Magnetism

In physics, magnetism is one of the phenomena 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.

Types of magnets

Electromagnets

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.

Related Topics:
Electric current - Helix - Circle - Dipole

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Permanent Magnets

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.

Related Topics:
Metal - Ore - Iron ore - Magnetite - Lodestone - Cobalt - Nickel

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

Related Topics:
Sintered - Composite - Ceramic - Electronic component - Radio antennas

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Alnico

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.

Related Topics:
Casting - Sintering - Aluminium

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

Related Topics:
Injection molded - Composite - Resin - Plastic

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 factor.

Related Topics:
Lanthanoid - Electron shell

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 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) and Single Chain Magnets (SCMs)

In the nineties 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:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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

Related Topics:
Wave - Magnon - Bose-Einstein condensate

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

See results from NIST published April 2005, http://www.spacedaily.com/news/nanotech-05zm.html or http://physorg.com/news3784.html

~ ~ ~ ~ ~ ~ ~ ~ ~ ~