Rare-earth magnet

Ferrofluid on glass, with a rare-earth magnet underneath.

Rare-earth magnets are strong permanent magnets made from alloys of rare earth elements. Developed in the 1970s and 80s, rare-earth magnets are the strongest type of permanent magnets made, substantially stronger than ferrite or alnico magnets. The magnetic field typically produced by rare-earth magnets can be in excess of 1.4 tesla, whereas ferrite or ceramic magnets typically exhibit fields of 0.5 to 1 tesla. There are two types: neodymium magnets and samarium-cobalt magnets. Rare earth magnets are extremely brittle and also vulnerable to corrosion, so they are usually plated or coated to protect them from breaking and chipping.

The term "rare earth" is often misunderstood by the chemical layman; these metals are not particularly rare or precious, and as of 2007 rare earth magnets give the best cost/field ratios of any permanent magnet.^{[citation needed]} Interest in rare earth compounds as permanent magnets began in 1966, when K. J. Strnat and G. Hoffer of the US Air Force Materials Laboratory discovered that YCo₅ had by far the largest magnetic anisotropy constant of any material then known.^[1]

Explanation of strength

The rare earth (lanthanide) elements are metals that are ferromagnetic, meaning that like iron they can be magnetized, but their Curie temperatures are below room temperature, so in pure form their magnetism only appears at low temperatures. However, they form compounds with the transition metals such as iron, nickel, and cobalt, and some of these have Curie temperatures well above room temperature. Rare earth magnets are made from these compounds.

The advantage of the rare earths and their compounds over other magnets is that their crystalline structures have very high magnetic anisotropy. This means that a crystal of the material is easy to magnetize in one particular direction, but resists being magnetized in any other direction.

The high anisotropy of a rare earth element, as well as its other magnetic properties, is related to its 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 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.

Magnetic properties

Some important properties used to compare permanent magnets are: remanence (B_r), which measures the strength of the magnetic field; coercivity (H_ci), the material's resistance to becoming demagnetized; energy product (BH_max), the density of magnetic energy; and Curie temperature (T_c), the temperature at which the material loses its magnetism. Rare earth magnets have higher remanence, much higher coercivity and energy product, but (for neodymium) lower Curie temperature than other types. The table below compares the magnetic performance of the two types of rare earth magnet, neodymium (Nd₂Fe₁₄B) and samarium-cobalt (SmCo₅), with other types of permanent magnets.

Types

Samarium-cobalt

Samarium-cobalt magnets (chemical formula: SmCo₅), the first family of rare earth magnets invented, are less used than neodymium magnets because of their higher cost and weaker magnetic field strength. However, samarium-cobalt has a higher Curie temperature, creating a niche for these magnets in applications where high field strength is needed at high operating temperatures. 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.

Neodymium

Neodymium magnets, invented in the 1980s, are the strongest and most affordable type of rare-earth magnet. Neodymium alloy (Nd₂Fe₁₄B) is 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 Curie temperature. Use of protective surface treatments such as gold, nickel, zinc and tin plating and epoxy resin coating can provide corrosion protection where required.

Traditionally, the high cost of these magnets has limited their use to applications requiring compactness together with high field strength. Both raw materials and patent licenses were expensive. Beginning in the 1990s, NIB magnets have become steadily less expensive and more popular in other applications such as children's magnetic building toys.

Hazards

The greater force exerted by rare earth magnets creates hazards that are not seen with other types of magnet. Magnets larger than a few centimeters are strong enough to cause injuries to body parts pinched between two magnets, or a magnet and a metal surface, even causing broken bones.^[2] Magnets allowed to get too near each other can strike each other with enough force to chip and shatter the brittle material, and the flying chips can cause injuries. There have even been cases where young children that have swallowed several magnets have had a fold of the digestive tract pinched between the magnets, causing injury or death.^[3] The stronger magnetic fields can be hazardous also, and can erase magnetic media such as hard disks and credit cards, and magnetize the shadow masks of CRT type monitors at a significant distance.

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.
• LED Throwies An LED throwie is a small LED attached to a coin battery and a rare earth magnet (usually with conductive epoxy or electrical tape), used for the purpose of creating non-destructive graffiti and light displays.
• Fender Musical Instruments use Samarium-Cobalt in a line of electric guitar pickups for their electric guitars and basses. They are higher end pickups that cut out much of the 60hz hum associated with single coil pickups.

Footnotes and references

Printed references

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

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

• MMPA 0100-00, Standard Specifications for Permanent Magnet Materials [1]

Further readings

The Dependence of Magnetic Properties and Hot Workability of Rare Earth-Iron-Boride Magnets Upon Composition