Neodymium magnet

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Neodymium magnet on a bracket from a hard drive

A neodymium magnet or NIB magnet (also, but less specifically, called a rare-earth magnet) is a powerful magnet made of a combination of neodymium, iron, and boron — Nd₂Fe₁₄B. They have replaced marginally weaker and significantly more heat-resistant samarium-cobalt magnets in most applications, due mainly to their lower cost. These magnets are very strong in comparison to their mass, but are also mechanically fragile and the most powerful grades lose their magnetism at temperatures above 176 degrees fahrenheit or 80 degrees Celsius. High-temperature grades will operate at up to 200 and even 230 °C but their strength is only marginally greater than that of samarium-cobalt. Neodymium magnets (or “neo” as they are known in the industry) are graded in strength from N24 to the strongest N54. The number after the N represents the magnetic energy product, in megagauss-oersteds (MGOe) (1 MG·Oe = 7,958 T·A/m = 7,958 J/m³). N48 has a remnant static magnetic field of 1.38 teslas and an H (magnetic field intensity) of 13,000 oersteds (1.0 MA/m). By volume one requires about 18 times as much ceramic magnet material for the equivalent magnet strength. The neodymium magnet industry is continually working to push the maximum energy product (strength) closer to the theoretical maximum of 64 MGOe. Scientists are also working hard to improve the maximum operating temperature for any given strength.

A neodymium magnet lifting 1300 times its own mass

Used for stabilization and angular head motors in computer hard drives, neodymium magnets are also popular with hobbyists, and a small magnet can have amazing properties — it exhibits magnetic braking when moved near a non-magnetic metal due to induced eddy currents. An excellent demonstration for students to see the effects of Lenz's Law in non-ferrous metals may be performed by dropping a strong neodymium magnet through a copper pipe. The magnet will travel through the pipe remarkably slowly as it falls, the effect may be greatly enhanced by immersing the pipe in liquid nitrogen (thus increasing its conductivity even further) prior to dropping the magnet through. A somewhat larger magnet interacts strongly enough with the magnetic field of the Earth to allow its tendency to align with that field to be perceived directly when holding it, essentially forming a compass. Cylinder- and disc-shaped neodymium magnets are especially responsive to the Earth's magnetic fields. Neodymium magnets are used for the transducers in many headphones.

A toy containing dozens of NIB magnets

As NIB magnets produced in China have become less expensive in the last few years, the toy industry has used millions of them in magnetic building sets and other products including magnetic jewelry. Rose Art Industries of New Jersey, now owned by Mega Brands, Inc. of Montreal, Canada, manufactures a popular line of Magnetix and Magna Man toys containing neodymium magnets the size and shape of aspirin tablets. The small cylindrical magnets are used at the ends or corners of plastic pieces in order to allow connections of multiple pieces. The Magnetix brand was the subject of a March, 2006 recall notice by the Consumer Product Safety Commission as well as numerous consumer lawsuits due to product safety concerns. In defective kits the NIB magnets became dislodged from their plastic housing, and many children of varying ages consumed the small magnets.

[edit] Health Hazards

Neodymium magnets should always be handled carefully. Some that are slightly larger than the size of a penny are powerful enough to lift over 10 kilograms. They are hazardous; able to interfere with pacemakers and implanted heart devices with deadly consequences ^[1]. If swallowed, neodymium magnets can cause lethal conditions by joining up inside the intestine ^[2].

An NIB's magnetic force increases with the size of the piece of ferromagnetic metal and larger neodymium magnets can severely pinch skin or fingers, or even break bones when suddenly attracted to a magnetic object. Operating a large neodymium magnet close to smaller magnetic objects (keys, pens, etc.) and larger magnetic surfaces (radiator or a car, for example) can be dangerous if the person is caught between the magnet and the magnetic object or surface. On an episode of MythBusters, it was demonstrated that large neodymium magnets cannot pull a paper clip (or other small metal objects) through a hand[1].

Neodymium magnets are made with special powders and coatings, so they are very fragile. They are often plated with a metal such as nickel. The magnets can fracture at temperatures over 150 °C, or under impact as a result of their own acceleration. When this happens, in some cases the magnets may break apart so suddenly that flying pieces can cause injury.

Caution must be taken when using neodymium magnets. A neodymium magnet is powerful enough to destroy the contents of a floppy disk to such an extent that the information is unrecoverable, a guarantee not present with techniques such as formatting the disk. It can also erase information contained on the magnetic stripes of credit cards, rendering them unreadable. Neodymium magnets are often strong enough to not only magnetize color CRT shadow masks, but also physically deform the mask itself. Such damage is typically not repairable by degaussing.