Transport and magnetic properties of digital superlattices of (LaMnO$_{3})_{2n}$/(SrMnO$_{3})_{n}$.

Superlattices of (LaMnO$_{3})_{2n}$/(SrMnO$_{3})_{n }$, $n$=1-5, were synthesized using O$_{3}$-assisted MBE. Both constituents are antiferromagnetic insulators at low temperatures. The overall stoichiometry is the same as for La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ random alloys, but the $A$-site disorder is eliminated. For $n$=1, 2 a metallic ferromagnetic state is obtained at low T. For $n$=1, all measured properties are nearly identical to those of the random alloy. The emergence of a metallic state is interpreted in terms of a Mott transition driven by the proximity between LaMnO$_{3}$/SrMnO$_{3}$ interfaces. For $n\ge $3, a transition to an insulating state occurs, with a suppression of $T_{C }$ and $M_{s}$ and an increase in $H_{c}$. Using neutron and resonant x-ray scattering, we observe a modulation of the ferromagnetism commensurate with the superlattice period for $n$=5. We propose that magnetic frustration at the AF/F interfaces drives the insulating state. Ack: DOE BES contract {\#}DE-AC02-06CH11357.