Magnetic Materials

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Magnetic materials

Materials exhibiting ferromagnetism. The magnetic properties of all materials make them respond in some way to a magnetic field, but most materials are diamagnetic or paramagnetic and show almost no response. The materials that are most important to magnetic technology are ferromagnetic and ferrimagnetic materials. Their response to a field H is to create an internal contribution to the magnetic induction B proportional to H, expressed as B = μH, where μ, the permeability, varies with H for ferromagnetic materials. Ferromagnetic materials are the elements iron, cobalt, nickel, and their alloys, some manganese compounds, and some rare earths. Ferrimagnetic materials are spinels of the general composition MFe2O4, and garnets, M3Fe5O12, where M represents a metal. See Ferrimagnetism, Ferromagnetism, Magnetism, Magnetization

Ferromagnetic materials are characterized by a Curie temperature, above which thermal agitation destroys the magnetic coupling giving rise to the alignment of the elementary magnets (electron spins) of adjacent atoms in a crystal lattice. Below the Curie temperature, ferromagnetism appears spontaneously in small volumes called domains. In the absence of a magnetic field, the domain arrangement minimizes the external energy, and the bulk material appears unmagnetized. See Curie temperature

Magnetic materials are further classified as soft or hard according to the ease of magnetization. Soft materials are used in devices in which change in the magnetization during operation is desirable, sometimes rapidly, as in ac generators and transformers. Hard materials are used to supply a fixed field either to act alone, as in a magnetic separator, or to interact with others, as in loudspeakers and instruments. See Electrical measurements, Inductor

Magnetic Materials


substances that substantially alter the value of the magnetic field in which they are placed.

Magnetite, a naturally magnetized mineral, has been known since antiquity. The needle of a magnetic compass was made from it more than 2,000 years ago in China. But magnetite is a weak magnet; the magnetic properties of iron are much stronger.

Iron was first used as a magnetic material in the 19th century, after the discovery of the laws of electromagnetism by H. C. Oersted, M. Faraday, and H. F. E. Lenz and after the invention of direct-current machinery by B. S. lakobi, of a transformer and an alternating-current generator by P. N. lablochkin, and of a three-phase current by M. O. Dolivo-Dobrovol’skii. Silicon-iron steel was used in electrical engineering after 1900. Nickel-iron alloys, which are easily magnetized in weak magnetic fields and widely used in communications engineering, were introduced not long thereafter.

The evolution of the theory of ferromagnetism significantly accelerated the development of new magnetic materials. Magnetic oxides, or ferrites, appeared in the mid-20th century. Although the ferrites are poor conductors of electric current, they have been used in high-frequency and ultrahigh-frequency engineering.

Many different magnetic materials are used in technology. Based on ease of magnetization and magnetic reversal, magnetic materials are classified as hard or soft. Although the vast majority of the materials falls into these categories, other groups have also been distinguished, including thermomagnetic alloys, magnetostrictive materials, and magnetodielectrics.

The quality of magnetic materials is continually being improved by using increasingly pure raw materials (charge materials) and by perfecting production processes (for example, heat-treating the materials in a protective medium and vacuum melting). Improvements in the crystalline and magnetic texture of magnetic materials make it possible to reduce their energy losses in magnetic reversal, a fact that is particularly important in producing transformer steels. It is possible to produce special types of magnetization curves and hysteresis loops by treating magnetic materials with magnetic fields, radioactive radiation, or heat. Rare-earth elements are potentially of great use for the development of magnetic materials (for example, magnetically soft materials with high saturation induction and narrow magnetic resonance). Materials are being developed that combine magnetic properties with many other properties (electric, optical, thermal).


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