Equilibrium conductivity fluctuations of mesoscopic domains are found in film and bulk single-crystal manganite colossal magnetoresistive material. Temperature and field dependences of the Boltzmann factors for a collection of two-state fluctuators give measures of the magnetic moment and entropy differences between the states, and of the fluctuator volumes. The large resistance step size implies dramatic current inhomogeneities. Occasional anomalous temperature dependences indicate that the film inhomogeneous phase is stabilized by a repulsive interaction between conducting regions.

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We report measurements of voltage fluctuations in magnetic tunnel junctions which exhibit both high and low magnetoresistance ͑MR͒. The voltage noise power normalized to the square of the junction bias voltage was 10 Ϫ14 /Hz at a frequency of 1 Hz in a high MR junction. Low MR junctions had significantly higher noise power at 1 Hz and the origin of the noise was not magnetic. In these junctions, random telegraph noise was observed over a wide range of temperatures and junction biases. The results are consistent with a two-channel model of conduction, one of which is spin independent and gives rise to large noise. A noise measuring technique provides evidence for bias-dependent current-path rearrangements. The data support the existence of an inhomogeneous ͑filamentary-like͒ current-flow pattern across the tunnel junction associated with the spin-independent channel.

The sensitivity of low-frequency noise to ac magnetic-field perturbations is explored in the plastic flow regime near the ''peak effect'' in the clean type-II superconductor NbSe 2. Very small ac magnetic fields ͑on the order of 1 Oe in a dc field of about 20 000 Oe͒ substantially reduce the low-frequency spectral density by inducing rapid switching among metastable configurations present in the pinned state. This sensitivity to ac fields shrinks rapidly as H increases in the peak-effect regime, indicating reduced static correlation lengths as the peak is approached. ͓S0163-1829͑98͒50902-2͔

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Epitaxial films of La 0.72 Sr 0.28 MnO 3 ͑LSMO͒ are found to show almost no excess conductivity fluctuations associated with the colossal-magnetoresistive transition, in contrast to the closely related material La 0.67 Ca 0.33 MnO 3 ͑LCMO͒. The result fits with the contrast between the suspected second-order transition in LSMO and the ͑locally͒ first-order transition and mixed phase in LCMO.

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Discrete two-state resistance fluctuations are found deep within the metallic phase of a colossalmagnetoresistive film. Thermodynamic derivatives show the ratios of entropy changes to magnetic moment changes of these fluctuators to be very small, with mixed signs, unlike fluctuators found near the transition temperature. These low ratios are a signature of magnetic orientation fluctuations, indicating an important role for domain-wall effects on the transport properties in the metallic regime.