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

Abstract

Possible origins of ‘‘colossal magnetoresistance’’ (CMR) behavior in the ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{MnO}}_3$ 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 ${\mathrm{CaMnO}}_3$ and ${\mathrm{LaMnO}}_3$, the very different structural and magnetic symmetries of their ground states are predicted correctly. The distortion from the cubic perovskite structure of the ${\mathrm{LaMnO}}_3$ 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 ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{MnO}}_3$ in the CMR regime x≊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 (${\mathrm{La}}^{3+}$ or ${\mathrm{Ca}}^{2+}$) 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.