Rare-earth magnets facts,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 teslas, 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" can be misleading as these metals are not particularly rare or precious; they are about as abundant as tin or lead,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.
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 earth 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 (Br), which measures the strength of the magnetic field; coercivity (Hci), the material's resistance to becoming demagnetized; energy product (BHmax), the density of magnetic energy; and Curie temperature (Tc), 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.
Magnet Br (T) Hci (kA/m) (BH)max (kJ/m3) Tc (°C)
Nd2Fe14B (sintered) 1.0–1.4 750–2000 200–440 310–400
Nd2Fe14B (bonded) 0.6–0.7 600–1200 60–100 310–400
SmCo5 (sintered) 0.8–1.1 600–2000 120–200 720
Sm(Co,Fe,Cu,Zr)7 (sintered) 0.9–1.15 450–1300 150–240 800
Alnico (sintered) 0.6–1.4 275 10–88 700–860
Sr-ferrite (sintered) 0.2–0.4 100–300 10–40 450
Types
Samarium-cobalt magnet
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 magnet
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.
Originally, the high cost of these magnets 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 the low cost has inspired new uses 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.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 who have swallowed several magnets have had a fold of the digestive tract pinched between the magnets, causing injury or death. 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
Since their prices became competitive in the 1990s, neodymium magnets have been replacing Alnico and ferrite magnets in the many applications in modern technology requiring powerful magnets. Their greater strength allows smaller and lighter magnets to be used for a given application.
Common applications
Common applications of rare-earth magnets include:
computer hard drives
audio speakers / headphones
bicycle dynamos
fishing reel brakes
permanent magnet motors in cordless tools
self-powered torches, employing rare earth magnets for generating electricity in a shaking motion
Other applications
Other applications of rare-earth magnets include:
Mag-lev trains.
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.
Electrodynamic bearings.
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.
Electric guitar pickups.
Miniature figure (gaming) in particular Warhammer 40,000 and Warhammer Fantasy Battle Rare Earth magnets have gained popularity in the miniatures gaming community for their small size and relative strength assiting in swapping weapons between models to adhere to WYSIWYG conventions.
(source:wikipedia)
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