  We examine, by means of ab initio pseudopotential calculations,
La_2/3Sr_1/3MnO_3/SrTiO_3 (LSMO/STO) heterojunctions in which one unit layer of
La_(1-x)Sr_xMnO_3 (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 MnO_2-interface layer relative to bulk LSMO. The optimal
doping is found to be close to x=1/3, which corresponds to an abrupt TiO_2
(001)-layer termination of STO. This is also the composition which gives the
largest p-type Schottky barrier height in our calculations.
