  Recent experiments have found new magnetic behaviors, which are different
from the parent bulk materials, at the interfaces between 3$d$ and 5$d$ oxides
such as SrMnO$_3$ (SMO) and SrIrO$_3$ (SIO). The system is of considerable
interest due to the strong spin-orbit coupling in the 5$d$ materials on one
hand and the double exchange physics in SMO on the other, which belongs to the
class of the colossal magnetoresistive (CMR) manganites. In order to gain
insight into the physics of the system, we have performed density-functional
studies on a selected interface structure, viz., the (SMO)$_1$(SIO)$_1$
superlattice, which has been experimentally grown and studied. Our
density-functional results show that the interfacial magnetism is controlled by
a net charge transfer at the interface from the SIO to the SMO side, turning
both of them into ferromagnetic metal from the original antiferromagnetic
insulating state in the bulk. The transferred electrons to the SMO side make it
ferromagnetic through the Anderson-Hasegawa double exchange interaction, while
the SIO part becomes ferromagnetic due to the doping of the half-filled
Mott-Hubbard insulator as suggested by the Nagaoka Theorem. Our results are
discussed in the context of the experiments for the same structure.
