Sankaranarayanan V

The influence of A-site disorder on the magnetic and transport properties of Pr0.7-yHoySr0.3MnO3 (y=0, 0.04, 0.08 and 0.1) polycrystalline samples is analyzed within the context of percolative transport and the existence of Griffiths phase. It is seen that the metal-insulator transition temperature is higher than the Curie temperature and coincides with the Griffiths temperature. The percolation threshold was found to increase with the increase in the A-site disorder. © 2004 Elsevier B.V. All rights reserved.

We have studied the effect of Ar annealing on the electrical and magnetic transport properties of Pr0.7Sr0.3MnO3 manganite. It is observed that, with increase in the annealing time there is a progressive decrease in TC and, resistivity, as functions of temperature become broader with the occurrence of two metal-insulator transitions. Electron-spin resonance analysis shows that for the Ar annealed sample, additional lines are present much above TC and the activated intensity behaviour and the paramagnetic linewidth behaviour points to the existence of spin clusters above TC. The metal-insulator transition was found to be percolative in nature with the high temperature transition having a temperature-dependent metallic volume fraction (p) while the low-temperature broad hump could be described by a bond percolation model with temperature independent p. © 2006 Elsevier B.V. All rights reserved.

In this paper, we report the influence of the disorder on the magnetization and magnetoresistance in Pr0.6 R0.1 Sr0.3 MnO3 (R = Tb, Y, Ho and Er) manganites at high and low temperatures. At high temperatures, it is seen that both these properties are governed by the lowering of average A-site radii and increase in variance of the A-site disorder while at low temperatures by the rare earth ion coupling with that of Mn sublattice. The rare earth sublattice was found to order at a temperature very similar to that of corresponding rare earth manganate even though their concentrations were small. The magnetoresistance was found to scale with both the lowering of average A-site radii and increase in variance of the A-site disorder while at low temperatures was found to be independent of variance but rather depends on whether the dopant ion is magnetic or non magnetic. This behaviour was attributed to the competition that exists between double exchange and Jahn-Teller mechanisms which govern the physics of these systems. © 2006 Elsevier B.V. All rights reserved.