Strongly inhomogeneous conduction in cobaltite films: Non-Gaussian resistance noise

Strongly non-Gaussian conductance fluctuations are found in relatively large (e.g., 1× 10^-10 cm³) samples of (001)-oriented ∼50-nm -thick epitaxial films of the nominally ferromagnetic and metallic perovskite cobaltite La_0.5Sr_0.5CoO₃. Field dependences of Boltzmann factors of individual fluctuators show them to be magnetic regions with of order 1× 10⁴ Bohr magnetons. Given the size of the fluctuating regions, the size of the discrete conductance fluctuations would be anomalously large unless the conductance were strongly inhomogeneous as expected for magnetoelectronic phase-separation effects known to occur in some bulk cobaltites at lower concentrations or with polycrystalline structure. Temperature dependences of Boltzmann factors indicate small energy and entropy changes, more like magnetic rotations than like fluctuations between phases. The films, like bulk samples of the same composition, show resistance increasing with temperature in Van der Pauw measurements; but resistance in strip-shaped regions weakly decreases with temperature, indicating particularly extreme effects of the inhomogeneous magnetic orientation dependent conductivity.