Rare-Earth Magnets

Whenever energy and power are needed in today’s age of miniaturization, rare-earth magnets are called upon to play a vital role. Applications abound, from the family car that uses on average 30 such magnets to powerful levitation systems on magnetic trains. All of this is made possible by elements with strange-sounding names: neodymium, lanthanum, samarium, yttrium and scandium – some of the “rare earths” or Lanthanide elements in the periodic table.

Neodymium as a Magnet Material

Neodymium-iron-boron (Nd-Fe-B) magnets are generally considered the strongest permanent magnets in the world. Their attributes include:

High-energy products (up to 56 MGOe)
Very high coercive forces (resistance to demagnetization)
Excellent energy-to-size ratio
Stability in ambient temperature conditions
Good mechanical characteristics

Common applications include linear actuators, microphones, servo motors, DC motors (automotive starters), computer disc drives, printers and stereo speakers. They are susceptible to progressive loss of magnetism when exposed to temperatures about 175°F (80°C) and will lose all their magnetic properties above its Curie temperature of 310°F (155°C).

Samarium Cobalt as a Magnet Material

Samarium-cobalt magnets have a number of interesting properties. Although the material is relatively expensive (cobalt is market-price sensitive), their attributes include:

High-energy products (up to 32 MGOe)
High coercive forces
Excellent magnetic strength for its size
Good temperature stability
Good mechanical characteristics

Common applications include: computer disc drives, sensors, traveling wave tubes, linear actuators, satellite systems and motors where temporary stability is vital.

Sintering, Heat Treating are Key

A rare-earth magnet’s performance hinges on its size and control of its microstructure, which is accomplished during the sintering and heat-treating processes. Sintering cycles for these materials are typically done in either very high-purity argon atmosphere retorts or in vacuum furnaces. Sintering temperatures are in the range of 1950°F (1065°C) to 2300°F (1290°C). Temperature uniformity is critical since an overshoot of as little as 15°F (8°C) may cause melting. Heat-treating cycles can be complex depending on what you are trying to achieve and may involve solution treatments followed by quenching or, in some alloys, aging to form intermetallic phases.