A substance that is diamagnetic repels a magnetic field. All materials have diamagnetic properties, but the effect is very weak, and is usually overcome by the object's paramagnetic or ferromagnetic properties, which act in the opposite manner. Any material in which the diamagnetic component is strongest will be repelled by a magnet.
Diamagnetic levitation can be used to levitate very light pieces of pyrolytic graphite or bismuth above a moderately strong permanent magnet. As water is predominantly diamagnetic, this technique has been used to levitate water droplets and even live animals, such as a grasshopper, frog and a mouse. However, the magnetic fields required for this are very high, typically in the range of 16 teslas, and therefore create significant problems if ferromagnetic materials are nearby.
The minimum criterion for diamagnetic levitation is , where:
- is the magnetic susceptibility
- is the density of the material
- is the local gravitational acceleration (−9.8 m/s2 on Earth)
- is the permeability of free space
- is the magnetic field
- is the rate of change of the magnetic field along the vertical axis.
Assuming ideal conditions along the z-direction of solenoid magnet: