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 Clarence Melvin Zener ( ; December 1, 1905 – July 2, 1993) was an American physicist who in 1934 was the first to describe the property concerning the breakdown of electrical insulators. These findings were later exploited by Bell Labs in the ...

, 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 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 Spin (physics), spins (on different sublattices) pointing in oppos ...

, 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 The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than 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 Mn⁴⁺, its vacant orbital can then be filled by an electron from Mn³⁺. 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 In atomic physics and quantum chemistry, Hund's rules refers to a set of rules that German physicist Friedrich Hund formulated around 1925, which are used to determine the term symbol that corresponds to the ground state of a multi-electron atom ...

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 Superexchange or Kramers–Anderson superexchange interaction, is a prototypical ''indirect'' exchange coupling between neighboring magnetic moments (usually next-nearest neighboring cations, see the schematic illustration of MnO below) by virtue ...

. 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 {{quantum-chemistry-stub es:Doble canje