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Register a new userBias Dependent 1/f Conductivity Fluctuations in Low-Doped La$_{1-x}$Ca$_{x}$MnO$_3$ Manganite Single Crystals
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Added by
Mikhail Belogolovskii
Publication date
2011
fields
Physics
and research's language is
English
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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.
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Conductivity noise in dc current biased La_{0.82}Ca_{0.18}MnO_{3} single crystals has been investigated in different metastable resistivity states enforced by applying voltage pulses to the sample at low temperatures. Noise measured in all investigated resistivity states is of 1/f-type and its intensity at high temperatures and low dc bias scales as a square of the bias. At liquid nitrogen temperatures for under bias exceeding a threshold value, the behavior of the noise deviates from above quasi- equilibrium modulation noise and depends in a non monotonic way on applied bias. The bias range of nonequilibrium 1/f noise coincides with the range at which the conductance increases linearly with bias voltage. This feature is 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 metastable states to variations in the energy landscape for charge carriers introduced by microcracks created by the pulse procedures employed
We report low temperature specific heat measurements of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ ($0.3leq x leq 0.5$) and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ with and without applied magnetic field. An excess specific heat, $C^{prime}(T)$, of non-magnetic origin associated with charge ordering is found for all the samples. A magnetic field sufficient to induce the transition from the charge-ordered state to the ferromagnetic metallic state does not completely remove the $C^{prime}$ contribution. This suggests that the charge ordering is not completely destroyed by a melting magnetic field. In addition, the specific heat of the Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ compounds exhibit a large contribution linear in temperature ($gamma T$) originating from magnetic and charge disorder.
We report new zero-field muon spin relaxation and neutron spin echo measurements in ferromagnetic (FM) (La,Ca)MnO3 which taken together suggest two spatially separated regions in close proximity possessing very different Mn-ion spin dynamics. One region corresponds to an extended cluster which displays critical slowing down near Tc and an increasing volume fraction below Tc. The second region possesses more slowly fluctuating spins and a decreasing volume fraction below Tc. These data are discussed in terms of the growth of small polarons into overlapping regions of correlated spins below Tc, resulting in a microscopically inhomogeneous FM transition.
1/f noise in current biased La0.82Ca0.18MnO3 crystals has been investigated. The temperature dependence of the noise follows the resistivity changes with temperature suggesting that resistivity fluctuations constitute a fixed fraction of the total resistivity, independently of the dissipation mechanism and magnetic state of the system. The noise scales as a square of the current as expected for equilibrium resistivity fluctuations. However, at 77 K at bias exceeding some threshold, the noise intensity starts to decrease with increasing bias. The appearance of nonequilibrium noise is interpreted in terms of bias dependent multi-step indirect tunneling.
We report on an a $mu$SR and $^{55}$Mn NMR investigation of the magnetic order parameter as a function of temperature in the optimally doped La$_{5/8}$(Ca$_y$Sr$_{1-y}$)$_{3/8}$MnO$_3$ and in the underdoped La$_{1-x}$Sr$_{x}$MnO$_3$ and La$_{1-x}$Ca$_{x}$MnO$_3$ metallic manganite families. The study is aimed at unraveling the effect of lattice distortions, implicitly controlled by the Ca-Sr isoelectronic substitution, from that of hole doping $x$ on the Curie temperature $T_c$ and the order of the magnetic transition. At optimal doping, the transitions are second order at all $y$ values, including the $y=1$ (La$_{5/8}$Ca$_{3/8}$MnO$_3$) end member. In contrast, they are first order in the underdoped samples, which show a finite (truncated) order parameter at the Curie point, including La$_{0.75}$Sr$_{0.25}$MnO$_3$ whose $T_c$ is much higher than that of La$_{5/8}$Ca$_{3/8}$MnO$_3$. The order parameter curves, on the other hand, exhibit a very minor dependence on $x$, if truncation is excepted. This suggests that the effective exchange interaction between Mn ions is essentially governed by local distortions, in agreement with the original double-exchange model, while truncation is primarily, if not entirely, an effect of under- or overdoping. A phase diagram, separating in the $x-y$ plane polaron-driven first order transitions from regular second order transitions governed by critical fluctuations, is proposed for the La$_{1-x}($Ca$_y$Sr$_{1-y}$)$_{x}$MnO$_3$ system.
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