Ferromagnetic zigzag chains and properties of the charge ordered perovskite manganites

摘要

The low-temperature properties of the so-called ''charge ordered'' state in50% doped perovskite manganites are described from the viewpoint of themagnetic spin ordering. In these systems, the zigzag antiferromagneticordering, combined with the double-exchange physics, effectively divides thewhole sample into the one-dimensional ferromagnetic zigzag chains and resultsin the anisotropy of electronic properties. The electronic structure of onesuch chain is described by an effective 3$imes$3 Hamiltonian in the basis ofMn($3de_g$) orbitals. We treat this problem analytically and consider thefollowing properties: (i) the nearest-neighbor magnetic interactions; (ii) thedistribution of the Mn($3de_g$) and Mn($4p$) states near the Fermi level, andtheir contribution to the optical conductivity and the resonant x-rayscattering near the Mn $K$-absorption edge. We argue that the anisotropy ofmagnetic interactions in the double-exchange limit, combined with the isotropicsuperexchange interactions, readily explains both the local and the globalstability of the zigzag antiferromagnetic state. The two-fold degeneracy of$e_g$ levels plays a very important role in the problem and explains theinsulating behavior of the zigzag chain, as well as the appearance of theorbital ordering in the double-exchange model. Importantly, however, the chargeordering itself is expected to play only a minor role and is incompatible withthe ferromagnetic coupling within the chain. We also discuss possible effectsof the Jahn-Teller distortion and compare the tight-binding picture withresults of band structure calculations in the local-spin-density approximation.