Residual magnetization testing, commonly referred to as remanence or Br testing, quantifies the magnetic flux density that remains in a permanent magnet or ferromagnetic component after the external magnetizing field is removed, making it one of the most decisive quality metrics in magnet manufacturing, motor design, and component acceptance. Accredited third-party laboratories perform these measurements in accordance with recognized standards such as IEC 60404-5, ASTM A977/A977M, and GB/T 3217, using hysteresisgraphs, vibrating sample magnetometers (VSM), and fluxmeter–Helmholtz coil systems. This article explains the physical principles, measurable parameters, sample requirements, instrumentation, industrial applications, and frequently asked questions related to residual magnetization testing.
Technical Overview
Residual magnetization is the magnetization retained by a ferromagnetic or permanent magnet material after it has been driven to technical saturation and the applied magnetic field has been reduced to zero. The corresponding material property is expressed either as the remanent flux density Br (measured in millitesla, mT, or tesla, T) or as the remanent magnetization Mr (measured in A/m), with the relationship Br = μ0 · Mr. Together with the coercive fields HcB and HcJ and the maximum energy product (BH)max, remanence defines the demagnetization curve in the second quadrant of the B–H hysteresis loop and therefore determines how much usable magnetic energy a material can deliver in an engineering application.
Remanence testing serves several distinct purposes. First, it verifies that a delivered magnet lot conforms to its declared magnetic grade, for example an N42 or N52 class sintered Nd-Fe-B material. Second, it provides validated input data for finite-element electromagnetic simulations used in motor and sensor design. Third, it evaluates degradation mechanisms such as thermal aging, corrosion, radiation exposure, hydrogen embrittlement, or mechanical damage, all of which typically manifest as a measurable reduction in remanence. Finally, residual magnetization measurements are used to confirm effective demagnetization of steel components after magnetic particle inspection or machining, where excessive residual fields can attract ferrous debris or interfere with nearby electronic instruments.
A critical technical distinction must be made between intrinsic material remanence and apparent residual magnetism. Intrinsic Br is measured under closed-circuit conditions where the internal demagnetizing field is minimized, whereas an open-circuit magnet or a finished component exhibits a lower apparent surface flux density because the geometry of the part introduces a self-demagnetizing field governed by its demagnetization factor. Professional residual magnetization testing therefore requires both the correct instrumentation and the correct measurement geometry to produce traceable, comparable results.
Testing Samples
Sintered Nd-Fe-B Magnets
Sintered neodymium-iron-boron magnets are the most frequently tested sample category because of their widespread use in traction motors, servomotors, and high-performance actuators. Standard test specimens are typically precision-ground cylinders or rectangular blocks with a preferred size in the range of 10 mm diameter by 10 mm height, machined with parallel end faces and aligned along the magnetization direction to satisfy the requirements of IEC 60404-5 and ASTM A977/A977M.
Samarium-Cobalt (SmCo) Magnets
SmCo 1:5 and 2:17 type magnets are tested extensively for aerospace, defense, and high-temperature applications. Because of their exceptional intrinsic coercivity, complete saturation requires substantially higher magnetizing field strengths, and testing laboratories must verify that the applied field is sufficient to saturate the specimen before the demagnetization curve is recorded.
Alnico and Hard Ferrite Magnets
Alnico alloys exhibit high remanence but low coercivity, which makes them strongly susceptible to self-demagnetization in open-circuit geometries; consequently, closed-circuit permeameter testing is essential. Hard ferrite (ceramic) magnets have lower remanence values, generally between 200 mT and 450 mT, and require measurement ranges and calibration standards appropriate to the lower flux levels.
Bonded, Injection-Molded Magnets, and Magnetic Powders
Compression-bonded and injection-molded magnets, as well as melt-spun ribbons and magnetic powders, are usually characterized on small specimens using vibrating sample magnetometers or by compacting powder into cylindrical holders. These materials contain a non-magnetic binder phase, so the measured remanence is normalized against the magnetic loading fraction when comparing against fully dense material properties.
Soft Magnetic Materials and Steel Components
Soft magnetic materials such as electrical steels, pure iron, and low-carbon steels are tested for residual induction and residual field after processing. Weakly magnetic materials, including austenitic stainless steels, are evaluated for residual permeability and residual induction in accordance with ASTM A342/A342M when low-magnetic-signature performance is required.
Magnetic Assemblies and Rotors
Finished rotors, magnetic couplings, sensor targets, and magnetized assemblies cannot be characterized by closed-circuit methods. Instead, their total magnetic moment or flux linkage is measured with a fluxmeter and Helmholtz coil, and their surface residual field is mapped with a calibrated Hall-effect gaussmeter.
Testing Items
A comprehensive residual magnetization test report typically includes the following measurable parameters:
- Remanent flux density Br and remanent magnetization Mr at room temperature
- Normal coercivity HcB and intrinsic coercive force HcJ
- Maximum magnetic energy product (BH)max
- Complete DC hysteresis loops, including both the B–H and J–H curves in the second quadrant
- Saturation flux density Bs and saturation magnetization Ms
- Recoil permeability μrec and the knee point of the demagnetization curve
- Total magnetic moment m and flux output measured with Helmholtz coil systems
- Surface residual magnetic field in millitesla or gauss for finished components
- Temperature coefficients of remanence α(Br) and of intrinsic coercivity β(HcJ)
- Residual field verification after demagnetization treatments
Testing Methods
Closed-circuit hysteresisgraph testing is the reference method for determining intrinsic remanence and coercivity of permanent magnet materials and is prescribed by IEC 60404-5, ASTM A977/A977M, and GB/T 3217. The specimen is clamped between the pole shoes of a high-field electromagnet so that a closed magnetic circuit is formed, and concentric B-coils and H-sensing devices record the induction and field strength as the material is cycled between positive and negative saturation. The loop intercept at H = 0 yields Br, the loop intercepts at B = 0 and J = 0 yield HcB and HcJ respectively, and the second-quadrant B–H product is evaluated to obtain (BH)max. Typical measurement uncertainty for a properly conditioned specimen is within approximately 1 to 2 percent for remanence.
Vibrating sample magnetometry is the preferred method for small specimens, research quantities of powder, thin films, and irregularly shaped materials. The specimen vibrates within a pickup coil system, inducing a voltage proportional to its magnetic moment, and calibration is performed against a standard reference such as a nickel sphere. VSM measurements are normally open-circuit, so the raw loop must be corrected for the demagnetizing field of the specimen geometry before material-level Br values are reported. For extremely low residual signals, such as weakly magnetic alloys or trace contamination analysis, SQUID magnetometry offers sensitivities several orders of magnitude higher than conventional VSM.
Fluxmeter methods with Helmholtz coils provide rapid and non-destructive measurement of the total magnetic moment or flux of finished magnets and assemblies. Because the coil constant is geometrically defined, the method offers excellent repeatability and is widely used for lot acceptance testing of production magnets. For soft magnetic materials, DC permeameter measurements in accordance with IEC 60404-4 and ASTM A341/A341M determine residual induction and coercivity using ballistic or electronic flux integration techniques.
Surface residual field measurement uses calibrated Hall-effect gaussmeters with axial or transverse probes. This approach is standard for verifying demagnetization after magnetic particle inspection, where specifications such as ASTM E1444/E1444M commonly require residual fields to be reduced to acceptably low levels, typically on the order of 3 gauss or less, to prevent artifact indications and ferrous debris collection. Probe orientation, standoff distance, and scanning path must be documented to ensure reproducible readings.
Applications
Residual magnetization testing is indispensable across a broad range of industries:
- Electric vehicle traction motors and industrial servo drives, where remanence directly determines torque density and thermal stability margins
- Wind turbine generators and hybrid excitation machines requiring verified magnet grade and thermal coefficient data
- Aerospace and defense systems, including demagnetization verification of flight hardware and magnetic signature control of naval components
- Magnetic sensors, encoders, and proximity targets, where flux consistency governs signal accuracy
- Oil and gas downhole measurement-while-drilling (MWD) tools exposed to vibration and elevated temperatures
- Consumer electronics, including loudspeakers, linear actuators, and magnetic fastening systems
- Incoming material inspection and supplier qualification programs for magnet purchasers
- Failure analysis of degraded, corroded, or thermally aged magnets in returned components
FAQ
What is the difference between remanence and residual magnetism?
Remanence (Br) is an intrinsic material property measured under closed-circuit saturation conditions, describing the flux density retained when the applied field returns to zero. Residual magnetism is an operational term describing any unwanted magnetic field remaining in a component or assembly, usually measured as a surface field in an open-circuit configuration. A part can therefore exhibit low residual surface field while its material remanence remains high.
What sample size is required for hysteresisgraph testing?
Preferred specimens are cylinders of approximately 10 mm diameter and 10 mm height, or rectangular blocks with comparable cross-section, ground flat and parallel on both pole-contact faces along the magnetization direction. Other sizes can be measured, but the demagnetization factor and pole contact quality must be accounted for in the reported uncertainty.
What accuracy can be expected?
Accredited laboratories typically achieve expanded uncertainties of approximately 1 to 2 percent for Br and HcB, and 2 to 3 percent for HcJ, provided the specimen is fully saturated, dimensionally accurate, and thermally stabilized to the reference temperature of 23 °C.
Can irregularly shaped or assembled magnets be tested?
Yes. Irregular parts are either machined into standard specimens, measured by VSM with demagnetization-factor correction, or evaluated by Helmholtz coil flux measurement to determine total magnetic moment. Surface field mapping with a Hall probe provides additional spatial information for finished assemblies.
Why is temperature control important?
Remanence decreases with temperature; sintered Nd-Fe-B materials, for example, exhibit a typical temperature coefficient α(Br) of roughly −0.11 to −0.13 percent per kelvin. Small deviations from the reference temperature therefore produce measurable changes in Br, and all results should be corrected to or reported at 23 °C.
How is demagnetization effectiveness verified?
After demagnetization treatment, the component is scanned with a calibrated Hall-effect gaussmeter, and the maximum residual field is compared against the applicable acceptance limit, commonly 3 gauss or less following magnetic particle inspection per ASTM E1444/E1444M. Test certificates document probe type, measurement positions, and the maximum recorded value.