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

History The familiar property of a magnet – its power of attracting iron – has been known since the time of Thales. The name magnet is derived from the town Magnesia (now Manisa) in Asia Minor, in whose neighbourhood a naturally magnetic mineral was found. This mineral, known as magnetite or lodestone, contains considerable quantities of the oxides of iron. It was used as a crude compass before artificial magnets were employed for that purpose. The well-known properties of a magnet, and the fact that the Earth is a magnet, were explained by the Elizabethan scientist William Gilbert, ‘the father of magnetic philosophy’. In 1820 Hans Oersted discovered the phenomenon of electromagnetism. The character of a magnetic field – the region of magnetic attractions and repulsions around a magnet – was investigated by Michael Faraday. Instead of magnetic poles attracting or repelling each other without the intervention of a medium, Faraday imagined the medium traversed by lines of magnetic force that gave the direction of the magnetic force at any point.

Magnetic properties All substances are magnetic to a greater or lesser degree, and their magnetic properties, however feeble, may be observed when they are placed in an intense magnetic field. Most substances are paramagnetic, that is they become magnetized with their magnetic axes (the lines joining the south pole to the north pole) parallel to the magnetizing force. A few, notably bismuth, are diamagnetic; these substances become magnetized with their axes making an angle of 180° with the magnetizing force. The ferromagnetic substances, iron, nickel, cobalt, and some of their alloys, are all paramagnetic, but the extent to which they are magnetized depends not only on the magnetizing force, but also on their previous magnetic history. Furthermore, if the magnetizing force is increased, a stage is reached when the magnet becomes saturated, that is its pole strength reaches a maximum value.

Molecular magnets When a magnet is broken in two, we do not obtain two halves, one with a north pole, the other with a south pole. Two new poles appear at the point of fracture. However often this process is repeated, the same result is obtained; every magnet has two poles. Wilhelm Weber suggested that every magnet was really composed of magnetic particles or magnetized domains that are now believed to be of molecular dimensions. J Alfred Ewing developed his theory and suggested that, since the act of magnetization did not change the chemical character nor the weight of the specimen, but simply endowed it with magnetic properties, magnetizable substances consisted of molecular magnets. According to this theory, an ordinary piece of iron is made up of molecular magnets arranged in haphazard fashion, so that they neutralize each other's effects on external bodies. This disorder disappears when the iron is placed in a magnetic field and the molecular magnets are set with their axes parallel to the field. Free poles appear at the ends of the magnet, while the central portions exhibit only feeble magnetic powers because equal and opposite poles neutralize each other's effects. This theory accounts for the appearance of new poles wherever the magnet is broken, and the state of saturation is reached when all the molecular magnets have been arranged in order. Subsequent loss of magnetism is explained by the partial return to disordered array.

The magneton theory Early in the 20th century, Pierre Weiss suggested the existence of the magneton or elementary magnet, an analogue of the electron, the elementary charge of electricity. This idea was developed by physicists, notably Albert Einstein, de Haas, and Niels Bohr. An electric current flowing around a circular coil has a magnetic field similar to that of a magnet whose axis coincides with that of the coil. The electrical theory of matter attempts to ascribe the magnetic properties of bodies to the orbital motions of the electrons in the atom. The quantum theory of the atom developed by Bohr supported the magneton theory, and subsequently direct experimental evidence of the existence of the magnetic moment associated with electron orbits was obtained by Otto Stern and Walther Gerlach in 1921.