Diamagnetic



Diamagnetism is a form of magnetism which is only exhibited by a substance in the presence of an externally applied magnetic field. It is the result of changes in the orbital motion of electrons due to the application of an externally applied magnetic field. Applying a magnetic field creates a magnetic force on a moving electron in the form of F = qv × B. This force changes the centripetal force on the electron, causing it to either speed up or slow down in its orbital motion. This changed electron speed modifies the magnetic moment of the orbital in a direction against the external field.

Consider two electron orbitals; one rotating clockwise and the other counterclockwise. An external magnetic field into the page will make the centripetal force on an electron rotating clockwise increase, which increases its moment out of the page. That field would make the centripetal force on an electron rotating counterclockwise decrease, decreasing its moment into the page. Both changes oppose a magnetic field into the page. The induced magnetic moment is very small in most everyday materials. All materials show a diamagnetic response in an applied magnetic field; however for materials which show some other form of magnetism (such as ferromagnetism or paramagnetism), the diamagnetism is completely swamped. Substances which only, or mostly, display diamagnetic behaviour are termed diamagnetic materials, or diamagnets. Materials that are said to be diamagnetic are those which are usually considered by non-physicists as "non magnetic", and include water, DNA, most organic compounds such as oil and plastic, and many metals such as gold and bismuth.

Diamagnets are repelled by magnetic fields. However, since diamagnetism is such a weak property its effects are not observable in every-day life. For example, the magnetic susceptibility of diamagnets such as water is χ = −9.05×10−6. The most strongly diamagnetic material is Bismuth χ = −166×10−6, although pyrolitic graphite may have a susceptibility of χ = −400×10−6 in one plane. Nevertheless these values are orders of magnitudes smaller than the magnetism exhibited by paramagnets and ferromagnets. Superconductors may be considered to be perfect diamagnets (χ = −1), since they expel all field from their interior due to the Meissner effect.

History
Brugmans (in 1778) was the first person to observe that certain materials were repelled by magnetic fields. However, the term "diamagnetism" was coined by Michael Faraday in September 1845, when he realised that all materials in nature possessed some form of diamagnetic response to an applied magnetic field.

Diamagnetic levitation


A particularly fascinating phenomenon involving diamagnets is that they may be levitated in stable equilibrium in a magnetic field, with no power consumption. Earnshaw's theorem seems to preclude the possibility of static magnetic levitation. However, Earnshaw's theory only applies to objects with permanent moments m, such as ferromagnets, whose magnetic energy is given by m&middot;B. Ferromagnets are attracted to field maxima, which do not exist in free space. Diamagnetism is an induced form of magnetism, thus the magnetic moment is proportional to the applied field B. This means that the magnetic energy of diamagnets is proportional to B2, the intensity of the magnetic field. Diamagnets are also attracted to field minima, and there can be a minimum in B2 in free space (in fact $$\nabla^2 \mathbf{B}^2\geq 0$$).

A thin slice of pyrolytic graphite, which is an unusually strongly diamagnetic material, can be stably floated in 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 Radboud University Nijmegen has conducted experiments where they have successfully levitated water and a live frog, amongst other things. 

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