MAGNETISM Magnetism, in physics, is a general term that refers to the effects originating from the electromagnetic interactions of particles. Magnetic phenomena have been recognized since ancient times, but understanding of magnetism began only with developments in physics in the 19th century. Basic Concepts The magnetic field is the central concept used in describing magnetic phenomena. If a moving, charged particle experiences a force always at right angles to its direction of motion (with the exception that the forces is zero when moving along one particular line), then it is moving in a magnetic field. The field lies along the particular line. This force is called the Lorentz force, after the Dutch physicist Hendrik LORENTZ. A magnetic field originates from moving charges or electric currents, according to the law discovered by Andre AMPERE (see ELECTRICITY). The Biot-Savart law, named for French physicist J.B. BIOT and Felix Savart, permits the calculation of the magnetic field from any arbitrarily shaped current paths. The laws of electricity and magnetism are summarized in elegant but simple fashion by Maxwell's equations, named for the 19th-century physicist James Clerk MAXWELL. Many important magnetic effects originate from the flow of charge in a circular loop. This flow gives rise to a characteristic distribution of magnetic lines of force, called a dipole. A dipole can be thought of as looking and behaving like a small bar MAGNET. One end, or pole, is called north, the other end south. Lines of magnetic force flow around the dipole from north to south, just as in a bar magnet. The so-called magnetic moment is the measure of the strength of the dipole. Thus far, all experimental evidence indicates that magnetic poles come in pairs that can never be separated, even though quantum theory predicts the existence of a unit magnetic charge called a MONOPOLE. At the atomic level, a magnetic dipole moment arises from the orbital motions of the electrons around the nucleus and from the intrinsic spin of the electron. The magnetic moments of atoms are expressed as multiples of BOHR MAGNETONS. A Bohr magneton has a value of 9.27 times 10 to the minus 24 joules/tesla. The effects that are more specifically associated with the terms magnetism and magnets and that will be treated in the following sections are those displayed by material objects when subjected to magnetic fields. The attraction between the unlike poles of two iron bar magnets is a consequence of the interaction of the magnetic moments of the atoms in each magnet with the field produced by atoms in the other magnet. The bar magnet, or horseshoe magnet, has the property of permanent magnetism and is an example of ferromagnetism. Other types of magnetism exist, called ferrimagentism, antiferromagnetism, paramagnetism, and diamagnetism. For ferromagnetism and ferrimagnetism, three important physical quantities can be defined: the magnetic induction, or B-field, which is the magnetic flux density within the magnetized material; the magnetization, which is the sum of moments over all the atoms in a unit volume; and the permeability, which is the ratio of the B-field to the applied field. For paramagnetism, antiferromagnetism, and diamagnetism, the important quantities are the magnetization and the magnetic susceptibility; the susceptibility, represented by the Greek lower case letter chi, is the ratio of magnetization to applied field. History The lodestone was the first permanent magnetic material to be identified and studied. The Greeks were aware of the power of this mineral, now called MAGNETITE, to attract pieces of iron. The name magnet is thought to be derived from Magnesia, a district in Thessaly where the lodestone was mined in ancient times. Credit for inventing the magnetic compass is variously given to the Chinese, the Arabs, and the Italians during the first ten centuries AD (see COMPASS, NAVIGATIONAL). By the 12th century mariners were using the instrument for navigation. In the 13th century Peter Perigrinus of France determined that the two poles of a spherical lodestone were the regions of strongest force, and he found that like poles repel whereas unlike poles attract. In 1600 the English scientist William Gilbert confirmed these discoveries and concluded, correctly, that the Earth itself is a magnet (see EARTH, GEOMAGNETIC FIELD OF). Another English scientist, John Michell (1724-93), discovered in 1750 that the attractive and repulsive forces between the poles of a magnet vary inversely as the square of the distance of separation (see INVERSE SQUARE LAW). Charles COULOMB of France verified Michell's experiments in 1785 and also showed that the same law is applicable to the forces between electrical charges. Coulomb concluded that regardless of how small the pieces into which a magnet might be subdivided, each piece would always retain a north and south pole. In 1820, Hans Christian OERSTED of Denmark discovered a direct relationship between electricity and magnetism by showing that an electric current flowing in a wire causes a nearby compass needle to be deflected. Two French physicists, Andre Ampere and Dominique ARAGO, demonstrated later that same year that an electric current flowing in a solenoid (a coil of wire carrying direct current) increases the permanent magnetism of an iron needle within the solenoid. In 1825, Ampere generalized that a current-carrying loop is equivalent to a magnet and he proposed that the origin of permanent magnetism resides in the many molecular-sized current whorls within the magnet. During the 1830s the English scientist Michael FARADAY introduced the idea of representing the magnetic field by lines of flux that extend through the space surrounding a magnet, running from the north pole to south. This pictorial model was to prove extremely useful in understanding ELECTROMAGNETIC INDUCTION--the generation of an ELECTROMOTIVE FORCE in a closed circuit when lines of flux passing through the circuit change. In the 1870s, James Clerk Maxwell of Scotland unified all electromagnetic phenomena under a single theory. In addition to the ferromagnetism of permanent magnets, other types of magnetism became known after the middle of the 19th century. In 1845 Michael Faraday found that bismuth and glass are repelled from magnetic fields. He classified this behavior as diamagnetism. Faraday also discovered that some substances clearly not permanent magnets are nevertheless attracted by magnetic fields, a behavior he called paramagnetism. These opposite characteristics stem from fundamental differences at the atomic level and require separate theoretical explanations. In the 1930s two other forms of magnetism were recognized and described: ferrimagnetism and antiferromagnetism. Theoretical explanations of the behavior of magnetic materials began with the formulation of the quantitative theories of paramagnetism and diamagnetism by Paul Langevin (1872-1946) and of ferromagnetism by another French scientist, Pierre-Ernest Weiss (1865-1940), in 1907. The subsequent application of quantum theory after 1925 has provided a more exact understanding of these phenomena of magnetic behavior. Diamagnetism A substance is diamagnetic if its magnetic susceptibility is negative. This property is displayed by a repulsion of the sample from a magnetic field. The theory of diamagnetism explains it as a consequence of an induced magnetization set up when lines of magnetic flux penetrate the electron loops around atoms. The direction of this induced magnetization is opposite to that of the external field, in accordance with LENZ's LAW. This makes the susceptibility negative. The magnetization persists only as long as the external field is present. Diamagnetism is very weak, compared to ferromagnetism, and it is virtually independent of temperature. Some metals are diamagnetic such as bismuth, copper, gold, silver and lead, with bismuth being the strongest. Water and most organic compounds are also diamagnetic, as are many nonmetals. All substances possess an inherent diamagnetism, so susceptibility measurements on other magnetic materials must always be corrected for the diamagnetic contributions of their constituent atoms. If a magnetic field is applied to a superconductor while it is being cooled through its superconducting transition temperature, the magnetic field is expelled from the sample and it behaves like a perfect diamagnet. This is called the Meissner effect for its co-discoverer, German physicist Walther Meissner. Paramagnetism A paramagnetic substance is characterized by a positive susceptibility. Like a diamagnet, it can acquire a magnetization only from induction by an external magnetic field. The magnetization, however, is in the same direction as the inducing field, and a sample will be attracted toward the strongest part of a field. In 1895, Pierre CURIE first determined experimentally that the paramagnetic susceptibility obeys the law Greek lower case letter chi = C/T, where C is a constant and T is the absolute temperature. Langevin's theory explained the phenomenon by assuming that the paramagnet consists of a large number of noninteracting atomic dipoles in thermal equilibrium. In zero field the magnetization of the system is zero, because the dipoles are randomly oriented. A magnetic field forces the dipoles to partially align along the field direction, like little magnetic compasses, against the randomizing effects of the thermal agitation of the atoms. The amount of alignment and hence the magnetization depend directly on the strength of the field and inversely on the absolute temperature. This accounts for the Curie-law behavior. A much closer description of the real behavior of paramagnetic materials is given by the Weiss-Curie law, as follows: Greek lower case letter chi = C/T plus or minus Greek letter theta. Quantum theory explains the individual atomic dipole moments as resulting from the adding together of the electron spin and orbital moments in unfilled interior electron shells. The unfilled 3d shells are the source of paramagnetism in the iron group, the so-called transition metals. Similar effects occur in the rare earth, palladium, platinum, and actinide groups in the periodic table. Almost all compounds and alloys containing elements from the above groups exhibit paramagnetic behavior at high temperatures but undergo phase transitions to the magnetically ordered states of ferromagnetism and antiferromagnetism at sufficiently low temperatures. Ferromagnetism Ferromagnetism is characterized by a spontaneous magnetism that exists in the absence of a magnetic field. The retention of magnetism distinguishes ferromagnetism from the induced magnetisms of diamagnetism and paramagnetism. When ferromagnets are heated above a critical temperature, the ability to possess permanent magnetism disappears. Curie first identified this effect in iron as occurring at 770 deg C, and his name is now attached to this characteristic temperature. Other ferromagnetic elements and their Curie temperatures include nickel (358 deg. C), cobalt (1,130 deg C), gadolinium (16 deg C), and dysprosium (- 188 deg C). Theories to explain ferromagnetism started with the molecular-field approach formulated by Weiss in 1907. His theory assumes that each atomic dipole is subject to a local field proportional to the net magnetization produced by all the other dipoles. Above a critical temperature identified with the Curie temperature, the susceptibility is given by Greek lower case letter chi = C/T minus Greek letter theta, where theta is a constant, as in a paramagnet, since the atomic moments are free to orient in any direction. Below the Curie temperature the local "molecular" field is sufficiently strong to cause a spontaneous parallel alignment of the moments, which overcomes the randomizing effects of the thermal motion. This represents a phase transition to the ordered ferromagnetic state. The theory predicts that with further decrease in temperature below the Curie temperature, an increase in magnetization will occur as the moments approach complete alignment at absolute zero. The application of quantum mechanics has led to considerable refinement of the molecular field theory, but the theoretical problems in calculating exactly the properties of ferromagnets from quantum principles are formidable. Ferromagnetism has been found in many compounds, including several europium compounds that are insulators, such as the oxide (- 196 deg C) and the hydride (- 244 deg C). Ferromagnetic glasses can also be prepared. One way is to cool, rapidly, a molten mixture of carbon (a ferromagnetic element) and a glass-former such as silicon or phosphorus. Evidence exists that the organic transfer salt composed of decamethyl ferrocene cations and tetracyanoethylene (TCNE) anions is ferromagnetic below - 286 deg C, and it is possible that plastic ferromagnetic materials will eventually become available. A distinctinve characteristic of a real ferromagnet is its HYSTERESIS CURVE. That is, the ferromagnet in its virgin state has zero magnetization. It will acquire a magnetization represented by the magnetic induction B only when an external field H is applied. The leveling off of the B-field as H continues to increase is called saturation. When the H-field is reduced to zero, however, the ferromagnet remains magnetized. The amount of induction remaining is called the remanence or retentivity. In order to eliminate the remanent induction completely, a field must be applied in the opposite direction; this is called the coercive field. The shape of the hysteresis loops can be explained by the domain theory first proposed by Weiss. A real ferromagnet, such as iron, contains many small regions called domains. Initially the magnetizations point in random directions and cancel each other out in these domains. The application of an external field first increases the size of those domains with components along the field direction through movement of domain walls, and then causes the domains to rotate toward the field direction. When the H-field is reduced to zero, the domains retain much of their high-field configuration and orientation, so the specimen remains magnetized. The response of domains to external fields is strongly influenced by the tendency of the magnetization to lie along particular crystalline axes. Ferromagnetic materials are generally put in two categories: those with high coercivities and retentivities, called hard; and those with low coercivities and large initial permeabilities, called soft. Hard magnets are used in those devices where a strong magnetic field is required for an indefinite period of time. This is what is popularly called a permanent magnet. Many commercial permanent magnets are of the Alnico group (composed of iron, nickel, cobalt, and aluminum). In recent years cobalt-samarium and neodymium-iron-boron magnets have also proved successful in permanent-magnet applications. Certain ferrites are also hard. The use of fine powders that are sintered at high temperatures has been found to enhance coercivity. Magnetically soft materials find application in such devices as lifting electromagnets and the power transformers that convert standard 60-cycle commercial electric power from one voltage to another. The cores of such transformers are usually made of iron-silicon alloys. The low coercivity keeps the hysteresis loop narrow and thus minimizes power loses. For high-frequency applications, where eddy current losses in metal cores are significant, ferrite cores are more effective because of their very much larger electrical resistance. An extensive technology has evolved to synthesize ferromagnetic and ferrimagnetic materials with specific properties to meet a wide variety of needs. Ferrimagnetism Ferrites are similar to ferromagnets in that they undergo transitions to ordered arrangements of magnetic moments below Curie temperatures. They form magnetic domains, have hysteresis curves, and possess sizable permanent magnetism. Above their Curie temperatures, they behave as paramagnets. The important difference from ferromagnetism is that not all the moments are aligned in the same direction in the ordered state. There are generally two sets of sites in the crystal lattice on which the moments are located. Even though some moments are aligned oppositely, the sum of all the moments still results in a net moment; hence the specimen possesses a net magnetization. A simple example is magnetite. Most ferrites are of two crystalline types: spinels and garnets. The excellent electrical insulating properties of ferrites have led to their use in components that operate in high-frequency devices with low electrical losses, such as transformers and phase shifters. Antiferromagnetism Antiferromagnetism is characterized by an antiparallel pattern of magnetic moments below a critical temperature called the Neel temperature, named for French physicist Louis Neel. Above the Neel point, the susceptibility follows a Weiss-Curie law, Greek lower case letter chi = C/T + Greek letter theta, as in paramagnetism. Below the Neel temperature, the susceptibility depends on the particular pattern of ordering and whether the specimen is single or polycrystalline. In general the susceptibility decreases with temperature after passing through a maximum at the Neel point. The theory of antiferromagnetism was developed in the 1930s by Neel and U.S. physicist Francis Bitter (1902-67). It followed the approach of the Weiss molecular-field theory. In the simplest case the magnetic atoms can be considered as lying on two interpenetrating sublattices. The molecular field acting on an atom of one sublattice causes its moment to point opposite to the moments on the other sublattice. Below the Neel temperature the two sublattices have oppositely directed magnetizations, and the net magnetization is zero. The application of a magnetic field causes a partial alignment of the moments; the alignment depends on the direction of field relative to crystalline axes and the crystalline anisotropy energy. Antiferromagnetism was first verified in 1939 in manganous oxide. Anitferromagnetic ordering patterns can be determined by neutron diffraction experiments, because the magnetic moments of the neutron interacts with the staggered array of moments in the antiferromagnet. A large number of compounds--many of the insulators--containing iron-group, rare earth, and actinide metals are antiferromagnets. Ferromagnetism, ferrimagnetism, and antiferromagnetism are all representative of more general phenomena in solids, called phase transitions. A phase transition occurs when cooling through a critical temperature is accompanied by a significant change in a physical property. Other examples of phase transitions are ferroelectricity, in which a spontaneous electrical polarization develops, and SUPERCONDUCTIVITY. Bibliography: Berkowitz, A.E., and Kneller, E., Magnetism and Metallurgy, 2 vols. (1969); Bozorth, R.M., Ferromagnetism (1951); Chen, Chi-Wen, Magnetism and Metallurgy of Soft Magnetic Materials (1986); Lee, E. W., Magnetism (1984); Mattis, D.C., The Theory of Magnetism II (1985); Morrish, A.H., The Physical Properties of Magnetism (1965); Sears, F.W., Zemansky, M.W., and Young, H.D., University Physics, 7th ed. (1986).