Types of permanent magnets
Types of permanent magnets
Magnetic metallic elements
Many materials have unpaired（不成对的） electron spins, and the majority of these materials are paramagnetic（顺磁性的）. When the spins interact with each other in such a way that the spins align spontaneously（自然，本能地）, the materials are called ferromagnetic (what is often loosely（不严谨的） termed as magnetic). Because of the way their regular crystalline atomic structure causes their spins to interact, some metals are (ferro)magnetic when found in their natural states, as ores（矿石）. These include iron ore (magnetite or lodestone（天然磁石）), cobalt and nickel, as well the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring (ferro)magnets were used in the first experiments with magnetism. Technology has since expanded the availability（实用性） of magnetic materials to include various man made products, all based, however, on naturally magnetic elements.
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Composites
Ceramic（si´ræmic）（陶瓷） or ferrite
Ceramic, or ferrite, magnets are made of a sintered(烧结的) composite of powdered（成粉的） iron oxide（氧化物） and barium［þεeriem］（钡）/strontium carbonate ceramic. Given the low cost of the materials and manufacturing methods, inexpensive magnets of various shapes can be easily mass-produced（MP）. The resulting magnets are non-corroding（不腐蚀）, but brittle(易碎的) and must be treated like other ceramics.
Alnico
Alnico magnets are made by casting or sintering a combination of aluminium, nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate（精致的） shapes. Alnico magnets resist corrosion(侵蚀) and have physical properties more forgiving(宽大的) than ferrite, but not quite as desirable as a metal.
Ticonal
Ticonal magnets are an alloy of titanium, cobalt, nickel, and aluminium, with iron and small amounts of other elements. It was developed by Philips for loudspeakers.
Injection（注模） molded
Injection molded magnets are a composite of various types of resin（树脂） and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength.
Flexible
Flexible magnets are similar to injection molded magnets, using a flexible resin or binder(粘合剂) such as vinyl(乙烯基), and produced in flat strips(条，带), shapes or sheets. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used. Flexible magnets can be used in industrial printers.
Rare earth magnets
Rare earth (lanthanoid) elements have a partially occupied f electron shell (which can accommodate up to 14 electrons.) The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore these elements are used in compact high-strength magnets where their higher price is not a concern. The most common types of rare earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets.
Costs
The current cheapest permanent magnets, allowing for field strengths, are flexible and ceramic magnets, but these are also among the weakest types. Neodymium-iron-boron（钕铁硼）(NIB) magnets are among the strongest. These cost more per kilogram than most other magnetic materials, but owing to their intense（强烈的） field, are smaller and cheaper in many applications.
Temperature
Temperature sensitivity varies, but when a magnet is heated to a temperature known as the Curie point, it loses all of its magnetism, even after cooling below that temperature. The magnets can often be remagnetized however. Additionally some magnets are brittle and can fracture（破裂） at high temperatures.
A ferromagnet, like a paramagnetic substance, has unpaired electrons. However, in addition to the electrons' intrinsic magnetic moments wanting to be parallel to an applied field, there is also in these materials a tendency for these magnetic moments to want to be parallel to each other. Thus, even when the applied field is removed, the electrons in the material can keep each other continually pointed in the same direction.
Every ferromagnetic substance has its own individual temperature, called the Curie temperature, or Curie point, above which it loses its ferromagnetic properties. This is because the thermal tendency to disorder overwhelms the energy-lowering due to ferromagneti
Fields of a magnet
Far away from a magnet, the magnetic field created by that magnet is almost always described (to a good approximation) by a dipole field characterized by its total magnetic moment. This is true regardless of the shape of the magnet, so long as the magnetic moment is non-zero. One characteristic of a dipole field is that the strength of the field falls off inversely with the cube of the distance from the magnet's center.
Closer to the magnet, the magnetic field becomes more complicated, and more dependent on the detailed shape and magnetization of the magnet. Formally, the field can be expressed as a multipole expansion: A dipole field, plus a quadrupole field, plus an octupole field, etc.
At close range, many different fields are possible. For example, for a long, skinny bar magnet with its north pole at one end and south pole at the other, the magnetic field near either end falls off inversely with the square of the distance from that pole.
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