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
 
MaterialEnergy ( 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 :

 
MaterialReversible Losses / Deg CWorking Temperature Deg CCurie 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

 
MaterialResistance to CorrosionAdditional 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

 
MaterialCost 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.

 
MaterialEase of DemagnetisationComments
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.

 
MaterialLength to Diameter Ratio
Ferrite 1
AlNiCo 5
SmCo 0.7
NdFeB 0.7