magnetism

Phenomena associated with magnetic fields. Magnetic fields are produced by moving charged particles. In electromagnets, electrons flow through a coil of wire connected to a battery; in permanent magnets, spinning electrons within the atoms generate the field.

Substances differ in the extent to which they can be magnetized by an external field (susceptibility). Materials that can be strongly magnetized, such as iron, cobalt, and nickel, are said to be ferromagnetic. This is due to the formation of areas called domains in which atoms, weakly magnetic because of their spinning electrons, align to form areas of strong magnetism. Magnetic materials lose their magnetism if heated to the Curie temperature. Most other materials are paramagnetic, being only weakly pulled towards a strong magnet. This is because their atoms have a low level of magnetism and do not form domains. Diamagnetic materials are weakly repelled by a magnet since electrons within their atoms act as electromagnets and oppose the applied magnetic force. Antiferromagnetic materials have a very low susceptibility that increases with temperature; a similar phenomenon in materials such as ferrites is called ferrimagnetism.

Application
Apart from its universal application in dynamos, electric motors, and switch gears, magnetism is of considerable importance in advanced technology – for example, in particle accelerators for nuclear research, memory stores for computers, tape recorders, and cryogenics.