Y. Kalyana Lakshmi

Magnetoelectric behavior of sodium doped lanthanum manganites

Nanocrystalline samples of sodium doped manganites with compositional formula La1−xNaxMnO3 (0.025⩽x⩽0.25) were prepared by polyvinyl alcohol assisted precursor method. After characterizing the samples by x-ray diffraction and transmission electron microscopy a systematic investigation of electrical, magnetic, and thermopower properties has been undertaken. The resistivity data were analyzed using effective medium approximation. From the analysis it has been found that the metallic fraction is increasing up to x=0.10 and remains constant with further doping. A close examination of the resistivity data clearly indicates that the sodium doped samples are slowly transformed from colossal magnetoresistance behavior to charge ordering behavior. Thermoelectric power data at low temperatures were analyzed by considering the magnon drag concept, while the high temperature data were explained by small polaron conduction mechanism.

Thermopower studies of Pr0.67D0.33MnO3 manganite system

With a view to explaining the structural and electrical transport behaviour of some rare earth manganites, having the general formula Pr0.67D0.33MnO3 (D = Ca, Sr, Pb and Ba), a systematic investigation of the electrical resistivity, thermopower and magnetic properties has been undertaken. These materials were prepared using the citrate–gel route by sintering at 900 ◦ C. All the materials were characterized by x-ray diffraction, scanning electron microscopy, etc measurements. The x-ray data were analysed using the Rietveld method and the variation of various parameters involved in the process as explained. The average crystallite size of the materials has been estimated using the peak broadening method, while TC and TP values were determined from ac magnetic susceptibility and electrical resistivity measurements, respectively. Finally, the magnetoresistance (MR) measurements were also carried out over a magnetic field of 1–7 T and the data clearly exhibit the extrinsic MR in ferromagnetic metallic region.

Thermopower and resistivity studies of Nd-Na-Mn-O manganites

The electrical transport properties of the polycrystalline Nd1−xNaxMnO3 (0.05 ≤ x ≤ 0.25) were investigated. All the samples are found to exhibit insulating behavior down to the lowest measured temperature. However, on application of 4 T magnetic field, metal-insulator transitions along with a low temperature resistivity upturn are observed for the samples with x ≥ 0.15. Thermopower values of samples with x = 0.05, 0.10, and 0.20 are negative in the entire temperature range of investigation, while they changes from negative to positive with decreasing temperature in the case of samples with x = 0.15 and 0.25. The resistivity and thermopower data in the high temperature region are explained within the framework of variable range and small polaron hopping models, while the resistivity minima are explained using the combined effect of electron-electron and electron-phonon scattering is considered.

Thermopower Studies of Polycrystalline Ag Doped LaMnO3 Manganites

The effects of silver doping on the magnetotransport and thermopower of La1−xAgxMnO3 (0.05≤x≤0.30) have been investigated. For the sample with x=0.05, temperature dependent resistivity exhibits an insulating behavior, while thermopower is found to be large and negative over the measured temperature range. An increase in the Ag doping enhances the conductivity and shifts the metal-insulator transition temperature toward high temperature side. The low temperature thermopower data has been explained in terms of diffusion, magnon drag, and phonon drag effects and found that the magnon drag effect dominates in this region. Finally, the electrical transport in the high temperature region has been analyzed by using adiabatic small polaron hopping mechanism.

Influence of Sintering Temperature on Magnetotransport Behavior of Some Nanocrystalline Manganites

The colossal magnetoresistance (CMR) in hole doped manganese oxides, widely known as manganites with formula Ln1-xAexMnO3 (where Ln is a trivalent rare earth and Ae is a divalent alkaline earth ion; x=0-1), has been intensively studied over the last two decades. In general, these systems comprise a strong competition between charge, orbital, lattice and spin degrees of freedom all of which make them an intriguing subject of research (Dogotto, 2003; Goodenough, 2003; Ramirez, 1997; Tokura, 2000). The end-members of this system are antiferromagnetic (AFI) insulators. Partial substitution of the Ln with divalent alkaline-earth ion of the ABO3 structure introduces mixed valence Mn4+/Mn3+ on the B site. With decreasing temperature the system undergoes a transition from the paramagnetic to the ferromagnetic state accompanied by a metal-insulator transition. A large number of studies on CMR materials of different forms such as single crystals (Okuda et al., 1998; Zhou et al., 1997), thin film (Kwon et al., 1997; Rao et al., 1998; Suzuki et al., 1997) and ceramics (Hwang et al., 1995; Mahendiran et al., 1996a, 1996b) for the basic research point of view and also the possible future device applications view point. The Zener (1951) has proposed a simplest model, known as double-exchange (DE) model to explain the electrical behavior in ferromagnetic metallic region below Curie Temperature (TC). However, due to various interactions among charge, spin and lattice makes these materials more complex and DE alone cannot explain the entire electrical transport behavior. Later on various theoretical models have been proposed by considering, electron-lattice and spin lattice interaction and even today there is no comprehensive model to explain transport phenomena in manganites (Millis et al., 1995; Tokura, 2000).