Determination of Residual Magnetization (Remanence) After Magnetization: Methods, Standards, and App

Determination of residual magnetization after magnetization—commonly expressed as remanence Br—is a fundamental magnetic characterization test that verifies whether permanent magnets, magnetic assemblies, and magnetized components reach full saturation and retain the expected magnetic performance. Accredited third-party laboratories perform this measurement in accordance with international standards such as IEC 60404-5, IEC 60404-14, ASTM A977/A977M, and GB/T 3217, providing manufacturers with traceable data for quality control, material qualification, and product reliability assurance.

Technical Overview

Residual magnetization is the magnetization that remains in a ferromagnetic or ferrimagnetic material after an external magnetizing field has been removed and no opposing field is applied. In engineering practice it is most often expressed as the remanent flux density Br, the point where the hysteresis loop intersects the B-axis at H = 0; the residual magnetization in A/m is given by Mr = Br / μ0, where μ0 is the permeability of free space.

Determination of residual magnetization after magnetization typically involves two steps: first, the specimen is magnetized to technical saturation using a pulsed-field magnetizer, a DC solenoid, or an electromagnet; second, the residual magnetic state is quantified with a calibrated measurement system. For permanent magnet materials, the measured remanence confirms that the material has been fully saturated, that the grade meets its specification, and that subsequent processing or handling has not degraded the magnetic state. Typical remanence ranges include approximately 1.0–1.5 T for sintered Nd-Fe-B, 0.85–1.15 T for Sm-Co, 0.2–0.45 T for hard ferrites, and up to about 1.3 T for Alnico, depending on grade and temperature.

Because open-circuit samples experience self-demagnetization, the measured flux density of a magnetized part can be lower than the intrinsic material remanence. Test methods therefore distinguish between material remanence, measured under closed-circuit conditions or corrected using the demagnetization factor, and the actual magnetic moment of a component, measured in open circuit. Reliable testing is performed per IEC 60404-5, IEC 60404-14, ASTM A977/A977M, GB/T 3217, and related standards, with calibration traceable to national metrology institutes.

Testing Samples

Residual magnetization measurements can be performed on virtually any magnetizable material or component. Common sample categories submitted to accredited third-party laboratories include:

Sintered and Bonded Nd-Fe-B Magnets

Neodymium-iron-boron magnets in block, disc, ring, arc, and custom geometries. Because of their high intrinsic coercivity (Hcj typically 800–3000 kA/m), saturation requires very strong pulsed fields, and accurate Br measurement is essential for grade verification, for example distinguishing N35 from N52 grades.

Samarium-Cobalt (Sm-Co) Magnets

SmCo5 and Sm2Co17 type magnets used in high-temperature and aerospace applications. Testing confirms remanence stability and verifies that thermal treatment, grinding, or other machining operations have not degraded magnetic properties.

Hard Ferrite (Ceramic) Magnets

Barium and strontium ferrite magnets in isotropic and anisotropic grades, widely used in motors, loudspeakers, and separators. Their lower remanence requires appropriately sensitive measurement coils, and their distinctive temperature behavior makes temperature control during testing important.

Alnico Magnets

Cast and sintered aluminum-nickel-cobalt alloys with high remanence but relatively low coercivity. These samples are highly susceptible to demagnetization during handling and transport, so protective keepers and standardized, low-disturbance measurement procedures are critical.

Magnetic Assemblies and Magnetized Components

Rotor assemblies, speaker magnets, magnetic couplings, sensor magnets, magnetic chucks, and steel parts exhibiting residual magnetism after machining, welding, or magnetizing operations. Measurements determine the effective magnetic moment or residual field of the finished assembly.

Soft Magnetic and Demagnetized Parts

Steel components after demagnetization treatment, where the residual magnetization level must be verified to be below a specified limit to prevent chip adhesion in machining, arc blow in welding, or interference with sensitive instruments.

Testing Items

Depending on the selected method and customer requirements, typical reported parameters include:

• Residual magnetization Mr and remanent flux density Br (T)
• Magnetic dipole moment m (A·m²)
• Normal coercivity Hcb and intrinsic coercivity Hcj (kA/m or Oe)
• Maximum magnetic energy product (BH)max (kJ/m³ or MGOe)
• Complete second-quadrant demagnetization curve and recoil behavior
• Saturation verification by comparing Br before and after re-magnetization
• Surface residual field and its distribution (mT or Gs) measured with a Hall probe
• Temperature-dependent remanence and reversible temperature coefficients

Testing Methods

Sample Magnetization and Saturation

Prior to measurement, specimens are magnetized to technical saturation. A general engineering rule is that the magnetizing field should be at least 3–5 times the intrinsic coercivity of the material; for sintered Nd-Fe-B, peak fields of roughly 2400–4800 kA/m (approximately 3–6 T) are commonly delivered by capacitor-discharge pulse magnetizers, while DC solenoids or electromagnets suffice for ferrites and Alnico. After magnetization, samples are conditioned at the reference temperature, typically 23 ± 5 °C, before measurement.

Closed-Circuit Hysteresisgraph Method

This is the preferred method under IEC 60404-5 and ASTM A977/A977M for determining intrinsic material remanence. The specimen, fitted with a B-coil and accompanied by H-measuring coils or Hall sensors, is clamped between the pole shoes of an electromagnet to form a closed magnetic circuit. The sample is saturated in place, the field is swept through the second quadrant, and the B–H curve is recorded. Br is obtained by extrapolating the measured curve to H = 0, with corrections applied for air-gap flux and self-demagnetization. The method simultaneously yields Hcb, Hcj, and (BH)max.

Helmholtz Coil with Fluxmeter (Open-Circuit Method)

Described in IEC 60404-14 using withdrawal and rotation techniques, this is the fastest approach for production control. The magnetized specimen is placed at the center of a calibrated Helmholtz coil pair connected to an integrating fluxmeter; the sample is then withdrawn from the coil or rotated 180 degrees, and the flux change is integrated. The result is directly proportional to the magnetic dipole moment m. For a uniformly magnetized specimen of volume V, the average remanence can be calculated as Br = μ0 · m / V along the coil axis, and multi-axis coil systems allow determination of the magnetization direction and magnitude.

Vibrating Sample Magnetometer (VSM)

In this laboratory-grade technique, the sample vibrates within a set of pickup coils located between electromagnet poles; the induced voltage is proportional to the magnetic moment and is calibrated against a nickel standard sphere. The VSM records the full M–H loop, from which Mr is read at H = 0. It is well suited to small specimens, research and development studies, and applications requiring high sensitivity or variable-temperature measurement.

Extraction and Search-Coil Techniques

In the extraction method, the sample is rapidly moved from inside a pickup coil to a remote position, and the integrated flux change yields the magnetic moment. Alternatively, a closely fitted wrapping coil can be slipped off or opened around a component to capture its total flux, which is useful for large or irregularly shaped assemblies.

Surface Field Mapping with Hall Probe Gaussmeters

Although it does not directly yield Br, mapping the residual flux density on pole faces and at defined distances is a widely used acceptance check for magnetized assemblies. Axial and transverse Hall probes, calibrated in traceable reference fields, verify pole orientation, peak surface field, and field uniformity.

Calibration, Traceability, and Uncertainty

Measurement systems are calibrated with reference magnets and standard samples, such as high-purity nickel spheres, traceable to national metrology institutes. Typical expanded uncertainties are approximately 1–3% for Br and magnetic moment under routine conditions, improving with temperature stabilization, careful sample centering, and demagnetization-factor correction. Test reports state the applied standard, sample geometry, magnetizing conditions, environmental conditions, and measurement uncertainty.

Applications

• Electric motors and generators: incoming inspection of rotor and stator magnets, verification of magnetization after assembly, and end-of-line quality control
• Automotive systems: EPS motor magnets, speed and position sensors, actuators, and electric drive assemblies requiring consistent magnetic performance
• Consumer electronics and acoustics: speaker magnets, vibration motors, and magnetic closures where remanence directly determines product performance
• Sensors and instrumentation: encoders, flow meters, and precision instruments requiring calibrated dipole moments
• Aerospace and defense: qualification of Sm-Co and high-temperature magnet assemblies, including remanence verification after thermal cycling
• Industrial equipment: magnetic separators, lifting magnets, magnetic couplings, and chucks
• Machining and welding: verification that demagnetization treatments reduce residual magnetism below specified limits
• Research and development: characterization of new magnet grades, grain-boundary-diffusion materials, and recycled magnet feedstock

FAQ

What is the difference between residual magnetization, remanence, and residual magnetism?

Residual magnetization (Mr) is the magnetization remaining in a material after the magnetizing field is removed. Remanence (Br) is the corresponding flux density, related by Br = μ0 · Mr, and is the standard reported quantity for permanent magnet materials. Residual magnetism usually refers to unwanted leftover magnetization in steel components after demagnetization, which is checked against a maximum allowable level.

Why can the measured flux density of a magnet be lower than its material remanence?

In open circuit, the magnet's own field opposes its magnetization through the demagnetization factor N, so the operating point lies below Br on the demagnetization curve. The Helmholtz-coil method measures the magnetic moment and calculates an average Br from the sample volume; deviations from datasheet values can also result from incomplete saturation, geometry effects, or surface damage.

How strong must the magnetizing field be to guarantee saturation?

As a guideline, the peak field should be at least 3–5 times the material's intrinsic coercivity. Sintered Nd-Fe-B typically requires roughly 2400–4800 kA/m, while ferrites and Alnico saturate at considerably lower fields. Saturation is confirmed when repeated re-magnetization produces no further increase in measured Br.

Which standards govern residual magnetization measurement?

Principal standards include IEC 60404-5 for closed-circuit measurement of magnetically hard materials, IEC 60404-14 for determining magnetic dipole moment by withdrawal and rotation methods, ASTM A977/A977M for the hysteresisgraph method applied to high-coercivity permanent magnets, and GB/T 3217 as the Chinese national standard for permanent magnet test methods.

Does temperature affect the measurement?

Yes. Remanence decreases reversibly with temperature for most magnet families—for example, at roughly −0.11 to −0.13 %/K for Nd-Fe-B—so measurements are performed at a controlled reference temperature, and reversible temperature coefficients are reported when thermal stability matters for the application.

What sample sizes and shapes can be tested?

Hysteresisgraph measurements typically use cylindrical or rectangular specimens with machined parallel faces, commonly 8–10 mm in diameter. The Helmholtz coil and extraction methods accommodate samples from a few millimeters up to large assemblies weighing several kilograms, provided the sample fits within the uniform region of the coil or the system is calibrated accordingly.

How should magnetized samples be shipped and handled?

Samples should be packed with magnetic shielding or keepers to prevent accidental demagnetization and field interference, protected from mechanical shock, especially for Alnico, and stabilized at laboratory temperature before testing. Low-coercivity materials may partially self-demagnetize during transport, so short-circuiting keepers are recommended.