The double-exchange mechanism is a type of a
magnetic exchange that may arise between ions in different
oxidation states
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
. First proposed by
Clarence Zener
Clarence Melvin Zener (December 1, 1905 – July 2, 1993) was the American physicist who first (1934) described the property concerning the breakdown of electrical insulators. These findings were later exploited by Bell Labs in the development of ...
,
this theory predicts the relative ease with which an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
may be exchanged between two species and has important implications for whether materials are
ferromagnetic,
antiferromagnetic
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. ...
, or exhibit spiral magnetism.
For example, consider the 180 degree interaction of
Mn-
O-Mn in which the Mn "e
g" orbitals are directly interacting with the O "2p" orbitals, and one of the Mn ions has more electrons than the other. In the
ground state, electrons on each Mn ion are aligned according to the
Hund's rule
Hund's rule of maximum multiplicity is a rule based on observation of atomic spectra, which is used to predict the ground state of an atom or molecule with one or more open electronic shells. The rule states that for a given electron configuration ...
:
If O gives up its spin-up electron to Mn
4+, its vacant orbital can then be filled by an electron from Mn
3+. At the end of the process, an electron has moved between the neighboring metal ions, retaining its spin. The double-exchange predicts that this electron movement from one species to another will be facilitated more easily if the electrons do not have to change
spin direction in order to conform with
Hund's rules when on the accepting species. The ability to hop (to delocalize) reduces the kinetic energy. Hence the overall energy saving can lead to ferromagnetic alignment of neighboring ions.
This model is superficially similar to
superexchange. However, in superexchange, a ferromagnetic or antiferromagnetic alignment occurs between two atoms with the same
valence (number of electrons); while in double-exchange, the interaction occurs only when one atom has an extra electron compared to the other.
References
External links
Exchange Mechanisms
in E. Pavarini, E. Koch, F. Anders, and M. Jarrell: Correlated Electrons: From Models to Materials, Jülich 2012,
Quantum chemistry
Magnetic exchange interactions
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