Remanent magnetization testing measures the residual magnetic flux density (Br) that remains in a ferromagnetic or permanent magnet material after an external magnetizing field is removed, making it a critical quality control parameter for magnets, magnetic assemblies, and machined steel components. This article provides a comprehensive technical overview of remanent magnetization test principles, applicable standards such as IEC 60404 and ASTM A977, common test methods, sample types, and typical industrial applications.
Technical Overview
Remanent magnetization, also referred to as remanence or residual magnetism, is the magnetization that remains in a magnetic material after the external magnetic field used to saturate it has been removed. For permanent magnet materials, remanence (denoted Br, measured in tesla or gauss) is a fundamental figure of merit that directly determines the magnetic flux a magnet can deliver in service. For soft magnetic and structural steel components, however, unintended residual magnetism can attract ferrous debris, interfere with sensitive electronics, disturb welding arcs, and degrade the performance of adjacent instruments.
Remanent magnetization testing quantifies this residual flux density or residual field strength under controlled, reproducible conditions. The measurement is typically performed by first saturating the sample in a pulsed or quasi-static magnetizing field, then measuring the magnetic properties with a calibrated sensing system while the applied field follows a complete hysteresis cycle. The result is a full hysteresis loop from which remanence Br, coercivity HcB, intrinsic coercivity HcJ, and the maximum energy product (BH)max are derived.
Testing is generally carried out by accredited third-party laboratories in accordance with international standards, including the IEC 60404 series (particularly IEC 60404-5 for permanent magnet materials and IEC 60404-10 for measurement methods), ASTM A977 and ASTM A341/A341M for DC hysteresis properties, and relevant national standards such as GB/T 3217 for permanent magnet materials and JIS C 2501. Traceable calibration of all magnetometers, fluxmeters, and field sources is essential for measurement uncertainty typically within one to three percent.
Testing Samples
Permanent Magnet Materials
Sintered and bonded permanent magnets are the most common samples submitted for remanence characterization. These include neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo), aluminum-nickel-cobalt (AlNiCo), and hard ferrite (ceramic) materials supplied as blocks, discs, rings, arc segments, or bonded compression-molded parts.
Magnetic Assemblies and Rotors
Complete magnetic assemblies, such as brushless motor rotors, magnetic couplings, sensor targets, and magnetic chucks, can be evaluated for effective remanent flux and pole integrity. Assembly-level testing verifies that magnetizing fixtures have fully saturated every pole and that no magnet was damaged during assembly.
Soft Magnetic Components
Electrical steel laminations, soft ferrite cores, pure iron poles, and finished electromagnetic components are tested to confirm effective demagnetization and to quantify residual flux density that may remain after annealing, grinding, or degaussing.
Structural and Machined Steel Parts
Pipeline sections, bearings, shafts, fasteners, machined weldments, and pressure vessel components that have been exposed to magnetic handling, magnetic particle inspection, or magnetic chucks are tested for residual surface field strength to confirm they fall within acceptance limits prior to assembly or shipment.
Testing Items
A remanent magnetization test program typically includes the following measurable quantities:
- Remanent flux density Br, the residual induction at the intersection of the hysteresis loop with the B axis after saturation.
- Normal coercivity HcB, the demagnetizing field required to reduce flux density to zero.
- Intrinsic coercivity HcJ, the demagnetizing field required to reduce the intrinsic magnetization to zero, a key indicator of thermal stability.
- Maximum energy product (BH)max, derived from the demagnetization curve in the second quadrant.
- Complete DC hysteresis loop trace, including saturation induction and loop squareness.
- Residual surface field strength on components, expressed in gauss or A/m at a defined standoff distance.
- Percentage of saturation achieved, or magnetization uniformity mapping across a magnet or rotor surface.
- Demagnetization verification, confirming residual levels below specified thresholds after degaussing.
Testing Methods
Closed-Circuit Hysteresisgraph (Permeameter) Method
Preferred for bulk permanent magnet samples, this method per IEC 60404-5 places a rectangular or cylindrical specimen in an electromagnet yoke that closes the magnetic circuit, minimizing demagnetizing effects. wound B-coils and a coercimeter measure the full hysteresis loop under quasi-static conditions, delivering the most accurate Br, HcB, HcJ, and (BH)max values for material certification.
Coil-Fluxmeter Method with Helmholtz Coil
The sample is withdrawn from a calibrated Helmholtz coil or a fixture-specific search coil, and the integrated induced voltage yields the magnetic moment. Dividing by sample volume gives an average remanence. This rapid open-circuit method is widely used for production lot inspection of permanent magnets and assemblies, with temperature coefficients applied when testing at non-reference temperatures.
Vibrating Sample Magnetometer (VSM)
A small specimen vibrates within pickup coils in a controlled applied field, and the induced signal is compared against a nickel reference standard. The VSM offers high sensitivity for small or irregular samples, thin films, and research quantities, providing full hysteresis loops over wide field and temperature ranges.
Pulsed Field Magnetization Followed by Field Mapping
Samples or assemblies are first saturated using a capacitive pulsed magnetizer, then scanned with a calibrated Hall-effect gaussmeter or a three-axis magnetometer to map residual flux density or surface field distribution. This approach is common for rotors, multi-pole rings, and finished magnetic assemblies.
Surface Residual Field Measurement
For machined steel parts and post-NDT components, a Hall probe or fluxgate sensor is systematically scanned over the surface to record peak residual field strength. Results are compared against acceptance criteria, often specified in the range of a few gauss, and re-demagnetization is performed if limits are exceeded.
SQUID Magnetometry
For ultra-low residual signals or high-precision research applications, superconducting quantum interference device magnetometers provide the highest available sensitivity, capable of resolving magnetization many orders of magnitude below conventional instruments.
Applications
Remanent magnetization testing supports quality assurance and product development across many industries:
- Permanent magnet manufacturing, including incoming material inspection, lot certification, and grade verification for NdFeB and SmCo magnets.
- Electric motors and generators, confirming rotor pole strength, magnetization uniformity, and consistency across production batches.
- Sensors and instrumentation, where stable and precisely controlled remanence is essential for Hall, magnetoresistive, and reed-based devices.
- Automotive systems, including EPS motors, actuators, and electric drive units that demand tight remanence tolerances over the operating temperature range.
- Aerospace components, verifying magnetic properties and controlling residual magnetism for avionics compatibility and space-grade assemblies.
- Welding and fabrication, ensuring machined parts and pipelines are demagnetized below limits that would cause arc blow or attract grinding debris.
- Post-magnetic-particle-inspection demagnetization verification in accordance with NDT acceptance procedures.
- Failure analysis, where degraded or thermally damaged magnets are compared against original remanence specifications.
FAQ
What is the difference between remanence and residual magnetism?
Remanence (Br) is a defined material property measured under standardized saturation conditions on a closed magnetic circuit, whereas residual magnetism generally refers to any unintended magnetic field remaining in a component, often measured as a surface field strength. The former characterizes permanent magnet quality; the latter is usually an unwanted condition to be controlled or eliminated.
What units are used to report results?
Remanent flux density is reported in tesla (T) or millitesla (mT) in the SI system, and in gauss (G) in the CGS system, where 1 T equals 10,000 G. Coercivity is reported in A/m in SI units or oersted (Oe) in CGS units, with 1 Oe corresponding to approximately 79.577 A/m.
How should samples be prepared and handled?
Samples for hysteresisgraph testing should be machined to the dimensional tolerances specified in the applicable standard, typically rectangular blocks or cylinders with parallel, clean faces. All samples must be kept away from strong stray fields, ferromagnetic tools, and elevated temperatures prior to testing, since partial demagnetization during handling can falsely reduce the measured remanence.
Why do open-circuit and closed-circuit measurements differ?
In open-circuit methods such as the Helmholtz coil test, the sample's own demagnetizing field reduces the effective remanence relative to the material value, so results depend on geometry and permeability. Closed-circuit measurements defined by IEC 60404-5 remove this self-demagnetization effect and are used for material data sheets and grade certification.
What residual field level is acceptable for machined parts?
Acceptance limits depend on the application and applicable specification, but common industrial practice requires residual surface fields below 3 to 5 gauss for general machining and below 1 to 2 gauss for components entering welding, bearings, or sensitive electronic systems. Specific thresholds should always be agreed with the customer or the governing standard.
Can temperature affect the measured remanence?
Yes. Remanence decreases with temperature, and NdFeB in particular exhibits significant temperature coefficients. Accredited laboratories can perform measurements at controlled elevated or sub-ambient temperatures and report normalized values referenced to standard laboratory conditions of 23 degrees Celsius.