When you choose permanent magnetic material, it’s necessary to consider the following aspects:
• Magnetic stability required;
• Maximum working temperature;
• Availability;
• Corrosion resistance;
• Cost;
• Size and/or weight limitations;
• Flux requirement for the particular application.
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1.Magnetic Performance
2.Effects of Temperature
3.Effects of Exposure on Magnetic Stability
4.Corrosion Resistance without Coating
5.Price Comparison
6.Properties of Magnetic Lines of Force
7.Useful Design Suggestion
8.Typical Applications of Permanent Magnets
There are many different types of magnet material, but we will just choose four major types in use today to make comparisons. They are NdFeB, Alnico, SmCo and Ferrite magnets.
Most magnets are anisotropic, and can only be magnetized in the orientation direction. Although isotropic magnets can be magnetized in any direction, they are generally lower in performance than anisotropic magnets.
1. Magnetic Performance of Different Magnets
BHmax is the point where a magnet provides most energy for the minimum volume. If you want to compare the magnetic performance of different types and grades of permanent magnets, the most convenient method is to consider their BHmax.
NdFeB(N38H) | 306 kJ/m³ | 38 MGO |
Alnico(anisotropic Alcomax III) | 42 kJ/m³ | 5.2 MGO |
SmCo(2:17) | 208 kJ/m³ | 26 MGO |
Ferrite(anisotropic) | 26 kJ/m³ | 3.3 MGO |
Another parameter that should be taken into consideration is the flux density on the pole face of a magnet. This figure is often mistaken for the Br, but actually it is purely the induction in a closed circuit. The following table shows typical pole face flux densities of the four grades when working at approximately their BHmax points.
NdFeB(N38H) | 450 mT (4500 Gauss) |
Alnico(anisotropic Alcomax III) | 130 mT (1300 Gauss) |
SmCo(2:17) | 350 mT (3500 Gauss) |
Ferrite(anisotropic) | 100 mT (1000 Gauss) |
Effects of Temperature can be classified into two categories, reversible and irreversible. The reversible changes with temperature have nothing to do with the shape, size or the working point on the demagnetization curve. They are dependant upon material composition. When a magnet is returned to its initial temperature, reversible losses will disappear completely without remagnetization. Irreversible losses won’t come up if a certain temperature isn’t exceeded. In addition, they can also be limited by operating at as high a working point as possible. But when the external temperature exceeds the Curie temperature of a magnet, metallurgical changes occur within the magnet and there will be unrecoverable losses.
Temperature Coefficient of Br (20ºC-150 ºC)
NdFeB(N38H) | - 0.12% ºC |
Alnico(anisotropic Alcomax III) | - 0.02% ºC |
SmCo(2:17) | - 0.03% ºC |
Ferrite(anisotropic) | - 0.19% ºC |
Maximum Working Temperature (No Irreversible Losses)
The working point in the circuit determines the maximum working temperature of a magnet. The higher the working point the higher the temperature the magnet can operate.
NdFeB(N38H) | 120 ºC |
Alnico(anisotropic Alcomax III) | 550 ºC |
SmCo(2:17) | 300 ºC |
Ferrite(anisotropic) | 250 ºC |
Curie Temperature (Unrecoverable Losses Occur)
When the Curie temperature is reached, metallurgical changes occur within the magnet structure and the individual magnetic domains break down. Once these losses come up they cannot be reversed by remagnetizing.
NdFeB(N38H) | 320 ºC |
Alnico(anisotropic Alcomax III) | 860 ºC |
SmCo(2:17) | 750 ºC |
Ferrite(anisotropic) | 460 ºC |
Effects of Sub-zero Temperature
Different material groups are affected by low temperature differently. In addition, the influence is closely connected with the magnet shape as well as its working point on the demagnetization curve.
NdFeB(N38H) | No irreversible losses down to 77K |
Alnico(anisotropic Alcomax III) |
Permanent losses of no more than 10% are to be expected down to 4K |
SmCo(2:17) | Minimal losses down to 4K |
Ferrite(anisotropic) | Large irreversible losses below - 60 ºC |
3. Effects of Exposure on Magnetic Stability
Although high temperature is the biggest threat to magnetic stability, exposure to high external fields also has an effect on certain types of magnets. The following table shows different degree of effects:
NdFeB(N38H) | Very Low |
Alnico(anisotropic Alcomax III) | High |
SmCo(2:17) | Very Low |
Ferrite(anisotropic) | Low |
Effects of Shock and Vibration
The earliest magnets were always affected by shock and vibration but now it has little effect on modern magnet materials, except for the most closely calibrated devices. However, mechanical impact will cause magnet materials to be brittle and fractured. SmCo is the most brittle one.
Effects of Radiation
Magnets are used within particle beam deflection applications and those with a higher Hci are more suitable to be used in such environments. According to some tests, SmCo has significant losses when it is exposed to high levels of radiation (109 to 1010 rads). For NdFeB, it losses 50% at 4x106 rads and 100% at 7x107 rads. Losses at low levels of radiation are basically the same as temperature losses. It’s remarkable that some magnet materials have Cobalt in them, and Cobalt can retain radiation after exposure.
Effects of Shape
The performance and stability of a magnet are also affected by its shape. The shape of the magnet determines its working point along the demagnetization curve. The higher the working point is the more difficult for the magnet to be demagnetized. Magnets that have a longer length or are used in a closed magnetic circuit have better performance and magnetic stability.
Some methods can be adopted to improve magnetic stability in performance, such as local demagnetization and high temperature aging treatment. After expose the magnet in advance to any possible detrimental influences, the unstable texture and magnetic domains disappear and the magnet can be magnetically more stable.
The total breakdown of composition will also cause loss of performance. Corrosion can break the magnet structure down, and for NdFeB, exposure to Hydrogen will lead to structural breakdown as well.
Effects of Time
Time has little effect on magnets and it is negligible. There is only a loss of less than 1 x 10 -5 per annum at 200 ºC on average. Actually, a period of 100,000 hour (11.4 years) causes no loss for SmCo while it just cause a loss of less than 3% for Alcomax III at low permeance coefficients.
4. Corrosion Resistance without Coating
Coating can prevent the magnets form being corroded. There are many protective coatings available. NdFeB magnets often have Nickel, Zinc, Lacquer, Epoxy or Parylene as a protective coating. Usually Alnico magnets don’t need coating, but powder coating and electroplating can be used when required.
NdFeB(N38H) | Poor |
Alnico(anisotropic Alcomax III) | Fair |
SmCo(2:17) | Excellent |
Ferrite(anisotropic) | Excellent |
There are several factors that affect the price of a magnet, such as shape, tolerances and quantity. However, the most significant effect is the cost of the basic raw material. When new sizes and volume production of magnets are required, tooling should be considered sometimes. Besides, fixtures are sometimes required for close tolerance machining.
NdFeB(N38H) | High (x10) |
Alnico(anisotropic Alcomax III) | Medium (x5) |
SmCo(2:17) | Very High(x20) |
Ferrite(anisotropic) | Low(x1) |
6. Properties of Magnetic Lines of Force
8. Typical Applications of Permanent Magnets
Permanent magnets have wide and various applications in lots of industries. However, they can all be divided into several categories as the following: