The Diamagnetism reference article from the English Wikipedia on 24-Jul-2004
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Diamagnetism is a very weak form of magnetism that is only exhibited in the presence of an external magnetic field. It is the result of changes in the orbital motion of electrons due to the external magnetic field. The induced magnetic moment is very small and in a direction opposite to that of the applied field. When placed between the poles of a strong electromagnet, diamagnetic materials are attracted towards regions where the magnetic field is weak. Diamagnetism is found in all materials; however, because it is so weak it can only be observed in materials that do not exhibit other forms of magnetism. Also, diamagnetism is found in elements with unpaired electrons. Oxygen was once thought to be diamagnetic, but a new revised molecular orbital (MO) model confirmed oxygen's paramagnetic nature.

An exception to the "weak" nature of diamagnetism occurs with the rather large number of materials that become superconducting, something that usually happens at lowered temperatures. Superconductors are perfect diamagnets and when placed in an external magnetic field expel the field lines from their interiors (depending on field intensity and temperature). Superconductors also have zero electrical resistance, a consequence of their diamagnetism. Superconducting structures have been known to tear themselves apart with astonishing force in their attempt to escape an external field. Superconducting magnets are the major component of most magnetic resonance imaging systems, perhaps the only important application of diamagnetism.

A thin slice of pyrolitic graphite, which is an unusually strongly diamagnetic material, can be stably floated on a magnetic field, such as that from rare earth permanent magnets. This can be done with all components at room temperature, making a visually effective demonstration of diamagnetism.

The University of Nijmegen has conducted experiments where they have successfully levitated water and a live frog. [1]

Diamagnetic materials have a relative magnetic permeability that is less than 1, and a magnetic susceptibility that is less than 0.

Diamagnetism was discovered and named in September 1845 by Michael Faraday.