diamagnetism
/ DY-uh-mag-net-iz-um /
Bring a magnet near a piece of copper, glass, or water and nothing seems to happen. But something incredibly faint is: the material pushes back, ever so slightly, against the magnet. That feeble, universal repulsion is diamagnetism. It is the weakest form of magnetism and, remarkably, every material has it, even ones we call non-magnetic.
The mechanism is a magnetic version of Lenz's law. When you switch on an external field, the orbiting electrons in every atom subtly change their motion to set up a tiny opposing field, like a reflex that resists the change. Because the induced moment points against the applied field, diamagnets are weakly repelled and have a small negative magnetic susceptibility, typically around minus 10^-5. This effect is present in all atoms but is so tiny that it is only visible when the atoms have no permanent moment of their own to swamp it.
Diamagnetism is usually a curiosity rather than a design tool, but it has spectacular demonstrations: a strong magnet will make a sheet of pyrolytic graphite or a live frog levitate, because their diamagnetic repulsion can balance gravity. Its extreme cousin is superconductivity, where a material becomes a perfect diamagnet and completely expels the field (the Meissner effect), floating a magnet above it. Do not confuse diamagnetism with being non-magnetic: a diamagnet is genuinely, if very weakly, repelled, whereas a truly non-interacting material would do nothing.
A thin flake of pyrolytic graphite floats above an array of neodymium magnets because its diamagnetic repulsion balances its weight.
Diamagnetism repels weakly; superconductors are the perfect-diamagnet extreme.
Diamagnetism opposes the field but is extremely weak (susceptibility about minus 10^-5); it is present in all materials but is masked whenever the atoms carry a permanent moment.