Metastability is a generic feature of phase separated systems with two or more competing ordering mechanisms. Mixed valence manganites are characterized by a complex phase diagram containing many magnetic and electronic phases. The multi-phase state stems from interplay of structural, charge, orbital, and spins degrees of freedom with comparable energy scales. Coexistence of phases with different orbital order and electronic properties leads to appearance of metastable states with markedly different resistivity. Metastable resistivity states can be induced by making relatively fast changes of experimental parameters such as magnetic field, electrical bias, or temperature. Metastable states are characterized by history dependent conductivity, pronounced relaxation of magnetization and resistivity, memory effects in electric resistance and magnetization, and strong 1/f-type conductivity noise frequently accompanied by non-Gaussian fluctuations.
In the talk we will discuss conductivity noise observed in dc current biased La0.82Ca0.18MnO3 (LCMO) single crystals. Despite pronounced changes in the magnetic state and dissipation mechanism with changing temperature, the noise spectra were found to be always of 1/f type and their intensity (except the lowest temperature studied) scaled as a square of the bias. At liquid nitrogen temperatures, and under bias exceeding some threshold value, the behavior of the noise deviates from the quasi-equilibrium modulation noise and starts to depend in a non monotonic way on bias. It has been verified that the observed noise obeys Dutta and Horn model of 1/f noise in solids. The appearance of nonequilibrium 1/f noise has been associated with changes in the underlying energy landscape, correlated with changes in the intrinsic tunneling mechanism which dominates dissipation in La0.82Ca0.18MnO3 at low temperatures.
Long living metastable resistivity states were enforced by applying voltage pulses to LCMO crystals at low temperatures. The bias range of nonequilibrium 1/f noise in all metastable states was found to coincide with the range at which sample conductance increases linearly with bias voltage. This feature has been attributed to a broad continuity of states enabling indirect inelastic tunneling across intrinsic tunnel junctions. The nonequilibrium noise has been ascribed to indirect intrinsic tunneling mechanism while resistivity changes in these particular metastable states to microcracks created by the pulse procedures employed.