Determination of Irreversible Loss of Permanent Magnet Materials

Irreversible loss testing quantifies the permanent degradation of magnetic performance that occurs when permanent magnet materials such as NdFeB and SmCo are exposed to elevated temperatures, demagnetizing fields, or adverse environments. This article explains the test principle, applicable standards, sample requirements, measurement procedures, and data analysis methods used by accredited third-party laboratories to evaluate the long-term magnetic stability of permanent magnets. Understanding irreversible loss is essential for engineers who must guarantee the performance of motors, generators, and sensors over the full service life of their products.

Technical Overview

Permanent magnet materials—including sintered and bonded neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo), aluminum-nickel-cobalt (AlNiCo), and hard ferrite—exhibit a characteristic loss of magnetization when exposed to elevated temperatures, opposing magnetic fields, or corrosive environments. This total loss consists of two distinct components: a reversible loss, which is governed by the temperature coefficients of remanence and intrinsic coercivity and is fully recovered when the magnet returns to ambient temperature, and an irreversible loss, which persists after cooling and can only be restored by complete remagnetization.

Irreversible loss occurs when thermal agitation causes individual magnetic domains to reverse permanently. From a magnetic-circuit perspective, as temperature rises, the demagnetization curve shifts downward and the intrinsic coercivity decreases; if the operating point of the magnet—determined by its load line and permeance coefficient—falls below the knee of the intrinsic demagnetization curve, a fraction of the domains flip orientation and the open-circuit flux drops permanently. The magnitude of the loss depends on the exposure temperature, the duration of exposure, the permeance coefficient (which is set by the magnet geometry and the surrounding magnetic circuit), the intrinsic coercivity grade of the material, and any externally applied demagnetizing fields.

Because irreversible loss directly reduces the air-gap flux of a motor or sensor and accumulates over service life, its determination is a critical reliability parameter. Relevant reference standards include IEC 60404-5 (measurement of magnetic properties of magnetically hard materials), IEC 60404-8-1 (specifications for magnetically hard materials), ASTM A977/A977M (hysteresisgraph measurement of high-coercivity permanent magnets), GB/T 3217, and GB/T 13560 for sintered NdFeB products.

Testing Samples

Sintered NdFeB Magnets

Sintered NdFeB samples covering the full commercial grade range (N, M, H, SH, UH, EH, and AH series) can be evaluated. Because NdFeB has the largest temperature sensitivity among rare-earth magnets, irreversible loss testing is particularly important for grades intended for high-temperature service such as traction motors and generators. Samples are typically supplied as magnetized blocks, discs, rings, or arc segments with the magnetization direction clearly marked.

Sintered SmCo Magnets

Both SmCo5 (1:5 type) and Sm2Co17 (2:17 type) materials are suitable for testing. SmCo grades offer superior thermal stability and intrinsic coercivity at elevated temperature, and testing verifies that irreversible losses remain within specification for aerospace, defense, and high-temperature industrial applications.

Bonded and Injection-Molded Magnets

Polymer-bonded NdFeB and bonded ferrite magnets, manufactured by compression bonding, injection molding, or calendaring, may be tested in finished or blank form. Because the binder system and lower density alter the demagnetization behavior, irreversible loss data for bonded materials must be established independently of sintered equivalents.

Hard Ferrite and AlNiCo Magnets

Anisotropic and isotropic hard ferrite (SrFe/CaFe) and cast AlNiCo magnets can also be assessed. AlNiCo in particular has very low coercivity, and irreversible loss evaluation under demagnetizing fields is often more relevant than pure thermal exposure for these materials.

Magnet Assemblies and Rotors

Complete magnet assemblies—such as motor rotors, magnetic couplings, sensor targets, and Halbach arrays—may be tested in their functional state. Testing at the assembly level captures the influence of the actual magnetic circuit, including steel flux paths and adjacent pole interaction, which strongly affects the effective permeance coefficient and therefore the irreversible loss.

Testing Items

The following quantities are typically determined during an irreversible loss evaluation program:

• Irreversible open-circuit flux loss (percentage) after exposure to a specified temperature for a specified duration
• Reversible flux loss and separation of total loss into reversible and irreversible components
• Irreversible loss as a function of exposure temperature (loss versus temperature profile)
• Irreversible loss versus exposure time, including logarithmic time-dependence characterization and long-term extrapolation
• Repeated thermal cycling stability across a defined number of cycles
• Post-exposure demagnetization curve verification, including changes in Br, HcB, HcJ, and (BH)max
• Temperature coefficients α(Br) and β(HcJ) as supplementary stability data
• Combined environmental aging (temperature plus humidity or salt spray) where corrosion is a contributing factor

Testing Methods

Open-Circuit Flux Method (Helmmholtz Coil and Fluxmeter)

The most widely used approach measures the magnetic flux linkage or magnetic moment of the magnet in an open-circuit configuration using a Helmholtz coil paired with an electronic integrating fluxmeter. The procedure follows the sequence below:

• Condition the sample to the reference temperature (typically 23 ± 5 °C) and record the initial flux reading Φ0
• Place the sample in a temperature-controlled oven set to the specified exposure temperature, with temperature stability typically within ±2 °C
• Hold the sample at temperature for the specified duration, commonly 1 to 10 hours for screening tests and up to 1000 hours or more for long-term qualification
• Remove the sample, allow it to cool naturally to the reference temperature without applying any external field, and record the recovered flux reading Φ1
• Compute the irreversible loss as: PIL (%) = (Φ0 − Φ1) / Φ0 × 100%
• Repeat for multiple temperatures or exposure times as required by the test plan

Because the sample is measured in open circuit, the test inherently reflects the self-demagnetizing field of the magnet, and results are specific to the sample geometry. Reporting must therefore include sample dimensions and the calculated permeance coefficient.

Hysteresisgraph (Closed-Circuit) Method

For a complete material-level characterization, full demagnetization curves are measured before and after thermal aging using a hysteresisgraph with a closed-circuit electromagnet and compensated measuring coils, following the general principles of ASTM A977/A977M. This method quantifies changes in Br, HcB, HcJ, and (BH)max directly, distinguishes true material degradation (such as oxidation-driven surface damage or aging of 2:17 SmCo phase structure) from purely operating-point-related domain reversal, and provides the data required for re-simulation of the magnetic circuit after aging.

Thermal Cycling and Long-Term Aging Protocols

For products subjected to cyclic service conditions, samples are repeatedly transitioned between a low temperature, ambient, and a high extreme for a defined number of cycles, with flux measured at ambient after each interval. For long-term stability, irreversible loss is measured at several cumulative exposure times; the loss typically grows approximately linearly with the logarithm of time, allowing extrapolation to the design life of the end product.

Measurement Assurance

Calibration of coils and fluxmeters is performed using traceable reference magnets, measurements are repeated to verify reproducibility, and demagnetizing fixtures are strictly avoided between readings so that only thermally induced domain reversal is captured. All test conditions—temperature, time, atmosphere, cooling method, and measurement orientation—are documented in the final report to ensure comparability of results.

Applications

Irreversible loss determination supports design, qualification, and quality control across a broad range of industries:

• Electric vehicle traction motors and hybrid drive systems, where magnets operate at elevated winding temperatures
• Wind turbine direct-drive and doubly-fed generators requiring decades of stable output
• Industrial servo motors, spindle motors, and variable-frequency drive motors
• Automotive position and speed sensors, EPS motors, and actuator magnets
• Aerospace and defense actuators, fuel pumps, and instrumentation with strict derating requirements
• Magnetic couplings, magnetic separators, and lifting systems exposed to self-heating and overload
• Magnet manufacturing quality control and incoming inspection for motor OEMs
• Reliability qualification programs and failure analysis of demagnetized assemblies

FAQ

What is the difference between reversible and irreversible loss?

Reversible loss is the temporary flux reduction caused by the intrinsic temperature dependence of remanence and coercivity; the magnet fully recovers when it cools back to ambient temperature. Irreversible loss is the portion that does not recover after cooling because magnetic domains have permanently reversed; it can only be eliminated by remagnetizing the sample.

Can irreversible loss be recovered?

Yes, irreversible loss caused by thermal exposure alone can usually be fully recovered by re-saturating the magnet in a strong pulsed or DC magnetizing field, provided the material itself has not degraded. However, unless the operating condition or the material grade is changed, the same loss will recur upon repeated exposure. Losses caused by corrosion, oxidation, or metallurgical aging generally cannot be recovered.

How are the test temperature and duration selected?

The test temperature is chosen based on the maximum expected operating temperature of the magnet in the end product, including self-heating, with a safety margin applied. Screening tests commonly use 1 to 10 hours of exposure, while qualification programs extend to 100, 500, or 1000 hours and multiple temperature points so that loss-versus-temperature and loss-versus-log-time trends can be established.

Why does magnet geometry affect the result?

The aspect ratio and the surrounding magnetic circuit determine the permeance coefficient, which sets the operating point on the demagnetization curve. Long magnets with a high length-to-diameter ratio operate far above the knee and show small irreversible losses, while thin or short magnets operating near or below the knee can suffer severe irreversible loss at the same temperature.

How can irreversible loss be reduced in design?

Effective countermeasures include selecting a higher-coercivity grade (for example, moving from an SH to a UH or EH grade), increasing the magnet thickness or using a flux-concentrating circuit to raise the permeance coefficient, minimizing demagnetizing current transients, applying protective coatings against corrosion-related aging, and performing a pre-aging stabilization treatment so that the small initial loss occurs before the magnet enters service.

Which standards govern irreversible loss testing?

Core references include IEC 60404-5 for measurement methods of magnetically hard materials, IEC 60404-8-1 for material specifications, ASTM A977/A977M for hysteresisgraph measurement of high-coercivity magnets, and the Chinese national standards GB/T 3217 and GB/T 13560. Test plans are frequently customized by accredited third-party laboratories to match the specific temperature, duration, and load-line conditions of the customer application.