Department of Physics

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Nanocrystalline pure α-Fe2O3 powder, with an average particle size of 35 nm, has been synthesized by using an aqueous solution-based synthetic route. DC electrical resistivity of the synthesized material was measured with respect to temperature by the two-probe method from 28° to 225°C. Room temperature resistivity of the nanopowder was ∼108 Ω·cm. Magnetic hysteresis measurement revealed that the synthesized α-Fe2O3 nanopowder exhibited ferromagnetic behavior at room temperature. The hysteretic features are high saturation magnetization of 5.1 emu/g, high remanence of 2.2 emu/g, and coercivity of 200.5 Oe. © 2009 The American Ceramic Society.

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Journal of the American Ceramic Society

DC electrical resistivity and magnetic property of single-phase α-Fe2O3 nanopowder synthesized by a simple chemical method

Boron Nitride Nanotubes (BNNTs) have alternating boron and nitrogen atoms in graphite like network and are strongly polar in nature due to a large charge on boron and nitrogen atoms. Hence electrostatic interactions are expected to play an important role in determining the elastic properties of BNNTs. In the absence of specific partial atomic charge information for boron and nitrogen, we have studied the elastic properties BNNTs varying the partial atomic charges on boron and nitrogen. We have computed Young modulus (Y) and Shear modulus (G) of BNNT as a function of the tube radius and number of walls using molecular mechanics calculation. Our calculation shows that Young modulus of BNNTs increases with increase in magnitude of the partial atomic charge on B and N and can be larger than the Young modulus of CNTs of same radius. This is in contrast to the earlier finding that CNTs has the largest tensile strength (PRL, 80, 4502, 1998). Shear modulus, on the other hand depends weakly on the magnitude of partial atomic charge and is less than the shear modulus of the CNT. The values obtained for Young modulus and Shear modulus are in excellent agreement with the available experimental results. Copyright © 2009 American Scientific Publishers.

Journal of Nanoscience and Nanotechnology

Elastic properties of Boron Nitride Nanotubes and their comparison with carbon Nanotubes

The intensity ratios of spectral lines of a single species are often used as diagnostics for inferring the electron temperature or density in plasmas. Helium is a well-known example. In this paper, the atomic data and analysis structure (ADAS) database has been used to identify line ratios in boron that might prove useful for plasmas created in tokamaks for the wall conditioning of plasma-facing-surfaces. Using the collisional radiative (CR) model-based photon emission rate coefficients, a few line intensity ratios selectively sensitive only to temperature and those that are sensitive to density are presented. © 2009 The Royal Swedish Academy of Sciences.

Physica Scripta

Boron spectral line ratios helpful to characterize wall conditioned tokamak plasmas

We have synthesized a range of transition-metal-doped BiFeO3 thin films on conducting silicon substrates using a spin-coating technique from metal-organic precursor solutions. Bismuth, iron and transition-metal-organic solutions were mixed in the appropriate ratios to produce 3% transition-metal-doped samples. X-ray diffraction studies show that the samples annealed in a nitrogen atmosphere crystallize in a rhombohedrally distorted BiFeO3 structure with no evidence for any ferromagnetic secondary phase formation. We find evidence for the disappearance of the 404cm -1 Raman mode for certain dopants indicative of structural distortions. The saturation magnetization of these BiFeO3 films has been found to increase on doping with transition metal ions, reaching a maximum value of 8.5emucm-3 for the Cr-doped samples. However, leakage current measurements find that the resistivity of the films typically decreases with transition metal doping. We find no evidence for any systematic variation of the electric or magnetic properties of BiFeO3 depending on the transition metal dopant, suggesting that these properties are determined mainly by extrinsic effects arising from defects or grainboundaries. © 2009 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Structural, magnetic, and electrical studies on polycrystalline transition-metal-doped BiFeO3 thin films

We have investigated the structural, optical, and electrical properties of both as-grown and vacuum annealed In2 O3 thin films. In contrast to the insulating as-prepared samples, vacuum annealed In2 O3 films exhibit a metallic electrical conductivity with increased carrier concentration and mobility. We attribute the excess carriers to an oxygen deficiency introduced during vacuum annealing. Remarkably, these carrier densities seem to be stable under ambient conditions for at least two years. Optical spectroscopy measurements show a large optical transparency, greater than 80%, for both the as-prepared and vacuum annealed In2 O 3 films. © 2009 American Institute of Physics.

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Applied Physics Letters

Undoped vacuum annealed In2 O3 thin films as a transparent conducting oxide

Fabrication and characterization of organic photodetectors based on copper phthalocyanine and fullerenes are presented. We study current-voltage and impedance characteristics of the devices under dark and varied illumination conditions. The current-voltage characteristics in the reverse bias show little or no dark current and large photocurrent. The results show that photocurrent in the devices is associated with a decrease in bulk resistance and an increase in dielectric constant of the active material. Copyright © 2009 American Scientific Publishers All rights reserved.

Organic photodetectors based on phthalocyanine and fullerenes: Dielectric properties

While exploring the random diffusion of 2 bit molecular switches (we define as molecular neuron) on an atomic flat Au (111) substrate, we have found that at least four molecules are required to construct a functional neural net. Surface electron density wave enables communication of one to many molecules at a time-a prerequisite for the parallel processing. Here we have shown that in a neural net of several molecules, some of them could dynamically store information as memory and consistently replicate the fundamental relationship that is found only in a collective and emergent computing system like our brain. © 2009 American Institute of Physics.

Smallest artificial molecular neural-net for collective and emergent information processing

Magnetic Barkhausen emissions and magnetic hysteresis measurements were carried out on water-quenched 5Cr-0.5Mo steel subjected to ageing at 600 °C up to 5000 h. During initial ageing, this steel exhibited magnetic softening, which was attributed to relaxation of quenching stress in the material as well as decrease in dislocation density and migration of interstitial carbon atoms towards the grain boundary. Further ageing resulted in magnetic hardening owing to the restricted movement of the domain wall by the precipitation of carbides such as M3C2, M2C, M7C3 where M stands for Fe, Cr or a combination of them. At longer ageing periods, magnetic behaviour was affected by a change in the composition and morphology of the carbides. Massive M23C6 types of carbides were formed during longer periods of ageing. The coarsening of carbides decreased the pinning density for the domain wall motion and affected the magnetic properties of the steel. The effect of demagnetizing field from voids and non-magnetic massive carbides also affected the magnetic behaviour. Magnetic behaviour and Vickers hardness measurements during ageing have been effectively supported by microstructural evaluations suggesting the capability of the magnetic techniques for assessment of damage during ageing in high temperature 5Cr-0.5Mo steel components. © 2009 IOP Publishing Ltd.

Journal of Physics D: Applied Physics

Magnetic properties evaluation of ageing behaviour in water-quenched 5Cr-0.5Mo steel

The effect of substituting Fe by Co on the crystallization, structural and magnetic behaviour of Fe 72-xCo xSi 4B 20Nb 4 (X = 10,20,36,50 at%) and CO 36Fe 36Si 4-yAl yB 20Nb 4 (Y = 0,1 at%) alloys prepared in the form of melt spun ribbons has been discussed. Alloys containing optimum content of cobalt = 36 at% showed consistent coercivity at elevated temperatures. This soft magnetic property was further improved with aluminium incorporation. Transmission electron microscopy (TEM) indicated that such enhancement in the properties was due to finer dispersions of (CoFe)SiAI nanoparticles in amorphous matrix. Nanocrystallisation also raised the Curie temperature of the aluminium contained alloy. © 2009 American Scientific Publishers.

Development of nanostructured CoFe-based alloys for high temperature magnetic applications

Large-area arrays of highly oriented Co-doped ZnO nanorods with pyramidal hexagonal structure are grown on silica substrates by wet chemical decomposition of zinc-amino complex in an aqueous medium. In case of undoped ZnO with an equi-molar ratio of Zn2+/hexamethylenetetramine (HMT), highly crystalline nanorods were obtained, whereas for Co-doped ZnO, good quality nanorods were formed at a higher Zn2+/HMT molar ratio of 4:1. Scanning electron microscope (SEM) studies show the growth of hexagonal-shaped nanorods in a direction nearly perpendicular to the substrate surface with a tip size of ∼50 nm and aspect ratio around 10. The XRD studies show the formation of hexagonal phase pure ZnO with c-axis preferred orientation. The doping of Co ions in ZnO nanorods was confirmed by observation of absorption bands at 658, 617 and 566 nm in the UV-vis spectra of the samples. The optical studies also suggest Co ions to be present both in +2 and +3 oxidation states. From the photoluminescence studies, a defect-related emission is observed in an undoped sample of ZnO at 567 nm. This emission is significantly quenched in Co-doped ZnO samples. Further, the Co-doped nanorods have been found to show ferromagnetic behavior at room temperature from vibrating sample magnetometer (VSM) studies. © 2009 Elsevier Ltd. All rights reserved.