Magnets

• The phenomenon most people associate with magnetism, i.e., a material that is attracted to certain metals such as iron, is referred to as ferromagnetism. A ferromagnetic material behaves as a magnet due to two factors: its configuration of electrons and its atomic structure.
  
  

Electrons

• Electrons spin. They are, in effect, moving electrical charges. The fundamental principles of physics hold that moving electrical charges generate magnetic fields. The spin of an electron can assume one of two configurations: up or down. Most electrons are paired with another electron of opposite spin, effectively canceling each other's magnetic field. Some materials, however, contain metal atoms with unpaired electrons, whose spins are not canceled. The magnetic field generated by each electron is very small. However, a piece of metal just the size of a penny contains trillions and trillions of atoms, each potentially containing one or more unpaired electrons. The unpaired electrons can collectively generate a substantial magnetic field if they happen to all spin the same direction.
  
  

Structure

• A magnetic field generated by an electron can potentially influence the spin of other nearby electrons. Physicists refer to this as induction. Spinning electrons generate magnetic fields, but they are also influenced by other magnetic fields. Thus, an electron can induce nearby electrons to "align" their spins and thereby generate a large magnetic field. In practice, the alignment of electron spins occurs in very few materials. For such an alignment to occur, the atoms must be arranged such that their unpaired electrons are close enough to each other that their fields can interact.
  
  

Rare-Earth Magnets

• The rare-earth magnets are one of the classes of compounds in which all of the requirements for strong ferromagnetism are present. Most rare-earth magnets fall into one of two systems: neodymium-iron-boron (Nd-Fe-B) and samarium cobalt (Sm-Co). The name "rare-earth" does not reflect the abundance of these elements; none are classified as precious metals. They are so named because one of their components (either neodymium or samarium) is classified as a "rare-earth metal" on the modern periodic table.
  
  Among the many different measurements used to characterize magnets, one of the most important is the magnetic energy product, which relates the strength of the field generated by a magnet to the magnet's size. Thus, ferromagnets with high magnetic energy products can generate relatively strong fields with relatively little material. Nd2Fe14B exhibits a magnetic energy product of 260 kilojoules per cubic meter (kJ/m^3), whereas that of Sm2Co17 is 215 kJ/m^3. For comparison, common ferrite magnets (such as those that might be found in a refrigerator magnet) are typically on the order of 25 kJ/m^3.
  
  

Applications

• The applications that benefit from rare-earth-magnet technology include magnetic levitation trains (maglevs), magnetic data storage (disc drives), high-performance electric motors, loudspeakers and microphones, magnetic resonance imaging (MRI), air conditioners and refrigerators.