We examine, by means of ab initio pseudopotential calculations, La2/3Sr1/3MnO3/SrTiO3 (LSMO/STO) heterojunctions in which one unit layer of La1−xSrxMnO3 (with 0<x<1) is inserted at the interface. The optimal interlayer doping x for a robust interface ferromagnetism is investigated by considering the energy differences between antiferromagnetic and ferromagnetic alignment of the MnO2-interface layer relative to bulk LSMO. The optimal doping is found to be close to x = 1/3, which corresponds to an abrupt TiO2 (001)-layer termination of STO. This is also the composition which gives the largest p-type Schottky barrier height in our calculations.