Antiferromagnetism

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Antiferromagnetic ordering
Magnetic orders : comparison between ferro, antiferro and ferrimagnetism

In materials that exhibit antiferromagnetism, the magnetic 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. The phenomenon of antiferromagnetism was first introduced by Lev Landau in 1933.[1]

Generally, antiferromagnetic order may exist at sufficiently low temperatures, but vanishes at and above the

Néel temperature – named after Louis Néel, who had first in the West identified this type of magnetic ordering.[2] Above the Néel temperature, the material is typically paramagnetic
.

Measurement

When no external field is applied, the antiferromagnetic structure corresponds to a vanishing total magnetization. In an external magnetic 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, resulting in a nonzero net magnetization. Although the net magnetization should be zero at a temperature of absolute zero, the effect of spin canting often causes a small net magnetization to develop, as seen for example in hematite.[citation needed]

The magnetic susceptibility of an antiferromagnetic material typically shows a maximum at the Néel 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 a 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 order will result. Geometrical frustration or competing ferro- and antiferromagnetic interactions may lead to different and, perhaps, more complicated magnetic structures.

The relationship between

residual magnetization
.

Antiferromagnetic materials

Antiferromagnetic structures were first shown through neutron diffraction of transition metal oxides such as nickel, iron, and manganese oxides. The experiments, performed by Clifford Shull, gave the first results showing that magnetic dipoles could be oriented in an antiferromagnetic structure.[4]

Antiferromagnetic materials occur commonly among

5-dehydro-m-xylylene
.

Antiferromagnets can couple to ferromagnets, for instance, through a mechanism known as

spin valves, which are the basis of magnetic sensors including modern hard disk drive
read heads. The temperature at or above which an antiferromagnetic layer loses its ability to "pin" the magnetization direction of an adjacent ferromagnetic layer is called the blocking temperature of that layer and is usually lower than the Néel temperature.

Geometric frustration

Unlike ferromagnetism, anti-ferromagnetic interactions can lead to multiple optimal states (ground states—states of minimal energy). In one dimension, the anti-ferromagnetic ground state is an alternating series of spins: up, down, up, down, etc. Yet in two dimensions, multiple ground states can occur.

Consider an equilateral triangle with three spins, one on each vertex. If each spin can take on only two values (up or down), there are 23 = 8 possible states of the system, six of which are ground states. The two situations which are not ground states are when all three spins are up or are all down. In any of the other six states, there will be two favorable interactions and one unfavorable one. This illustrates

Kagome lattice or hexagonal lattice
.

Other properties

Synthetic antiferromagnets (often abbreviated by SAF) are artificial antiferromagnets consisting of two or more thin ferromagnetic layers separated by a nonmagnetic layer.[5] Dipole coupling of the ferromagnetic layers results in antiparallel alignment of the magnetization of the ferromagnets.

Antiferromagnetism plays a crucial role in

Nobel prize winners Albert Fert and Peter Grünberg
(awarded in 2007) using synthetic antiferromagnets.

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; 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
.

See also

  • Exchange bias – occurs in bilayers (or multilayers) of magnetic materials where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the soft magnetization curve of a ferromagnetic film
  • Giant magnetoresistance – Phenomenom involving the change of conductivity in metallic layers
  • Geometrically frustrated magnet
     – phenomenon where atoms tend to stick to non-trivial positions; set of degrees of freedom incompatible with the space occupied
  • Ising model – Mathematical model of ferromagnetism in statistical mechanics
  • ANNNI model – Variant of the Ising model
  • Mottness
     – Materials classically predicted to be conductors, that are actually insulators
  • Quantum spin liquid – Phase of matter
  • Ferromagnetism – Mechanism by which materials form into and are attracted to magnets
  • Diamagnetism – Magnetic property of ordinary materials

References

  1. ^ Landau, L. D. (1933). A possible explanation of the field dependence of the susceptibility at low temperatures. Phys. Z. Sowjet, 4, 675.
  2. S2CID 126111103
    .
  3. .
  4. .
  5. .

External links