Diffusion and segregation effects in doped manganite/titanate heterostructures

For the properties of devices based on oxide multilayers, interfaces play a crucial role. For ferromagnetic tunnel junctions the sign of the spin polarization depends on the barrier material and the interface termination. Here, we present a transmission electron microscopy study of heteroepitaxial trilayer systems consisting of a $SrTiO3$ insulating barrier sandwiched between $La2/3D1/3MnO3$ $(D=Ca$ or Ba) electrodes grown on $SrTiO3$ substrates. Intermixing during growth depends on the earth alkaline ion: Ca segregates to the surface of the growing epitaxial thin film, leading to a Ca rich interface layer between the bottom electrode and the barrier. In contrast, no Ba segregation could be detected in the well lattice-matched Ba-doped film. In addition, a significant diffusion of both Ca and Ba into the $SrTiO3$ tunneling barrier is found, whereas the diffusion of Mn is negligibly small. Our results explain the different tunneling magnetoresistance in various manganite based tunnel junctions.