Sankaranarayanan V

Structural, magnetic and magnetocaloric properties of sol-gel prepared, nanocrystalline oxides Pr 1-xA xMn1-yCo yO 3 (A = Ca, Sr) (x = 0.3; y = 0.5) (cubic, space group Fm3̄m) have been studied. From the X-ray data, the crystallite size of Pr 0.7Ca 0.3Mn 0.5Co 0.5O 3 and Pr 0.7Sr 0.3Mn 0.5Co 0.5O 3 samples is found to be ∼24 nm and ∼15 nm respectively. High resolution transmission electron microscopy image shows average particle size of ∼34 nm and ∼20 nm. Magnetization measurements indicate a Curie temperature of ∼153 K and ∼172 K in applied magnetic field of 100 Oe for Pr 0.7Ca 0.3Mn 0.5Co 0.5O 3 and Pr 0.7Sr 0.3Mn 0.5Co 0.5O 3 compounds. The magnetization versus applied magnetic field curves obtained at temperatures below 150 K show significant hysteresis and magnetization is not saturated even in a field of 7 T. The magnetocaloric effect is calculated from M versus H data obtained at various temperatures. Magnetic entropy change shows a maximum near T C for both the samples and is of the order ∼2.5 J/kg/K. © 2011 American Scientific Publishers.

Nanocrystalline samples with an average particle size of 40 and 52 nm have been synthesized by citrate-complex auto-ignition method. Magnetic properties of the samples show para- to ferromagnetic transition at around 135 K. The electron magnetic resonance (EMR) study on these samples indicates the presence of coexistence of two magnetic phases below 290 K. Electrical resistivity follows variable range hopping (VRH) mechanism in the paramagnetic regime. The magnetoresistance (MR) data has been analysed by spin dependent hopping between the localized spin clusters together with the phase-separation phenomenon. These clusters are assumed to be formed by distribution of canted spins and defects all over the nanoparticle. In addition, the hopping barrier depends on the magnetic moment orientation of the clusters. The magnetic moments of the clusters are narrowly oriented in ferro- and are randomly oriented in paramagnetic phase. The ferromagnetic phase contributes to the total MR at low applied magnetic fields whereas the paramagnetic phase contributes at relatively high fields in both the samples. The average cluster size in ferromagnetic phase is bigger than that in paramagnetic phase. It is also observed that the cluster size, in ferromagnetic phase, in 52 nm sample is bigger than that in the 40 nm sample. However, the average cluster size in paramagnetic phase is almost same in both the samples. © 2006 Springer Science+Business Media, Inc.

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.