Showing papers by "Chandran Sudakar published in 2014"

Effect of Microstrain on the Magnetic Properties of BiFeO3 Nanoparticles

Abstract: We report on size induced microstrain-dependent magnetic properties of BiFeO3 nanoparticles. The microstrain is found to be high (e > 0.3%) for smaller crystallite sizes (d < 30 nm), and shows a sharp decrease as the particle size increases. The presence of pseudo-cubic symmetry is evidenced for these nanoparticles. Raman spectral studies suggest straightening of the Fe-O-Fe bond angle accompanied by a decrease in FeO6 octahedral rotation for d < 65 nm. The magnetization shows a dip around 30 nm, half the size of spin cycloid length for BiFeO3, due to a decrease in rhombohedral distortion with crystallite size. We also observe a similar trend in the TN with respect to size indicating that the microstrain plays a significant role in controlling the magnetic property of BiFeO3.

Raman spectral signature of Mn-rich nanoscale phase segregations in carbon free LiFe1−xMnxPO4 prepared by hydrothermal technique

Abstract: Mn-rich nanoscale secondary phases were identified in LiFe1−xMnxPO4, despite the known complete solubility for the LiFePO4–LiMnPO4 system and the observed linear increase in the lattice parameters of LiFe1−xMnxPO4 with increasing Mn concentration. Carbon free LiFe1−xMnxPO4 (x = 0, 0.05, 0.10, 0.25) were prepared by the sequential precipitation of Li3PO4 and (Fe1−xMnx)3(PO4)2, followed by hydrothermal treatment. At low doping concentration (x ≤ 0.05), Li–Mn–O secondary phases were discerned by Raman spectra, which corroborated with the inductively coupled plasma elemental analysis. Though energy dispersive elemental mapping with scanning transmission electron microscopy do not show segregation of Mn at low concentrations, Mn-rich phases were clearly discerned at high doping concentration (x = 0.25). The kinetics of Mn-rich phase formation during hydrothermal synthesis of carbon free LiFe1−xMnxPO4, which was attributed to the difference in the solubility constant of the intermediate products of Li3PO4 and (Fe1−xMnx)3(PO4)2, and its implications on the capacity of LiFe1−xMnxPO4 cathode material were discussed. Our results present how de-convoluted Raman peaks show clear signatures of nanophase impurity segregations and how an increase in the lattice constant with Mn doping concentration can be decisive.

Experimental and Theoretical Investigations of Dopant, Defect, and Morphology Control on the Magnetic and Optical Properties of Transition Metal Doped ZnO Nanoparticles

Abstract: The control of size, shape, and physical properties by surface modifications are of immense interest in materials which are of technological importance. The ZnO-based wide bandgap semiconductor nanoparticles have gained significant interest in the research community due to its large exciton binding energy (60 meV). Further substantial renewed interest in ZnO-based compounds is due to the possible realization of p-type conduction and ferromagnetic behavior when doped with transition metals. In this report we present interesting results on the ZnO nanoparticle system in which the control of dopants, morphology, and the surface modification can influence significantly the physical properties of the ZnO nanoparticles. First, we present the methods to control the morphology of the ZnO particle to obtain nanorods. As an example we show the effect of Li dopant on the morphology control of Co and Ni doped ZnO. The effect of morphology on the magnetic properties of these compounds is discussed further. We also demonstrate the effect of the n-type charge carriers on the magnetic and optical properties by doping aliovalent cations in Zn(Co)O. Following this we comment on the magnetic property manipulations by surfactant treatment of transition metal (TM) doped ZnO and defect stabilization in ZnO by Mg doping. The magnetic coupling is RKKY-like both with and without Li co-doping. Finally, we provide the significant implications of these results on the nanorods structures of room temperature ferromagnetic materials by first-principles modeling. These theoretical analyses demonstrate that Li co-doping has primarily two effects in bulk Zn1−x M x O (with M = Co or Ni). First, the Li-on-Zn acceptors increase the local magnetic moment by depopulating the M 3d minority spin-states. Second, Li-on-Zn prefer to be closer to the M atoms to compensate the M–O bonds and to locally depopulate the 3d states, and this will help in forming high aspect nanostructures. The observed room temperature ferromagnetism in Li co-doped Zn1−x M x O nanorods can therefore be explained by the better rod morphology in combination with locally ionizing the magnetic M atoms.