux density (Br): the residual flux density remaining in a closed magnetic circuit after the saturating field is removed, expressed in tesla (T) or millitesla (mT), indicating the strength of the magnet.
• Magnetic polarization coercivity (Hcj): the opposing field strength required to reduce the intrinsic magnetic polarization to zero, measured in kA/m or Oe, reflecting resistance to demagnetization.
• Flux density coercivity (Hcb): the opposing field strength required to reduce the external flux density to zero, always lower than Hcj.
• Maximum energy product ((BH)max): the maximum value of the product of B and H along the demagnetization curve, in kJ/m³ or MGOe, representing the energy available per unit volume.
• Demagnetization curve shape factor (Hk/Hcj): the ratio of the field at which intrinsic polarization falls to 90% of Br (Hk) to Hcj, indicating the squareness of the demagnetization curve.
• Recoil permeability (μrec): the effective relative permeability along the recoil line, important for circuit design.
• Temperature coefficient of Br (α) and of Hcj (β): percentage changes per kelvin (%/K), quantifying reversible thermal effects between reference and elevated temperatures.
• Reversible and irreversible flux losses: changes in magnetic flux after thermal cycling or exposure to adverse fields, critical for motor reliability.
• Curie temperature (Tc): the temperature at which ferromagnetic ordering disappears, defining the absolute upper operating boundary.
• Surface flux density and total magnetic flux: measured on fully magnetized components using Hall probes, Helmheld coils, or fluxmeters for functional acceptance.

Testing Methods

Closed-Circuit Hysteresisgraph Method (Permeameter Method)

The reference method described in IEC 60404-5 uses an electromagnet with pole pieces forming a closed magnetic circuit around the specimen. Search coils wound around the sample measure flux density B, while a surrounding coil or Hall sensor measures the internal field H. The sample is first pulse-magnetized to saturation, then the applied field is swept to trace the complete hysteresis loop, from which the second-quadrant demagnetization curve and all derived parameters are computed. This method delivers the highest accuracy for Br, Hcb, Hcj, and (BH)max and is the standard approach for grading production magnets.

Pulse Field Magnetometer (PFM) Method

For high-coercivity grades, particularly NdFeB with Hcj exceeding 2000 kA/m, an electromagnetic yoke cannot reach the saturation field. A pulsed field magnetometer generates fields of 4000–8000 kA/m in a capacitor-discharge coil, and induced voltages in detection coils are integrated to reconstruct the hysteresis loop. Corrections for eddy-current effects in electrically conductive samples are applied according to established algorithms, making PFM the dominant industrial tool for high-grade material verification.

Vibrating Sample Magnetometer (VSM) Method

Following ASTM A977/A977M, the VSM method vibrates a small specimen between pickup coils in a controlled applied field, detecting the induced emf proportional to magnetic moment. VSM offers high sensitivity and is suitable for small samples, material research, temperature-dependent measurements, and anisotropy studies. Absolute calibration against nickel reference standards ensures traceability.

Flux and Field Mapping on Finished Components

Fully magnetized products are evaluated using Helmheld coil fluxmeters to measure total magnetic flux, and calibrated Hall-effect gaussmeters to map surface flux density at defined points. These functional tests verify assembly-level performance, consistency across production lots, and conformance to customer drawings.

Environmental and Aging Tests

Thermal cycling chambers, humidity cabinets, and salt-spray equipment are combined with flux measurements to determine irreversible loss after heat exposure, structural aging, and corrosion-related degradation, in line with IEC 60068 environmental testing procedures and material-specific specification standards such as GB/T 13560 for sintered NdFeB.

Applications

Permanent magnet magnetic performance testing serves a broad range of industries and product categories:

• Traction and servo motors: verification of NdFeB and SmCo magnet grades for electric vehicles, industrial servo drives, and robotics, where thermal stability and demagnetization resistance under high current loads are safety-critical.
• Wind power generators: qualification of large-volume magnet blocks used in direct-drive turbines, including long-term flux stability and corrosion protection assessment.
• Aerospace and defense: evaluation of SmCo magnets in actuators, gyros, and traveling wave tubes, emphasizing high-temperature demagnetization curves and radiation tolerance.
• Consumer electronics: incoming inspection of tiny magnetized components in speakers, vibration motors, sensors, and magnetic connectors.
• Medical devices: characterization of magnets in MRI assemblies, implantable pumps, and magnetic therapy devices, where performance consistency and biocompatibility-related coatings are validated.
• Automation and logistics: testing of magnetic chucks, lifting magnets, separators, and magnetic couplings used in material handling and process equipment.

FAQ

Why must samples be fully saturated before measurement?

All reported intrinsic properties (Br, Hcj) are defined for the saturated magnetic state. If a sample is not driven to technical saturation before the measurement sweep, the measured loop will be a minor loop, and remanence and coercivity values will be underestimated, leading to incorrect material grading.

What is the difference between Hcb and Hcj?

Hcb is the field at which the external flux density B reaches zero, while Hcj is the field at which the intrinsic magnetic polarization J reaches zero. Hcj is always larger than Hcb and represents the true resistance of the material to complete demagnetization; it is the key parameter for high-temperature and adverse-field applications.

How are measurement results made traceable?

Accredited third-party laboratories calibrate instruments against certified reference materials, such as nickel standards for VSM calibration and reference magnets for hysteresisgraph verification, following the calibration requirements embedded in IEC 60404-5 and laboratory accreditation frameworks. Reported results include measurement uncertainty budgets.

Can bonded magnets be tested using the same methods as sintered magnets?

Yes, the same hysteresisgraph and VSM principles apply, but sample geometry, demagnetization factor corrections, and saturation field requirements differ. Bonded magnet specimens are often measured as rings or arc segments relevant to motor applications, requiring fixture-specific corrections.

How does temperature affect the reported values?

Br decreases reversibly with temperature (negative coefficient α of roughly −0.11%/K for NdFeB), and Hcj falls more steeply (coefficient β around −0.6 to −0.8%/K). Reference demagnetization curves are therefore reported at specified temperatures such as 20 °C, 80 °C, 100 °C, 150 °C, or 180 °C to support motor designers in selecting the appropriate grade.

What sample dimensions are typically required?

Closed-circuit hysteresisgraph testing generally uses cylinders or rectangular blocks with a length-to-diameter or length-to-width ratio near one, sized to fit the pole pieces. VSM testing accepts much smaller specimens, often a few milligrams to tens of milligrams. Dimensional accuracy of the sample directly affects the volume-based calculation of (BH)max and must be measured with calibrated micrometers.