Antiferromagnetism
In materials that exhibit antiferromagnetism, the magetic moments of atoms or molecules, usually
related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism.  Generally, antiferromagnetic order may exist at sufficiently low temperatures, vanishing at and above a certain temperature, the Neel temperature (named after 
Louis Eugene Felix Neel, who had first identified this type of magnetic ordering). Above the Neel temperature, the material is typically paramagnetic.
When no external field is applied, the antiferromagnetic structure corresponds to a vanishing
total magnetization. In a field, a kind of ferrimagnetic behavior may be displayed
in the antiferromagnetic phase, with the absolute
value of one of the sublattice magnetizations differing from that of the
-ther sublattice.
The magnetic susceptibility of an antiferromagnetic material typically shows a maximum at
the Neel temperature. In contrast, at the transition between the ferromagnetic
to the paramagnetic phases the susceptibility will diverge. In the antiferromagnetic
case, a divergence is observed in the staggered susceptibility.
Various microscopic (exchange) interactions between the magnetic moments or spins
may lead to antiferromagnetic structures. In the simplest case, one may consider an Ising model on
an bipartite lattice, e.g. the simple cubic lattice, with couplings between spins at nearest neighbor sites. Depending on
the sign of that interaction, ferromagnetic or antiferromagnetic
-rder will result. Geometrical frustration or competing interactions may lead to different
and, perhaps, more complicated magnetic structures.
Antiferromagnetic materials occur less frequently in nature than ferromagnetic ones. An example is the heavy-fermion superconductor URu2Si2.  Better known examples include metals such as chromium, alloys such as iron manganese (FeMn), and oxides such as nickel oxide (NiO).  There are also numerous examples among high nuclearity metal clusters.  Organic molecules can also exhibit antiferromagnetic coupling under rare circumstances, as seen in radicals such as 5-dehydro-m-xylylene.
Antiferromagnets can couple to ferromagnets, for instance, through a mechanism known as exchange bias, in which the ferromagnetic film is either grown upon the antiferromagnet or annealed in an aligning magnetic field, causing the surface atoms of the ferromagnet to align with the surface atoms of the antiferromagnet. This provides the ability to "pin" the orientation of a ferromagnetic film, which provides one of the main uses in so-called spin valves, which are the basis of magnetic sensors including modern hard drive read heads.
There are also examples of disordered materials (such as iron phosphate glasses) that become antiferromagnetic below their Neel temperature. These disordered networks 'frustrate' the antiparallelism of adjacent spins; i.e. it is not possible to construct a network where each spin is surrounded by opposite neighbour spins. It can only be determined that the average correlation of neighbour spins is antiferromagnetic. This type of magnetism is sometimes called speromagnetism.