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Udaipur, India

Singhal R.K.,University of Rajasthan | Singhal R.K.,Brazilian Center for Research in Physics (CBPF) | Kumari P.,University of Rajasthan | Samariya A.,University of Rajasthan | And 4 more authors.
Applied Physics Letters | Year: 2010

This article reports on reversible manipulation of room temperature ferromagnetism (RTFM) in nondoped bulk CeO2. The magnetization measurements establish that paramagnetic CeO2 is driven to a ferromagnetic state, without change in structure, when vacuum annealed at 600 °C. The Ce ions transform from 4+ to 3+ state, accompanied by evolution of oxygen vacancies (VO) during the RTFM transition, as determined by x-ray photoemission. The F+ centers (the electrons in singly occupied oxygen vacancies) play key role in the exchange mechanism. The transition shows complete reversibility where the RTFM is removed by removing the vacancies through re-heating the vacuum-annealed CeO2 in air. © 2010 American Institute of Physics. Source

Singhal R.K.,University of Rajasthan | Samariya A.,University of Rajasthan | Kumar S.,Ml Sukhadia University | Xing Y.T.,Brazilian Center for Research in Physics (CBPF) | Saitovitch E.,Brazilian Center for Research in Physics (CBPF)
Materials Letters | Year: 2010

Many dilute magnetic semiconductors, when annealed in hydrogen atmosphere, have been reported to show a giant ferromagnetism. The H-induced lattice defects, mainly the oxygen vacancies, have been suggested to mediate this ferromagnetic coupling. We have also observed huge magnetic induction in paramagnetic Co and Fe-doped ZnO systems, upon hydrogenation; however, the re-heating caused this magnetism to vanish quickly. In the present work, we have furthered these studies on their longevity aspect i.e. the ageing effect. Alarmingly, the magnetic properties of hydrogenated samples stored in dry atmosphere degrade gradually with ageing. On the other hand, the 2% Mn doped ZnO, which depicts only a weak ferromagnetism, and no enhancement in magnetization upon hydrogenation, exhibits no such ageing effect. © 2010 Elsevier B.V. All rights reserved. Source

Singhal R.K.,University of Rajasthan | Kumar S.,Ml Sukhadia University | Samariya A.,University of Rajasthan | Dhawan M.,University of Rajasthan | And 2 more authors.
Materials Chemistry and Physics | Year: 2012

We report a systematic structural, electronic, and magnetic investigation on occurrence of ferromagnetism and its "switch" action in non-doped bulk ceria (CeO 2). The magnetization measurements establish that the pristine CeO 2 having a paramagnetic ground state can be driven to a ferromagnetic state at room temperature, when hydrogenated at 600 °C. The observed H-induced ferromagnetism is closely related to the oxygen vacancies and the Ce valence state. X-ray photoemission results depict that Ce ions reduce from 4+ to 3+ state along with creation of oxygen vacancies during the ferromagnetic transition. A parallel variation of carrier concentration, revealed by resistance measurements, seems to be a secondary effect of the oxygen vacancies creation. The F + centers, i.e. the electrons in singly occupied oxygen vacancies, seem to play the key role in establishing the ferromagnetism in CeO 2, in the framework of bound magnetic polaron model. The exchange mechanism shows a "switch" action such that one could remove the oxygen vacancies through re-heating the H 2-treated CeO 2 and the ferromagnetism is subsequently vanished. © 2011 Elsevier B.V. All rights reserved. Source

Vyas B.M.,Ml Sukhadia University | Sunda S.,Airports Authority of India
Advances in Space Research | Year: 2012

An annular solar eclipse occurred over the Indian subcontinent during the afternoon hours of January 15, 2010. This event was unique in the sense that solar activity was minimum and the eclipse period coincides with the peak ionization time at the Indian equatorial and low latitudes. The number of GPS receivers situated along the path of solar eclipse were used to investigate the response of total electron content (TEC) under the influence of this solar eclipse. These GPS receivers are part of the Indian Satellite Based Augmentation System (SBAS) named as 'GAGAN' (GPS Aided Geo Augmented Navigation) program. The eight GPS stations located over the wide range of longitudes allows us to differentiate between the various factors induced due to solar eclipse over the equatorial and low latitude ionosphere. The effect of the eclipse was detected in diurnal variations of TEC at all the stations along the eclipse path. The solar eclipse has altered the ionospheric behavior along its path by inducing atmospheric gravity waves, localized counter-electrojet and attenuation of solar radiation intensity. These three factors primarily control the production, loss and transport of plasma over the equatorial and low latitudes. The localized counter-electrojet had inhibited the equatorial ionization anomaly (EIA) in the longitude belt of 72°E-85°E. Thus, there was a negative deviation of the order of 20-40% at the equatorial anomaly stations lying in this 'inhibited EIA region'. The negative deviation of only 10-20% is observed for the stations lying outside the 'inhibited EIA region'. The pre-eclipse effect in the form of early morning enhancement of TEC associated with atmospheric gravity waves was also observed during this solar eclipse. More clear and distinctive spatial and temporal variations of TEC were detected along the individual satellite passes. It is also observed that TEC starts responding to the eclipse after 30 min from start of eclipse and the delay of the maximum TEC deviation from normal trend with respect to the maximum phase of the eclipse was close to one hour in the solar eclipse path. © 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. Source

Dashora A.,University of Mumbai | Patel N.,University of Trento | Kothari D.C.,University of Mumbai | Ahuja B.L.,Ml Sukhadia University | Miotello A.,University of Trento
Solar Energy Materials and Solar Cells | Year: 2014

First principles calculations are performed to study the electronic and optical properties of Cu-doped, N-doped and (Cu+2N)-co-doped anatase TiO 2. Strong hybridization between Cu 3d and N 2p orbitals above the valence band leads to the formation of an isolated intermediate band (IB) deep in the band gap of pure TiO2. The new energetic features, predicted by DFT, have been experimentally confirmed using UV-vis-NIR spectroscopy. In particular, the diffuse reflectance spectrum of the co-doped TiO2 sample shows the presence of two edges which confirm the existence of IB in the band gap. This IB in the band gap of TiO2 is responsible for high visible light absorption through a two-step optical transition between valence and conduction bands via the IB. In mono-doped samples, only a reduction of the band gap is observed which is consistent with the first-principles calculations. X-ray photoelectron spectroscopy of mono- and co-doped TiO2 samples establishes the chemical states of several atomic elements and especially clarifies the key presence of O-vacancies leading to a new position of conduction band minima. The presence of broad IB and the absence of dopant energy levels close to the conduction band minimum in (Cu+2N)-co-doped TiO 2 qualify it to be an efficient material for photovoltaic conversion, photocatalytic water splitting, and photocatalytic degradation of pollutants. © 2014 Elsevier B.V. Source

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