In thin-film La_0.6Ca_0.4MnO₃, conducting-tip and magnetic force microscopy reveal a pattern of nanoscale phase separation that is reproducible across cooling runs. This pattern represents the intersection of buried three-dimensional filamentary ferromagnetic metallic pathways with the sample surface. As an interlayer in current-perpendicular-to-the-plane trilayer devices, this phase-separated material magnetically decouples ferromagnetic metallic La_0.7Ca_0.3MnO₃ electrodes which switch sharply. This yields sharp two-state low-field magnetoresistance that is also reproducible across cooling runs. The reproducibility and the magnitude of the resistance jump are linked to highly resistive (~10^-12 Ωm²) constrained domain walls in the pathways of the phase-separated interlayer. Phase separation is normally associated with high-field colossal magnetoresistance and, therefore, its exploitation here to produce low-field effects is unusual.