Electronic structure and half-metallic transport in the La1-xCaxMnO3 system

Possible origins of ''colossal magnetoresistance'' (CMR) behavior in the La1-xCaxMnO3 system are studied using the local spin-density method. These calculations allow the quantification of the effects of Mn d-O p hybridization that have been largely neglected in previously published work. As regards the end-point compounds CaMnO3 and LaMnO3, the very different structural and magnetic symmetries of their ground states are predicted correctly. The distortion from the cubic perovskite structure of the LaMnO3 lattice is necessary to produce an antiferromagnetic insulating ground state. The distortion also strengthens the Mn magnetic moments. Application to ferromagnetic and constrained ferrimagnetic phases of La1-xCaxMnO3 in the CMR regime x approximate to 1/4-1/3 suggests, as observed, that magnetic coupling switches from antiferromagnetic to ferromagnetic. Hybridization between Mn d states and O p states is found to be strongly spin dependent, because the majority Mn d bands overlap the O p bands while the minority Mn d bands are separated by a gap from the O p bands. Both ferromagnetic and ferrimagnetic orderings are obtained and compared. We identify strong local environment effects arising from neighboring cation charge differences (La3+ or Ca2+) that suggest localization of the low density of minority carriers, leading to effective half-metallic ferromagnetism in the CMR regime. This behavior supports in some respects the popular ''double exchange'' picture of Zener but indicates the Mn d-O p hybridization is much too strong to be considered perturbatively. Half-metallic character promotes the possibility of very large magnetoresistance, and may well be an essential ingredient in the CMR effect.