Double-exchange mechanism

The double-exchange mechanism is a theory that predicts the relative ease with which an electron may be exchanged between two species. For example, consider the 180 degree interaction of Mn-O-Mn in which the Mn "eg" orbitals are directly interacting with the O "2p" orbitals.



Hence, double-exchange predicts that 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.

Having correct spin direction will allow for easier conduction of electrons. For example, in the case of a superconductor just above the critical temperature (Tc) needed to display superconductivity, electrons are not fully aligned and therefore have a high resistance to electrical current. At just below the Tc electrons are aligned by the magnetic effect of the super-conductor and we see a reduction in the electrical resistance (dropping to ~zero).