Phase separation at the La$_{1-x}$Sr$_{x}$CoO$_{3}$ / SrTiO$_{3}$ (001) interface from thickness and doping dependent magnetotransport*

Bulk La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO) has received considerable attention with regard to nanoscale magnetic phase separation. Fabrication of epitaxial films provides a means to study this phase separation under dimensional confinement and at interfaces. Moreover, the characteristic intercluster ``GMR'' effect observed in the phase-separated state of this material provides a simple means to probe phase separation even in very thin films. We have found that even at x = 0.5 (a homogeneous ferromagnetic metal in bulk), sufficiently thin films ($<$ 60 {\AA}) grown on SrTiO$_{3}$ (001) show a crossover to a reduced moment insulating phase, with the characteristic intercluster MR effect, i.e. phase separation occurs. By measuring the thickness dependence of the magnetotransport as a function of doping we have assembled a three-dimensional phase diagram in temperature-doping-thickness space. The interfacial phase-separated region increases dramatically as x is decreased (to over 250 {\AA} at x = 0.2) and the non-ferromagnetic phase boundary is approached. The increased prominence of phase separation as proximity to the non-ferromagnetic phase is increased provides a strong hint to the origin of this interfacial phase separation.