Electronic structure of zigzag ferromagnetic chains and conditions for the existence of insulating state in manganites

The tight-binding method and the double-exchange Hamiltonian with infinite on-site Hund interaction are used to calculate the spectrum of e g electrons in manganites for various magnetic structures composed of ferromagnetic zigzag spin chains that have two to eight spins per one-dimensional unit cell. The values of the carrier concentration x are found at which the band insulator state can arise in antiferromagnetic manganites La 1 − x Ca x MnO 3 containing the above chains. In addition to the known spectra of (1 × 1) and (2 × 2) chains, the spectrum of a (3 × 3) zigzag chain is obtained for the first time. A three-dimensional unit cell consisting of this type of chains and having 24 manganese atoms permits a correct qualitative description of the atomic and magnetic structures of the manganite La 1/3 Ca 2/3 MnO 3 .