Magnetoresistive dynamics and noise in low-strain manganite films

ac magnetoresistance measurements of low-strain epitaxial films of La_0.7Ca_0.3MnO₃ on NdGaO₃ substrate show an out-of-phase response which switches sign near the metal-insulator transition, indicating at least two types of slow contributors to the magnetoresistance and suggesting the role of a third phase. These films show noise properties very distinct from films under tensile strain. A narrow peak in the noise found at the transition is non-Gaussian but lacks distinct persistent few-state fluctuators. Such fluctuators only appear some 10 K below the transition, indicating that conduction in the nominally metallic phase remains highly inhomogeneous. Comparison of discrete fluctuator sizes in a range of materials indicates that strain constraints limit the sizes in films.