Permanent magnet remanence testing measures the residual flux density (Br) that remains in a magnetized material after the external field is removed, providing a decisive indicator of magnet quality, grade conformity, and long-term reliability. Accurate Br determination, performed in accordance with standards such as IEC 60404-5, ASTM A977, and GB/T 3217, is essential for incoming inspection, process control, and design validation of NdFeB, SmCo, ferrite, and AlNiCo magnets used in motors, sensors, and precision assemblies. This article explains the testing principles, sample requirements, measured parameters, methods, and typical applications of remanence testing for permanent magnet materials.
Technical Overview
Remanence, denoted Br and also called residual induction or remanent flux density, is the magnetic flux density that remains in a permanently magnetizable material when the externally applied magnetizing field strength H is reduced to zero after the specimen has been driven to technical saturation. On the hysteresis loop, Br corresponds to the intersection of the saturation loop with the B-axis at the boundary of the second quadrant. Remanence is reported in tesla (T) or millitesla (mT) in SI units and in gauss (G) in CGS units, where 1 T equals 10,000 G.
For permanent magnet materials, remanence is a primary figure of merit. Together with the normal coercivity Hcb, the intrinsic coercivity Hcj, and the maximum energy product (BH)max, it defines the material grade and determines the maximum flux a magnet of given geometry and magnetization direction can deliver. Br depends sensitively on composition, microstructure, and processing history, including powder alignment, sintering, heat treatment, machining, and any thermal or corrosive exposure the magnet has undergone. It also decreases reversibly with temperature, characterized by the temperature coefficient of remanence.
Typical remanence ranges for common permanent magnet families are approximately as follows:
- Sintered neodymium-iron-boron (NdFeB): 1.0 to 1.5 T
- Sintered samarium-cobalt (SmCo 1:5 and 2:17): 0.9 to 1.15 T
- Anisotropic hard ferrites (BaFe or SrFe): 0.36 to 0.45 T
- Cast and sintered AlNiCo: 0.6 to 1.35 T
- Bonded NdFeB (compression bonded): 0.5 to 0.8 T; injection-molded grades: 0.2 to 0.5 T
Remanence testing is routinely performed for grade verification against specifications such as IEC 60404-8-1, incoming and lot-release inspection, supplier qualification, research and development, failure analysis, and thermal aging studies.
Testing Samples
Sintered NdFeB and SmCo Specimens
High-energy-product rare-earth magnets are usually tested as machined cylinders or rectangular blocks with ground, parallel end faces. Closed-circuit hysteresisgraph testing favors specimens with a well-defined uniform cross-section, for example a 10 mm diameter by 10 mm long cylinder. High-coercivity grades, particularly NdFeB with Hcj above 2,000 kA/m, require very high magnetizing fields to reach saturation, which may necessitate pulsed or superconducting excitation during the saturation step.
Hard Ferrite and AlNiCo Specimens
Ceramic ferrite magnets exhibit low remanence but high resistance to demagnetization, so larger specimen volumes are tested to obtain adequate signal levels. AlNiCo materials combine relatively high Br with low coercivity, meaning they are easily demagnetized by their own internal demagnetizing field in open circuit; consequently, AlNiCo must be measured in a closed magnetic circuit or while attached to a keeper.
Bonded and Polymer-Bonded Magnets
Compression-bonded, injection-molded, and calendared magnet materials can be tested as rings, arcs, discs, or blocks. Because the magnetic phase is diluted by the polymer binder, the effective remanence is proportional to the loading fraction, and the magnet's isotropic or anisotropic character must be recorded together with the result.
Finished Magnets and Assemblies
Production magnets in their final geometry, including multipole ring and arc magnets, as well as magnet sub-assemblies such as rotor stacks and magnetic rollers, are commonly evaluated using open-circuit flux methods. Demagnetized machined components and steel parts are also checked for unwanted residual magnetism using a Hall-effect gaussmeter.
Testing Items
A comprehensive remanence test campaign for permanent magnet materials typically includes the following parameters:
- Remanence Br: residual flux density at H = 0 after saturation
- Normal coercivity Hcb: the negative field at which flux density B falls to zero
- Intrinsic coercivity Hcj: the negative field at which intrinsic polarization J falls to zero, indicating resistance to irreversible demagnetization
- Maximum energy product (BH)max: the extreme value of the product B·H on the demagnetization curve
- Full second-quadrant demagnetization curve B(H) and intrinsic curve J(H)
- Squareness factor, often expressed as Hk/Hcj, where Hk is the knee field at 90 percent of Br
- Recoil permeability, relevant to magnet operation under dynamic load changes
- Temperature coefficients of remanence and coercivity, typically measured between –40 °C and 200 °C as required by the application
- Open-circuit total magnetic flux and surface field of finished magnets
- Residual magnetism level of demagnetized or post-processed components
Testing Methods
Closed-Circuit Hysteresisgraph Method
The reference method for permanent magnet materials is specified in IEC 60404-5, ASTM A977, and GB/T 3217. The specimen is clamped between the pole shoes of an electromagnet so that the magnetic circuit is closed with minimal air gap. A B-coil wound directly on the specimen measures flux density, a J-coil may be used to obtain intrinsic polarization, and the field strength H is measured with a calibrated H-coil or derived from the magnetizing current. The specimen is first pulse-saturated or saturated in the electromagnet, the field is then swept through zero into the second quadrant, and the complete demagnetization curve is recorded. Remanence is read directly as the flux density at zero applied field. Proper method control includes verifying complete saturation, avoiding pole-shoe saturation, maintaining a quasi-DC sweep rate to suppress eddy-current errors, and stabilizing the laboratory temperature, typically at 23 °C plus or minus 5 °C.
Search-Coil Extraction and Helmholtz Coil Method
For finished magnets and assemblies, open-circuit flux measurement per the draw-out principle of IEC 60404-14 is widely used. The magnet is centered in a Helmholtz coil pair or a search coil connected to an integrating fluxmeter, then rapidly withdrawn along the magnetization axis. The integrated flux change is proportional to the magnetic moment of the magnet; combined with an accurate volume measurement, the moment yields the effective remanence, since Br equals the permeability of free space multiplied by the moment per unit volume for a uniformly magnetized body. This method is fast, non-destructive, and ideal for 100 percent production screening, with measurement uncertainty dominated by dimensional tolerances and coil calibration.
Vibrating Sample Magnetometry and SQUID Magnetometry
Small specimens in the milligram range can be characterized on a vibrating sample magnetometer (VSM) or a SQUID magnetometer. These open-circuit techniques offer very high sensitivity and are calibrated using reference specimens traceable to national metrology institutes. Because the sample self-demagnetizes in open circuit, a demagnetization-factor correction based on the specimen geometry is required to recover intrinsic parameters, including corrected Br. VSM and SQUID methods are favored in research, grade screening of small or irregular samples, and temperature- and field-dependent studies.
Pulsed-Field Magnetometry
Pulsed-field magnetometers generate capacitor-discharged field pulses of several tesla inside a solenoid with an integrated measuring coil, enabling rapid generation of full hysteresis loops for ultra-high-coercivity grades that cannot be saturated by conventional electromagnets. For electrically conductive sintered magnets, eddy currents induced during the pulse must be suppressed through low field-sweep rates, laminated or sliced specimens, or numerical correction, otherwise the apparent Br and Hcj can be distorted.
Surface Residual Field Measurement
A Hall-effect gaussmeter with a thin active-area probe is used to map the surface field of finished magnets, verify multipole magnetization patterns, and confirm that machined or stress-relieved steel components have been adequately demagnetized. Surface field values depend strongly on geometry and probe positioning, so they are treated as comparative quality indicators rather than as direct measurements of Br.
Temperature-Dependent Remanence Measurement
By fitting a cryostat or furnace to a hysteresisgraph or VSM, Br and Hcj are measured at controlled temperature setpoints spanning the intended service range. The reversible temperature coefficient of remanence is derived from the slope of Br versus temperature, typically about minus 0.11 percent per kelvin for NdFeB and minus 0.03 percent per kelvin for SmCo. Thermal cycling also reveals irreversible remanence losses, which are critical for automotive, aerospace, and downhole applications.
Key Measurement Considerations
- Saturation verification: re-magnetize with progressively higher fields until the measured Br changes by less than one percent
- Geometry control: parallel ground faces, accurate volume determination, and an appropriate length-to-diameter ratio
- Circuit integrity: clean pole faces, minimal air gap, and pole shoes large enough to avoid saturation
- Temperature recording: report the test temperature together with all magnetic values
- Calibration traceability: fluxmeters, coils, and reference magnets traceable to national metrology institutes
- Typical expanded measurement uncertainty for Br is 1 to 3 percent at a coverage factor of k = 2
Applications
Remanence testing of permanent magnet materials underpins quality assurance and design across a broad range of industries, including:
- Electric vehicle traction motors, hybrid drive systems, and industrial servo and stepper motors
- Direct-drive wind turbine generators and energy-storage flywheel machines
- Position, speed, and current sensors based on Hall-effect and magnetoresistive principles
- Loudspeakers, headphones, acoustic transducers, and voice-coil actuators
- Magnetic separation equipment, lifting and holding systems, magnetic couplings, and magnetic gears
- Medical devices, including magnetic resonance imaging and implantable actuator assemblies
- Aerospace actuators, relays, traveling-wave tubes, and other microwave devices
- Grade conformity assessment and lot release at magnet manufacturers and accredited third-party laboratories
- Failure analysis of degraded magnets affected by corrosion, oxidation, overheating, or improper handling
FAQ
What is the difference between remanence Br and the surface field measured with a gaussmeter?
Br is an intrinsic material property determined under closed-circuit conditions at zero applied field, while surface field is an external quantity influenced by magnet geometry, magnetization pattern, permeance coefficient, and probe distance. Surface field readings can be compared against acceptance limits but cannot be converted directly into Br without magnetic modeling.
Why must the specimen be fully saturated before remanence is measured?
If the magnetizing field is insufficient, the specimen does not reach its maximum achievable magnetization and the measured Br will be artificially low. Saturation is confirmed by re-magnetizing at successively higher fields until the remanence value stabilizes within the measurement repeatability.
Which standards govern permanent magnet remanence testing?
The principal standards are IEC 60404-5 for closed-circuit hysteresisgraph measurement, IEC 60404-14 for the draw-out magnetic moment method, ASTM A977 for hysteresisgraph testing of high-coercivity permanent magnet materials, GB/T 3217 for equivalent Chinese national method requirements, and IEC 60404-8-1 for material grade specifications.
What is the difference between Hcb and Hcj?
Hcb is the field at which the external flux density B reaches zero, whereas Hcj is the field at which the intrinsic polarization J reaches zero, representing true magnetic reversal. A magnet with Hcb equal to Hcj has a perfectly square demagnetization curve, and Hcj is always greater than or equal to Hcb.
What sample forms can be accepted for testing?
Closed-circuit hysteresisgraph testing is best suited to ground cylinders or blocks, commonly around 10 mm in characteristic dimension. Larger or complex finished magnets, rings, arcs, and assemblies are evaluated with Helmholtz coil flux methods, while very small or irregular specimens are characterized by VSM or SQUID magnetometry with appropriate corrections.
How is the temperature coefficient of remanence determined?
Br is measured at a reference temperature, typically 20 °C or 23 °C, and again at specified elevated or sub-zero temperatures. The relative change in Br divided by the temperature difference gives the coefficient, which allows designers to predict flux output over the full operating range and to select SmCo or high-temperature NdFeB grades where required.