The magnetotransport properties of La0.7Sr0.3MnO3/BaTiO3 superlattices grown by pulsed laser deposition technique

We have investigated the magnetotransport properties of La0.7Sr0.3MnO3/BaTiO3 superlattices, grown on SrTiO3 substrate by pulsed laser deposition technique, both with current-in-plane and current-perpendicular-to-the-plane directions. Several features indicate the presence of magnetic inhomogeneities at the interfaces which is independent of BaTiO3 layer thickness variation. First, the magnetic property in the superlattices decreases. Second, a hysteresis in magnetoresistance due to the relaxation of the resistive state is observed. Third, a threshold under an applied magnetic field in the magnetoresistance is seen. Such behaviors are in agreement with the phase separation scenario which could be the possible reason for these magnetic inhomogeneities at the interfaces. On the contrary, the magnetoresistance with the current-perpendicular-to-the-plane direction is mostly attributed to the tunneling effect along with the ordering of the spin at the interface. This study confirms the importance of the interfaces in superlattices that can be used to control novel physical properties in oxide materials.Comment: 12 pages, 4 figures, to be published in Jour. Appl. Phy

Atmospheric aerosol formation and its growth during the cold season in India

The effects of molecular diffusivity of H2SO4 and NH3 vapours on nucleated particles of SO2-4 and NO-3 species are reported. Condensation sink and source rate of H2SO4 and NH3 vapours, growth rates and ratios of real to apparent nucleation rates are calculated for SO2-4 and NO2-3 aerosols using fractional contributions of them in total aerosol size-distribution during the measurement period at Pune, reported in Chate and Pranesha (2004). The percentage of nucleated SO2-4and NO-3 aerosols of mid-point diameter 13 nm are 2 and 3 respectively of the total particles (13 nm Dp750 nm) for both H2SO4 and NH3 diffusion. In the diameter range 75 nm Dp 133 nm, it is 48 and 45 of SO2-4 and NO-3 aerosols, respectively for NH3 diffusion and 43 and 36 of SO2-4and NO-3 for H2SO4 diffusion. Increase in percentage of nucleated particles of these species corresponding to mid-point diameter 133 nm around 0900 h IST is significantly higher than that of mid-point diameter 13 nm and it is due to photo-chemical nucleation, coagulation and coalescence among nucleated clusters. The ratios of real to apparent formation rates for SO2-4 and NO-3 aerosols are 12 and 11 respectively, corresponding to mid-point diameter 13 nm, 17 and 13, for midpoint diameter 133 nm and 12 and 9.5, for mid-point diameter 750 nm. The results indicate that nucleation involving H2SO4 and acidic NH3 diffusion on SO2-4and NO-3 particles is the most relevant mechanism in this region

Changes in the concentration and size-distribution of the sub-micron particles associated with the sea- and land-breezes at a coastal station

Surface measurements of the size-distribution of submicron aerosol particles in the range of 0.003 to 1 μm diameter have been made on 6-8 January 1998, at Thiruvananthapuram during an inter-comparison campaign of the Indian Ocean experiment (INDOEX). The results are studied with respect to the setting-in of the sea- and land-breezes at the station. Observations show an increase of up to an order of magnitude in aerosol concentrations of all size categories with the setting-in of the land breeze at 1800-1900 IST. High concentrations of aerosol particles prevail throughout the period of the land breeze at night-time. Aerosol concentrations remarkably decrease at about 1000 IST with the arrival of much cleaner air with the sea breeze. During the land breeze at night-time, the size-distributions of aerosol particles is bimodal with the maximum at 0.075 and 0.024 μm diameters. During the sea breeze, especially in the afternoon, the maximum in the accumulation mode shifts to a slightly higher size (0.133 μm) and the maximum in the nucleation mode seems to shift to a smaller size (0.013 μm or smaller). The size-distribution curves during the daytime are mostly open-ended at the small particle-size end. The enhanced coagulation of aerosol particles and the gas-to-particle conversion processes have been proposed to explain the shift of maxima in the accumulation mode and the enhanced generation of small particles in the nucleation mode in the afternoon, respectively

The role of ferroelectric-ferromagnetic layers on the properties of superlattice-based multiferroics

A series of superlattices and trilayers composed of ferromagnetic and ferroelectric or paraelectric layers were grown on (100) SrTiO3 by the pulsed laser deposition technique. Their structural and magneto-electric properties were examined. The superlattices made of ferromagnetic Pr0.85Ca0.15MnO3 (PCMO) and a ferroelectric, namely Ba0.6Sr0.4TiO3 (BST) or BaTiO3, showed enhanced magnetoresistance (MR) at high applied magnetic field, whereas such enhancement was absent in Pr0.85Ca0.15MnO3/SrTiO3 superlattices, which clearly demonstrates the preponderant role of the ferroelectric layers in this enhanced MR. Furthermore, the absence of enhanced MR in trilayers of PCMO/BST indicates that the magneto-electric coupling which is responsible for MR in these systems is stronger in multilayers than in their trilayer counterparts.Comment: to be published in J. Appl. Phy

Magnetotransport properties of ferromagnetic Pr0.85Ca0.15MnO3/ferroelectric Ba0.6Sr0.4TiO3 superlattice films

Artificial superlattices designed with ferromagnetic Pr0.85Ca0.15MnO3 insulating layer and ferroelectric Ba0.6Sr0.4TiO3 layer were grown on (100) SrTiO3 substrates. The magnetotransport properties were measured with the current perpendicular to the plane geometry. An increase in magnetoresistance (MR), with no significant low field effect, was observed as the number of ferroelectric Ba0.6Sr0.4TiO3 layer thickness increases even up to 9 unit cells. For example, the superlattice [(Pr0.85Ca0.15MnO3)10(Ba0.6Sr0.4TiO3)9]25 shows 35 % MR at 100 K, though the Pr0.85Ca0.15MnO3 film was a robust insulator with negligible MR even at high applied magnetic field. This observed large MR cannot be explained by simple interfacial ferromagnetism or by the tunneling magnetoresistance. One possible explanation could be the effect due to the ferroelectric spacer layer and the associated magnetoelectric coupling.Comment: To be published in Appl. Phys. Let

New particle formation by ion-induced nucleation during dissipation stage of thunderstorm

A case of new particle formation observed during dissipation stage of a thunderstorm at a tropical station, Pune, India on 3 June 2008 is reported. The flash rate and rainfall intensity increased as high as 110 flashes per 5 minutes and 150 mm hour-1 respectively during the active stage of thunderstorm, and then gradually decreased during the dissipation stage. The number concentration of particles in the size range of 10-100 nm sharply increased from 350 particles cm-3 to ~8000 particles cm-3 during the dissipation stage of a thunderstorm and grew to larger diameter subsequently. Observations suggest that the atmospheric conditions such as (i) reduced background aerosol concentration after heavy rain, (ii) high humidity condition, and (iii) increased ion concentration during the dissipation stage by corona discharges, favoured generation of new particles by ion-induced nucleation (IIN). Observations also suggest that generation of unipolar ions by corona discharges may be more favourable for IIN and subsequent growth of the particles

Anomalous electric field changes and high flash rate beneath a thunderstorm in northeast India

In spite of many experimental and theoretical studies the relationships between storm dynamics, severe weather, and lightning activity have been least understood. Measurements of electric field made under a severe thunderstorm at a northeastern Indian station, Guwahati, India are reported. Lightning flash rate increases drastically to about 84 flashes per minute (fpm) during the active stage which lasted for about 7 minutes, from about 15 flashes per minute during the initial phase of thunderstorm. Sudden increase in lightning flash rate ('lightning jump') of about 65 fpm/min is also observed in the beginning of the active stage. The dissipating stage is marked by slow and steady decrease in lightning frequency. Despite very high flash rate during the active stage, no severe weather conditions are observed at the ground. It is proposed that the short duration of the active stage might be the reason for the non-observance of severe weather conditions at the ground. Analysis of Skew-t graph at Guwahati suggests that vertical distribution of Convective Available Potential Energy (CAPE) also may play some role in non-occurrence of severe weather at ground in spite of large lightning flash rate and lightning jump observed in this thunderstorm. Further, all electric field changes after a lightning discharge indicates the presence of strong Lower Positive Charge Centers (LPCC) in the active and dissipation stages. This suggests that LPCC plays an important role in initiation of lightning discharges in these stages

Estimation of small ion concentration near the Earth's surface

Atmospheric ions produced by radon gas exhalation from the Earth's surface can play a vital role in the electrification of atmosphere, especially during nights when the gases are trapped in a stable layer close to the surface. The measurements of concentration of radon and its progeny, air conductivity and aerosol size distribution made at Pune, India, have been analyzed. The concentrations of radon and its progeny show maxima during night and early morning hours, between 0500 and 0700. IST when atmosphere is more stable and mixing is low and start decreasing after sunrise and attain minima during 1000-1800. h when air is unstable. The diurnal variation of the ionization rate, calculated using the concentrations of radon and its progeny, follows the variations of concentrations of radon and its progeny. The ion-aerosol balance equations are solved to study the effect of aerosols on small ion concentration in the lower atmosphere. It has been found that during daytime when aerosol concentration is high, 20-30 reduction in small ion concentration can occur due to aerosols. The small ion concentration estimated using measured air conductivity is compared with small ion concentration estimated by solving ion-aerosol balance equations and both are found to be in good agreement with each other. © 2011 Elsevier Ltd

Background aerosol concentration derived from the atmospheric electric conductivity measurements made over the Indian Ocean during INDOEX

Measurements of the atmospheric electric conductivity on board ORV Sagarkanya during her three cruises over the Indian Ocean (17° N to 20° S, 57° E to 79° E) during the periods of December to March 1996-1997, 1998, and 1999 are reported. The results show that the values of atmospheric conductivity over the southern hemisphere are 2 to 3 times of that over the northern Indian Ocean and the north-to-south gradients extend up to the Intertropical Convergence Zone (ITCZ) and have large interseasonal and intraseasonal variations. The values of electric conductivity have been used to calculate the aerosol concentrations. The latitudinal variations of the aerosol concentration have been observed to have positive gradients from the Indian coastline to the ITCZ, and the gradients are different during the three cruises. The aerosol concentrations attain their pristine level only at 15°-20° S in this season. Because of the large interseasonal variability of the aerosol concentration observed over the northern Indian Ocean, it is concluded that estimating any secular change in the background aerosol pollution may be a futile exercise in this area

Nonlinear Ionic Conductivity of Thin Solid Electrolyte Samples: Comparison between Theory and Experiment

Nonlinear conductivity effects are studied experimentally and theoretically for thin samples of disordered ionic conductors. Following previous work in this field the {\it experimental nonlinear conductivity} of sodium ion conducting glasses is analyzed in terms of apparent hopping distances. Values up to 43 \AA are obtained. Due to higher-order harmonic current density detection, any undesired effects arising from Joule heating can be excluded. Additionally, the influence of temperature and sample thickness on the nonlinearity is explored. From the {\it theoretical side} the nonlinear conductivity in a disordered hopping model is analyzed numerically. For the 1D case the nonlinearity can be even handled analytically. Surprisingly, for this model the apparent hopping distance scales with the system size. This result shows that in general the nonlinear conductivity cannot be interpreted in terms of apparent hopping distances. Possible extensions of the model are discussed.Comment: 7 pages, 6 figure