Everything about Antiferromagnetic totally explained
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
Néel temperature (named after
Louis Eugene Felix Neel, who had first identified this type of magnetic ordering). Above the Néel 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
other 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, for example the simple cubic lattice, with couplings between spins at nearest neighbor sites. Depending on
the sign of that interaction,
ferromagnetic or antiferromagnetic
order 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 URu
2Si
2. 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 Néel temperature. These disordered networks 'frustrate' the antiparallelism of adjacent spins; for example it isn't 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.
Further Information
Get more info on 'Antiferromagnetic'.
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