Diamagnetism
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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 paired 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.
Perhaps the substance that displays the strongest diamagnetism is bismuth, used in guns. Melting down bismuth and then molding it is a very efficient way of capturing the diamagnetic properties.
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 Radboud University Nijmegen has conducted experiments where they have successfully levitated water and a live frog. [1] (http://www.hfml.sci.kun.nl/froglev.html)
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.
Diamagnetic levitation
Diamagnets can be used for levitation. The materials required for this set up are two diamagnets, one small rare-earth magnet, and one larger rare earth magnet.
Begin by placing one of the diamagnets on a flat surface. Set the small rare earth magnet on top and in the center of it. On the edges of the diamagnet, place spacers that will hold up the second diamagnet (pennies or something similar will work fine), and place the second diamagnet on top of the spacers.
At this point, there should be the two diamagnets stacked on top of one another with enough room for the small magnet to float up and down in between them. While not necessary, it can be helpful to tape the entire assembly down to the surface it is sitting on.
To levitate the small magnet, slowly bring the larger magnet down from above. At some point, the small magnet will lift off of the bottom diamagnet and float in mid air. If the larger magnet gets closer, the small magnet will fly up; if the larger magnet moves farther away, the small magnet will drop.
It is easiest to understand why this set up is stable if it is examined in individual pieces. To begin with, there are two main forces acting on the small magnet: gravity and magnetism. Gravity from the Earth is pulling down on the magnet, and the bottom diamagnet is pushing up on the magnet. The force of gravity is far greater, so the magnet stays down.
As the large magnet is lowered down toward the small magnet, a point is reached where its magnetic field is pulling up on the small magnet with a force that is equal to the force of gravity (which is pulling down). At this point, the magnet is effectively weightless, and the only remaining force acting upon it is from the diamagnets.
Once the force of gravity is effectively negated by the large magnet, the small magnet can now respond to the small force from the bottom diamagnet. This force moves the small magnet up until the force from the bottom and top diamagnets equals one another. At this point, the magnet is stably levitated.
In summation, the large magnet negates the force of gravity on the small magnet, and the diamagnets create a magnetic field where the small magnet can stably rest in between them.
See also
- Antiferromagnetism
- Ferrimagnetism
- Ferromagnetism
- Paramagnetism
- Magnetic levitation
- Superdiamagnetism
External links
- Videos of frogs and other diamagnets levitated in a strong magnetic field (http://www.hfml.science.ru.nl/froglev.html)ca:Diamagnetisme