R. Nithya

Indira Gandhi Centre for Atomic Research, Chennai, Tamil Nādu, India

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Publications (30)26.87 Total impact

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    ABSTRACT: Single crystals of pure and Eu2+ doped LiBaF3 have been grown from melt by using a vertical Bridgman–Stockbarger method. Effects induced by irradiation on europium doped LiBaF3 (lithium barium fluoride) single crystals were monitored by optical absorption, photoluminescence and thermoluminescence studies. The absorption bands of Eu2+ ions with peaks at 240, 290 and 320 nm were observed in the LiBaF3:Eu2+ crystal. Drastic increase in absorption was noted below 600 nm after gamma irradiation, which was dependent on the radiation dose. The additional absorption peak at around 570 nm was observed in irradiated crystal due to the ionization process Eu2+(−)e−→Eu3+. Photoluminescence of Eu2+ doped LiBaF3 single crystal shows sharp line peaked at ~359 nm and a broad band extending between 370 and 450 nm which shows a considerable reduction in Eu2+ PL intensity after gamma irradiation. Irradiated LiBaF3:Eu2+ sample has revealed three intense TL glow peaks at 128 °C (peak-1), 281 °C (peak-2) and 407 °C (peak-3). Activation energy (E) and frequency factor (s) of the latter two peaks were determined by various heating rate (VHR) method and graphical method.
    Radiation Physics and Chemistry 01/2014; 96:135–139. · 1.38 Impact Factor
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    ABSTRACT: Polycrystalline YBaCo4O7 compounds are synthesized by solid state reaction in air atmosphere. X-ray powder diffraction at room temperature indicates its single phase nature. Rietveld refinement of X-ray diffraction data shows distortions in bond lengths in the cobalt-oxygen tetrahedra. Electrical resistivity of YBaCo4O7 suggests a variable range hopping type of conductivity from 80 K up to room temperature. Raman scattering spectra exhibit several broad phonon modes at ambient temperature. Raman spectroscopic measurements are carried out as a function of temperature to look for any structural phase transitions from 80 to 823 K. Raman scattering results show no phase transitions, supporting low temperature conductivity measurements. From temperature variation of Raman spectra, it is found that the structure is stable in the temperature range of present study.
    AIP Advances. 02/2013; 3(2).
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    ABSTRACT: Single crystals of pure and Eu2+ doped LiBaF3 have been grown from melt by using a vertical Bridgman-Stockbarger method. Absorption and luminescence spectra for pure and rare-earth-doped LiBaF3 were studied. At ambient conditions the photoluminescence spectra consisted of sharp lines peaked at ~359 nm attributed to the 6P7/2-->8S7/2 transitions in the 4f7 electronic configuration of Eu2+ and a broad band extending between 370 and 450 nm attributed to Eu2+ trapped exciton recombination. The effect of 60Co gamma irradiation has also been investigated.
    02/2013;
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    ABSTRACT: We have carried out Magnetization, Raman spectroscopy and IR spectroscopy studies as a function of temperature to investigate the spin state transition in NdCoO3 single crystal. The crystal has been grown by Optical Float Zone technique and characterized by Synchrotron X-ray Diffraction. Our results indicate that the spin crossover from low spin state (LS) to intermediate spin state (IS) occurs continuously over a wide range of temperature above 200K. The wide temperature range of the spin crossover reflects possible electronic or magnetic in-homogeneity at the microscopic level, which is a common feature of some cobalt perovskites. The magnetization studies indicate a spin gap which is higher than that of LaCoO3 (180 K) and Raman spectroscopy studies reveal a Jahn Teller vibration mode characteristic of the intermediate spin state similar to that reported for LaCoO3 in literature.
    02/2013;
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    ABSTRACT: Titanium nitride (TiN) thin films were deposited by reactive pulsed laser deposition (RPLD) technique. For the first time, the composition evaluated from proton elastic backscattering spectrometry, in a quantitative manner, revealed a dependence on the partial pressure of nitrogen from 1 to 10 Pa. Grazing incidence-XRD (GI-XRD) confirmed the formation of predominantly nanocrystalline TiN phase with a crystallite size of around 30 nm. The hardness showed maximum value of ~30 GPa when the composition is near stoichiometric and the friction coefficient was found to be as low as 0.3. In addition, a systematic optical response was observed as a function of deposition pressure from the surface of the TiN films using spectroscopic ellipsometry.
    Journal of Materials. 01/2013; 2013.
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    ABSTRACT: The perovskite-like NaMgF 3 polycrystalline samples were synthesized by standard solid state reaction method. The perovskite material was doped with 0.2mol% of EuF 3 and CeF 3 impurity. Phase purity of synthesized compounds was analyzed by powder X-ray diffraction technique. The polycrystalline samples were β-irradiated by Sr 90 -Y 90 source with a dose rate 6 Gy/min and their thermoluminescence (TL) properties were analyzed. The TL glow peaks were found around 140 0 C and 220 0 C. The TL intensity was found to increase linearly up to 24 Gy. The emission spectrum showed peaks at 435nm corresponding to Eu 2+ and 536 nm corresponding to Eu 3+ ions. A broad spectrum peaking around 477 nm that of host lattice was also observed.
    NSRP-19, Mamallapuram; 12/2012
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    ABSTRACT: Behavior of cubic spinel phase of Ag2MoO4 is investigated at high pressure using X-ray diffraction and Raman spectroscopy. The P–V data are fitted to a third order Birch–Murnaghan equation of state using a value of B0=113 GPa and B′0=4. The compound is also found to exhibit a phase transition around 5 GPa to a tetragonal structure and the two phases are found to coexist over a range of pressures. Raman spectra exhibit dramatic changes across the phase transition. Increase of X-ray background scattering and broadening of the Raman peaks associated with MoO4 tetrahedral ions in the high pressure phase suggest evolution of positional disorder. However, no evidence of pressure-induced amorphization was found up to 47 GPa.
    Journal of Solid State Chemistry 12/2012; 196:391–397. · 2.04 Impact Factor
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    ABSTRACT: A thin film bilayer of Pr0.5Ca0.5MnO3 and YBa2Cu3O7 has been deposited on LaAlO3 substrate by pulsed laser deposition. XRD pattern confirms the formation of the bilayer. Current and magnetic field dependent resistance measurements have been carried out to study the proximity effect. These measurements have shown that the superconducting transition temperature (Tc) of YBa2Cu3O7 is suppressed to lower temperatures with a progressive increase in current. In addition, a hump like feature appears in resistivity near Tc with increase in applied current. The results are explained on the basis of the formation and growth of ferromagnetic clusters on account of charge order melting in Pr0.5Ca0.5MnO3 upon application of high current and magnetic field.
    Journal of Applied Physics 06/2012; 111(11). · 2.21 Impact Factor
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    ABSTRACT: Sodium Zirconium Phosphate (NaZr2(PO4)3), a low thermal expansion material is synthesized by aqueous sol gel method. Raman spectroscopic studies are carried out as a function of temperature from 80 K to 860 K. The modes at 72 and 423 cm-1 exhibit a positive temperature coefficient which points to their negative role in thermal expansion. These modes are due to the translational motion of Na atoms and rotational motion of PO4 ions respectively, and it is conceivable that they contribute to negative thermal expansion in this material.
    06/2012;
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    ABSTRACT: Bulk YBaCo4O7 was synthesized using a ceramic route in air. Thin films of this compound were grown on (100) oriented LaAlO3 substrate using a Pulsed Laser Deposition technique. X-ray diffraction results of thin film indicate that the film has (110) orientation. Electrical resistivity measurements, in 300K to 100K temperature range showed that the film is insulating, similar to that of the bulk. Spectroscopic ellipsometric measurements on thin films in the energy range 1.5 to 5 eV and at temperatures from 300 K to 400 K showed that the direct band gap s 2.45 eV at ambient temperature while its value increases with increase in temperature. The increase in band gap with temperature is attributed to the Moss-Burstein effect.
    06/2012;
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    ABSTRACT: A thin film bilayer of Pr0.5Ca0.5MnO3 and YBa2Cu3O7 has been deposited on LaAlO3 substrate by pulsed laser deposition. X-ray diffraction analysis confirms the formation of the bilayer. Current dependent magnetoresistance measurements show that the superconducting transition temperature (Tc) of YBa2Cu3O7 is suppressed to lower temperatures and a hump like feature appears in resistivity near Tc with increase in measuring current. The results are explained on the basis of formation of ferromagnetic clusters upon application of high current and magnetic field.
    06/2012;
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    ABSTRACT: The effect of strain on charge order melting in half doped manganite Pr0.5Ca0.5MnO3 thin films of different thicknesses has been investigated. The films were deposited on (100) oriented MgO substrates by pulsed laser deposition technique and were characterized by x‐ray diffraction, electrical resistivity and magnetoresistance measurements. A field induced charge order melting is observed for films with very small thickness. The charge order transition temperature and the magnetic field induced charge order melting appears to be governed by the nature of strain that is experienced by the film.
    AIP Conference Proceedings. 06/2011; 1347(1):199-201.
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    ABSTRACT: Thin films of the half-doped manganite Pr0.5Ca0.5MnO3 were grown on (100) oriented MgO substrates by pulsed laser deposition technique. In order to study the effect of strain on the magnetic field induced charge order melting, films of different thicknesses were prepared and their properties were studied by X-ray diffraction, electrical resistivity and magnetoresistance measurements. A field induced charge order melting is observed for films with very small thicknesses. The charge order transition temperature and the magnetic field induced charge order melting are observed to depend on the nature of strain that is experienced by the film.
    Journal of Magnetism and Magnetic Materials 01/2011; 323(22):2823-2827. · 1.83 Impact Factor
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    ABSTRACT: Titanium nitride surface layers were prepared by gas-phase thermal nitridation of pure titanium in an ammonia atmosphere at 1373 K for different times. In addition to the surface nitride layer, nitride/hydride formation was observed in the bulk of the specimen. The cross-section of the specimen was characterized by various techniques such as optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, secondary ion mass spectrometry and nanomechanical testing, and the mechanism of formation of these phases is discussed.
    Scripta Materialia. 08/2009;
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    ABSTRACT: High quality TiN films were synthesised from elemental metallic target using reactive and plasma assisted pulsed laser deposition (RPLD and PAPLD) techniques. In these processes, a high pure titanium target is ablated using a nanosecond pulsed Nd:YAG laser operating at 1064 nm wavelength at different nitrogen pressures. In RPLD process, the titanium plume reacts directly with the nitrogen gas at pressures less than 0·07 mbar to yield TiN films. In PAPLD process, additional nitrogen plasma was generated and confined by a DC coil positioned between target and substrate. Resultant films were characterised for phase, composition and morphology using glancing incidence X-ray diffraction, Auger Electron Spectroscopy (AES), Rutherford back-scattering and Atomic Force Microscope (AFM). Reactive pulsed laser deposition grown TiN films were found to contain traces of unreacted titanium. The crystallite size is estimated to be 15 and 50 nm respectively by using X-ray diffraction and AFM. Rutherford backscattering investigations helped the authors in arriving at the stoichiometry and AES analysis revealed the formation of TiN with low oxygen contamination.
    Surface Engineering 03/2009; 25(3):218-222. · 1.55 Impact Factor
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    ABSTRACT: Systematic studies on the structural, transport and magnetic properties of SrRu1−xCuxO3 (x=0.0–0.2) compounds have been performed. SrRu0.8Cu0.2O3 shows a tetragonal structure unlike the other compositions which exhibit a pseudo-cubic structure. Low temperature powder X-ray diffraction data of SrRu0.8Cu0.2O3 collected at a synchrotron beam line reveals that the tetragonal structure is stable down to 8 K. Ferromagnetic transition temperatures (Tc) are significantly reduced from 160 to 34 K with Cu doping. All the compositions exhibit irreversibilities in MZFC(T) and MFC(T) curves ascribable to the presence of domain structures. Magnetic susceptibility measurements show that the copper ions are anti-ferromagnetically coupled for concentrations higher than x=0.16. The antiparallel arrangement of Ru5+ ions with its neighboring cations also contributes to the large reduction in the observed magnetic moment. X-ray photoemission spectroscopy measurements show evidence for both tetravalent and pentavalent Ru ions while copper is in a divalent state. We conclude from our resistivity data that Cu2+ substitution promotes a polaronic type of conductivity.
    Solid State Communications 01/2009; · 1.53 Impact Factor
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    ABSTRACT: The electron-spin-resonance measurement for the double perovskite, La <sub>1.2</sub> Sr <sub>1.8</sub> Mn <sub>2</sub> O <sub>7</sub> single crystal is carried out from 80 to 450 K. Magnetic phase transitions from the paramagnetic to a mixed state are identified at T<sub>p</sub>∼335 K and from mixed to a ferromagnetic state at T<sub>c</sub>∼117 K . Two intermediate lines with 2≪g≪4 are observed within a narrow temperature range (280–340 K), which could be attributed to the magnetic interactions within the double layered perovskite via the interlayer (Mn–O–Mn) and interbilayer (Mn–O–O–Mn) couplings. On the other hand, a strong ferromagnetic resonance line with g=3–6 persists at a wide range of temperature from 340 to 100 K, which linearly varies with temperature and thus attributed to the intralayer (Mn–O–Mn) interaction.
    Journal of Applied Physics 10/2008; · 2.21 Impact Factor
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    ABSTRACT: Structural and magnetic studies have been carried out on SrRu <sub>1-x</sub>M<sub>x</sub>O<sub>3</sub> ( M= Ga :x=0.1 ; Cu and Zn: x=0.1 and 0.2) in order to understand the dependence of the ferromagnetic transition temperature of SrRuO <sub>3</sub> on heterovalent magnetic and nonmagnetic cation substitution at the Ru site. Powder x-ray diffraction has been carried out to analyze the crystal structure using the Rietveld refinement procedure. T<sub>c</sub> decreases more rapidly for divalent cation dopants as compared to the trivalent cations. The refinement results show that there are distortions of the RuO <sub>6</sub> octahedra in the doped compounds, with little change in the lattice parameters. However, the direction and magnitudes of the tilt of the octahedra for all the compounds do not show correlation with the T<sub>c</sub> . Therefore, the change of T<sub>c</sub> is attributed to the charge and spin disorders introduced by the heterovalent dopant cations. The filled e<sub>g</sub> orbitals in Ga, Cu, and Zn ions are expected to have weak hybridization with the oxygen p orbitals as compared to the pristine compound, leading to a weakening of the overall ferromagnetic interactions between the Ru ions.
    Journal of Applied Physics 08/2007; · 2.21 Impact Factor
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    ABSTRACT: Gas phase nitridation of nc-TiO2 was carried out for obtaining nc-TiN. The nc-TiO2 precursor itself was derived from a Ti(OH)x precipitate formed through hydrolysis of titanium nitrate solution by aqueous ammonia reagent. Calcination of titanium hydroxide at 1000 °C yielded nc-TiO2 powder with an average particle size of ~5 nm. Both nc-Ti(OH)x as well as nc-TiO2 were separately nitrided in an environment of argon and ammonia for synthesizing nc-TiN. A home-built thermogravimetric analysis-mass spectrometer apparatus was used for the nitridation experiment. The nanocrystalline phases of Ti(OH)x, TiO2 and TiN were characterized by XRD, TEM, IR and XPS techniques. The powders' XRD patterns indicated the formation of nanocrystalline TiN by ammonia bases nitridation of nc-TiO2. These studies also revealed sequential crystallo-chemical transformation of the intermediate precursor. FTIR studies revealed the existence of IR active modes in nc-TiO2 and the formation of a chemically pure TiN phase. The TEM image of TiN powder depicted the formation of nanocrystalline phase. Ti(2p3/2) photoelectron spectra indicated complete conversion of nc-TiO2 to nc-TiN.
    Journal of Physics D Applied Physics 07/2007; 40(15):4597. · 2.53 Impact Factor
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    ABSTRACT: Nanocrystalline ceria has been synthesized by a non-hydrolytic method using organic solvent and precipitant. The effect of the source compound on final nanocrystalline powder was investigated. The cerium nitrate hexahydrate subjected to different vacuum/thermal treatments to get cerium source compound having different extent of water of crystallization. The nanoceria, synthesized from these compounds was characterized by TGA-MS, XRD, HR-TEM and AFM.
    Powder Technology. 01/2006; 161(2):145–149.