On the role of exchange interaction in magnetic ordering and conduction of manganates

A model of chemical bonds in manganates that allows for one-electron covalent σ bonding between manganese and oxygen ions is suggested. One-electron covalent bonding results in the strongly correlated state of electrons due to exchange interaction between the electrons when they are shared by the cation and anion orbitals. The correlated state shows up as the spatial ordering of electrons and the ordering of their spins, causing the spin-ordered electron lattice to form. In this model, electrical conduction in manganates takes place when the electron lattice (more precisely, its part) shifts from one site of localization to another. The conductivity of the material depends on the type of the spin order of electrons in the electron lattice and on the energy of localization, which is defined by the energy of one-electron σ bonding. The model also implies the strong cationic polarization of anions, which facilitates the 3s2p hybridization of anions and the transition of one of the pairs of 2p electrons from the singlet state to the triplet one. The 3s2p hybridization of anions favors the formation of the spin-polarized electron lattice (the electron spins are parallel) and the ferromagnetic ordering of manganese ions. Under these assumptions, the effect of giant magnetoresistance is explained by a change in the conduction mechanism when an external voltage is applied. In this case, the conduction mechanism typical of ionic crystals changes to that specified by the spin-polarized electron lattice.