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IEC TR 62517 – Magnetizing Behavior of Permanent Magnets: From Ferrites to Rare Earth
Permanent magnets are essential components in modern electromechanical systems ranging from motors and generators to sensors and actuators. However, the full performance of a permanent magnet can only be realized if it is magnetized properly to saturation. IEC TR 62517, published in 2009, provides a comprehensive technical analysis of the magnetizing behavior of modern permanent magnet materials, including sintered ferrites, rare earth magnets (Nd-Fe-B, SmCo5, Sm2Co17), and single domain particle magnets.
Key Scope: This Technical Report covers effective magnetizing field strength requirements, initial magnetization states, coercivity mechanisms, and recommended saturation field strengths for achieving complete magnetization in various permanent magnet types.
Magnetization Mechanisms and Coercivity Types
IEC TR 62517 classifies permanent magnets by their coercivity mechanism, which fundamentally determines their magnetizing behavior:
Nucleation-type magnets (sintered ferrites, Nd-Fe-B, SmCo5) exhibit a coercivity mechanism where reverse domains nucleate at grain boundaries or surface defects. These materials are relatively easy to magnetize because once a magnetic field is applied, domain walls move freely until saturation is achieved. Their initial magnetization curve shows sharp magnetization increase at relatively low applied fields.
Pinning-type magnets (Sm2Co17) have their coercivity determined by the pinning of domain walls at precipitates within the crystal grains. These materials require significantly higher magnetizing fields to achieve saturation because domain wall movement is impeded by the pinning sites. The initial magnetization curve of pinning-type magnets rises much more gradually than nucleation-type materials.
| Magnet Type | Coercivity Mechanism | Recommended Hmag (kA/m) | Ease of Magnetization |
|---|---|---|---|
| Sintered Ferrite | Nucleation | 800 – 1200 | Easy |
| Nd-Fe-B (sintered) | Nucleation | 1600 – 2400 | Moderate |
| SmCo5 | Nucleation | 2400 – 3200 | Moderate |
| Sm2Co17 | Pinning | 3200 – 4800 | Difficult (requires high field) |
| Single Domain Particle | Magnetocrystalline anisotropy | 4800 – 8000 | Very difficult |
Engineering Insight: The required magnetizing field for complete saturation can be 3 to 5 times the intrinsic coercivity (HcJ) value. This has major implications for production line design — the magnetizing fixture must generate sufficient field in the correct orientation. For large Sm2Co17 magnets, pulsed field magnetizers with capacitor banks exceeding 100 kJ may be necessary.
Approach to Saturation and Reversal Behavior
The standard provides detailed analysis of the approach-to-saturation process for each magnet type. For nucleation-type magnets, the approach follows a smooth curve described by the law of approach to saturation (LAS), expressed as M = Ms(1 – a/H – b/H^2) + chi_p * H, where Ms is saturation magnetization, a and b are material-specific coefficients, and chi_p is the paramagnetic susceptibility.
For pinning-type magnets, the approach is more complex, often showing a two-stage behavior: initial slow magnetization increase as domain walls bow between pinning sites, followed by a rapid increase once the applied field exceeds the pinning field strength. Understanding this behavior is critical for designing magnetization fixtures that can deliver adequate magnetomotive force across the full magnet volume. The coercivity field (HcJ) at which 50% of magnetization is reversed provides important insight into the quality of the magnet’s microstructure — higher HcJ indicates finer and more uniformly distributed pinning sites, which translates to better resistance to demagnetization in application.
Design Recommendation: When designing magnetization coils for permanent magnets, consider the demagnetizing factor (N) of the magnet geometry. Long, thin magnets (high L/D ratio) require higher applied fields than short, stubby magnets of the same material because their self-demagnetizing field opposes the applied field more strongly. A magnet with L/D < 0.5 may require 50% higher applied field than one with L/D > 2.
Practical Magnetization Fixture Design
The technical report offers guidance on practical magnetization system design including: selecting between DC and pulsed field magnetizers based on magnet size and material, optimizing coil geometry for uniform field distribution, handling thermal effects during repeated magnetization cycles, and verifying magnetization completeness through surface field measurement or Hall probe scanning. For large-scale production environments, the report recommends automated magnetization systems with closed-loop field control and real-time quality monitoring to ensure each magnet achieves consistent saturation levels.
Frequently Asked Questions
Q: What happens if a permanent magnet is not fully saturated?
A: An unsaturated magnet will not deliver its maximum energy product (BHmax), resulting in lower flux density in the application. The magnet may also exhibit greater temperature sensitivity and long-term flux drift. Partial magnetization leads to unpredictable performance that varies with minor field perturbations.
A: An unsaturated magnet will not deliver its maximum energy product (BHmax), resulting in lower flux density in the application. The magnet may also exhibit greater temperature sensitivity and long-term flux drift. Partial magnetization leads to unpredictable performance that varies with minor field perturbations.
Q: Can a magnet be over-magnetized?
A: No. Once a magnet is fully saturated, increasing the applied field beyond the saturation point does not increase the remanent flux density (Br) any further. However, excessively high fields can cause mechanical stress in the magnet due to Lorentz forces, potentially causing cracking in large or brittle magnets.
A: No. Once a magnet is fully saturated, increasing the applied field beyond the saturation point does not increase the remanent flux density (Br) any further. However, excessively high fields can cause mechanical stress in the magnet due to Lorentz forces, potentially causing cracking in large or brittle magnets.
Q: How do I verify that a magnet has been fully magnetized?
A: Verification methods include: measuring open-circuit flux with a Helmholtz coil or fluxmeter, Hall probe mapping of the surface field distribution, or measuring the magnetic moment in a hysteresisgraph. The most reliable method is to compare the measured Br against the material specification value after correcting for the demagnetizing factor of the magnet geometry.
A: Verification methods include: measuring open-circuit flux with a Helmholtz coil or fluxmeter, Hall probe mapping of the surface field distribution, or measuring the magnetic moment in a hysteresisgraph. The most reliable method is to compare the measured Br against the material specification value after correcting for the demagnetizing factor of the magnet geometry.
