WordNet

relating to antiferromagnetism
magnetic field creates parallel but opposing spins; varies with temperature

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/08/19 03:49:37」(JST)

wiki en

[Wiki en表示]

Antiferromagnetic ordering

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. Generally, antiferromagnetic order may exist at sufficiently low temperatures, vanishing at and above a certain temperature, the Néel temperature (named after Louis Néel, who had first identified this type of magnetic ordering).^[1] 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.

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 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 order will result. Geometrical frustration or competing ferro- and antiferromagnetic interactions may lead to different and, perhaps, more complicated magnetic structures.

Antiferromagnetic materials

Antiferromagnetic materials occur commonly among transition metal compounds, especially oxides. Examples include hematite, 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. 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 frustration: the inability of the system to find a single ground state. This type of magnetic behavior has been found in minerals that have a crystal stacking structure such as a 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. Due to dipole coupling of the ferromagnetic layers which results in antiparallel alignment of the magnetization of the ferromagnets.

Antiferromagnetism plays a crucial role in giant magnetoresistance, as had been discovered in 1988 by the 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.

References

^ L. Néel, Propriétées magnétiques des ferrites; Férrimagnétisme et antiferromagnétisme, Annales de Physique (Paris) 3, 137–198 (1948).

External links

Magnetism: Models and Mechanisms in E. Pavarini, E. Koch, and U. Schollwöck: Emergent Phenomena in Correlated Matter, Jülich 2013, ISBN 978-3-89336-884-6

English Journal

Triple-stranded ferric helices: a π-π interaction-driven structural hierarchy of Fe5, Fe7, and Fe17 clusters.

Shiga T, Noguchi M, Sato H, Matsumoto T, Newton GN, Oshio H.SourceDepartment of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571, Japan. oshio@chem.tsukuba.ac.jp.
Dalton transactions (Cambridge, England : 2003).Dalton Trans.2013 Dec 7;42(45):16185-93. doi: 10.1039/c3dt51480c. Epub 2013 Sep 24.
Three polypyridine ligand-supported multinuclear iron complexes, [Fe5], [Fe7] and [Fe17], were synthesized and their physical properties were investigated. The complexes had triple-stranded helical structures with pseudo threefold symmetry, and were stabilized by varying degrees of intramolecular π
PMID 24061338

Magnetic anisotropy of FeO and CoO: the influence of gradient corrections on exchange and correlation.

Schrön A, Bechstedt F.SourceInstitut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität, Max-Wien-Platz 1, D-07743 Jena, Germany. European Theoretical Spectroscopy Facility (ETSF).
Journal of physics. Condensed matter : an Institute of Physics journal.J Phys Condens Matter.2013 Dec 4;25(48):486002. doi: 10.1088/0953-8984/25/48/486002. Epub 2013 Oct 31.
We study the formation of local magnetic moments and the magnetic anisotropy of the antiferromagnetic transition-metal compounds FeO and CoO in the framework of the density functional theory. Exchange and correlation are described within two (semi-)local approximations, the local spin-density approx
PMID 24177337

Origin of anomalous magnetite properties in crystallographic matched heterostructures: Fe3O4(111)/MgAl2O4(111).

Gilks D, Lari L, Naughton J, Cespedes O, Cai Z, Gerber A, Thompson SM, Ziemer K, Lazarov VK.SourceDepartment of Physics, University of York, Heslington, York YO10 5DD, UK.
Journal of physics. Condensed matter : an Institute of Physics journal.J Phys Condens Matter.2013 Dec 4;25(48):485004. doi: 10.1088/0953-8984/25/48/485004. Epub 2013 Oct 31.
Magnetite films grown on crystallographically matched substrates such as MgAl2O4 are not expected to show anomalous properties such as negative magnetoresistance and high saturation fields. By atomic resolution imaging using scanning transmission electron microscopy we show direct evidence of anti-p
PMID 24177186