Diamagnetic refers to the ability of a material to create an opposing magnetic field when exposed to a strong one. The effect is created by a change in the orbit of electrons, which generate small currents to oppose magnetism from external sources. Many non-magnetic materials possess the qualities of diamagnetism, such as water, wood, plants, animals, and human beings. Millions of times weaker than a regular magnetic force, diamagnetism can cause levitation under the right circumstances.
Graphite and bismuth are the strongest diamagnetic materials. Organic compounds, such as petroleum, and heavy metals, such as gold and mercury, are strong as well. Such materials are repelled by outside magnetic forces because of eddy currents that form in their magnetic field. The best diamagnets are superconductors, which resist a magnetic field while transforming into a superconducting state, as explained by the Meissner effect.
One of the phenomena associated with this weak magnetic force is diamagnetic levitation. Stable equilibrium in a given magnetic field results in objects floating in free space, when the overall magnetic field strength is at a minimum. The molecules in living things, including water and proteins, are diamagnetic, and have only gravity as the resisting force when diamagnetism is present. Small animals, such as frogs, can be levitated in this way, which has been demonstrated by experiments in small tubes.
Theories related to diamagnetic materials include the Bohr-Leeuwen theorem, which states that a system cannot depend on a magnetic field if it is at a stable temperature. Diamagnetism is most persistent at high temperatures. Earnshaw’s theorem explains the phenomenon by saying that a magnetic field focused one way must not be as focused in another direction. It only applies to fixed magnets, while diamagnets can repel fields and levitate in free space.
Diamagnetism was first observed in the late-1700s, but the term was first derived in the mid-1800s by Michael Faraday, who discovered all materials show diamagnetic properties when exposed to a magnetic field. Strong superconductors make use of opposing magnetic forces today. Small samples of water can be levitated, and magnetic objects have been suspended for hours in vacuum environments without adding power. The concept has also been extensively studied by the National Aeronautics and Space Administration (NASA), and is expected to aid microgravity experiments on human bone and muscle as well as the development of magnets that counteract the gravity of Earth.