Analysis of Remanent Magnetization Characteristics

Remanent magnetization characteristics analysis is a specialized magnetic testing service that evaluates the residual magnetic properties of permanent magnet and magnetic materials after the removal of an external magnetizing field. This analysis is essential for verifying key parameters such as remanence flux density (Br), coercivity, and maximum energy product, ensuring material quality and performance consistency across industries such as electric motors, sensors, aerospace, and consumer electronics.

Technical Overview

Remanent magnetization, commonly referred to as remanence, is the magnetization remaining in a ferromagnetic material after an external magnetic field has been removed. It represents a fundamental property of permanent magnet materials and is graphically characterized by the intersection of the hysteresis loop with the magnetization axis. The magnitude and stability of remanence directly determine how much magnetic flux a magnet can deliver in practical applications such as electric machines, magnetic couplings, and sensors.

Remanent magnetization characteristics analysis involves magnetizing a specimen to saturation and subsequently measuring the residual magnetic parameters along the demagnetization curve. The evaluation typically covers the remanence flux density (Br), the normal coercivity (HcB), the intrinsic coercivity (HcJ), and the maximum energy product ((BH)max). These parameters collectively define the performance envelope of a magnetic material and are critical for design engineers selecting magnets for demanding operating environments.

The analysis also encompasses the assessment of remanence stability under external influences, including elevated temperature, reverse fields, mechanical stress, and time-dependent aging. Because modern applications increasingly expose magnets to harsh thermal and dynamic conditions, understanding irreversible remanence loss and temperature coefficients of Br is indispensable for long-term reliability prediction.

Testing Samples

Neodymium-Iron-Boron (NdFeB) Magnets

Sintered and bonded NdFeB magnets, the most widely used rare-earth permanent magnets, are routinely analyzed to verify grade compliance, sintering quality, and surface degradation effects on remanence. Both block, disc, ring, and arc-segment geometries can be accommodated.

Samarium-Cobalt (SmCo) Magnets

SmCo5 and Sm2Co17 magnets are evaluated for high-temperature remanence retention, making them essential candidates for aerospace and automotive powertrain applications where thermal stability is paramount.

Hard Ferrite Magnets

Sintered barium and strontium ferrite magnets are tested for remanence and coercivity to confirm conformance with commercial grades, particularly in cost-sensitive applications such as motors and magnetic assemblies.

Aluminum-Nickel-Cobalt (AlNiCo) Magnets

AlNiCo alloys, known for high remanence but low coercivity, are characterized to assess their susceptibility to demagnetization and to validate remanence after magnetization and stabilization treatments.

Soft Magnetic and Other Materials

Electrical steel, soft ferrite components, amorphous and nanocrystalline ribbons, magnetic powders, and geological or archaeological specimens can be analyzed to determine residual magnetization levels and magnetic history.

Testing Items

• Remanence flux density (Br) — the residual flux density measured at the intercept of the demagnetization curve
• Normal coercivity (HcB) — the reverse field strength required to reduce flux density to zero
• Intrinsic coercivity (HcJ) — the reverse field strength required to reduce intrinsic magnetization to zero
• Maximum energy product ((BH)max) — the peak energy density available from the magnet material
• Complete demagnetization curve and recoil permeability
• Full hysteresis loop tracing (saturation magnetization, loop squareness)
• Temperature coefficients of Br and HcJ over specified temperature ranges
• Irreversible remanence loss and reversible temperature-dependent flux variation
• Long-term aging and time stability of remanence
• Saturation magnetization field determination and magnetization uniformity

Testing Methods

Remanent magnetization characteristics are measured using internationally recognized test protocols, including IEC 60404-5 for the measurement of magnetic properties of permanent magnet materials, ASTM A977/A977M for hysteresis properties of permanent magnet materials using ring and permeameter methods, GB/T 3217 for closed-circuit magnetic property measurement of permanent magnet materials, and ASTM A341/A341M for permeameter-based induction measurements. Commonly employed techniques include:

• Hysteresisgraph measurement with closed-circuit permeameter: The specimen is clamped between pole pieces of an electromagnet, magnetized to saturation, and the complete hysteresis loop is recorded using calibrated sensing coils. This method provides the highest accuracy for Br, HcB, HcJ, and (BH)max determination.
• Vibrating Sample Magnetometry (VSM): A small specimen vibrates within detection coils while a controlled field is swept, enabling precise hysteresis loop measurement of irregular or miniature samples, powders, and thin films.
• Helmholtz coil with fluxmeter method: The open-circuit magnetic flux moment of a magnetized sample is measured by withdrawing or rotating it within the coil, allowing calculation of remanence after geometric correction, ideal for production quality control.
• Pulsed field magnetization and subsequent flux measurement: Samples requiring very high saturation fields are pulse-magnetized, and residual flux is verified using a fluxmeter and search coil or Hall probe arrangement.
• Temperature-controlled remanence testing: Specimens are held at specified elevated or sub-ambient temperatures within environmental chambers, and irreversible remanence losses are quantified after thermal cycling.

All measurements are performed in accredited third-party laboratories using calibrated instrumentation traceable to national metrology institutes, with test conditions, specimen preparation, and data reduction procedures aligned with the applicable IEC, ASTM, and GB standards.

Applications

Remanent magnetization characteristics analysis serves a broad spectrum of engineering and scientific purposes:

• Incoming quality inspection of permanent magnets for electric vehicle traction motors, industrial servo motors, and wind turbine generators
• Verification of magnet grade compliance and supplier consistency for procurement and certification programs
• Thermal derating design and irreversible demagnetization risk assessment for motors, actuators, and magnetic couplings
• Failure analysis of demagnetized magnets recovered from returned motors, sensors, or magnetic assemblies
• Research and development of novel magnet compositions, including grain-boundary-diffused and heavy-rare-earth-free materials
• Calibration and validation of permanent magnet standards and reference materials
• Geophysical and archaeological studies of natural remanent magnetization in rocks and artifacts
• Demagnetization verification for components requiring low residual magnetism prior to assembly or shipment

FAQ

What is the difference between remanence (Br) and coercivity?

Remanence is the residual flux density that remains in a fully magnetized material after the applied field is removed, indicating how strongly magnetized the material stays. Coercivity, in contrast, is the opposing magnetic field strength needed to reduce the magnetization to zero, indicating how resistant the material is to demagnetization. Both parameters must be evaluated together to fully characterize a permanent magnet.

Why does remanence decrease at elevated temperatures?

Rising temperature increases thermal agitation of magnetic domains, which weakens the alignment of magnetic moments and lowers remanence. This change is partially reversible when the temperature returns to ambient, but if the intrinsic coercivity becomes insufficient, irreversible demagnetization occurs and the original remanence cannot be recovered without re-magnetization.

How is remanence measured accurately?

For the highest accuracy, closed-circuit permeameter or hysteresisgraph measurement in accordance with IEC 60404-5 or ASTM A977 is used, because the closed magnetic circuit minimizes demagnetizing effects from the sample geometry. For routine production control, the Helmholtz coil and fluxmeter method offers rapid, repeatable results when proper geometric correction factors are applied.

What sample sizes and shapes can be tested?

Closed-circuit measurements typically require cylindrical or block specimens with specific aspect ratios, while VSM accommodates very small or irregular samples. Helmholtz coil testing can handle a wide variety of production shapes, including discs, rings, blocks, and arc segments, provided the geometry is well defined.

Can a magnet that has lost remanence be restored?

If the remanence loss is purely due to reversible thermal effects, the magnet recovers once it cools. If irreversible demagnetization has occurred, the magnet can often be re-magnetized to saturation provided no structural degradation, corrosion, or metallurgical change has taken place. Re-magnetized magnets should be re-tested to confirm restoration of the specified remanence.