ELECTRONIC-STRUCTURE AND HALF-METALLIC TRANSPORT IN THE LA1-XCAXMNO3 SYSTEM

Possible origins of ''colossal magnetoresistance'' (CMR) behavior in t he 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 publis hed work. As regards the end-point compounds CaMnO3 and LaMnO3, the ve ry different structural and magnetic symmetries of their ground states are predicted correctly. The distortion from the cubic perovskite str ucture of the LaMnO3 lattice is necessary to produce an antiferromagne tic insulating ground state. The distortion also strengthens the Mn ma gnetic moments. Application to ferromagnetic and constrained ferrimagn etic phases of La1-xCaxMnO3 in the CMR regime x approximate to 1/4-1/3 suggests, as observed, that magnetic coupling switches from antiferro magnetic to ferromagnetic. Hybridization between Mn d states and O p s tates 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 separa ted by a gap from the O p bands. Both ferromagnetic and ferrimagnetic orderings are obtained and compared. We identify strong local environm ent effects arising from neighboring cation charge differences (La3+ o r Ca2+) that suggest localization of the low density of minority carri ers, leading to effective half-metallic ferromagnetism in the CMR regi me. This behavior supports in some respects the popular ''double excha nge'' picture of Zener but indicates the Mn d-O p hybridization is muc h 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.