Materials spontaneously order the spins of their component atoms in the presence of a magnetic field. My understanding is that all materials exhibit either paramagnetism (creation of a weak magnetic field that works in parallel with the external field, attracting the object to the source) or diamagnetism (creation of a weak magnetic field opposing the external field, which causes the source to repel the object). In both cases, the spin alignment of the atoms dissolves quickly after the external field is removed, due simply to thermodynamic effects.
This is in contrast with ferromagnetism, in which an object such as iron aligns the spins of its atoms with the external magnetic field and maintains this alignment after the magnetic field is removed, which allows one to make a (weak) permanent magnet out of a paper clip, for instance.
But it turns out that some very special materials have a fourth spin-ordered state, called antiferromagnetism. In this state, the spins of adjacent atoms are opposite, so moving along the lattice we see atoms that are spin-up, spin-down, spin-up, spin-down, and so on. These materials only attain this ordered state at temperatures below what's called the Neel temperature, and it is believed that learning more about antiferromagnetism could lead to the development of room-temperature superconductors.
Cool, huh?
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