Permanent Magnets (NdFeB) Core Magnetic Property
Permanent magnets, particularly high-performance NdFeB magnets, are widely used in critical applications such as industrial servo motors, EV traction motors, EPS systems, wind turbine generators, and MRI equipment. To ensure their reliability under demanding operational conditions, the following key magnetic properties must be evaluated
Remanence (Br)
Coercivity (Hcj and Hcb)
Maximum Energy Product (BHmax)
Curie temperature (Tc)
Squareness Ratio Hk/Hcj
Temperature coefficient of remanence ( αBr)
Remanence Br
+Physical Significance
Determines the static magnetic output capability without external field, directly affecting motor torque (T ∝ Br) and power density. High Br enables smaller motor size and higher efficiency (e.g., EV traction motors typically require Br≥1.2T).
Material Comparison
| Material Type | Br (T) | Applications |
|---|---|---|
| Sintered NdFeB | 1.0-1.4 | High-end motors, MRI |
| Ferrite | 0.2-0.4 | Consumer motors, Speakers |
Key Influencing Factors
Material Composition
Nd2Fe14B main phase content (theoretical Br≈1.6T)
Microstructure
Grain orientation (Sintered NdFeB > Bonded NdFeB)
Processing
Hot deformation can improve orientation >95%
Measurement Methods
- Closed-circuit test: Using electromagnet/superconducting magnet to apply saturation field, then measure Br via Hall probe (per IEC 60404-5)
- Open-circuit test: Requires demagnetization field correction (for anisotropic materials)
Curie Temperature Tc
+Physical Mechanism
Above Tc, thermal agitation disrupts magnetic domain alignment, causing spontaneous magnetization to vanish. NdFeB's Tc is determined by Nd2Fe14B phase (theoretical 312°C), which can be increased to 400°C with Co addition.
Engineering Implications
- Maximum operating temperature should be ≥50°C below Tc (e.g., wind turbines require Tc>350°C)
- High-temperature applications (e.g., aerospace) use SmCo (Tc=700-800°C)
Measurement Techniques
Susceptibility-Temperature
SQUID magnetometer measures χ(T) discontinuity
Thermogravimetric Analysis
TGA with applied field detects magnetization loss
Coercivity Hcj/Hcb
+- Intrinsic coercivity (Hcj): Reverse field required to reduce magnetization M to zero (kA/m or Oe)
- Coercive force (Hcb): Reverse field to reduce flux density B to zero
Technical Significance
Hcj determines resistance to demagnetization in dynamic conditions (e.g., servo motors). Automotive-grade NdFeB requires Hcj ≥ 2000 kA/m for thermal stability.
Material Grades
| Material | Hcj (kA/m) | Characteristics |
|---|---|---|
| Low-Dy NdFeB | 800-1200 | Cost-effective for mild environments |
| High-Dy NdFeB | 2000-3000 | High temperature stability for EV motors |
Squareness Ratio Hk/Hcj
+Performance Impact
Values closer to 1 indicate rectangular demagnetization curves (ideal magnets achieve ≥0.95). Low squareness (e.g., ferrites at 0.7) causes operational point drift.
Maximum Energy Product (BH)max
+Design Importance
Determines volumetric efficiency - 20% increase in (BH)max enables 15% motor weight reduction. Theoretical limit for NdFeB is 512 kJ/m³ (64 MGOe), with commercial grades reaching 415 kJ/m³.
Temperature Coefficient αBr
+Typical Values
- NdFeB: -0.12%/°C (standard), -0.08%/°C (with Dy)
- SmCo: -0.04%/°C (superior thermal stability)
Analysis of 42SH High-Temperature Magnetic Properties
Hysteresis Loop Characteristics
The chart displays multiple closed curves representing the relationship between magnetic flux density (B) and magnetic field strength (H) at different temperatures (20°C, 120°C, 130°C, 150°C). These curves exhibit typical hysteresis loop shapes, reflecting energy loss and magnetic characteristics during magnetization and demagnetization processes.
Curve Interpretation
X-axis (H): Magnetic field strength in kOe (kilo-Oersteds), indicating external magnetic field magnitude
Y-axis (B): Magnetic flux density in kGs (kilo-Gauss), showing magnetization degree
Color-coded curves: Temperature-dependent hysteresis loops showing downward/leftward shifts with increasing temperature
Key Parameters and Significance
Remanence (Br)
Change: Decreases from ~13.00 kGs (20°C) to ~10.99 kGs (150°C)
Significance: Indicates reduced ability to retain magnetism at high temperatures, critical for high-temperature motor applications
Coercivity (Hcb)
Change: Drops from ~12.78 kOe (20°C) to ~7.129 kOe (150°C)
Significance: Shows decreased resistance to demagnetization at elevated temperatures, important for magnetic storage devices
Intrinsic Coercivity (Hcj)
Change: Gradually decreases with temperature
Significance: Reflects reduced domain structure stability under thermal stress, crucial for aerospace applications
Temperature Effects
Increasing temperature causes deterioration in Br, Hcb and Hcj due to intensified atomic thermal motion disrupting magnetic moment alignment. This impacts high-precision applications requiring stable performance.
Black Slope Lines Interpretation
These lines represent approximate linear magnetization behavior before saturation:
Slope significance: Indicates material permeability (μ)
Steeper slope = higher permeability (easier magnetization)
Shallower slope = lower permeability
Temperature impact: Slope changes demonstrate temperature’s effect on permeability
Practical importance: Critical for designing electromagnetic devices operating at various temperatures