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 minerallodestone; 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 Biot-Savart's 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'.
In an atom the orbital magnetic moments of some electron pairs cancel each other. The same is true for the spin magnetic moments. The overall magnetic moment of the atom is thus the sum of all of the magnetic moments of the individual electrons, accounting for moment cancellation between properly paired electrons. For the case of a completely filled electron shell or subshell, the magnetic moments completely cancel each other out. Thus only atoms with partially filled electron shells have a magnetic moment. The magnetic properties of materials are in large part determined by the nature and magnitude of the atomic magnetic moments.
Magnetic material may be modelled by a system of spins located at positions in a lattice, where the interaction of neighboring spins contributes to the total energy of the system and the states of the spins change according to some non-deterministic (probabalistic) rule (the dynamics of the system). In the Ising model spins have only two possible states (up and down), whereas in the Potts model they may have more than two possible states. This is discussed in detail in Spin Models, particularly in the section Modelling Magnetic Material and subsequent sections.
Several forms of magnetic behavior have been observed including:
Electromagnets are useful in cases where a magnetic 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, then the magnetic field will wrap around the wire in the direction indicated by the fingers of the right hand. If a loop is formed, such that the charged particles are traveling in a circle, then all of the field lines in the center of the loop are directed in the same direction. The result is called a magnetic dipole. When placed in a magnetic field, a magnetic dipole will tend to align itself with that field. For the case of a loop, if the fingers of the right hand are directed in the direction of current flow, the thumb will point in the direction corresponding to the North pole of the dipole.
Ceramic, or ferrite, magnets are made of a sinteredcomposite of powdered iron oxide and barium/strontium carbonate ceramic. Due to the low cost of the materials and manufacturing methods, low cost magnets (or nonmagnetized ferromagnetic cores, for use in electronic components such as radio antennas, for example) of various shapes can be easily produced. The resulting magnets are noncorroding, but brittle and must be treated like other ceramics.
Alnico magnets are made up of aluminum, nickel and cobalt with small amounts of other elements added to enhance the properties of the magnet. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as a metal. They are resistant to demagnetization by temperature or mechanical shock but they are easily demagnetized by magnetic fields. Alnico magnets are produced by casting or sintering. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes.
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.
Rare earth elements have an felectron shell, filled with 14 electrons. Having the spin of these electrons aligned results in very strong magnetic fields, therefore these elements are used in compact high strength magnets where their higher price is not a factor.
