Modulation-doped ferromagnetism in digitally synthesized manganite superlattices

We have digitally synthesized ordered analogs of La$_{1-x}$Sr$_x$MnO$_3$ by interleaving single unit cell layers of LaMnO$_3$ and SrMnO$_3$ with atomic layer precision using ozone-assisted molecular beam epitaxy. In our neutron diffraction experiments on these epitaxial superlattices near $x=0.5$, we confirmed the $A$-type antiferromagnetic spin structure with an enhanced ordering temperature. We found that in superlattices with composition at the ferromagnetic/antiferromagnetic phase boundary, inserting an additional single unit cell layer of LaMnO$_3$ causes a significant increase of the net magnetic moment while still retaining the $A$-type spin structure. Our polarized neutron reflectometry experiments revealed a highly modulated moment commensurate with the structural periodicity of the superlattice, with higher moment in the region of the extra LaMnO$_3$ layer. Thus, introducing a single La dopant layer results in a localized enhancement of double exchange along the c-axis and a canted moment in an otherwise antiferromagnetic structure. PNR analysis reveals the length scale over which these modulation-doped charges extend normal to the interfaces. Supported by DOE, Basic Energy Sciences, contract No. DE-AC02-06CH11357.