A. Karmakar

Polaron relaxation and hopping conductivity in LaMn 1 − x Fe x O 3

dc and ac transport properties as well as electric modulus spectra have been investigated for the samples

Tuning A-site ionic size in R0.5Ca0.5MnO3 (R = Pr, Nd and Sm): robust modulation in dc and ac transport behavior

{\text{LaMn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}{\text{O}}_{3}

A Griffiths-like phase in antiferromagnetic R0.5Eu0.5MnO3 (R = Pr, Nd, Sm)

with compositions

Intragrain electrical inhomogenities and compositional variation of static dielectric constant in LaMn1?xFexO3

0\ensuremath{\le}x\ensuremath{\le}1.0

Nonlinearity exponent of ac conductivity in disordered systems

. The bulk dc resistivity shows a temperature variation consistent with the variable range hopping mechanism at low temperature and Arrhenius mechanism at high temperatures. The ac conductivity has been found to follow a power-law behavior at a limited temperature and frequency region where Anderson localization plays a significant role in the transport mechanism for all the compositions. At low temperatures large dc resistivity and dielectric relaxation behavior for all the compositions are consistent with the polaronic nature of the charge carriers. Scaling of the modulus spectra shows that the charge transport dynamics is independent of temperature for a particular composition but depends strongly on different compositions possibly due to different charge carrier concentrations and structural properties.

Structural and magnetic properties of spontaneously phase-separated Eu0.5Sm0.5MnO3

Structural investigations on Pr(0.5)Ca(0.5)MnO(3), Nd(0.5)Ca(0.5)MnO(3) and Sm(0.5)Ca(0.5)MnO(3) display a systematic tuning in orthorhombic distortion which appears due to a change in A-site ionic size. Comprehensive investigations on dc conduction using various models indicate strong correlation with structural changes. Careful analysis of the ac conductivity mechanism as a function of frequency and temperature brings out a pronounced effect of structural modifications on the polaron conduction mechanism. Contrary to typical manifestations, dc conductivity and relaxation of charge carriers reveal non-Arrhenius behavior. This work provides a detailed and systematic addition to the phase diagram of electronic transport in the rare-earth manganite system doped with a divalent alkali atom.

Observation of Griffiths phase in antiferromagnetic La0.32Eu0.68MnO3

We report a Griffiths-like phase in isovalent doped rare-earth manganites R(0.5)Eu(0.5)MnO(3) (R = Pr, Nd, Sm). Rietveld refinement of the structural data reveals strong orthorhombic and Jahn-Teller distortions. The dc and ac magnetic studies nicely demonstrate aspects of Griffiths phase-like behaviour. The presence of short range ferromagnetically correlated spin clusters is observed above the antiferromagnetic transition temperature. A disorder driven phase inhomogeneity arising from strong structural distortion is the possible origin of this behaviour.

Glassiness in charge dynamics of half-doped manganite: A study on oxygen-deficient charge-ordered R0.5Ca0.5MnO3−δ (R = Pr, Nd and Sm)

Bulk dc resistivity and dielectric constant measurements at different temperatures and frequencies have been performed in LaMn1?xFexO3 (0 ? x ? 1.0) as a result of Fe substitution both for as-synthesized and oxygen annealed samples. Temperature dependence of the real part of the dielectric constant at different frequencies show a frequency and temperature independent value (S) at low temperature for x ? 0.15 where S rises with x reaching a maximum at x = 0.5 and then decreases sharply at x = 0.70 showing a further increasing trend with a further increase in x. The sharp drop of S at x = 0.70 is correlated with the structural change from rhombohedral to the orthorhombic structure. A considerable increase in S ~ 43% is observed for an increase in x from 0.15 to 0.50. Furthermore, S is increased considerably (up to ~17% at x = 0.5) due to the oxygen annealing for x ? 0.50. The low temperature resistivities satisfying the variable range hopping model are found to be related to the increase in S with x for x ? 0.50. The analysis of the complex impedance and modulus (M) planes at low temperature indicates electrical inhomogeneities in the grain interior of the compounds where the capacitance obtained from the high-frequency complex-M arc shows an anomaly at the onset of the short range ferromagnetic ordering, suggesting a possible magnetoelectric coupling for x = 0.3.

Orthorhombic distortion and novel magnetic phase separation in Pr0.5Eu0.5MnO3

We measured the real part of ac conductance Σ(x,f) or Σ(T,f) of iron-doped mixed-valent polycrystalline manganite oxides LaMn(1-x)Fe(x)O(3) as a function of frequency f by varying initial conductance Σ(0) by quenched disorder x at a fixed temperature T (room) and by temperature T at a fixed quenched disorder x. At a fixed temperature T, Σ(x,f) of a sample with fixed x remains almost constant at its zero-frequency dc value Σ(0) at lower frequency. With increase in f, Σ(x,f) increases slowly from Σ(0) and finally increases rapidly following a power law with an exponent s at high frequency. Scaled appropriately, the data for Σ(T,f) and Σ(x,f) fall on the same universal curve, indicating the existence of a general scaling formalism for the ac conductivity in disordered systems. The characteristic frequency f(c) at which Σ(x,f) or Σ(T,f) increases for the first time from Σ(0) scales with initial conductance Σ(0) as f(c) ~ Σ(0)(x(f)), where x(f) is the onset exponent. The value of x(f) is nearly equal to one and is found to be independent of x and T. Further, an inverse relationship between x(f) and s provides a self-consistency check of the systematic description of Σ(x,f) or Σ(T,f). This apparent universal value of x(f) is discussed within the framework of existing theoretical models and scaling theories. The relevance to other similar disordered systems is also highlighted.

Engineering of exchange bias by A-site disorder in Pr1−xEuxMnO3

We have studied structural and magnetic properties of Eu(0.5)Sm(0.5)MnO(3). The system shows strong orthorhombic distortion. The static Jahn-Teller effect modifies the low temperature magnetic properties, creating a complicated anisotropic magnetic ground state. A low temperature magnetic phase separation has also been realized that builds magnetic interfaces exhibiting strong exchange bias. Careful analysis of the magnetic behaviour indicates the origin of the exchange bias to be a phase separation between an A-type antiferromagnetic phase and a low temperature highly anisotropic phase originating from another ordering at the rare-earth sublattice.