Eur. Phys. J. B 30, 501-510 (2002)
https://doi.org/10.1140/epjb/e2002-00407-3

Phase evolution of layered cobalt oxides versus varying corrugation of the cobalt-oxygen basal plane

Max-Planck-Institut für Physik komplexer Systeme, Nöthnitzer Str. 38, 01187 Dresden, Germany

Corresponding author: ^a wu@fhi-berlin.mpg.de

Received: 28 July 2002
Revised: 10 October 2002
Published online: 31 December 2002

Abstract

A general spin-state model and a qualitative physical picture have been proposed for a class of lately synthesized layered cobalt oxides (LCOs) by means of density functional calculations. As the plane corrugation of the cobalt-oxygen layer decreases, the LCOs evolve from a high-spin (HS) superexchange-coupled antiferromagnetic (AFM) insulator to an almost-HS AFM/ferromagnetic (FM) competing system where the FM coupling is mediated via the p-d exchange by an increasing amount of delocalized holes having mainly the planar O 2p character. It is tentatively suggested that the delocalized holes more than 0.3 per CoO₂ basal square are likely necessary for the insulator-metal and/or AFM-FM transitions in the corrugation-weakened LCOs. A phase control may be realized in LCOs by varying the plane corrugation (thus modifying the hole concentration) through an ionic-size change of the neighboring layers on both sides of the cobalt-oxygen layer. In addition, a few experiments are suggested for a check of the present model and picture.