Bias Dependent 1/f Conductivity Fluctuations in Low-Doped La$_{1-x}$Ca$_{x}$MnO$_3$ Manganite Single Crystals

Abstract

Low frequency noise in current biased La 0.82 Ca 0.18 MnO 3 single crystals has been investigated in a wide temperature range from 79 K to 290 K. Despite pronounced changes in magnetic properties and dissipation mechanisms of the sample with changing temperature, the noise spectra were found to be always of the 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 and its dependence on bias have been associated with changes in the distribution of activation energies in the underlying energy landscape. These changes have been correlated with bias induced changes in the intrinsic tunneling mechanism dominating dissipation in La 0.82 Ca 0.18 MnO 3 at low temperatures.

On origin of 1/f noise in manganites: memoryless transport against mysterious slow fluctuators

“…In the mentioned temperature range, resistance of the L = 0.3 mm long part of rectangular crystal with crosssection A = 2×3 mm 2 changed between R(300 o K) ≈ 2 Ohm and R(80 o K) ≈ 200 Ohm [5]. At that, the power spectral density (PSD) of relative resistance fluctuations was practically independent on temperature,…”

“…If, nevertheless, they really take place and, -as authors of [5] do allude, -represent more or less local structural rearrangements or switchings between coexisting phases, then we can write…”

“…The so-called perovskite manganites, or colossal magneto-resistance manganites [1,2], are materials known as "1/f-noise champions". For proper references see [3][4][5], since just their extremely interesting experimental results stimulated my present communication. Namely, first, observation of very high level of 1/f noise in good bulk crystals (instead of thin films as usually).…”

“…Second, very weak dependence of this noise (expressed in standard relative units via S R (f )/R 2 ) on temperature in wide from the room one down to 79 • K . At 79 • K transition to strongly non-ohmic regime was found and attracted most authors' "theoretical interest" in [3][4][5].…”

“…In my opinion, however, discussion of the wide Ohmic region may be much more useful for understanding nature of 1/f-noise. Below I will try to suggest a principal alternative to the hypotheses seemingly accepted by authors of [3][4][5] .…”