Rare-earth magnet

Rare-earth magnets are strong, permanent magnets made from alloys of rare earth elements. Rare-earth magnets are substantially stronger than ferrite or alnico magnets. The magnetic field typically produced by rare-earth magnets can be in excess of 1.2 teslas. Ferrite or ceramic magnets typically exhibit fields of 50 to 100 milliteslas. Because rare earth magnets are extremely brittle, they are usually nickel-coated to protect them from breaking and chipping, hence their typically shiny, corrosion resistant nature.

Magnetic properties of rare earths

Lanthanides are elements with an incompletely filled f-shell. Electrons in such orbitals are strongly localized and therefore easily retain their magnetic moments and function as paramagnetic centers. Magnetic moments in other orbitals are often lost ("quenched") due to strong overlap with the neighbors. In addition, the f-shell can contain up to 7 unpaired electrons, enhancing the size of the magnetic moment. The presence of large magnetic moments is however not sufficient. There also is a strong ferromagnetic coupling between them in these materials.

Rare earths are common in the Earth's crust and depending on the element, not particularly rare or precious, and as of 2007 give the best cost/field ratios of any permanent magnet.

Applications

Common Applications

Common applications of rare-earth magnets include:
*Computer hard drives
*Audio speakers
*Bicycle dynamos.
*Fishing Reel brakes.

Other Applications

Other applications of rare-earth magnets include:
*Mag-lev wind turbines.
*Stop motion animation as tie-downs when the use of traditional screw and nut tie-downs is impractical
*Diamagnetic levitation experimentation, the study of magnetic field dynamics and superconductor levitation.
*Launched roller coaster technology found on roller coaster and other thrill rides.

Types of rare-earth magnets

Neodymium

Neodymium magnets (chemical formula: Nd₂Fe₁₄B) are the strongest and most affordable type of rare-earth magnet. Neodymium magnets are made of neodymium, iron and boron. Neodymium magnets are typically used in most computer hard drives and a variety of audio speakers. They have the highest magnetic field strength, but are inferior to samarium-cobalt in resistance to oxidation and temperature. This type of magnet has traditionally been expensive, due to both the cost of raw materials and licensing of the patents involved. This high cost limited their use to applications where such high strengths from a compact magnet are critical. Use of protective surface treatments such as gold, nickel, zinc and tin plating and epoxy resin coating can provide corrosion protection where required. Beginning in the 1980s, NIB magnets have increasingly become less expensive and more popular in other applications such as children's magnetic building toys. Even tiny neodymium magnets are very powerful, and can cause blood blisters and other injuries, by severe pinching, if skin or other body parts are allowed to come between them. [Magnet Man, Magnet Basics - [http://www.coolmagnetman.com/magsafe.htm Safety Considerations] accessed 6 October 2006.]

Samarium-cobalt

Samarium-cobalt magnets (chemical formula: SmCo₅) are less common than Neodymium magnets because they are more expensive to produce and create a weaker magnetic field than Neodymium magnets. However, Samarium-cobalt magnets have a relatively high Curie point, which makes them suitable for high-temperature environments. They are highly resistant to oxidation, but sintered samarium-cobalt magnets are brittle and prone to chipping and cracking and may fracture when subjected to thermal shock.

Footnotes and References

Printed References

1. Edward P. Furlani, "Permanent Magnet and Electromechanical Devices: Materials, Analysis and Applications", Academic Press Series in Electromagnetism (2001). ISBN 0-12-269951-3.

2. Peter Campbell, "Permanent Magnet Materials and their Application" (Cambridge Studies in Magnetism)(1996). ISBN-13: 978-0521566889.