In physics, magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials. Some well known materials that exhibit easily detectable magnetic properties are iron, some steels, and the mineral lodestone; however, all materials are influenced to one degree or another by the presence of a magnetic field, although in most cases the influence is too small to detect without special equipment.
Magnetic forces are fundamental forces that arise due to the movement of electrical charge. Maxwell's equations describe the origin and behavior of the fields that govern these forces (see also the Biot-Savart Law). Thus, magnetism is seen whenever electrically charged particles are in motion. This can arise either from movement of electrons in an electric current, resulting in 'electromagnetism', or from the constant subatomic movement of electrons, resulting in what are known as 'permanent magnets'.
Normally, magnetic fields are seen as dipoles, having a 'South pole' and a 'North pole'; terms dating back to the use of magnets as compasses, interacting with the Earth's magnetic field to indicate North and South on the globe.
A magnetic field contains energy, and physical systems will stabilize into the configuration with the lowest energy. Therefore, when placed in a magnetic field, a magnetic dipole will tend to align itself in opposed polarity to that field, thereby canceling the net field strength as much as possible and lowering the energy stored in that field to minimum. For instance, two identical bar magnets will normally line up North to South resulting in no net magnetic field, and be resistant to any attempts to reorient them to point in the same direction. The energy required to reorient them in that configuration is then stored in the resulting magnetic field, which is double the strength of the field of each individual magnet. (This is, of course, why the magnet used as a compass will interact with the Earth's magnetic field to indicate North and South).
Contrary to normal experience, some theoretical physics models predict the existence of Magnetic monopoles. Paul Dirac observed in 1931 that, because electrical theory and magnetic theory show a certain symmetry, just as quantum theory predicts that individual positive or negative electric charges can be observed without the opposing charge, isolated South or North magnetic poles should be observable. In practice, however, although charged particles like protons and electrons can be easily isolated as individual electrical charges, magnetic south and north poles do not appear in isolation. Using quantum theory Dirac showed that if magnetic monopoles exist, then one could explain why the observed elementary particles carry charges that are integral multiples of the charge of the electron. Quarks carry fractional electric charge, but they do not appear as free particles.
In modern elementary particle theory, the "quantization" of charge is realised in a spontaneous breakdown of a non-abelian gauge symmetry. It should be noted that the monopoles predicted in certain Grand unified theories are different from the one originally thought of by Dirac. These monopoles, unlike that of elementary particles are solitons, namely localised energy packets. These monopoles, if at all they exist, contradict cosmological observations. A solution to this monopole problem in Cosmology gave rise to the currently interesting idea of inflation.
Atomic magnetic dipoles
The physical cause of the magnetism of objects, distinct from electrical currents, is the atomic magnetic dipole. Magnetic dipoles, or magnetic moments, result on the atomic scale due to the two kinds of movement of electrons. The first is the orbital motion of the electron around the nucleus; this can be considered as a current loop, resulting in a magnetic moment along the axis of the nucleus. The second, much stronger, source of electronic magnetic moment is due to a quantum mechanical property called spin (although current quantum mechanical theory states that electrons do not actually physically spin, or orbit the nucleus for that matter).
The overall magnetic moment of the atom is the net sum of all of the magnetic moments of the individual electrons. Because of the tendency of magnetic dipoles to oppose each other to reduce the net energy, in an atom the opposing magnetic moments of some pairs of electrons will cancel each other, both in orbital motion and in spin magnetic moments. Thus, in the case of an atom with a completely filled electron shell or subshell, the magnetic moments normally completely cancel each other out and only atoms with partially filled electron shells will have a magnetic moment, whose strength depends on the number of unpaired electrons.
The differences in configuration of the electrons in various elements thus determine the nature and magnitude of the atomic magnetic moments, which in turn determine the differing magnetic properties of various materials. Several forms of magnetic behavior have been observed in different materials, including:
Highly magnetic stars called magnetars are also believed to exist.
Types of magnets
Electromagnets are useful in cases where a magnet must be switched on or off; for instance, large cranes to lift junked automobiles.
For the case of electric current moving through a wire, the resulting field is directed according to the "right hand rule." If the right hand is used as a model, and the thumb of the right hand points along the wire from positive towards the negative side ('conventional current', the reverse of the direction of actual movement of electrons), then the magnetic field will wrap around the wire in the direction indicated by the fingers of the right hand. As can be seen geometrically, if a loop or helix of wire is formed such that the current is traveling in a circle, then all of the field lines in the center of the loop are directed in the same direction, resulting in a magnetic dipole whose strength depends on the current around the loop, or the current in the helix multiplied by the number of turns of wire. In the case of such a loop, if the fingers of the right hand are directed in the direction of conventional current flow (i.e. positive to negative, the opposite direction to the actual flow of electrons), the thumb will point in the direction corresponding to the North pole of the dipole.
Magnetic metallic elements
Due to their unpaired electron spins, some metals are magnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt, and nickel. Such naturally occurring magnets were used in the first experiments with magnetism. Technology has expanded the availability of magnetic materials to include various manmade products, all based, however, on naturally magnetic elements.
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 components 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 magnets are made by casting or sintering a combination of aluminium, nickel and cobalt with 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 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 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 elements have an f electron shell, filled with 14 electrons. The spin of these electrons can be aligned, resulting in very strong magnetic fields, therefore these elements are used in compact high strength magnets where their higher price is not a factor.
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 exposed to thermal shock.
Neodymium iron boron
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 is expensive, due to both the cost of raw materials and licensing of the patents involved. This high cost limits 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 and thermal protection where required.
Single molecule magnets (SMMs)
It has recently been discovered that certain molecules containing paramagnetic metal ions are capable of storing a magnetic moment at very low temperatures. These are very different to conventional magnets that store information at a "domain" level and theoretically could provide a far denser storage medium than conventional magnets. Very briefly, the attributes for an SMM are:
- a large ground state spin value (S), which is provided by ferromagnetic or ferrimagnetic coupling between the paramagnetic metal centres.
- a negative value for the anisotropy of the zero field splitting (D)
- It is also required for the ground state to be relatively well separated from its excited states.
Most SMM's contain manganese, but can also be found with vanadium, iron, nickel and cobalt clusters.
SI magnetism units
Other magnetism units