Diamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect. It is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field.
Diamagnetism is generally a quite weak effect in most materials, although superconductors exhibit a strong effect.
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All materials show a diamagnetic response in an applied magnetic field. In fact, diamagnetism is a very general phenomenon, because all paired electrons, including the core electrons of an atom, will always make a weak diamagnetic contribution to the material's response. However, for materials that show some other form of magnetism (such as ferromagnetism or paramagnetism), the diamagnetism is completely overpowered. Substances that mostly display diamagnetic behaviour are termed diamagnetic materials, or diamagnets. Materials that are said to be diamagnetic are those that are usually considered by non-physicists to be "non-magnetic", and include water, wood, most organic compounds such as petroleum and some plastics, and many metals including copper, particularly the heavy ones with many core electrons, such as mercury, gold and bismuth.
Additionally, all conductors exhibit an effective diamagnetism when they move through a magnetic field. The Lorentz force on electrons causes them to circulate around forming eddy currents. The eddy currents then produce an induced magnetic field which opposes the applied field, resisting the conductor's motion.
In 1778 S. J. Bergman was the first person to observe that bismuth and antimony were repelled by magnetic fields. However, the term "diamagnetism" was coined by Michael Faraday in September 1845, when he realized that all materials in nature possessed some form of diamagnetic response to an applied magnetic field.
If a thin (under 0.5 cm) layer of water is placed on top of a powerful magnet (such as a supermagnet) then the field of the magnet repels the water. This causes a slight dimple in the water's surface that may be seen by its reflection.
A thin slice of pyrolytic graphite, which is an unusually strong 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, the Netherlands, has conducted experiments where water and other substances were successfully levitated. Most spectacularly, a live frog (see figure) was levitated.
Recent experiments with studying the growth of protein crystals has led to a technique that utilizes powerful magnets to allow growth in ways that counteract Earth's gravity.
A simple homemade device for demonstration can be constructed out of bismuth plates and a few permanent magnets that will levitate a permanent magnet.
Material | Diamagnetism χm=Km-1 (x 10-5) |
---|---|
Bismuth | -16.6 |
Carbon (diamond) | -2.1 |
Carbon (graphite) | -1.6 |
Copper | -1.0 |
Lead | -1.8 |
Mercury | -2.9 |
Silver | -2.6 |
Water | -0.91 |