Material Selection Guide
Selecting a Magnet
- Stage 1 - Which Material - Choose the best material for your Application.
- Stage 2 - What Shape - Choose a shape which suits your design.
- Stage 3 - What Size - Use the web site to find the product code of the exact size or nearest we have in Stock.
Which Material?
Choice of magnetic materials is dependent on :
- Magnetic Strength
- Operating Temperature
- Corrosion Resistance
- Cost
- Stability
For each of the categories above we will compare the four main material groups Ferrite, AlNiCo, Samarium Cobalt (SmCo), Neodymium Iron Boron ( NdFeB ).
- The Energy stored in the material
- Flux Density on the surface of the magnet
- Pull required to remove the magnet from a flat piece of steel
| Material | Energy ( MGOe ) | Flux Density (Gauss) | Pull ( kg/cm2 ) |
|---|---|---|---|
| Ferrite | 3.3 | 1400 | 0.25 |
| AlNiCo | 5.0 | 1300 | 0.20 |
| SmCo | 26.0 | 3500 | 1.5 |
| NdFeB | 35.0 | 4500 | 2.5 |
Operating Temperature
All magnet materials lose magnetism as temperature rises :
Reversible Losses will return as the magnet cools as long as the working Temperatures are not exceeded. Exceeding working temperatures will mean that the magnet needs to be re-magnetised on cooling. Exceeding Curie temperatures will damage the magnet beyond repair.
Sub zero temperature affects are detailed below :
| Material | Reversible Losses / Deg C | Working Temperature Deg C | Curie Temperature Deg C | |
|---|---|---|---|---|
| Ferrite | 0.19% | 250 | 460 | Large irreversible losses below -40 deg C |
| AlNiCo | 0.02% | 550 | 860 | Losses < 10% down to 4 K |
| SmCo | 0.03% | 300 | 750 | Minimal loses down to 4 K |
| NdFeB | 0.12% | 120 | 320 | No losses down to 77 K |
Corrosion
| Material | Resistance to Corrosion | Additional Comments |
|---|---|---|
| Ferrite | Excellent | |
| AlNiCo | Fair | |
| SmCo | Excellent | Will break down if exposed to Hydrogen |
| NdFeB | Poor | Ni,Zn coatings protect in stores conditions. Will break down if exposed to Hydrogen |
Cost
| Material | Cost Comparison |
|---|---|
| Ferrite | Low ( x 1 ) |
| AlNiCo | Medium ( x 5 ) |
| SmCo | Very High ( x 20 ) |
| NdFeB | High ( x 10 ) |
Stability
External magnetic fields
External magnetic fields can demagnetise materials to different degrees.
| Material | Ease of Demagnetisation | Comments |
|---|---|---|
| Ferrite | Low | Pushing magnets together in repulsion can demagnetise by 10% |
| AlNiCo | High | Pushing magnets together in repulsion can demagnetise by 50% |
| SmCo | Very Low | |
| NdFeB | Very Low |
Time/Shock/Vibration
With modern materials magnetic losses with time and vibration are minimal. All magnet materials are brittle if subjected to impact, especially SmCo.
Radiation
It is recommended that magnets used in particle deflection systems have high Hci values. Tests have shown that SmCo shows significant losses when exposed to high levels of radiation ( 109 – 1010 rads ) and NdFeB also has high losses at 4 x 106 rads ( 50% ) and 7 x 107 rads ( 100 % ) Cobalt will also retain radiation after exposure.
Shape
For optimum performance it is recommended that you use the magnet materials in excess of the following Length to Diameter ratios. At ratios below this Gauss levels and pull could be reduced.
| Material | Length to Diameter Ratio |
|---|---|
| Ferrite | 1 |
| AlNiCo | 5 |
| SmCo | 0.7 |
| NdFeB | 0.7 |