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Dhaka R.S.,UGC-DAE Consortium for Scientific Research | Barman S.R.,UGC-DAE Consortium for Scientific Research
Physical Review Letters | Year: 2010

Aluminum bulk, surface, and multiple plasmons have been observed in the core-level spectra of rare gas (Ne, Ar, and Xe) nanobubbles in Al, whose intensities are even higher than those of Al metal. Both intrinsic and extrinsic bulk plasmons are detected, but they exhibit diametrically opposite intensity variation due to change in the size and implantation depth of the bubbles. Furthermore, the existence of bubble surface plasmon is demonstrated. © 2010 The American Physical Society.


Mishra A.K.,UGC-DAE Consortium for Scientific Research | Das D.,UGC-DAE Consortium for Scientific Research
Materials Science and Engineering B: Solid-State Materials for Advanced Technology | Year: 2010

Zn 1-xFe xO (x = 0.03, 0.05 and 0.07) nanoparticles synthesized by a chemical route were characterized by different techniques. The structural characterization by XRD and TEM confirmed the phase purity of the samples and indicated a reduction in particle size with increase in the dopant (Fe) concentration in ZnO. The optical characterization of the nanoparticles by FTIR, PL and UV-visible spectroscopy confirmed the formation of wurtzite structure and incorporation of Fe in the ZnO lattice. Magnetization measurements by VSM and Faraday balance techniques indicate presence of room temperature ferromagnetism in the Fe-doped ZnO samples. Local environment around the Fe atoms has been probed by 57Fe Mössbauer spectroscopy and the measured isomer shifts confirmed the charge state of iron as Fe 3+. Positron annihilation lifetime spectroscopy (PALS) measurements confirm the presence of cation vacancies in the nanoparticles and indicate a reduction of overall defect concentration with incorporation of Fe atoms in the ZnO structure. © 2010 Elsevier B.V. All rights reserved.


Pramanik A.K.,UGC-DAE Consortium for Scientific Research | Pramanik A.K.,Leibniz Institute for Solid State and Materials Research | Banerjee A.,UGC-DAE Consortium for Scientific Research
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The magnetic properties and the effects of interparticle interaction on them have been studied in nanoparticles of half-doped Pr0.5 Sr 0.5 MnO3. Three samples consisting of nanoparticles of different average particle sizes are synthesized to render the variation in interparticle interaction. Though all the samples crystallize in the same structure to that of their bulk compound, the low-temperature ferromagnetic-antiferromagnetic transition, which is present in bulk compound, is not evident in the nanoparticles. Linear as well as nonlinear ac susceptibility coupled with dc magnetic measurements have shown the superparamagnetic behavior of these nanoparticles where the blocking temperature increases with the increasing particle size. Presence of interparticle interaction is confirmed from the temperature variation in coercive field and the analysis of frequency-dependent ac susceptibility. We have identified the nature of this interaction to be of dipolar type and show that its strength decreases with the increasing particle size. The effect of this dipolar interaction on magnetic properties is intriguing as the compounds exhibit crossover from de Almeida-Thouless- to Gabay-Toulouse-type critical lines on field-temperature plane above their respective interaction field. In agreement with theoretical prediction, we infer that this crossover is induced by the unidirectional anisotropy arising from interparticle interaction, and this is confirmed from the presence of exchange bias phenomenon. © 2010 The American Physical Society.


Pramanik A.K.,UGC-DAE Consortium for Scientific Research | Banerjee A.,UGC-DAE Consortium for Scientific Research
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We show Griffiths phase (GP)-like behavior in half-doped Pr0.5 Sr0.5 MnO3 having ferromagnetic (FM) spin correlation above FM transition (TC) without any spontaneous magnetization. Temperature range and exponent of GP increase with Ga substitution at Mn site which is not related with structural disorder as Ga does not affect the structure, moreover, reduces Jahn-Teller active Mn3 + ions. FM clusters both above and below TC reduces their size with Ga. Hence, it is considered that GP originates from phase inhomogeneity and evolves with addition of disorder due to change in its length scale. © 2010 The American Physical Society.


Pandey S.K.,UGC-DAE Consortium for Scientific Research
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The electronic and magnetic properties of Sr2CoO4 compound have been studied using ab initio electronic-structure calculations. As opposed to a generalized gradient approximation (GGA) calculation, which gives ferromagnetic metallic solution, GGA+U calculations provide two kinds of ferromagnetic solutions: (i) half-metallic and (ii) metallic. The half-metallic solution is a ground state of the system and the metallic one is a metastable state. The strong hybridization between Co3d and O2p orbitals decides the electronic and magnetic properties of the compound. The total magnetic moment per formula unit is found to be ∼3 μB (S=3/2). Our calculations give the magnetocrystalline anisotropy energy of ∼2.7 meV, which provides a good description of experimentally observed large magnetocrystalline anisotropy. The Heisenberg exchange parameters up to fourth nearest neighbors are also calculated. The mean-field theory gives TC =887 K. The possible physical implications of the ferromagnetic half-metallic ground state are also discussed. © 2010 The American Physical Society.


Kumar D.,UGC-DAE Consortium for Scientific Research | Banerjee A.,UGC-DAE Consortium for Scientific Research
Journal of Physics Condensed Matter | Year: 2013

We report detailed dc magnetization and linear and nonlinear ac susceptibility measurements on the hole doped disordered cobaltite La 0.5Ba0.5CoO3. Our results show that the magnetically ordered state of the system consists of coexisting non-ferromagnetic phases along with percolating ferromagnetic clusters. The percolating ferromagnetic clusters possibly start a magnetic ordering at the Curie temperature of 201.5(5) K. The non-ferromagnetic phases mainly consist of antiferromagnetic clusters with size smaller than the ferromagnetic clusters. Below the Curie temperature the system exhibits an irreversibility in the field cooled and zero field cooled magnetization and a frequency dependence in the peak of ac susceptibility. These dynamical features indicate the possible coexistence of spin-glass phase along with ferromagnetic clusters similar to La1-xSrxCoO3 (x ≥ 0.18), but the absence of field divergence in the third harmonic of ac susceptibility and zero field cooled memory clearly rule out any such possibility. We argue that the spin-glass phase in La1-xSrxCoO3 (x ≥ 0.18) is associated with the presence of incommensurate antiferromagnetic ordering in non-ferromagnetic phases, which is absent in La0.5Ba 0.5CoO3. Our analysis shows that the observed dynamical features in La0.5Ba0.5CoO3 may be due to progressive thermal blocking of ferromagnetic clusters, which is further confirmed by Wohlfarth's model of superparamagnetism. The frequency dependence of the peak of ac susceptibility obeys the Vogel-Fulcher law with τ0 ≈ 10-9 s. This together with the existence of an AT-line in H-T space indicates the presence of significant inter-cluster interaction among these ferromagnetic clusters. © 2013 IOP Publishing Ltd.


Pandey S.K.,UGC-DAE Consortium for Scientific Research
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

In the light of recent interesting experimental work on MgV 2O4 we employ the density functional theory to investigate the crucial role played by different interaction parameters in deciding its electronic and magnetic properties. The strong Coulomb correlation in presence of antiferromagnetic (AFM) coupling is responsible for the insulating ground state. In the ground state, the dxz and dyz orbitals are ordered and intrachain vanadium ions are antiferromagnetically coupled. The calculation gives small spin-orbit coupling (SOC), which provides a tilt of ∼11.3 to the magnetic moment from the z axis. In the presence of weak SOC and strong exchange coupling, the experimentally observed small magnetic moment and low AFM transition temperature appear to arise from spin fluctuation due to activeness of geometrical frustration. © 2011 American Physical Society.


Okram G.S.,UGC-DAE Consortium for Scientific Research
AIP Advances | Year: 2012

We have reinvestigated the thermopower of group VB metals in polycrystalline forms in the temperature range of 6-300K, taking into account the critical nature of the sample surface and heat treatment especially for niobium. Strikingly small magnitude, negative sign, phonon drag dip and superconductivity not reported previously were observed in surface-cleaned single crystalline Nb. However, while thermopower magnitudes are small, mixed signs were found in the polycrystalline V, Nb and Ta samples. These properties were therefore interpreted as their intrinsic properties and were briefly discussed taking into account of the existing theory by fitting also the data that give the Fermi energies of 10.94 eV, 5.08 eV and 1.86eV, respectively. Copyright © 2012 Author(s).


Ahlawat A.,UGC-DAE Consortium for Scientific Research | Sathe V.G.,UGC-DAE Consortium for Scientific Research
Journal of Raman Spectroscopy | Year: 2011

Nanoparticles, bulk and thin films of NiFe2 O4 compounds are studied by micro-Raman spectroscopy. The effect of varying the incident laser power up to 40 mW was studied in all forms of the samples. The spectra showed a large magnitude of red shift and line broadening as a result of high incident laser power. It is shown that the inverse spinel structure remains robust, and no trace of laser-induced oxidation was observed. The low-temperature study of the bulk and nanoparticles has also been carried out for elucidating thermal effects due to the high incident laser power. The rise in temperature for maximum incident laser power of 40 mW was estimated to be ∼625 °C. Copyright © 2011 John Wiley & Sons, Ltd.


Gupta S.,Indian Institute of Technology Bombay | Rawat R.,UGC-DAE Consortium for Scientific Research | Suresh K.G.,Indian Institute of Technology Bombay
Applied Physics Letters | Year: 2014

Large magnetocaloric effect (MCE) and magnetoresistance (MR) together with negligible hysteresis loss have been observed in ErNiSi compound, which undergoes metamagnetic transition at low temperatures. Magnetization, heat capacity, and resistivity measurements confirm the metamagnetic transition. Both MCE and MR follow H2 dependence in the paramagnetic regime. The maximum value of isothermal entropy change (Δ SM) and MR for a field change of 50 kOe are found to be 19.1 J/kg K and ∼ 34%, respectively. Large MCE with negligible magnetic hysteresis loss could make this material promising for low temperature magnetic refrigeration. © 2014 AIP Publishing LLC.

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