History of Magnets

Dictionary:  magnet

  (n. mag'nit)


   1. An object that is surrounded by a magnetic field and that has the property, either natural or induced, of attracting iron or steel.

   2. An electromagnet.

   3. A person, a place, an object, or a situation that exerts attraction.


[Middle English, from Old French magnete, from Latin magnes, magnet-, from Greek Magnes (lithos), Magnesian (stone), magnet, from Magnesia, Magnesia, an ancient city of Asia Minor.]




How Magnets are Made: How is a magnet made?


Background


A magnet is a material that can exert a noticeable force on other materials without actually contacting them. This force is known as a magnetic force and may either attract or repel. While all known materials exert some sort of magnetic force, it is so small in most materials that it is not readily noticeable. With other materials, the magnetic force is much larger, and these are referred to as magnets. The Earth itself is a huge magnet.


Some magnets, known as permanent magnets, exert a force on objects without any outside influence. The iron ore magnetite, also known as lodestone, is a natural permanent magnet. Other permanent magnets can be made by subjecting certain materials to a magnetic force. When the force is removed, these materials retain their own magnetic properties. Although the magnetic properties may change over time or at elevated temperatures, these materials are generally considered to be permanently magnetized, hence the name.


Other magnets are known as electromagnets. They are made by surrounding certain materials with a coil of wire. When an electric current is passed through the coil, these materials exert a magnetic force. When the current is shut off, the magnetic force of these materials drops to nearly zero. Electromagnet materials retain little, if any, magnetic properties without a flow of electric current in the coil.


All magnets have two points where the magnetic force is greatest. These two points are known as the poles. For a rectangular or cylindrical bar magnet, these poles would be at opposite ends. One pole is called the north-seeking pole, or north pole, and the other pole is called the south-seeking, or south pole. This terminology reflects one of the earliest uses of magnetic materials such as lodestone. When suspended from a string, the north pole of these first crude compasses would always "seek" or point towards the north. This aided sailors in judging the direction to steer to reach distant lands and return home.


In our present technology, magnet applications include compasses, electric motors, microwave ovens, coin-operated vending machines, light meters for photography, automobile horns, televisions, loudspeakers, and tape recorders. A simple refrigerator note holder and a complex medical magnetic resonance imaging device both utilize magnets.


History


Naturally occurring magnetic lodestone was studied and used by the Greeks as early as 500 B.C. Other civilizations may have known of it earlier than that. The word magnet is derived from the Greek name magnetis lithos, the stone of Magnesia, referring to the region on the Aegean coast in present-day Turkey where these magnetic stones were found.


The first use of a lodestone as a compass is generally believed to have occurred in Europe in about A.D. 1100 to A.D. 1200. The term lodestone comes from the Anglo-Saxon meaning "leading stone," or literally, "the stone that leads." The Icelandic word is leider-stein, and was used in writings of that period in reference to the navigation of ships.


In 1600, English scientist William Gilbert confirmed earlier observations regarding magnetic poles and concluded that the Earth was a magnet. In 1820, the Dutch scientist Hans Christian Oersted discovered the relationship between electricity and magnetism, and French physicist Andre Ampere further expanded upon this discovery in 1821.


In the early 1900s, scientists began studying magnetic materials other than those based on iron and steel. By the 1930s, researchers had produced the first powerful Alnico alloy permanent magnets. Even more powerful ceramic magnets using rare earth elements were successfully formulated in the 1970s with further advances in this area in the 1980s.


Today, magnetic materials can be made to meet many different performance requirements depending on the final application.


Raw Materials


When making magnets, the raw materials are often more important than the manufacturing process. The materials used in permanent magnets (sometimes known as hard materials, reflecting the early use of alloy steels for these magnets) are different than the materials used in electromagnets (some-times known as soft materials, reflecting the use of soft, malleable iron in this application).


Permanent Magnet Materials


Permanent magnet lodestones contain magnetite, a hard, crystalline iron ferrite mineral that derives its magnetism from the effect the earth's magnetic field has on it. Various steel alloys can also be magnetized. The first big step in developing more effective permanent magnet materials came in the 1930s with the development of Alnico alloy magnets. These magnets take their name from the chemical symbols for the aluminum-nickel-cobalt elements used to make the alloy. Once magnetized, Alnico magnets have between 5 and 17 times the magnetic force of magnetite.


Ceramic permanent magnets are made from finely powdered barium ferrite or strontium ferrite formed under heat and pressure. Their magnetic strength is enhanced by aligning the powder particles with a strong magnetic field during forming. Ceramic magnets are comparable to Alnico magnets in terms of magnetic force and have the advantage of being able to be pressed into various shapes without significant machining.


Flexible permanent magnets are made from powdered barium ferrite or strontium ferrite mixed in a binding material like rubber or a flexible plastic like polyvinyl chloride.


In the 1970s, researchers developed permanent magnets made from powdered samarium cobalt fused under heat. These magnets take advantage of the fact that the arrangement of the groups of atoms, called magnetic domains, in the hexagonal crystals of this material tend to be magnetically aligned. Because of this natural alignment, samarium-cobalt magnets can be made to produce magnetic forces 50 times stronger than magnetite. Headphones for small, personal stereo systems use samarium-cobalt permanent magnets. Samarium-cobalt magnets also have the advantage of being able to operate in higher temperatures than other permanent magnets without losing their magnetic strength.


Similar permanent magnets were made in the 1980s using powdered neodymium iron boron which produces magnetic forces almost 75 times stronger than magnetite. These are the most powerful permanent magnets commercially available today.


Electromagnet Materials


Pure iron and iron alloys are most commonly used in electromagnets. Silicon iron and specially treated iron-cobalt alloys are used in low-frequency power transformers.


A special iron oxide, called a gamma iron oxide, is often used in the manufacture of magnetic tapes for sound and data recording. Other materials for this application include cobalt-modified iron oxides and chromium dioxide. The material is finely ground and coated on a thin polyester plastic film.


Other Magnetic Materials


Magnetic fluids can be made by encapsulating powdered barium ferrite particles in a single layer of molecules of a long-chain polymer plastic. The particles are then held in suspension in a liquid like water or oil. Because of the plastic encapsulation, the magnetic particles slide over each other with almost no friction. The particles are so small that normal thermal agitation in the liquid keeps the particles from settling. Magnetic fluids are used in several applications as sealants, lubricants, or vibration damping materials.


The Manufacturing

Process


Just as the materials are different for different kinds of magnets, the manufacturing processes are also different. Many electromagnets are cast using standard metal casting techniques. Flexible permanent magnets are formed in a plastic extrusion process in which the materials are mixed, heated, and forced through a shaped opening under pressure.


Some magnets are formed using a modified powdered metallurgy process in which finely powdered metal is subjected to pressure, heat, and magnetic forces to form the final magnet. Here is a typical powdered metallurgy process used to produce powerful neodymium-iron-boron permanent magnets with cross-sectional areas of about 3-10 square inches (20-65 sq cm):


Preparing the powdered metal


    * The appropriate amounts of neodymium, iron, and boron are heated to melting in a vacuum. The vacuum prevents any chemical reaction between air and the melting materials that might contaminate the final metal alloy.

    * Once the metal has cooled and solidified, it is broken up and crushed into small pieces. The small pieces are then ground into a fine powder in a ball mill.


Pressing


    * The powdered metal is placed in a mold, called a die, that is the same length and width (or diameter, for round magnets) as the finished magnet. A magnetic force is applied to the powdered material to line up the powder particles. While the magnetic force is being applied, the powder is pressed from the top and bottom with hydraulic or mechanical rams to compress it to within about 0.125 inches (0.32 cm) of its final intended thickness. Typical pressures are about 10,000 psi to 15,000 psi (70 MPa to 100 MPa). Some shapes are made by placing the powdered material in a flexible, air-tight, evacuated container and pressing it into shape with liquid or gas pressure. This is known as isostatic compaction.


Heating


    * The compressed "slug" of powdered metal is removed from the die and placed in an oven. The process of heating compressed powdered metals to transform them into fused, solid metal pieces is called sintering. The process usually consists of three stages. In the first stage, the compressed material is heated at a low temperature to slowly drive off any moisture or other contaminants that may have become entrapped during the pressing process. In the second stage, the temperature is raised to about 70-90% of the melting point of the metal alloy and held there for a period of several hours or several days to allow the small particles to fuse together. Finally, the material is cooled down slowly in controlled, step-by-step temperature increments.


Annealing


    * The sintered material then undergoes a second controlled heating and cooling process known as annealing. This process removes any residual stresses within the material and strengthens it.


Finishing


    * The annealed material is very close to the finished shape and dimensions desired. This condition is known as "nearnet" shape. A final machining process removes any excess material and produces a smooth surface where needed. The material is then given a protective coating to seal the surfaces.


Magnetizing


    * Up to this point, the material is just a piece of compressed and fused metal. Even though it was subjected to a magnetic force during pressing, that force didn't magnetize the material, it simply lined up the loose powder particles. To turn it into a magnet, the piece is placed between the poles of a very powerful electromagnet and oriented in the desired direction of magnetization. The electromagnet is then energized for a period of time. The magnetic force aligns the groups of atoms, or magnetic domains, within the material to make the piece into a strong permanent magnet.


Quality Control


Each step of the manufacturing process is monitored and controlled. The sintering and annealing processes are especially critical to the final mechanical and magnetic properties of the magnet, and the variables of time and temperature must be closely controlled.


Hazardous Materials,

Byproducts, and

Recycling


Barium and the barium compounds used to make barium ferrite permanent magnets are poisonous and are considered toxic materials. Companies making barium ferrite magnets must take special precautions in the storage, handling, and waste disposal of the barium products.


Electromagnets can usually be recycled by salvaging the component iron cores and copper wiring in the coil. Partial recycling of permanent magnets may be achieved by removing them from obsolete equipment and using them again in similar new equipment. This is not always possible, however, and a more comprehensive approach to recycling permanent magnets needs to be developed.


The Future


Researchers continue to search for even more powerful magnets than those available today. One of the applications of more powerful permanent magnets would be the development of small, high-torque electric motors for battery-powered industrial robots and laptop computer disk drives. More powerful electromagnets could be used for the levitation and propulsion of high-speed trains using pulsed magnetic fields. Such trains, sometimes called maglev trains, would be supported and guided by a central, magnetic "rail." They would move without ever contacting the rail, thus eliminating mechanical friction and noise. Pulsed magnetic fields could also be used to launch satellites into space without relying on expensive and heavy booster rockets.


More powerful magnets could also be used as research tools to develop other new materials and processes. Intense, pulsed magnet fields are currently being used in nuclear fusion research to contain the hot, reacting nuclear plasma that would otherwise melt any solid material vessel. Magnetic fields can also be used in materials research to study the behavior of semiconductors used in electronics to determine the effects of making micro-sized integrated circuits.


Where To Learn More


Books


Brady, George S. and Henry R. Clauser. Materials Handbook, 12th Ed. McGraw-Hill, 1986.


Braithwaite, Nicholas and Graham Weaver, eds. Electronic Materials. Butterworths, 1990.


Campbell, Peter. Permanent Magnet Materials and Their Design. Cambridge University Press, 1994.


Verschuur, Gerrit L. Hidden Attraction: The History and Mystery of Magnetism. Oxford University Press, 1993.


Periodicals


Boebinger, Greg, Al Passner, and Joze Bevk. "Building World-Record Magnets." Scientific American, June 1995, pp. 58-66.


Duplessis, John. "An Attractive Proposition." Machine Design, June 11, 1993, p. 46.


[Article by: Chris Cavette]




Sci-Tech Encyclopedia: Magnet


An object or device that produces a magnetic field. Magnets are essential for the generation of electric power and are used in motors, generators, labor-saving electromechanical devices, information storage, recording, and numerous specialized applications, for example, seals of refrigerator doors. The magnetic fields produced by magnets apply a force at a distance on other magnets, charged particles, electric currents, and magnetic materials. See also Generator; Magnetic recording; Motor.


Magnets may be classified as either permanent or excited. Permanent magnets are composed of so-called hard magnetic material, which retains an alignment of the magnetization in the presence of ambient fields. Excited magnets use controllable energizing currents to generate magnetic fields in either electromagnets or air-cored magnets. See also Electromagnet; Ferromagnetism; Superconductivity.


The essential characteristic of permanent-magnet materials is an inherent resistance to change in magnetization over a wide range of field strength. Resistance to change in magnetization in this type of material is due to two factors: (1) the material consists of particles smaller than the size of a domain, a circumstance which prevents the gradual change in magnetization which would otherwise take place through the movement of domain wall boundaries; and (2) the particles exhibit a marked magnetocrystalline anisotropy. During manufacture the particles are aligned in a magnetic field before being sintered or bonded in a soft metal or polyester resin. Compounds of neodymium, iron, and boron are used. See also Iron alloys.


Electromagnets rely on magnetically soft or permeable materials which are well annealed and homogeneous so as to allow easy motion of domain wall boundaries. Ideally the coercive force should be zero, permeability should be high, and the flux density saturation level should be high. Coincidentally the hysteresis energy loss represented by the area of the hysteresis curve is small. This property and high electrical resistance (for the reduction of eddy currents) are required where the magnetic field is to vary rapidly. This is accomplished by laminating the core and using iron alloyed with a few percent silicon that increases the resistivity.


Electromagnets usually have an energizing winding made of copper and a permeable iron core. Applications include relays, motors, generators, magnetic clutches, switches, scanning magnets for electron beams (for example, in television receivers), lifting magnets for handling scrap, and magnetic recording heads. See also Cathode-ray tube; Clutch; Electric switch; Relay.


Special iron-cored electromagnets designed with highly homogeneous fields are used for special analytical applications in, for example, electron or nuclear magnetic resonance, or as bending magnets for particle accelerators. See also Magnetic resonance; Particle accelerator.


Air-cored electromagnets are usually employed above the saturation flux density of iron (about 2 T); at lower fields, iron-cored magnets require much less power because the excitation currents needed then are required only to generate a small field to magnetize the iron. The air-cored magnets are usually in the form of a solenoid with an axial hole allowing access to the high field in the center. The conductor, usually copper or a copper alloy, must be cooled to dissipate the heat generated by resistive losses. In addition, the conductor and supporting structure must be sufficiently strong to support the forces generated in the magnet. See also Solenoid (electricity).


In pulsed magnets, higher fields can be generated by limiting the excitation to short pulses (usually furnished by the energy stored in a capacitor bank) and cooling the magnet between pulses. The highest fields are generally achieved in small volumes. A field of 75 T has been generated for 120 microseconds.


Large-volume or high-field magnets are often fabricated with superconducting wire in order to avoid the large resistive power losses of normal conductors. The two commercially available superconducting wire materials are (1) alloys of niobium-titanium, a ductile material which is used for generating fields up to about 9 T; and (2) a brittle alloy of niobium and tin (Nb3Sn) for fields above 9 T. Practical superconducting wires use complex structures of fine filaments of superconductor that are twisted together and embedded in a copper matrix. The conductors are supported against the electromagnetic forces and cooled by liquid helium at 4.2 K (-452?F). A surrounding thermal insulating enclosure such as a dewar minimizes the heat flow from the surroundings.


Superconducting magnets operating over 20 T have been made with niobium-titanium outer sections and niobium-tin inner sections. Niobium-titanium is used in whole-body nuclear magnetic resonance imaging magnets for medical diagnostics. Other applications of superconducting magnets include their use in nuclear magnetic resonance for chemical analysis, particle accelerators, containment of plasma in fusion reactors, magnetic separation, and magnetic levitation. See also Magnetic levitation; Magnetic separation methods; Medical imaging; Nuclear fusion; Nuclear magnetic resonance (NMR); Superconducting devices.


The highest continuous fields are generated by hybrid magnets. A large-volume (lower-field) superconducting magnet that has no resistive power losses surrounds a water-cooled inner magnet that operates at the highest field. The fields of the two magnets add. Over 35 T has been generated continuously.



Britannica Concise Encyclopedia: magnet


Any material capable of attracting iron and producing a magnetic field outside itself. By the end of the 19th century, all known elements and many compounds had been tested for magnetism, and all were found to have some magnetic property. However, only three elements — iron, nickel, and cobalt — exhibit ferromagnetism. See also compass, electromagnet.


For more information on magnet, visit Britannica.com.



Science Dictionary: magnet


An object that attracts iron and some other materials. Magnets are said to generate a magnetic field around themselves. Every magnet has two poles, called the north and south poles. Magnetic poles exert forces on each other in such a way that like poles repel and unlike poles attract each other. A compass is a small magnet that is affected by the magnetic field of the Earth in such a way that it points to a magnetic pole of the Earth. (See magnetic field and magnetism.)



Veterinary Dictionary: magnet


An object having polarity and capable of attracting iron.


    * oral dose m. — see reticular magnet.

    * reticular m. — a magnet placed in the reticulum to attract and isolate sharp metal and help to prevent traumatic reticuloperitonitis in ruminants.





Electronics Dictionary: magnet


Body that can be used to attract or repel magnetic materials.




Devil's Dictionary: magnet

A cynical view of the world by Ambrose Bierce



n.


Something acted upon by magnetism.



Word Tutor: magnet

pronunciation


IN BRIEF: A piece of some material such as the mineral iron oxide that is able to attract iron.


Memory is a magnet. It will pull to it and hold only material nature has designed it to attract. — Jassamyn West, American novelist, Quaker, best known for her novel, The Friendly.




Wikipedia: magnet

Iron filings in a magnetic field generated by a bar magnet

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Iron filings in a magnetic field generated by a bar magnet


A magnet is a material or object that produces a magnetic field. A "hard" or "permanent" magnet is one which stays magnetized for a long time, such as magnets often used in refrigerator doors. Permanent magnets occur naturally in some rocks, particularly lodestone, but are now more commonly manufactured. A "soft" or "impermanent" magnet is one which loses its memory of previous magnetizations. "Soft" magnetic materials are often used in electromagnets to enhance (often hundreds or thousands of times) the magnetic field of a wire that carries an electrical current and is wrapped around the magnet; the field of the "soft" magnet increases with the current.


Two measures of a material's magnetic properties are its magnetic moment and its magnetization. A material without a permanent magnetic moment can, in the presence of magnetic fields, be attracted (paramagnetic), or repelled (diamagnetic). Liquid oxygen is paramagnetic; graphite is diamagnetic. Paramagnets tend to intensify the magnetic field in their vicinity, whereas diamagnets tend to weaken it. "Soft" magnets, which are strongly attracted to magnetic fields, can be thought of as strongly paramagnetic; superconductors, which are strongly repelled by magnetic fields, can be thought of as strongly diamagnetic.


Qualities


The magnetic field, magnetic moment, and magnetization are vectors, meaning they have direction and magnitude. The magnetic moment and magnetization are properties only of the magnet, while the magnetic field it produces depends on the position relative to the magnet. The magnetic moment points from its south pole to its north pole. Also, its north pole points towards the Earth's geographic north pole, which is a magnetic south pole. A compass needle is approximately a bar magnet.




A magnetic field can be measured using a good magnetic compass (this is a small permanent magnet). The direction of the field at a point in space is the direction in which the compass needle points when it passes through that point and is in equilibrium. The magnitude (or strength, usually denoted by the symbol B) of a magnetic field can also be measured using a compass, if the field is, like the Earth's, nearly uniform over the volume occupied by the needle. The needle is rotated about its center, and this makes it oscillate about its equilibrium position. The period t of oscillation is measured. For small oscillation angles, the frequency of the oscillation, 1/t, is proportional to the square root of B. This is a result from the theory of rotational motion and the theory of the torque on a magnet, and can be tested by creating an electromagnet, which makes a magnetic field proportional to the electric current that it carries. The common unit of magnetic field is the tesla, denoted "T", equal to one N/(A?m) (Force/(Current?Distance)), or Wb/m? (magnetic flux per area), and about 20,000 times the Earth's magnetic field. Technically, B should be called the magnetic induction field, because changing B induces an electric field, by Faraday's Law of electromagnetic induction.


Magnetic moment


The magnetic moment ? of a magnet is the magnetic strength of the field at a distance r from the magnet. At large distances, the magnetic field B is proportional to ? and inversely proportional to r?. So, ? can be obtained by measuring B at a distance r. The common unit for magnetic moment is A?m?. A wire in the shape of a circle with area A and carrying current I has a magnetic moment equal to IA.


Magnetization


Magnetization of an object is its magnetic moment per unit volume. It is usually denoted M and has the units A/m. A good bar magnet may have a magnetic moment of 0.1 A?m? and a volume of 1 cm?, or 0.000001 m?, and therefore a magnetization of 100,000 A/m. Iron can have a magnetization of around a million A/m.




The magnetic moment of atoms in a ferromagnetic material cause them to behave something like tiny permanent magnets. They stick together and align themselves into small regions of more or less uniform alignment called magnetic domains or Weiss domains. Magnetic domains can be observed with Magnetic force microscope to reveal magnetic domain boundaries that resemble white lines in the sketch.There are many scientific experiments that can physically show magnetic fields.

Effect of a magnet on the domains.

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Effect of a magnet on the domains.


When a domain contains too many molecules, it becomes unstable and divides into two domains aligned in opposite directions so that they stick together more stably as shown at the right.


When exposed to a magnetic field, the domain boundaries move so that the domains aligned with the magnetic field grow and dominate the structure as shown at the left. When the magnetizing field is removed, the domains may not return to a unmagnetized state. This results in the ferromagnetic material being magnetized, forming a permanent magnet.


When magnetized strongly enough that the prevailing domain overruns all others to result in only one single domain, the material is magnetically saturated. When a magnetized ferromagnetic material is heated to the Curie point temperature, the molecules are agitated to the point that the magnetic domains lose the organization and the magnetic properties they cause cease. When the material is cooled, this domain alignment structure spontaneously returns as the material develops its crystalline structure.


Physical origin of magnetism


Magnetism, ultimately, is due to the motion of electric charge. For a macroscopic object, like a wire loop, an electric current flowing through it has a magnetic moment. Far from the loop there is a magnetic field proportional in strength to its magnetic moment.


For a microscopic object, the physical picture is more complex. An electron within an atom can have orbital angular momentum and a magnetic moment proportional to that orbital angular momentum; the electron also has intrinsic angular momentum, or spin, and a magnetic moment proportional to that spin angular momentum. The orbital and spin angular momentum of an electron are comparable in magnitude, as are their magnetic moments. Far from the electron there is a magnetic field proportional in strength to its magnetic moment.


In addition, within the atomic nucleus are both neutrons and protons, and these too have orbital and spin angular momentum, and associated magnetic moments. However, the nuclear magnetic moment typically is much smaller than the electron magnetic moment, because although the magnetic moment is proportional to its angular momentum (comparable to that of the electron) it is also inversely proportional to its mass. Nevertheless, it is the nucleus's relatively small nuclear magnetic moment that is responsible for nuclear magnetic resonance (NMR), which is the basis for magnetic resonance imaging (MRI).


Although most atoms and molecules have a net magnetic moment at temperatures well below room temperature, at room temperature they typically have no net magnetic moment. However, they can often be magnetized. If the orbital magnetic properties dominate, the response typically will be diamagnetic; if the intrinsic magnetic properties dominate, the response typically will be paramagnetic.


Solids are collections of atoms and molecules. At room temperature most solids are either diamagnetic or paramagnetic.


Although for many purposes it is convenient to think of a magnet as having magnetic poles, it must be remembered that no isolated magnetic pole has ever been observed. As indicated above, the proper description is ultimately one due to electrical currents. For a magnet, these currents should be thought of as circulating about its atoms, and flowing without any electrical resistance. This physical picture is due to Andr?-Marie Amp?re, and these atomic currents are known as Amperian currents. For a uniformly magnetized bar magnet in the shape of a cylinder, the net effect of the atomic currents is to make the magnet behave as if there is a sheet of current flowing around the cylinder, with local flow direction normal to the cylinder axis. A right-hand-rule due to Amp?re tells us how the currents flow, for a given magnetic moment. Align the thumb of your right hand along the magnetic moment, and with that hand grasp the cylinder. Your fingers will then point along the direction of current flow.


Permanent magnets


A few elements -- especially iron, cobalt, and nickel -- are ferromagnetic at room temperature. When quantum mechanics and the Pauli Exclusion Principle are accounted for, the electrical energy within these atoms is found to be lower if the magnetic moments of the valence electrons are aligned. This makes them ferromagnetic. Every ferromagnet has its own individual temperature, called the Curie temperature, or Curie point, above which it loses its ferromagnetic properties. This is because the thermal tendency to disorder overwhelms the energy lowering due to ferromagnetic order. A perfectly aligned ferromagnet is said to have long-range order because all of its atoms have their magnetic moments pointing in the same direction. Real ferromagnets are not perfectly aligned, but rather contain perfectly aligned regions, called magnetic domains, which have their own magnetization directions.


A long bar magnet appears to have a north pole at one end and a south pole at the other. Near either end the magnetic field falls off inversely with the square of the distance from that pole.


For a magnet of any shape, at distances large compared to its size, the strength of the magnetic field falls off inversely with the cube of the distance from the magnet's center.


Electromagnets


An electromagnet in its simplest form, is a wire that has been coiled into one or more loops, known as a solenoid. When electric current flows through the wire, a magnetic field is generated. It is concentrated near the coil, and its field lines are very similar to those for a magnet. The orientation of this effective magnet is determined via the right hand rule. The magnetic moment and the magnetic field of the electromagnet are proportional to the number of loops of wire, to the cross-section of each loop, and to the current passing through the wire.


If the coil of wire is wrapped around a material with no special magnetic properties (i.e., cardboard), it will tend to generate a very weak field. However, if it is wrapped around a "soft" ferromagnetic material, such as an iron nail, then the net field produced can result in a several hundred- to thousandfold increase of field strength.


Uses for electromagnets include particle accelerators, electric motors, junkyard cranes, and magnetic resonance imaging machines. Some applications involve configurations more than a simple magnetic dipole; for example, quadrupole magnets are used to focus particle beams.


Characteristics


Permanent magnets and dipoles


All magnets appear to have at least one north pole (reckoned positive) and at least one south pole (reckoned negative), and the net pole strength of every magnet is zero. Despite their apparent reality, as suggested by the image at the top of the page, where iron filings concentrate in regions of large magnetic field, poles are not physical objects on or in the magnet. They are simply a useful concept for describing magnets. Rather than poles being the fundamental unit, it is the magnetic dipole that is the fundamental unit. A magnetic dipole can be thought of as a combination of a positive and a negative pole that are microscopically close to one another and inseparable. This is not a bad description of the magnetic dipole of an electron in a magnetic material.


The effect of aligning many dipoles and placing them head-to-tail in a line is that there appears a north pole at one end and a south pole at the other, with all the intermediate north and south poles canceling out. The net effect is a very long dipole that appears to have poles only at its ends. Alternatively, aligning many dipoles and placing them on a sheet producing an object whose magnetic field is like that of a wire carrying current around the perimeter of the sheet.




A standard naming system for the poles of magnets is important. Historically, the terms north and south reflect awareness of the relationship between magnets and the earth's magnetic field. A freely suspended magnet will eventually orient itself north-to-south, because of its attraction to the north and south magnetic poles of the earth. The end of a magnet that points (approximately) toward the Earth's geographic North Pole is labeled as the north pole of the magnet; correspondingly, the end that points south is the south pole of the magnet. (The actual geographic north pole is in a slightly different location than the corresponding magnetic pole; see Magnetic North Pole.)


The Earth's present geographic north is thus actually its magnetic south. Confounding the situation further, magnetized rocks on the ocean floor show that the Earth's magnetic field has reversed itself in the past, so this system of naming is likely to be incorrect at some time in the future.


Fortunately, by using an electromagnet and the right hand rule relating the electromagnet's current and the magnetic field it produces, the orientation of the field of a magnet can be defined without reference to the Earth's geomagnetic field.


To avoid the confusion between geographic and magnetic north and south poles, the terms positive and negative are sometimes used for the poles of a magnet. The positive pole is that which seeks geographical north.


Common uses

Magnets have many uses in toys. M-tic uses magnetic rods connected to metal spheres for construction



    * Magnetic recording media: Common VHS tapes contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. Common audio cassettes also rely on magnetic tape. Similarly, in computers, floppy disks and hard disks record data on a thin magnetic coating.


    * Credit, debit, and ATM cards: All of these cards have a magnetic strip on one of their sides. This strip contains the necessary information to contact an individual's financial institution and connect with their account(s).


    * Common televisions and computer monitors: TV and computer screens using vacuum tube technology employ an electromagnet to guide electrons to the screen, in order to produce an image -- see the article on cathode ray tubes. Plasma screens and LCDs use different technologies.


    * Speakers and microphones: Most speakers employ a permanent magnet and a current-carrying coil to convert electric energy (the signal) into mechanical energy (the sound). The coil is wrapped around the speaker cone, and carries the signal, producing a changing magnetic field that interacts with the field of the permanent magnet. The low mass coil feels a magnetic force and in response moves the cone and the neighboring air, thus generating sound. Standard microphones employ the same concept, but in reverse. A microphone has a cone or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as well. As the coil moves through the magnetic field, a voltage is generated in the coil (see Lenz's Law). This voltage drives current in the wire that is characteristic of the original sound.



    * Electric motors and generators: Some electric motors (much like loudspeakers) rely upon a combination of an electromagnet and a permanent magnet, and much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy.


    * Transformers: Transformers are devices that transfer electric energy between two windings that are electrically isolated but are linked magnetically.


    * Chucks: Chucks are used in the metalworking field to hold objects. If these objects can be held securely with a magnet then a permanent or electromagnetic chuck may be used. Magnets are also used in other types of fastening devices, such as the magnetic base, the magnetic clamp and the refrigerator magnet.


    * A compass (or mariner's compass) is a navigational instrument for finding directions on the Earth. It consists of a magnetized pointer free to align itself accurately with Earth's magnetic field, which is of great assistance in navigation. The cardinal points are north, south, east and west. A compass can be used in conjunction with a marine chronometer and a sextant to provide a very accurate navigation capability. This device greatly improved maritime trade by making travel safer and more efficient. An early form of the compass was invented in China in the 11th century. The familiar mariner's compass was invented in Europe around 1300, as was later the liquid compass and the gyrocompass which does not work with a magnetic field.


    * Magic: Naturally magnetic Lodestones as well as iron magnets are used in conjunction with fine iron grains (called "magnetic sand") in the practice of the African-American folk magic known as hoodoo. The stones are symbolically linked to people's names and ritually sprinkled with magnetic sand to reveal the magnetic field. One stone may be utilized to bring desired things to a person; a pair of stones may be manipulated to bring two people closer together in love.


    * Art: 1 mm or thicker vinyl magnet sheets may be attached to paintings, photographs, and other ornamental articles, allowing them to be stuck to refrigerators and other metal surfaces.


    * Science Projects: Many topic questions are often based on magnets. For example; how is the strength of a magnet affected by glass, plastic, and cardboard?


    * Toys: Due to their ability to counteract the force of gravity at very close range, magnets are often employed in children's toys such as the Magnet Space Wheel to amusing effect.


    * Magnets can be used to make jewelry. Necklaces and bracelets can have a magnetic clasp. Necklaces and bracelets can be made from small but strong, cylindrical magnets and slightly larger iron or steel balls connected in a pattern that is repeated until it is long enough to fit on the wrist or neck. These accessories may be fragile enough to accidentally come apart, but they also can be disassembled and reassembled with a different design. When connected as a necklace or a bracelet, magnets lose their attraction to other pieces of iron steel because they are already attached to their own iron and steel balls. Magnetic lip-rings and earrings are sometimes employed to avoid piercing.


    * Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are either too small, too hard to reach, or too thin for fingers to hold.


    * Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains) via electromagnetic force. This method can be faster than wheeled mass transit systems, potentially reaching velocities comparable to turboprop and jet aircraft (900km/h, 559 mph). The maximum recorded speed of a maglev train is 581km/h (361 mph), achieved in Japan in 2003.


    * A recently developed use of magnetism is to connect portable computer power cables. Such a connection will occasionally break by accidentally pushing against the cable, but the computer battery prevents interruption of service, and the easy disconnection protects the cable from serious jerks or from being stepped on.



Magnetization and demagnetization


Ferromagnetic materials can be magnetized in the following ways:


    * Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the earth's magnetic field and which are subject to vibration (e.g. frame of a conveyor) have been shown to acquire significant residual magnetism.

    * Placing the item in a solenoid with a direct current passing through it.

    * Stroking - An existing magnet is moved from one end of the item to the other repeatedly in the same direction.

    * Placing a steel bar in a magnetic field, then heating it to a high temperature and then finally hammering it as it cools. This can be done by laying the magnet in a North-South direction in the Earth's magnetic field. In this case, the magnet is not very strong but the effect is permanent.


Permanent magnets can be demagnetized in the following ways:


    * Heating a magnet past its Curie point will destroy the long range ordering.

    * Contact through stroking one magnet with another in random fashion will demagnetize the magnet being stroked, in some cases; some materials have a very high coercive field and cannot be demagnetized with other permanent magnets.

    * Hammering or jarring will destroy the long range ordering within the magnet.

    * A magnet being placed in a solenoid which has an alternating current being passed through it will have its long range ordering disrupted, in much the same way that direct current can cause ordering.


In an electromagnet which uses a soft iron core, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis.




Magnetic metallic elements


Many materials have unpaired electron spins, and the majority of these materials are paramagnetic. When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as "magnetic"). Due to the way their regular crystalline atomic structure causes their spins to interact, some metals are (ferro)magnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt and nickel, as well the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring (ferro)magnets were used in the first experiments with magnetism. Technology has since expanded the availability of magnetic materials to include various manmade products, all based, however, on naturally magnetic elements.


Composites


Ceramic or ferrite


Ceramic, or ferrite, magnets are made of a sintered composite of powdered iron oxide and barium/strontium carbonate ceramic. Due to the low cost of the materials and manufacturing methods, inexpensive magnets (or nonmagnetized ferromagnetic cores, for use in electronic component such as radio antennas, for example) of various shapes can be easily mass produced. The resulting magnets are noncorroding, but brittle and must be treated like other ceramics.


Alnico


Alnico magnets are made by casting or sintering a combination of aluminium, nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as a metal.


Injection molded


Injection molded magnets are a composite of various types of resin and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength and resemble plastics in their physical properties.


Flexible


Flexible magnets are similar to injection molded magnets, using a flexible resin or binder such as vinyl, and produced in flat strips or sheets. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used.


Rare earth magnets


'Rare earth' (lanthanoid) elements have a partially occupied f electron shell (which can accommodate up to 14 electrons.) The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore these elements are used in compact high-strength magnets where their higher price is not a concern.


Samarium-cobalt


Samarium-cobalt magnets are highly resistant to oxidation, with higher magnetic strength and temperature resistance than alnico or ceramic materials. Sintered samarium-cobalt magnets are brittle and prone to chipping and cracking and may fracture when subjected to thermal shock.


Neodymium-iron-boron (NIB)


Neodymium magnets, more formally referred to as neodymium-iron-boron (NdFeB) magnets, have the highest magnetic field strength, but are inferior to samarium cobalt in resistance to oxidation and temperature. This type of magnet has traditionally been expensive, due to both the cost of raw materials and licensing of the patents involved. This high cost limited their use to applications where such high strengths from a compact magnet are critical. Use of protective surface treatments such as gold, nickel, zinc and tin plating and epoxy resin coating can provide corrosion protection where required. Beginning in the 1980s, NIB magnets have increasingly become less expensive and more popular in other applications such as children's magnetic building toys. Even tiny neodymium magnets are very powerful and have important safety considerations.[1]


Single-molecule magnets (SMMs) and single-chain magnets (SCMs)


In the 1990s it was discovered that certain molecules containing paramagnetic metal ions are capable of storing a magnetic moment at very low temperatures. These are very different from conventional magnets that store information at a "domain" level and theoretically could provide a far denser storage medium than conventional magnets. In this direction research on monolayers of SMMs is currently under way. Very briefly, the two main attributes of an SMM are:


   1. a large ground state spin value (S), which is provided by ferromagnetic or ferrimagnetic coupling between the paramagnetic metal centres.

   2. a negative value of the anisotropy of the zero field splitting (D)


Most SMM's contain manganese, but can also be found with vanadium, iron, nickel and cobalt clusters. More recently it has been found that some chain systems can also display a magnetization which persists for long times at relatively higher temperatures. These systems have been called single-chain magnets.


Nano-structured magnets


Some nano-structured materials exhibit energy waves called magnons that coalesce into a common ground state in the manner of a Bose-Einstein condensate.



Magnetic behaviors



There are many forms of magnetic behavior, and all materials exhibit at least one of these behaviors. Magnets vary in the permanency of their magnetization and the strength of the magnetic field that is created.


Paramagnetism


Most popularly found in paper clips, paramagnetism is exhibited in substances which do not emit fields by themselves, but when exposed to a magnetic field, its electrons will begin to spin in such a manner that the substance emits a field of its own. A good analogy for this behavior can be found in a bucket of nails - if you pick up a single nail, you can expect that other nails will not follow. However, you can apply an intense magnetic field to the bucket, pick up one nail, and find that many will come with it.



Diamagnetism


Unscientifically referred to as 'non-magnetic,' diamagnets actually exhibit some magnetic behavior - just to very small magnitudes. While paramagnetism is affected more by the direction of the spin of electrons, diamagnetism is affected by electrons' centripetal forces. Under the influence of a field, electrons of opposite spin will see opposite effects to their centripetal force: one will increase and one will decrease. This results in a very small magnetic force. All materials exhibit this type of magnetism, however, when diamagnetism pairs with a stronger type of magnetic behavior, the diamagnetic effect is severely overshadowed.




Ferromagnetism


This is the 'popular' perception of a magnet. Ferromagnetic materials have a high retainment for magnetization, and a common example is a traditional refrigerator magnet. By technicality, ferromagnetism exists when all of the atoms contribute to the magnetic force emitted. The mechanical explanation of this is similar to that of paramagnetism - the electrons' spins align such it creates a magnetic force. However, unlike paramagnetic substances, a ferromagnet will retain this spin alignment.




Ferrimagnetism


Like ferromagnetism, ferrimagnets retain their magnetization in the absence of a field. However, they are arranged such that some of its atoms oppose the magnetic moment. These atoms are said to be anti-aligned. The first discovered magnetic substance, magnetite, was originally believed to be a ferromagnet; Louis N?el disproved this, however, with the discovery of ferrimagnetism.





Antiferromagnetism


When all atoms are arranged in a substance so that they are anti-aligned, the substance is antiferromagnetic. Antiferromagnets have a zero net magnetic moment, meaning no field is emitted by them. Antiferromagnets are less common compared to the other types of behaviors, and are mostly observed at low temperatures. In varying temperatures, antiferromagnets can be seen to exhibit diamagnetic and ferrimagnetic properties.





Units and calculations in magnetism


How we write the laws of magnetism depends on which set of units we employ. For most engineering applications, MKS or SI (Syst?me International) is common. Two other sets, Gaussian and CGS-emu, are the same for magnetic properties, and are commonly used in physics.


In all units it is convenient to employ two types of magnetic field, B and H, as well as the magnetization M, defined as the magnetic moment per unit volume.


(1) The magnetic induction field B is given in SI units of T (tesla). B is the true magnetic field, whose time-variation produces, by Faraday's Law, circulating electric fields (which the power companies sell). B also produces a deflection force on moving charged particles (as in TV tubes). The tesla is equivalent to the magnetic flux (in webers) per unit area (in meters squared), thus giving B the unit of a flux density. In CGS the unit of B is G (gauss). One T equals 104 G.


(2) The magnetic field H is given in SI units of ampere-turns/meter (A-turn/m). The "turns" appears because when H is produced by a current-carrying wire, its value is proportional to the number of turns of that wire. In CGS the unit of H is Oe (oersted). One A-turn/m equals 4p x 10-3 Oe.


(3) The magnetization M is given in SI units of ampere/meter (A/m). In CGS the unit of M is the emu, or electromagnetic unit. One A/m equals 10-3 emu. A good permanent magnet can have a magnetization as large as a million A/m. Magnetic fields produced by current-carrying wires would require comparably huge currents per unit length, one reason we employ permanent magnets and electromagnets.


(4) In SI units, the relation B=?0(H+M) holds, where ?0 is the permeability of space, which equals 4p x 10-7 tesla?meter/ampere. In CGS it is written as B=H+4pM.


Materials that are not permanent magnets usually satisfy the relation M=?H in SI, where ? is the (dimensionless) magnetic susceptibility. Most non-magnetic materials have a relatively small ? (on the order of a millionth), but soft magnets can have ?'s on the order of hundreds or thousands. For materials satisfying M=?H, we can also write B=?0(1+?)H=?0?rH=?H, where ?r=1+? is the (dimensionless) relative permeability and ? = ?0?r is the magnetic permeability. Both hard and soft magnets have a more complex, history-dependent, behavior described by what are called hysteresis loops, which give either B vs H or M vs H. In CGS M=?H, but ?(SI) = 4p?(CGS), and ? = ?r.


Caution: In part because there are not enough Roman and Greek symbols, there is no commonly agreed upon symbol for magnetic pole strength and magnetic moment. The symbol m has been used for both pole strength (unit = A-m, where here "m is for meter") and for magnetic moment (unit = A-m?). The symbol ? has been used in some texts for magnetic permeability and in other texts for magnetic moment. We will use ? for magnetic permeability and m for magnetic moment. For pole strength we will employ qm. For a bar magnet of cross-section A with uniform magnetization M along its axis, the pole strength is given by qm=MA, so that M can be thought of as a pole strength per unit area.


Calculating the magnetic force


Calculating the attractive or repulsive force between two magnets is, in the general case, an extremely complex operation, as it depends on the shape, magnetization, orientation and separation of the magnets.


Force between two magnetic poles


The force between two magnetic poles is given by:


    F={{mu q_{m1} q_{m2}}over{4pi r^2}} [1]


where


    F is force (SI unit: newton)

    qm1 and qm2 are the pole strengths (SI unit: ampere-meter)

    ? is the permeability of the intervening medium (SI unit: tesla meter per ampere or henry per meter)

    r is the separation (SI unit: meter).


The pole description is useful to practicing magneticians who design real-world magnets, but real magnets have a pole distribution more complex than a single north and south. Therefore, implementation of the pole idea is not simple. In some cases, one of the more complex formulae given below will be more useful.


Force between two nearby attracting surfaces of area A and equal but opposite magnetizations M


    F=frac{mu_0}{2}AM^2 [2]


where


    A is the area of each surface, in m2

    M is their magnetization, in ampere/m.

    ?0 is the permeability of space, which equals 4p x 10-7 tesla?meter/ampere


Force between two bar magnets


The force between two identical cylindrical bar magnets placed end-to-end is given by:


    F=left[frac {B_0^2 A^2 left( L^2+R^2 right)} {pimu_0L^2}right] left[{frac 1 {x^2}} + {frac 1 {(x+2L)^2}} - {frac 2 {(x+L)^2}} right] [3]


where


    B0 is the magnetic flux density very close to each pole, in T,

    A is the area of each pole, in m2,

    L is the length of each magnet, in m,

    R is the radius of each magnet, in m, and

    x is the separation between the two magnets, in m


B0=frac{mu_0}{2}M relates the flux density at the pole to the magnetization of the magnet.



Online references


    * HyperPhysics E/M, good complete tree diagram of electromagnetic relationships with magnets

    * Maxwell's Equations and some history...

    * Detailed Theory on Designing a Solenoid or a Coil Gun


Printed references


1. "positive pole n." The Concise Oxford English Dictionary. Ed. Catherine Soanes and Angus Stevenson. Oxford University Press, 2004. Oxford Reference Online. Oxford University