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 other atoms are fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. Concep ...
. First proposed by
Clarence Zener,
this theory predicts the relative ease with which an
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
may be exchanged between two species and has important implications for whether materials are
ferromagnetic
Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagne ...
,
antiferromagnetic, 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:
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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