Inhomogeneities In Manganites Research Articles

Effect of Iron Doping on Structural, Magnetic, and Electrical Characteristics of Manganites in La0.7Sr0.3Mn0.9Zn0.1 –xFexO3 (0 ≤ x ≤ 0.1) System

Experimental data on the structure and properties of manganites of La0.7Sr0.3Mn0.9Zn0.1 –xFexO3 (0 ≤ x ≤ 0.1) system synthesized by ceramic processing and annealed under conditions ensuring the achievement of stoichiometric oxygen concentrations are presented. All obtained manganites have a rhombohedral structure. The substitution of iron for zinc leads to an increase in the unit cell volume and average cation–anion distance in the octahedral sublattice, which is determined by the increase in the concentration of Mn3+ ions due to the decrease in the concentration of Mn4+ as a result of charge compensation processes. The angles between the Mn–O–Mn bonds change slightly. Curie point, magnetization, and temperature of the metal–semiconductor transition as a function of the concentration of iron are characterized by the presence of a maximum. The maximum value of the resistance of manganite with x = 0.1 is a factor of 800–1000 higher than the maximum values of the resistance of other samples in the studied range of temperatures. Temperature dependences of magnetic permeability are indicative of the existence of magnetic inhomogeneities in manganites, especially in the samples with x = 0 and 0.1, which may be due to the nonuniform distribution of Zn2+ ions and the presence of ferromagnetic and paramagnetic clusters. These data are in agreement with the results of the investigation of the electron paramagnetic resonance (EPR) spectra containing additional and/or broadened lines. Characteristic features of the dependences of the properties of studied manganites on the concentration of iron are explained by the influence of a series of competing factors, namely, the decrease in the concentration of Mn4+ ions and probability of antiferromagnetic interaction between them, change in the number of disturbed exchange bonds between Mn4+ and Mn3+ ions, and variations of the characteristics of magnetic inhomogeneities.

Order and dynamics of intrinsic nanoscale inhomogeneities in manganites

Neutron elastic, inelastic, and high-energy x-ray scattering techniques are used to explore the nature of the polaron order and dynamics in the colossal magnetoresistive (CMR) system ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$. Polaron correlations are known to develop within a narrow temperature regime as the Curie temperature is approached from low temperatures, with a nanoscale correlation length that is only weakly temperature dependent. The static nature of these short-range polaron correlations indicates the presence of a glasslike state, very similar to the observations for the bilayer manganite in the metallic-ferromagnetic doping region. In addition to this elastic component, inelastic scattering measurements reveal dynamic correlations with a comparable correlation length and with an energy distribution that is quasielastic. The elastic component disappears at a higher temperature ${T}^{*}$, above which the correlations are purely dynamic. These observations are identical to the polaron dynamics found in the bilayer manganite system in the CMR regime, demonstrating that they are a general phenomenon in the manganites.

Coulomb Interactions and Nanoscale Electronic Inhomogeneities in Manganites

We study electronic inhomogeneities in manganites using simulations on a microscopic model with Coulomb interactions amongst two electronic fluids-one localized (polaronic), the other extended-and dopant ions. The long range Coulomb interactions frustrate phase separation induced by the large on site repulsion between the fluids. A single phase ensues which is inhomogeneous at a nanoscale, but homogeneous on mesoscales, with many features that agree with experiments. This, we argue, is the origin of nanoscale inhomogeneities in manganites, rather than phase competition or disorder effects.

Transverse resistance measurements: a very sensitive probe to charge inhomogeneities in manganites

Transverse resistance measurements are reported on patterned epitaxial thin films of optimally doped La2/3Ca1/3MnO3 oxides. We show that the voltage drop measured across a micron-size patterned track displays a temperature dependence that, particularly in a temperature region around the Curie temperature, cannot be explained solely by the contribution of a longitudinal resistivity component. We argue that this response reflects inhomogeneous current flow, which sets in dramatically around the Curie temperature, likely related to charge trapping at microstructural defects. Indeed, we show that appropriate thermal annealing allows reducing to some extent their contribution. Thus we conclude that this experimental set-up allows for the monitoring of how electronic phase separation develops in optimally doped manganites. The impact of this effect on the determination of Hall coefficients is discussed.

Giant cluster coexistence in doped manganites and other compounds

Computational studies of models for manganese oxides show the generation of large coexisting metallic and insulating clusters with equal electronic density, in agreement with the recently discovered micrometer-sized inhomogeneities in manganites. The clusters are induced by disorder on exchange and hopping amplitudes near first-order transitions of the nondisordered strongly coupled system. The random-field Ising model illustrates the qualitative aspects of our results. Percolative characteristics are natural in this context. The conclusions are general and apply to a variety of compounds.