Now showing 1 - 10 of 128
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    Near white light and near-infrared luminescence in perovskite Ga:LaCrO3
    (15-03-2022)
    Rajalekshmi, T. R.
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    Mishra, Vikash
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    Dixit, Tejendra
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    Miryala, Muralidhar
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    Rao, M. S.Ramachandra
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    We report the realization of near-white light emission (near-WLE) and the selective tuning to the near-infrared (NIR) luminescence from Ga substituted lanthanum chromite perovskite with electron-beam excitation for the first time. The near-WLE in Cathodoluminescence (CL), arises from the combination of 4T1(F)→4A2(blue), 4T1(P)→4A2(green) and 4T2→4A2(red) spin allowed transitions and 2E→4A2 spin forbidden transitions from the Cr3+ ion. The spin allowed transitions from the Cr3+ ions is rare, however can be accomplished by adjusting the Cr3+/Ga3+ ion concentration in the host lattice. The NIR emission originates from the selective 2E→4A2 transition, which increases with the increasing concentration of Ga3+ions. Moreover, the Density Functional Theory (DFT) calculations point out the presence of extra electronic states in the NIR region with Ga substitution, which elucidates the observation of NIR emission in Ga substituted samples.
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    Large magnetoresistance and Jahn-Teller effect in Sr2FeCoO6
    (01-08-2012)
    Pradheesh, R.
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    Nair, Harikrishnan S.
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    Neutron diffraction measurement on the spin glass double perovskite Sr2FeCoO6 reveals site disorder as well as Co3+ intermediate spin state. In addition, multiple valence states of Fe and Co are confirmed through M̈ossbauer and X-ray photoelectron spectroscopy. The structural disorder and multiple valence lead to competing ferromagnetic and antiferromagnetic interactions and subsequently to a spin glass state, which is reflected in the form of an additional T-linear contribution at low temperatures in specific heat. A clear evidence of Jahn-Teller distortion at the Co3+-O6 complex is observed and incorporating the physics of Jahn-Teller effect, the presence of localized magnetic moment is shown. A large, negative and anomalous magnetoresistance of ≈63% at 14 K in 12 T applied field is observed for Sr2FeCoO6. The observed magnetoresistance could be explained by applying a semi-empirical fit consisting of a negative and a positive contribution and show that the negative magnetoresistance is due to spin scattering of carriers by localized magnetic moments in the spin glass phase. © Springer-Verlag 2012.
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    Magnetic and magnetocaloric properties of nanocrystalline Pr1 xA xMn 1 yCo yO 3 (A = Ca, Sr) (x = 0.3; y = 0.5) manganites
    (01-03-2011)
    Mahato, Rabindra Nath
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    ; ; ;
    Malik, S. K.
    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.
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    High-pressure behavior of superconducting boron-doped diamond
    (25-05-2017)
    Abdel-Hafiez, Mahmoud
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    Kumar, Dinesh
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    Thiyagarajan, R.
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    Zhang, Q.
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    Howie, R. T.
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    Volkova, O.
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    Vasiliev, A.
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    Yang, W.
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    Mao, H. K.
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    This work investigates the high-pressure structure of freestanding superconducting (Tc=4.3 K) boron-doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30 GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients, and the grain boundary components undergo an irreversible phase change at 14 GPa. We show that the blueshift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of Tc in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of B0=510±28 GPa. The comparative analysis of high-pressure data clarified that the sp2 carbons in the grain boundaries transform into hexagonal diamond.
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    Magnetic and magnetotransport properties of Ce doped nanocrystalline LaMnO3
    (12-07-2007)
    Krishnamoorthy, C.
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    ; ; ;
    Malik, S. K.
    The magnetic and electrical transport properties of Ce doped LaMnO3 bulk samples have already been reported for various Ce concentrations. However there are few reports on the magnetotransport properties of these compounds. In this paper, we report the magnetic and magnetotransport properties of nanocrystalline La0.8Ce0.2MnO3 and La0.7Ce0.3MnO3 samples with different particle size. The nanocrystalline samples with different particle size were prepared through citrate-complex method by calcining the precursor at different temperatures. The transmission electron microscopy analysis of both the compounds reveal that the particle size increases with calcination temperature. Temperature dependence of magnetization of all the samples shows ferro- to paramagnetic transition. The zero field cooled and field cooled magnetization curves show irreversibility just below the magnetic transition temperature. The temperature dependence of resistivity of all the samples exhibits a metal to insulator transition without an impurity peak around 250 K, which is generally observed in multiphase samples. The results indicate that magnetoresistance (MR) increases with decreasing particle size. The observed MR will be discussed by spin dependent tunneling and enhancement of double exchange mechanism upon application of magnetic field. The physical properties suggest that the samples are single phase in nature. © 2006 Elsevier B.V. All rights reserved.
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    Spin reorientation induced anisotropic magnetoresistance switching in LaCo0.5Ni0.5 O3-δ thin films
    (01-06-2023)
    Sreejith, P. K.
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    Suraj, T. S.
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    Vasili, Hari Babu
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    Sreya, Suresh
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    Gargiani, Pierluigi
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    Cespedes, Oscar
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    Realization of diverse functionalities by tuning magnetic interactions in rare-earth perovskite oxide thin films opens up exciting technological prospects. Strain-induced tuning of magnetic interactions in rare-earth cobaltates and nickelates is of central importance due to their versatility in electronic transport properties. Here we report the spin reorientation induced switching of anisotropic magnetoresistance (AMR) and its tunability with strain in epitaxial LaCo0.5Ni0.5O3-δ thin films across the ferromagnetic transition. Moreover, with strain tuning, we observe a twofold to fourfold symmetry crossover in AMR across the magnetic transition temperature. The magnetization measurements reveal a ferromagnetic transition around 50 K. At temperatures below this transition, there is a subtle change in the magnetization dynamics, which reduces the ferromagnetic long-range ordering in the system. X-ray absorption and x-ray magnetic circular dichroism spectroscopy measurements at the Co and Ni L edges reveal a Co spin state transition below 50 K, leading to the AMR switching and also the presence of Ni2+ and Co4+ ions evidencing the charge transfer from Ni to Co ions. Our work demonstrates the tunability of magnetic interactions mediated electronic transport in cobaltate-nickelate thin films, which is relevant in understanding Ni-Co interactions in oxides for their technological applications such as in AMR sensors.
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    Effective Bandgap Engineering in Perovskite Ferroelectrics by Successive Multiple Doping
    (01-01-2020)
    Samanta, Shibnath
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    Rath, Martando
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    Rao, M. S.Ramachandra
    A very effective method of bandgap engineering in perovskite ferroelectrics by successive multiple doping (SMD) is proposed, tested, and compared with conventional incremental solitary doping (ISD). The application of SMD of La, Nb, and Fe (2% each) in Pb(Zr,Ti)O3 (PZT) tailors the bandgap to 2.74 from 3.53 eV (bandgap of pure PZT). This finds potential applications in ferroelectrics-based photovoltaics and optoelectronics. In contrast, only La doping even up to 10% curtails the bandgap to 3.10 eV. The X-ray photoelectron spectroscopy study confirms the presence of all elements doped and their chemical states. Analysis of photoluminescence shows that the emissions corresponding to the incorporations of La, Nb, and Fe are in the forbidden region of the energy band. In the case of only La doping, the emission corresponding to La gets broadened as its concentration increases. These observations are summarized and the mechanism of a larger bandgap reduction by SMD compared with solitary doping is explained in a systematic way. Comparing the ferroelectric hysteresis, true switchable polarization, and fatigue behaviors, it is found that ferroelectricity is affected marginally by SMD of La, Nb, and Fe compared with that by the ISD of La up to10%.
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    Effect of Successive Multiple Doping of La, Nb and Fe on Structure and Lattice Vibration of MPB PZT
    (01-01-2018)
    Samanta, Shibnath
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    Morphotropic phase boundary (MPB) Lead zirconate titanate [i.e Pb(Zr0.52Ti0.48)O3, PZT] is modified by incorporation of i) La in Pb-site, ii) simultaneously La in Pb-site and Nb in Zr/Ti site and iii) simultaneously La in Pb-site, Nb and Fe in Zr/Ti-site. Samples are prepared by alkoxide sol-gel synthesis. XRD study reveals peak shift towards lower 2á upon La addition but the shift is towards higher 2á for both La and Nb are incorporated. Addition of Fe makes peak shift towards higher angle but the amount of shift is very small with respect to that of La and Nb doping. The XRD data is further analyzed with the help of Rietveld refinement which reveals change in lattice parameters. It is observed that successive doping of La, La-Nb and La-Nb-Fe decrease both lattice parameters a and c.However, for the case of Nb and La co-doped PZT the value of c increases with respect to the c of only La doped PZT. The charge difference in Nb (5+) and Fe (3+) ions have compensating effect as they are doped in the same atomic site (i.e. B-site of the ABO3) which is having charge 4+. Raman spectroscopic study reveals more complex effect of successive multiple doping of La, Nb and Fe. Some of the observations in Raman spectroscopy are supportive to the observations made in XRD. Raman spectra are analyzed by cumulative fitting of de-convoluted peaks, which represent various individual vibrational modes: Reveal changes happening in the lattice due to the systematic successive doping of La, Nb and Fe.
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    Tc suppression and impurity band structure in overdoped superconducting Boron-doped diamond films
    (15-12-2018)
    Kumar, Dinesh
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    Chandran, Maneesh
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    Shukla, D. K.
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    Phase, D. M.
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    Ramachandra Rao, M. S.
    Our work investigates the impurity band and the core excitonic level evolution of a series of systematically boron doped diamond (BDD) films using Raman and X-ray absorption (XAS) spectroscopies. The resistivity versus temperature data shows that Tc of the BDD films achieves a peak at a maximum gaseous phase boron to carbon ratio, [[Formula presented]]max, beyond which Tc drops gradually as a result of excess defect formation, revealing a dome-shaped behaviour. Study of lattice expansion clearly shows the unambiguous presence of substitutional boron atoms beyond [[Formula presented]]max. Direct probe of the impurity band using XAS show a systematic increase in the absorption intensity of the bandgap states which ultimately degrades as the limit of [Formula presented]>[[Formula presented]]max is exceeded. The binding energy of the BDD 1s core exciton reveal a shallow measured value of 0.11 eV. This is less than earlier found values of 0.19 - 1.2 eV, in various reports for intrinsic diamond.
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    Large magnetic entropy change in nanocrystalline [formula omitted]
    (01-05-2010)
    Mahato, Rabindra Nath
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    Nanocrystalline [formula omitted] sample has been prepared by sol-gel method. The room temperature powder x-ray diffraction data show single phase nature of the sample and confirm the cubic crystal structure with [formula omitted] space group. The average crystallite size is calculated using Scherrer formula, and it is found to be [formula omitted]. Transmission electron microscopy image shows that the particles are spherical in shape and the average particle size is [formula omitted]. The sample undergoes ferromagnetic ordering at 235 K [formula omitted] and obeys the Curie–Weiss law in the paramagnetic region. The maximum value of the magnetic entropy change [formula omitted] is [formula omitted], and the relative cooling power is [formula omitted] for a field change of 50 kOe. The Arrott plot confirms that the magnetic ordering is of second order nature. The experimentally observed magnetic entropy change of the sample obeys Landau theory of phase transition well. © 2010, American Institute of Physics. All rights reserved.