Antiferromagnetism

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Hematite is antiferromagnetic
Magnetic orientations of disordered antiferromagnetic crystallites

The antiferromagnetism (from ancient Greek αντί anti , German , against ' ; latin ferrum , iron ' ; ancient Greek μαγνῆτις MAGN (lithos) , German , stone from Magne Sien ' ) is a variant of the magnetic order within materials in which atoms with magnetic moments available are. It occurs when the respective neighboring elementary magnets carry the same magnetic moment in terms of their absolute value, but their alignment is opposite to one another ( antiparallel ). Antiferromagnetically ordered materials have no external permanent magnetic moment due to the anti-parallel orientation of the elementary magnets. As with ferromagnets , Weiss domains form in antiferromagnets , within which the magnetic moments have the same spatial position . The phenomenon was u. a. thoroughly examined by Louis Néel .

overview

With this effect, the magnetic moments or spins of the atoms are aligned with one another with a constant, non-zero angle of rotation and are precisely compensated for over the entire crystal. In the simplest case this angle of rotation is 180 °, so that the spins of neighboring atoms are antiparallel to one another.

With the more well-known ferromagnetism , on the other hand, the spins are aligned parallel to each other, which creates a macroscopic magnetization, while with antiferromagnetism without an applied magnetic field, the resulting magnetization is zero. Both orders are only stable at sufficiently low temperatures. In antiferromagnetism, the permeability shows a maximum at the phase transition temperature , the Néel temperature (named after Louis Néel ). Above the Néel temperature, in the magnetically disordered phase with largely randomly oriented spins, the behavior of the material is paramagnetic , and the permeability decreases with increasing temperature.

The antiferromagnetic structure, just like other magnetic configurations, can be explained in particular with the help of an exchange interaction . Depending on their sign, one can easily describe ferromagnetic or antiferromagnetic arrangements of the magnetic moments within the framework of an Ising model with couplings that are limited to neighboring spins . Longer-range or competing interactions can lead to more complicated magnetic structures (e.g. spiral structures).

Clarification

Strictly speaking , the Néel state described above (with alternating spin directions ) is not the basic state of the system, but only a quasi-classical approximation for it, which is particularly suitable for describing the excitation states , the so-called spin waves , while the exact quantum mechanical ground state , except in special cases, is unknown, in any case extremely complicated in the special cases mentioned (e.g. Bethe approach ). In contrast, in the ferromagnetic case the classical ground state (e.g. all spins upwards ) is also exact in the quantum mechanical formalism, and the description of the excitation states ( spin waves) in the case of ferromagnetism corresponds almost entirely to the classical picture of precessing vectors.

materials

Antiferromagnetism occurs with many transition metals and especially their oxides.

The following materials or minerals are z. B. antiferromagnetic:

Some organic compounds are also antiferromagnetic, e.g. B.

See also

Specialist literature

  • Horst Stöcker: Pocket book of physics. 4th edition, Verlag Harry Deutsch, Frankfurt am Main 2000, ISBN 3-8171-1628-4
  • Hans Fischer: Materials in electrical engineering. 2nd edition, Carl Hanser Verlag, Munich Vienna, 1982 ISBN 3-446-13553-7
  • Daniel Mattis: The theory of magnetism , two volumes, Berlin, Springer-Verlag, 1985 and 1988; ISBN 3-540-10611-1 (there is also an older German version)

Individual evidence

  1. ↑ a b c d e f g http://www.geodz.com/deu/d/Antiferromagnetismus
  2. ^ S. Murphy, Sf Ceballos, G. Mariotto, N. Berdunov, K. Jordan: Atomic scale spin-dependent STM on magnetite using antiferromagnetic STM tips . In: Microscopy Research and Technique . tape 66 , no. 2-3 , February 1, 2005, ISSN  1097-0029 , p. 85–92 , doi : 10.1002 / jemt . 20148 .
  3. ↑ a b c Inorganic Chemistry II ( Memento from March 27, 2016 in the Internet Archive )
  4. ↑ https://elearning.physik.uni-frankfurt.de/data/FB13-PhysikOnline/lm_data/lm_324/daten/kap_25/node155.htm  ( page no longer available , search in web archives )  Info: The link was automatically defective marked. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Toter Link / elearning.physik.uni-frankfurt.de  
  5. ↑ PS Silinsky, MS Seehra "Principal magnetic susceptibilities and uniaxial stress experiments in CoO", in: Phys. Rev. B , 1981 , 24 , pp. 419-423; doi: 10.1103 / PhysRevB.24.419
  6. ↑ Archived copy ( Memento from October 31, 2015 in the Internet Archive )
  7. ↑ JA Vergés, G. Chiappe, E. Louis, L. Pastor-Abia, E. SanFabián: Magnetic molecules created by hydrogenation of polycyclic aromatic hydrocarbons. In: Physical Review B. 79, 2009, doi: 10.1103 / PhysRevB.79.094403 . arxiv : 0807.4908 .

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