Transport and Magnetotransport Studies on Sol–Gel Grown Nanostructured La_0.7Pb_0.3MnO₃ Manganites

Abstract

Sol-Gel synthesis offers relatively inexpensive scale processing of mixed oxide materials with a good control over the stoichiometry and morphology which helps to tailor the required materials on atomic scale to suit specific applications. Nanophasic polycrystalline samples of La0.7Pb0.3MnO3 (LPMO) manganites having perovskite type structure synthesized by novel Sol-Gel technique using acetate precursor route were sintered at various temperatures in the range 950-1150 degrees C for studying the effect of grain size modifications on their structural, transport and magnetotransport properties. X-ray diffraction (XRD) studies show that the samples exhibit rhombohedral structure crystallizing in space group R-3C. Microstructural investigations using Scanning Electronic Microscopy (SEM), Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFM) measurements reveal the observation of secondary grain growth behavior starting in the sample sintered at 1000 degrees C. The d.c. four probe resistivity measurements with and without applied magnetic field in the temperature range of 2-380 K, show the effect of secondary grain growth on the magnetoresistance (MR) behavior of LPMO manganites. The microstructural studies show the atomic scale engineering at nanoscale which is reflected in the improvement of surface to volume ratio (D(-1)) which in turn modifies the physical properties of samples under investigation. All the samples exhibit resistivity minima at approximately 30 K which can be explained in terms of e-e interaction at 0 Tesla field. There is a correlation between the parameters derived from e-e scattering model and the secondary grain growth present in the samples. The results of the microstructural and MR measurements on the nanostructured LPMO manganites have been discussed in detail.

Structure and microstructure dependent transport and magnetic properties of sol–gel grown nanostructured La 0.6 Nd 0.1 Sr 0.3 MnO 3 manganites: Role of oxygen

“…resistivity increases with decrease in temperature, well below 50K, as shown in figure 7. Similar observation has been reported earlier for nanostructured La 0.7 Pb 0.3 MnO 3 manganites synthesized using the same sol-gel method [16].…”

“…process temperature and upon annealing is attributed to the increased particle size and hence decreased number of boundaries resulting in the suppression in charge carrier scattering at the boundaries [11, 12, 16 -18]. It is worth to note that with increase in process temperature, T P increases which can be understood as -with increase in process temperature (for both the sintering and annealing processes), particle size increases with the sharp and clear boundaries and hence non-magnetic phase fraction gets suppressed at the boundaries due to decrease in number of Mn-O dangling bonds at the boundaries [16]. This in turn results in the enhancement in transport of e g electrons causing an effective zener double exchange mechanism with increase in T P .…”

“…They have suggested that resistivity upturn is a consequence of electron-electron elastic scattering exists at low temperature in manganites [16,37]. This upturn can also be related to an electrostatic blockade of charge carriers (eg electrons) at boundaries [38] originated mainly due to the competition between the elastic and inelastic scattering of the charge carriers below and above upturn temperature (T up ), respectively [12].…”

“…Sol-Gel method has proved to be one of the most successful and simple methods for the synthesis of nanostructured mixed valent manganites [16,18] …”

Structural, transport and magnetic properties of monovalent doped La1−xNaxMnO3 manganites

“…Variation in exponent n indicates a continuous decrease with monovalent substitution which may be correlated with the increased hole density in La 1-x Na x MnO 3 manganites.To explore the magnetoresistance (MR) behavior of La 1-x Na x MnO 3 manganites, MR has been calculated using the formula: MR (%) = [(ρ H -ρ 0 ) / ρ 0 ] × 100 from ρ -T plots of LNMO samples.Fig.15shows MR vs. T plots under 1, 5 and 8T fields for LNMO samples. It can be seen that, all the samples exhibit appreciably large negative MR at low temperature ~ 5K (extrinsic contribution) and ~ T P (intrinsic contribution)[33,34]. Peak in MR (intrinsic MR) shifts towards higher temperature with increase in field which may be due to the field induced suppression in scattering of charge carriers at thegrain boundaries and Mn -O -Mn bonds leading to enhancement in transfer integral.…”