Abstract
Physicists have long known that magnetic behaviour originates from the tiny magnetic moments or "spins" that are carried by electrons. These spins can be either up or down and, depending on the spin-spin interactions, materials can be ferromagnetic (i.e. all the spins pointing in the same direction) or antiferromagnetic (i.e. neighbouring spins pointing in opposite directions). Iron, for example, is a ferromagnet, while the parent compounds of the high-temperature superconducting cuprates are antiferromagnets. There is also a tendency for the spins to align as the temperature decreases.