Temperature Coefficients and Thermal Stability Analysis of Alnico Magnets
1. Introduction to Alnico Magnets
Alnico (Aluminum-Nickel-Cobalt) is a family of permanent magnet materials developed in the 1930s, composed primarily of iron (Fe), aluminum (Al), nickel (Ni), and cobalt (Co), with trace amounts of copper (Cu) and titanium (Ti). Known for its high remanence (Br) and excellent thermal stability, Alnico was once the dominant permanent magnet material before being surpassed by ferrite and rare-earth magnets in the late 20th century. However, it remains indispensable in applications requiring stable magnetic performance under extreme temperatures, such as aerospace, military, and precision instrumentation.
This analysis focuses on Alnico’s temperature coefficients (remanence temperature coefficient αBr and coercivity temperature coefficient αHcj) and explains why it is considered the most thermally stable permanent magnet material.
2. Temperature Coefficients of Alnico Magnets
2.1 Remanence Temperature Coefficient (αBr)
The remanence temperature coefficient (αBr) quantifies the reversible change in remanence (Br) with temperature, expressed as:
Where:
- ΔBr = Change in remanence
- Br = Initial remanence at reference temperature
- ΔT = Temperature change
For Alnico magnets:
- Typical αBr range: -0.02% to -0.01%/°C
- Implication: For every 1°C increase in temperature, Br decreases by only 0.02% (reversibly).
Comparison with other magnets:
| Magnet Type | αBr (%/°C) | Thermal Stability Implication |
|---|---|---|
| Alnico | -0.02 ~ -0.01 | Best (minimal Br loss) |
| SmCo (2:17) | -0.03 ~ -0.02 | Good |
| NdFeB (N35) | -0.12 ~ -0.11 | Poor (high Br loss) |
| Ferrite (SrFe12O19) | -0.20 ~ -0.18 | Very poor |
Alnico’s exceptionally low αBr means it retains 98% of its Br even at 500°C, making it ideal for high-temperature applications.
2.2 Coercivity Temperature Coefficient (αHcj)
The coercivity temperature coefficient (αHcj) measures the reversible change in intrinsic coercivity (Hcj) with temperature:
For Alnico magnets:
- Typical αHcj range: +0.01% to +0.03%/°C
- Implication: Hcj increases slightly with temperature (unlike most magnets where Hcj decreases).
Comparison with other magnets:
| Magnet Type | αHcj (%/°C) | Thermal Stability Implication |
|---|---|---|
| Alnico | +0.01 ~ +0.03 | Unique (Hcj increases) |
| SmCo (2:17) | -0.30 ~ -0.20 | Moderate (Hcj decreases) |
| NdFeB (N35) | -0.55 ~ -0.45 | Poor (Hcj drops sharply) |
| Ferrite | -0.60 ~ -0.50 | Very poor |
Alnico’s positive αHcj is a key advantage, as it prevents irreversible demagnetization at elevated temperatures, unlike NdFeB and ferrite magnets.
3. Why Alnico is the Most Thermally Stable Permanent Magnet
3.1 Exceptionally Low αBr and Positive αHcj
- Minimal Br loss: Alnico’s αBr is 10–20 times lower than NdFeB and ferrite, ensuring stable magnetic output over wide temperature ranges.
- Hcj increases with temperature: Unlike other magnets, Alnico’s coercivity improves at higher temperatures, reducing the risk of demagnetization.
3.2 High Curie Temperature (Tc)
- Curie temperature (Tc): The temperature at which a magnet loses all magnetism.
- Alnico’s Tc: 800–900°C (highest among permanent magnets).
- Comparison:
- SmCo: ~750°C
- NdFeB: ~310–370°C
- Ferrite: ~450°C
Alnico’s high Tc ensures it remains magnetic even at extreme temperatures.
3.3 Low Reversible Temperature Coefficient (RTC)
- Reversible Temperature Coefficient (RTC): Combines αBr and αHcj effects.
- Alnico’s RTC: Near-zero due to compensating effects (low αBr + positive αHcj).
- Implication: Minimal irreversible demagnetization after thermal cycling.
3.4 Stable Microstructure
- Spinodal decomposition: Alnico’s unique microstructure forms elongated α-Fe rods in a Ni-Al matrix, providing high remanence and coercivity.
- Thermal aging resistance: The structure remains stable even after prolonged exposure to high temperatures.
3.5 Resistance to Demagnetization
- Low coercivity (Hcj): While Alnico’s Hcj is lower than SmCo/NdFeB (~160–320 kA/m vs. 800–2400 kA/m), its positive αHcj prevents demagnetization under thermal stress.
- Non-linear demagnetization curve: Alnico’s B-H curve is flatter at high temperatures, reducing flux loss under external fields.
4. Performance Comparison with Other Magnets
4.1 Temperature Stability (Br vs. Temperature)
| Magnet Type | Br at 20°C (T) | Br at 500°C (T) | Br Retention (%) |
|---|---|---|---|
| Alnico 5 | 1.35 | 1.22 | 90.4% |
| SmCo 2:17 | 1.09 | 0.93 | 85.3% |
| NdFeB N35 | 1.23 | 0.59 | 48.0% |
| Ferrite | 0.38 | 0.15 | 39.5% |
Alnico retains 90% of Br at 500°C, while NdFeB loses over half.
4.2 Coercivity Stability (Hcj vs. Temperature)
| Magnet Type | Hcj at 20°C (kA/m) | Hcj at 500°C (kA/m) | Hcj Change (%) |
|---|---|---|---|
| Alnico 5 | 160 | 180 | +12.5% |
| SmCo 2:17 | 800 | 560 | -30.0% |
| NdFeB N35 | 960 | 430 | -55.2% |
| Ferrite | 240 | 96 | -60.0% |
Alnico’s Hcj increases by 12.5% at 500°C, while others degrade severely.
5. Applications Leveraging Alnico’s Thermal Stability
5.1 Aerospace & Defense
- Gyroscopes & Inertial Navigation: Alnico’s stable magnetic field ensures precision in high-vibration, high-temperature environments.
- Missile Guidance Systems: Used in magnetometers and actuators where temperature fluctuations are extreme.
5.2 Industrial & Motor Applications
- High-Temperature Motors: Alnico retains torque in motors operating at 400–500°C.
- Magnetic Clutches & Brakes: Used in steel mills and foundries where heat resistance is critical.
5.3 Sensors & Instrumentation
- Fluxgate Magnetometers: Alnico’s stability enables accurate magnetic field measurements in geophysical surveys.
- Hall Effect Sensors: Provides a stable reference field in automotive and aerospace sensors.
5.4 Electric Guitars & Audio Equipment
- Pickups: Alnico’s warm, stable tone is preferred in high-end guitars (e.g., Fender Stratocaster).
- Loudspeakers: Used in tweeters and mid-range drivers for consistent sound quality.
6. Limitations of Alnico Magnets
Despite its superior thermal stability, Alnico has drawbacks:
- Low coercivity (Hcj): Prone to demagnetization if exposed to strong reverse fields.
- Lower energy product (BHmax): 5–10 MGOe vs. NdFeB’s 40–55 MGOe, limiting use in high-power applications.
- Brittleness: Difficult to machine into complex shapes (requires casting or sintering).
- Cost: Higher than ferrite but lower than SmCo/NdFeB.
7. Conclusion: Why Alnico is the Best for Thermal Stability
Alnico magnets are the gold standard for thermal stability due to:
- Exceptionally low αBr (-0.02%/°C) → Minimal Br loss at high temperatures.
- Positive αHcj (+0.01–0.03%/°C) → Hcj increases with temperature, preventing demagnetization.
- Highest Curie temperature (800–900°C) → Retains magnetism at extreme heat.
- Stable microstructure → Resistant to thermal aging and degradation.
While NdFeB and SmCo offer higher energy products, no other magnet matches Alnico’s thermal stability, making it irreplaceable in aerospace, military, and high-temperature industrial applications.
For designers seeking reliable magnetic performance under extreme heat, Alnico remains the best choice despite its limitations in coercivity and energy density.
