Remanence, or residual magnetization (Br), is one of the most critical magnetic parameters for permanent magnet materials, directly determining flux output, energy product, and the electromagnetic performance of motors, sensors, and magnetic assemblies. Precise remanence testing requires saturated magnetization, controlled environmental conditions, calibrated instrumentation, and strict adherence to internationally recognized methods such as IEC 60404-5 and ASTM A977. This article provides a comprehensive technical overview of remanence measurement principles, applicable sample types, testing items, measurement methods, standards, and typical industrial applications.
Technical Overview
Remanence, also known as residual magnetization or remanent flux density (Br), is the magnetization retained by a ferromagnetic or ferrimagnetic material after an external saturating magnetic field has been removed. On a hysteresis loop, remanence corresponds to the intercept of the magnetization curve with the B (or J) axis at the point where the applied field H equals zero. In SI units, remanent flux density is expressed in tesla (T) or millitesla (mT), while remanent magnetization (Mr) is expressed in amperes per meter (A/m); the two quantities are related by Br = μ0 × Mr, where μ0 is the vacuum permeability of 4π × 10⁻⁷ H/m.
Accurate determination of Br is essential because this single parameter governs the open-circuit flux available from a magnet, the theoretical maximum energy product (BH)max, the torque constant of permanent magnet machines, and the output stability of magnetic devices over time and temperature. A measurement bias of only 1–2 % in Br propagates directly into flux density, back-EMF, and torque predictions, which can cause significant discrepancies between simulation models and physical prototypes. Reliable testing therefore demands careful control of every stage of the measurement chain, including specimen preparation, magnetization to full technical saturation, temperature stabilization, geometry correction, and instrument calibration.
- Complete saturation: the magnetizing field must typically reach 3–5 times the intrinsic coercivity HcJ to ensure the specimen reaches its saturated remanent state.
- Temperature control: sintered NdFeB exhibits a reversible temperature coefficient of Br of approximately −0.11 to −0.13 %/K, so measurements are usually performed at 23 ± 3 °C or with compensation to a reference temperature of 20 °C.
- Geometry and demagnetizing effects: open-circuit measurements require correction for the demagnetization factor, which depends on the length-to-diameter (L/D) ratio of the specimen.
- Closed versus open magnetic circuit: closed-circuit (permeameter) methods minimize demagnetizing effects, while open-circuit methods (coils, VSM, SQUID) require numerical correction or specific specimen shapes.
- Traceability: instruments are calibrated using certified nickel reference specimens or standard magnets traceable to national metrology institutes.
Testing Samples
Sintered NdFeB and SmCo Permanent Magnets
Sintered neodymium-iron-boron (NdFeB) and samarium-cobalt (SmCo5, Sm2Co17) magnets are the most frequently tested sample categories. Specimens are typically machined into cylinders or rectangular blocks with flat, parallel end faces and a favorable L/D ratio to reduce shape demagnetization effects. Testing verifies grade conformity, lot-to-lot consistency, and long-term thermal stability of Br and HcJ.
Bonded Magnets and Magnetic Powders
Injection-molded, compression-bonded, and calendered magnets, as well as raw magnetic powders such as rapidly quenched NdFeB ribbons, SmFeN, and hard ferrite powders, can be characterized for remanence after compaction into standard cylindrical pellets or by encapsulating a measured mass of powder in nonmagnetic holders.
Hard Ferrite and Alnico Magnets
Sintered strontium and barium ferrite (BaFe12O19, SrFe12O19) magnets, Alnico alloys, and FeCrCo materials are tested to confirm remanence grades and to evaluate the effects of composition, sintering parameters, and heat treatment on magnetic performance.
Magnetic Assemblies and Magnetized Components
Halbach arrays, multipole rotors, magnetic couplings, speaker magnet assemblies, and sensor magnets can be evaluated for effective remanent moment, pole flux distribution, and consistency using search coil and fluxmeter techniques, complemented by surface flux density mapping.
Soft Magnetic and Residually Magnetized Parts
Steel components, soft magnetic alloys, and machined parts that must remain essentially demagnetized (for example, precision tooling, bearing components, and vacuum-system hardware) are tested for residual magnetization levels to verify the effectiveness of demagnetization processes and to meet residual field acceptance limits.
Thin Films and Magnetic Recording Media
Magnetic thin films, multilayer stacks, recording media, and research specimens with weak remanent moments are characterized using high-sensitivity VSM or SQUID magnetometers capable of resolving signals at the 10⁻⁶ emu level and below.
Testing Items
Remanence testing programs typically report the following measured and derived quantities, either as single-point values or as complete second-quadrant demagnetization curves:
- Remanent flux density Br (mT, T, or gauss) and remanent magnetization Mr (A/m or emu/cm³)
- Normal coercivity HcB and intrinsic coercivity HcJ (kA/m or Oe)
- Maximum energy product (BH)max (kJ/m³ or MGOe)
- Complete B–H and J–H demagnetization curves in the second quadrant
- Knee field Hk (the field at which J has fallen by 10 %) and squareness ratio Hk/HcJ
- Recoil permeability μrec of the demagnetization curve
- Magnetic dipole moment m (A·m² or Wb·cm) of specimens and assemblies
- Surface and open-circuit flux density at defined air gaps
- Temperature coefficients of Br and HcJ, and irreversible remanence loss after thermal cycling
Testing Methods
Closed-Circuit Hysteresisgraph (Permeameter) Method
The hysteresisgraph method is the reference technique for characterizing permanent magnet materials, in accordance with IEC 60404-5, ASTM A977/A977M, and GB/T 3217. The specimen is clamped between the pole pieces of an electromagnet or water-cooled permeameter yoke to form a closed magnetic circuit, minimizing internal demagnetizing fields. B- and H-sensing coils or Hall probes record the full magnetization and demagnetization cycle, yielding Br, HcB, HcJ, and (BH)max with typical expanded uncertainties of approximately 1–2 % (k = 2). Magnetization to saturation is achieved with pulsed or DC fields of 3–5 × HcJ prior to tracing the demagnetization curve.
Vibrating Sample Magnetometer (VSM)
VSM measurements detect the voltage induced in pickup coils by a specimen vibrating in a uniform applied field. The method offers high sensitivity (down to about 10⁻⁶ emu), rapid sweep capability, and convenient operation over wide temperature ranges when equipped with cryostats or furnaces. Because VSM is an open-circuit technique, the measured remanence must be corrected for the self-demagnetizing field using the specimen demagnetization factor, effective medium methods, or iterative solvers, particularly for materials with high HcJ and non-ideal shapes.
SQUID Magnetometry
Superconducting quantum interference device (SQUID) magnetometers provide the highest available sensitivity, resolving moments in the 10⁻⁸ emu range. They are preferred for research-grade characterization of thin films, nanoparticles, and weakly magnetic specimens where accurate remanence and coercivity values at very low signal levels are required.
Search Coil and Helmholtz Coil Fluxmeter Method
In accordance with IEC 60404-14, the magnetic dipole moment of a specimen is determined by withdrawing or rotating it within a Helmholtz coil or search coil while an integrating fluxmeter records the induced voltage. This method is well suited to production quality control of fully magnetized magnets and multipole assemblies, providing rapid, repeatable moment measurements from which effective Br can be derived when specimen geometry and magnetization direction are known.
Pulsed-Field Magnetometry (PFM)
Pulsed-field magnetometers subject specimens to capacitor-discharge field pulses of up to several tesla, enabling complete hysteresis measurements of high-coercivity grades within milliseconds. PFM is used when DC field strength is insufficient for saturation or when high-throughput screening is required; eddy-current effects are corrected analytically for electrically conductive specimens.
Measurement Assurance and Calibration
Regardless of the method, credible results depend on a rigorous measurement assurance program. This includes calibration against certified nickel or standard-magnet reference materials traceable to national metrology institutes, verification of coil constants, thermally stabilized test environments, documented uncertainty budgets, and validated software for demagnetization correction. Interlaboratory comparisons and compliance with ISO/IEC 17025 quality requirements ensure that reported Br values are accurate, reproducible, and defensible.
Applications
Precise remanence testing supports a broad spectrum of industries and engineering activities:
- Electric vehicle traction motors, industrial servo motors, and wind turbine generators, where Br determines torque density and efficiency predictions
- Permanent magnet incoming inspection and supplier quality control for magnet manufacturers and motor OEMs
- Magnetic sensors, encoders, and position detectors requiring tightly controlled moment and pole uniformity
- Magnetic couplings, separators, holding systems, and magnetic chucks
- MRI, NMR, and beam-focusing assemblies that demand stable, characterized remanence over time and temperature
- Aerospace and defense components requiring lot acceptance testing and thermal-stability qualification
- Research and development of new magnet alloys, bonded magnet formulations, and recycled magnet materials
- Failure analysis, including irreversible remanence loss after thermal overload, corrosion, or mechanical damage
FAQ
What is the difference between remanence Br and residual magnetization Mr?
Both describe the magnetization remaining after the removal of a saturating field. Br is the remanent flux density expressed in tesla, while Mr is the remanent magnetization expressed in A/m; they are related by the equation Br = μ0 × Mr. In permanent magnet datasheets, Br is the conventional reported parameter.
What measurement uncertainty can be expected for Br?
With calibrated hysteresisgraph systems and compliant specimens, expanded uncertainties of approximately 1–2 % (k = 2) for Br and 2–3 % for HcJ are typical. VSM and coil-based methods may achieve comparable or better precision when demagnetization corrections and geometry are well characterized.
Why must the specimen be magnetized to saturation before testing?
If the applied field does not fully align the magnetic domains, the measured remanence will be lower than the intrinsic material value, producing systematically biased results. For sintered NdFeB, fields of at least 3–5 times HcJ are generally applied before the demagnetization curve is recorded.
How does temperature affect remanence measurements?
Br of NdFeB decreases reversibly by roughly 0.11–0.13 % per kelvin of temperature increase. Testing is therefore performed in a controlled laboratory environment, and results are corrected or referenced to a standard temperature, typically 20 °C, to ensure comparability between laboratories and lots.
Can irregularly shaped or assembled parts be tested?
Yes. While hysteresisgraph methods require machined specimens with parallel faces, search coil and fluxmeter techniques can measure the magnetic moment of finished parts and multipole assemblies directly. For complex geometries, results are often reported as magnetic moment or surface flux density rather than intrinsic Br.
How should a testing laboratory be selected?
Choose an accredited third-party laboratory or calibration facility that operates in accordance with ISO/IEC 17025, applies recognized standards such as IEC 60404-5, IEC 60404-14, ASTM A977/A977M, ASTM A341/A341M, and GB/T 3217, maintains traceable reference materials, and provides documented measurement uncertainty with every reported result.