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Magnets
Magnets are materials that produce a magnetic field of their own. Extreme examples of magnets are (1) "hard", or "permanent" magnets (like refrigerator magnets), which remember how they have been magnetized, and (2) "soft", or "impermanent" magnets (like the material of the refrigerator door), which lose their memory of previous magnetizations. "Soft" magnets are often used in electromagnets to enhance (often by factors of hundreds or thousands) the magnetic field of a current-carrying wire that has been wrapped around the magnet; when the current increases, so does the field of the "soft" magnet, which is much larger than the field due to the current. Permanent magnets occur naturally in some rocks, particularly lodestone, but they are now more commonly manufactured.
Materials without a permanent magnetic moment can, in the presence of magnetic fields, be attracted (paramagnetic), or repelled (diamagnetic). Liquid oxygen is paramagnetic; graphite is diamagnetic. "Soft" magnets, which are strongly attracted to magnetic fields, can be thought of as strongly paramagnetic; superconductors, which are strongly repelled by magnetic fields, can be thought of as strongly diamagnetic.
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Rare-earth magnets
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.
Magnetic fields produced by rare-earth magnets can be in excess of 1.2 teslas. Ferrite or ceramic magnets typically exhibit 50 to 100 milliteslas. Common applications of rare-earth magnets include computer hard drives, audio speakers and bicycle dynamos. Rare-earth magnets are used in stop motion animation as tie-downs when the use of traditional screw and nut tie-downs is impractical. Rare-earth magnets are used for diamagnetic levitation experimentation, the study of magnetic field dynamics and superconductor levitation. LSM launch technology found on roller coaster and other thrill rides utilize rare-earth magnets.
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Neodymium, Iron, and Boron — Nd2Fe14B Magnets
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.
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Samarium-cobalt Magnets, SmCo5
Samarium-cobalt
Samarium-cobalt magnets (SmCo5) are less common than neodymium magnets. Samarium-cobalt magnets are more expensive to produce and are not as strong as Neodymium magnets. Samarium-cobalt magnets have a relatively high Curie point. The high Curie point makes Samarium-cobalt magnets suitable for high-temperature applications.
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Alnico Magnets, Aluminium, Nickel and Cobalt Magnets
Alnico alloys are composed primarily of aluminium, nickel and cobalt (hence the term al-ni-co) with the addition of iron, copper, and sometimes titanium.
Alnico alloys make strong permanent magnets. They can be magnetized to produce strong magnetic fields. Of the more commonly available magnets, only rare-earth magnets such as neodymium and samarium-cobalt are stronger. Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss (0.15 tesla), or about 3000 times the strength of Earth's magnetic field.
Alnico alloys have some of the highest Curie points of any magnetic material, around 800 °C. This property, as well as its brittleness and high melting point, is the result of the strong tendency toward order due to intermetallic bonding between aluminium and its other constituents.
Alnico magnets are used in electric motors, electric guitar pickups, sensors, loudspeakers, and cow magnets. Alnico is produced by casting or sintering processes.
Some types of Alnico are isotropic, meaning they can be efficiently magnetized in any direction. Other types, such as Alnico 5 and Alnico 8, are anisotropic, meaning that they have a preferred direction of magnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in their preferred orientation than isotropic types. Anisotropic Alnico magnets are oriented by heating them above a critical temperature, and cooling them in the presence of a magnetic field.
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Permanent magnets and dipoles
All magnets appear to have at least one north pole (reckoned positive) and at least one south pole (reckoned negative), and the net pole strength of every magnet is zero. Despite their apparent reality, as suggested by the image at the top of the page, where iron filings concentrate in regions of large magnetic field, poles are not physical objects on or in the magnet. They are, rather, a useful concept for describing magnets. Rather than poles being the fundamental unit, it is the magnetic dipole that is the fundamental unit. A magnetic dipole can be thought of as a combination of a positive and a negative pole that are microscopically close to one another and inseparable. This is not a bad description of the magnetic dipole of an electron in a magnetic material.
By aligning a large number of these dipoles (say a million), and placing them head-to-tail in a line, we find that there is a north pole at one end and a south pole at the other, but all the intermediate north and south poles cancel out one another. The net effect is a very long dipole that appears to have poles only at its ends. Theories have been developed involving the possibility of north and south magnetic monopoles, but no magnetic monopole has yet been found.
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Ferrite Magnets
Ferrites are a class of chemical compounds with the formula AB2O4, where A and B represent various metal cations, usually including iron. These ceramic materials are used in applications ranging from magnetic components in microelectronics.Ferrites are a class of spinels, materials that adopt a crystal motif consisting of cubic close-packed (FCC) oxides (O2-) with A cations occupying one eighth of the octahedral holes and B cations occupying half of the octahedral holes. The magnetic material known as "ZnFe" has the deceptively simple formula ZnFe2O4, with Fe3+ occupying the octahedral sites and half of the tetrahedral sites. The remaining tetrahedral sites in this spinel are occupied by Zn2+.[1]
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