Source Citation: "Magnetism." DISCovering Science. Gale
Research, 1996. Reproduced in Discovering Collection. Farmington Hills, Mich.:
Gale Group. December, 2000.
Source Database: DISCovering Science
Table of Contents
Further Readings | Key Terms |
Magnetism is a force generated in matter by the motion of electrons within its atoms. Magnetism and electricity represent different aspects of the force of electromagnetism, which is one part of Nature's fundamental electroweak force. The region in space that is penetrated by the imaginary lines of magnetic force describes a magnetic field. The strength of the magnetic field is determined by the number of lines of force per unit area of space. Magnetic fields are created on a large scale either by the passage of an electric current through magnetic metals or by magnetized materials called magnets. The elemental metals-iron, cobalt, nickel, and their solid solutions or alloys with related metallic elements-are typical materials that respond strongly to magnetic fields. Unlike the all-pervasive fundamental force field of gravity, the magnetic force field within a magnetized body, such as a bar magnet, is polarized-that is, the field is strongest and of opposite signs at the two extremities or poles of the magnet.
The history of magnetism dates back to earlier than 600 b.c., but it is only in the twentieth century that scientists have begun to understand it, and develop technologies based on this understanding. Magnetism was most probably first observed in a form of the mineral magnetite called lodestone, which consists of iron oxide-a chemical compound of iron and oxygen. The ancient Greeks were the first known to have used this mineral, which they called a magnet because of its ability to attract other pieces of the same material and iron.
The Englishman William Gilbert (1540-1603) was the first to investigate the phenomenon of magnetism systematically using scientific methods. He also discovered that the Earth is itself a weak magnet. Early theoretical investigations into the nature of the Earth's magnetism were carried out by the German Carl Friedrich Gauss (1777-1855). Quantitative studies of magnetic phenomena initiated in the eighteenth century by Frenchman Charles Coulomb (1736-1806), who established the inverse square law of force, which states that the attractive force between two magnetized objects is directly proportional to the product of their individual fields and inversely proportional to the square of the distance between them. Danish physicist Hans Christian Oersted(1777-1851) first suggested a link between electricity and magnetism. Experiments involving the effects of magnetic and electric fields on one another were then conducted by Frenchman Andre Marie Ampere (1775-1836) and Englishman Michael Faraday (1791-1869), but it was the Scotsman, James Clerk Maxwell (1831-1879), who provided the theoretical foundation to the physics of electromagnetism in the nineteenth century by showing that electricity and magnetism represent different aspects of the same fundamental force field. Then, in the late 1960s American Steven Weinberg (1933- ) and Pakistani Abdus Salam (1926- ), performed yet another act of theoretical synthesis of the fundamental forces by showing that electromagnetism is one part of the electroweak force. The modern understanding of magnetic phenomena in condensed matter originates from the work of two Frenchmen: Pierre Curie (1859-1906), the husband and scientific collaborator of Madame Marie Curie (1867-1934), and Pierre Weiss (1865-1940). Curie examined the effect of temperature on magnetic materials and observed that magnetism disappeared suddenly above a certain critical temperature in materials like iron. Weiss proposed a theory of magnetism based on an internal molecular field proportional to the average magnetization that spontaneously align the electronic micromagnets in magnetic matter. The present day understanding of magnetism based on the theory of the motion and interactions of electrons in atoms (called quantum electrodynamics) stems from the work and theoretical models of two Germans, Ernest Ising (1900- ) and Werner Heisenberg (1901-1976). Werner Heisenberg was also one of the founding fathers of modern quantum mechanics.
Magnetism arises from two types of motions of electrons in atoms-one is the motion of the electrons in an orbit around the nucleus, similar to the motion of the planets in our solar system around the sun, and the other is the spin of the electrons around its axis, analogous to the rotation of the Earth about its own axis. The orbital and the spin motion independently impart a magnetic moment on each electron causing each of them to behave as a tiny magnet. The magnetic moment of a magnet is defined by the rotational force experienced by it in a magnetic field of unit strength acting perpendicular to its magnetic axis. In a large fraction of the elements, the magnetic moment of the electrons cancel out because of the Pauli exclusion principle , which states that each electronic orbit can be occupied by only two electrons of opposite spin. However, a number of so-called transition metal atoms, such as iron, cobalt, and nickel, have magnetic moments that are not cancelled; these elements are, therefore, common examples of magnetic materials. In these transition metal elements the magnetic moment arises only from the spin of the electrons. In the rare earth elements (that begin with lanthanum in the sixth row of the Periodic Table of Elements), however, the effect of the orbital motion of the electrons is not cancelled, and hence both spin and orbital motion contribute to the magnetic moment. Examples of some magnetic rare earth elements are: cerium, neodymium, samarium, and europium. In addition to metals and alloys of transition and rare earth elements, magnetic moments are also observed in a wide variety of chemical compounds involving these elements. Among the common magnetic compounds are the metal oxides, which are chemically bonded compositions of metals with oxygen.
The Earth's geomagnetic field is the result of electric currents produced by the slow convective motion of its liquid core in accordance with a basic law of electromagnetism which states that a magnetic field is generated by the passage of an electric current. According to this model, the Earth's core should be electrically conductive enough to allow generation and transport of an electric current. The geomagnetic field generated will be dipolar in character, similar to the magnetic field in a conventional magnet, with lines of magnetic force lying in approximate planes passing through the geomagnetic axis. The principle of the compass needle used by the ancient mariners involves the alignment of a magnetized needle along the Earth's magnetic axis with the imaginary south pole of the needle pointing towards the magnetic north pole of the Earth. The magnetic north pole of the Earth is inclined at an angle of 11 degrees away from its geographical north pole.
Five basic types of magnetism have been observed and classified on the basis of the magnetic behavior of materials in response to magnetic fields at different temperatures. These types of magnetism are: ferromagnetism, ferrimagnetism, antiferromagnetism, paramagnetism, and diamagnetism.
Ferromagnetism and ferrimagnetism occur when the magnetic moments in a magnetic material line up spontaneously at a temperature below the so-called Curie temperature, to produce net magnetization. The magnetic moments are aligned at random at temperatures above the Curie point, but become ordered, typically in a vertical or, in special cases, in a spiral (helical) array, below this temperature. In a ferromagnet magnetic moments of equal magnitude arrange themselves in parallel to each other. In a ferrimagnet, on the other hand, the moments are unequal in magnitude and order in an antiparallel arrangement. When the moments are equal in magnitude and ordering occurs at a temperature called the Neel temperature in an antiparallel array to give no net magnetization, the phenomenon is referred to as antiferromagnetism. These transitions from disorder to order represent classic examples of phase transitions. Another example of a phase transition is the freezing of the disordered molecules of water at a critical temperature of 32°F (0°C) to form the ordered structure of ice. The magnetic moments-referred to as spins-are localized on the tiny electronic magnets within the atoms of the solid. Mathematically, the electronic spins are equal to the angular momentum (the rotational velocity times the moment of inertia) of the rotating electrons. The spins in a ferromagnetic or a ferrimagnetic single crystal undergo spontaneous alignment to form a macroscopic (large scale) magnetized object. Most magnetic solids, however, are not single crystals, but consist of single crystal domains separated by domain walls. The spins align within a domain below the Curie temperature, independently of any external magnetic field, but the domains have to be aligned in a magnetic field in order to produce a macroscopic magnetized object. This process is effected by the rotation of the direction of the spins in the domain wall under the influence of the magnetic field, resulting in a displacement of the wall and the eventual creation of a single large domain with the same spin orientation.
Paramagnetism is a weak form of magnetism observed in substances which display a positive response to an applied magnetic field. This response is described by its magnetic susceptibility per unit volume, which is a dimensionless quantity defined by the ratio of the magnetic moment to the magnetic field intensity. Paramagnetism is observed, for example, in atoms and molecules with an odd number of electrons, since here the net magnetic moment cannot be zero. Diamagnetism is associated with materials that have a negative magnetic susceptibility. It occurs in nonmagnetic substances like graphite, copper, silver and gold, and in the superconducting state of certain elemental and compound metals. The negative magnetic susceptibility in these materials is the result of a current induced in the electron orbits of the atoms by the applied magnetic field. The electron current then induces a magnetic moment of opposite sign to that of the applied field. The net result of these interactions is that the material is shielded from penetration by the applied magnetic field.
The magnetic field or flux density is measured in metric units of a gauss (G) and the corresponding international system unit of a tesla (T). The magnetic field strength is measured in metric units of oersteds (Oe) and international units of amperes per meter (A/m). Instruments called gaussmeters and magnetometers are used to measure the magnitude of magnetic fields.
One form of the gaussmeter that is used commonly in the laboratory consists of a current carrying semiconducting element called the Hall probe, which is placed perpendicular to the magnetic field being measured. As a consequence of the so-called Hall effect, a voltage perpendicular to the field and to the current is generated in the probe. This induced voltage is proportional to the magnetic field being measured and can be simply measured using a voltmeter.
Magnetometers are extremely sensitive magnetic field detectors. In one commonly used form the magnetic force is detected by means of a sensitive electronic balance. In this instrument the magnetic substance is placed on one arm of a balance, which in turn is placed in a magnetic field. The magnetic force on the sample is then determined by the weight required to balance the force generated by the magnetic field. The most sensitive magnetometer in a modern physics laboratory utilizes a magnetic sensing element called the SQUID (which stands for Superconducting QUantum Interference Device). A SQUID consists of an extremely thin electrically resistive junction (called a Josephson junction) between two superconductors . Superconductors are materials which undergo a transition at low temperatures to a state of zero electrical resistance and nearly complete exclusion of magnetic fields. In its direct current mode of operation, a SQUID is first cooled down to its superconducting state, and then a current is passed through it while the voltage across the junction is monitored. When the junction senses a magnetic field, the flow of current is altered due to an interference phenomenon at the quantum level between two electron wave fronts through the junction, resulting in a change in voltage. Interference is a phenomenon that occurs generally due to the mixing of two wave fronts; the waves add up in some regions and cancel out in others depending on the location of the crest and trough of each wave in space. For example, the interference between the sound waves from two simultaneously played musical instruments tuned at somewhat different frequencies results in the occurrence of beats or modulations in the sound intensity.
A variation of the SQUID magnetometer is the SQUID gradiometer which measures differences in magnetic fields at different positions. Using this type of instrument magnetic field variations in the femtotesla (10-15 tesla) range can be detected. Devices of this type have been used to map the tiny magnetic signals from the human brain.
Electromagnets are utilized as key components of transformers in power supplies that convert electrical energy from a wall outlet into direct current energy for a wide range of electronic devices, and in motors and generators. High field superconducting magnets (where superconducting coils generate the magnetic field) provide the magnetic field in MRI (magnetic resonance imaging) devices that are now used extensively in hospitals and medical centers.
Magnetic materials that are difficult to demagnetize are used to construct permanent magnets. Permanent magnet applications are in loudspeakers, earphones, electric meters, and small motors. A loudspeaker consists of a wire carrying an alternating current . When the wire is in the magnetic field of the permanent magnet it experiences a force that generates a sound wave by alternate compression and rarefaction of the surrounding air when the alternating frequency of the current is in the audible range.
The more esoteric applications of magnetism are in the area of magnetic recording and storage devices in computers, and in audio and video systems. Magnetic storage devices work on the principle of two stable magnetic states represented by the 0 and 1 in the binary number system. Floppy disks have dozens of tracks on which data can be digitally written in or stored by means of a write-head and then accessed or read by means of a read-head. A write-head provides a strong local magnetic field to the region through which the storage track of the disk is passed. The read-head senses stray magnetic flux from the storage track of the disk as it passes over the head. Another example of digital magnetic storage and reading is the magnetic strip on the back of plastic debit and credit cards. The magnetic strip contains identification data which can be accessed through, for example, an automatic teller machine.
Ideally pure magnetic systems have provided the most extensively investigated models of the large scale collective behavior of atoms and electrons that occur in the vicinity of the critical point of phase transitions. More recent studies have unearthed fascinating effects caused by the intentional introduction of impurities and defects into random locations in the atomic lattice of a magnetic material. For example, these random magnetic systems display transitions to states of order that have no counterparts in pure systems, because pure systems are, by necessity, always close to thermodynamic equilibrium or stability. For these reasons there is now intense interest and research activity in disordered systems, and random magnets provide ideal model systems for such investigations.
An area of intense current activity centers around the search for a likely magnetic pairing force in the high temperature ceramic superconductors that were discovered in 1987 by the German-Swiss team of Georg Bednorz and Karl Alexander Muller. A superconductor achieves a zero resistance state by means of a force field that pairs up the conducting electrons within its atoms. The new ceramic materials are antiferromagnets in their undoped state, but on doping start to superconduct at temperatures that are over 182°F (83°C) warmer than conventional pure metal and alloy superconductors.
The effects of extremely high magnetic fields on the properties of condensed matter continues to be an area of high interest. New research areas, such as the search and study of magnetism in organic matter, and the study of diamagnetism and novel magnetic effects in the recently synthesized nanometer-sized (a nanometer is equal to 10-9 meter) carbon tubes, are of increasing interest to physicists and material scientists.
Source Citation: "Magnetism." DISCovering Science. Gale Research, 1996. Reproduced in Discovering Collection. Farmington Hills, Mich.: Gale Group. December, 2000. http://galenet.galegroup.com/servlet/DC/
Document Number: CD2103201551