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The dynamics of the release of stored energy during annealing along two different crystallographic planes i.e.{111} and {220} in deformed copper have been investigated using in-situ X-ray diffraction measurements at 458K and 473K respectively. The study has been carried out on 50% and 80% rolled Cu sheets. The microstructures of the rolled samples have been characterised using optical microscope and Electron Back Scattered Diffraction measurements. The microstructural parameters were evaluated using Scherrer equation and the modified Rietveld technique from X-ray diffractogram. The stored energy along different planes were determined using the modified Stibitz formula from the X-ray peak broadening and the bulk stored energy has been evaluated using differential scanning calorimetry. The process dynamics of recovery and recrystallization as observed through the release of stored energy has been modelled as a second order and first order process respectively., Comment: 11 figures

In the present study, an attempt has been made to understand the effect of different competing mechanisms controlling the overall degree of sensitization (DOS) of deformed austenitic stainless steel at the early stage of sensitization. The Double Loop Electrochemical Potentiokinetic Reactivation (DL-EPR) studies were performed to characterize the Degree of Sensitization (DOS) as functions of pre-defined strain and sensitization temperature. X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were used to explain the phenomena qualitatively. A non monotonous behaviour in the variation of DOS has been observed with deformation and sensitization temperature. The presence of Deformation Induced Martensites (DIM) and their transformation into tempered martensites (alpha + Fe3C) at higher temperatures was found to play major roles in controlling the overall sensitization and desensitization processes., Comment: 6 figures

In this report we have pointed out that magnetic hysteresis loop can be used as a probe to distinguish a single layer from a many layer graphitic structure. Chemically we have synthesized graphitic oxide (GO) and reduced graphitic oxide (RGO) for this investigation. We observe ferromagnetic like hysteresis loops for both GO and RGO below a certain applied critical magnetic field and above this critical field we observe cross-over of the positive magnetization to negative magnetization leading to diamagnetic behaviour. This cross-over is more dominant for the case of many layer graphitic structure. Upon annealing of GO in air the critical cross-over field decreases and the magnetization increases for multilayer graphitic structure. Possible reasons for all these observations and phenomena is presented here., Comment: 12 pages writeup and 4 pages figures (twelve figures)

We employ the methods of atomistic simulation to investigate the climb of edge dislocation at nanovoids by analyzing the energetics of the underlying mechanism. A novel simulation strategy has been demonstrated to estimate the release of surface energy of the nanovoid during the void induced climb. The curvature of the pinned dislocation segment is found to play a key role in mediating this unique mechanism of dislocation climb. Our study reveals that the kinetics of void-induced climb process is fundamentally distinct from the conventional diffusion-mediated climb., Comment: 26 pages, 8 figures

Tensile tests were carried out by deforming polycrystalline samples of Al-2.5%Mg alloy at four different temperatures in an intermediate strain rate regime of 2x10-4s-1 to 2x10-3s-1. The Portevin-Le Chatelier (PLC) effect was observed throughout the strain rate and temperature region. The mean cumulative stress drop magnitude and the mean reloading time exhibit an increasing trend with temperature which is attributed to the enhanced solute diffusion at higher temperature. The observed stress-time series data were analyzed using the nonlinear dynamical methods. From the analyses, we could establish the presence of deterministic chaos in the PLC effect throughout the temperature regime. The dynamics goes to higher dimension at a sufficiently high temperature of 425K but the complexity of the dynamics is not affected by the temperature., Comment: 18 pages, 8 figures; accepted in Met. Mater. Trans. A

The mechanism of the evolution of the deformed microstructure at the earliest stage of annealing where the existence of the lowest length scale substructure paves the way to the formation of the so-called subgrains, has been studied for the first time. The study has been performed at high temperature on heavily deformed Ti-modified austenitic stainless steel using X-ray diffraction technique. Significant changes were observed in the values of the domain size, both with time and temperature. Two different types of mechanism have been proposed to be involved during the microstructural evolution at the earliest stages of annealing. The nature of the growth of domains with time at different temperatures has been modelled using these mechanisms. High-resolution transmission electron microscopy has been used to view the microstructure of the deformed and annealed sample and the results have been corroborated successfully with those found from the X-ray diffraction techniques., Comment: 23 pages including 8 figures

We carry out the molecular statics simulations of depinning of an edge dislocation at voids of diameters of the order of ~1 nm. We show that the dislocation-void interactions are non-trivial as the applied shear load is found to enhance the pinning strength of the nanovoid itself. This leads to the surprising observation of multiple CRSS values for a given nanovoid., Comment: 4 figures

Dislocation pinning plays a vital role in the plastic behaviour of a crystalline solid. Here we report the first observation of the damped oscillations of a mobile dislocation after it gets pinned at an obstacle in the presence of a constant static shear load. These oscillations are found to be inertial, instead of forced as obtained in the studies of internal friction of solid. The rate of damping enables us to determine the effective mass of the dislocation. Nevertheless, the observed relation between the oscillation frequency and the link length is found to be anomalous, when compared with the theoretical results in the framework of Koehler's vibrating string model. We assign this anomaly to the improper boundary conditions employed in the treatment. Finally, we propose that the inertial oscillations may offer a plausible explanation of the electromagnetic emissions during material deformation and seismic activities., Comment: 28 pages, 4 figures

In this paper, we have tried to find out the origin of magnetism in Gold nanoparticles (Au- NPs). We observe that upon incorporating Gold nanoparticles (Au-NPs) in Fe3O4 nanoparticle medium the net magnetisation increases compared to the pure Fe3O4 nanoparticle medium. This increase of magnetization can be attributed to the large orbital magnetic moment formation at the Au/magnetic particle interface indicating that magnetism observed in Au-NPs is an interfacial effect. This interfacial effect has been supported by the observation of sudden transition from positive saturated magnetisation to a negative diamagnetic contribution as a function of magnetic field on citrate coated gold Au-NPs., Comment: 7 pages double column, 6 figures, This manuscript contains more quantitative analysis of magnetization of arXiv:0906.1497v2 and additional data on citrate coated Au has been added as figure 6

Tensile tests have been carried out by deforming polycrystalline samples of substitutional Al-2.5%Mg alloy at room temperature at a range of strain rates. The Portevin - Le Chatelier (PLC) effect was observed. From an analysis of the experimental stress versus time series data we have inferred that the dynamics of the PLC effect in a local finite time is controlled by a finite number of degrees of freedom and this effective dimension becomes reduced with increasing strain., Comment: 12 pages, 5 figures

Plastic deformation of crystals is a physical phenomenon, which has immensely driven the development of human civilisation since the onset of the Chalcolithic period. This process is primarily governed by the motion of line defects, called dislocations. Each dislocation traps a quantum of plastic deformation expressible in terms of its Burgers vector[1,2]. Theorising the mechanisms of dislocation motion at the atomistic scales of length and time remains a challenging task on account of the extreme complexities associated with the dynamics. We present a new concept of modelling a moving dislocation as the dynamic distribution of the elastic field singularity within the span of the Burgers vector. Surprisingly, numerical implementation of this model for the periodic expansion-shrinkage cycle of the singularity is found to exhibit an energetics, which resembles that of a dislocation moving in the presence of the Peierls barrier[1-4]. The singularity distribution is shown to be the natural consequence under the external shear stress. Moreover, in contrast to the conventional assumption, here the calculations reveal a significant contribution of the linear elastic region surrounding the core towards the potential barrier., Comment: 10 pages, 5 figures

In this report we present systematic magnetic studies of pure iron oxide nanoparticles and gold iron oxide nanocomposite with increasing Au particle size/content. For the magnetic studies of these samples we have measured: (1) zero field cooled (ZFC) and field cooled (FC) magnetization, (2) ac susceptibility, (3) magnetization vs field at various temperatures, (4) thermoremanant magnetization relaxation (TRM) and zero field cooled magnetization relaxation (ZFCM) at fixed temperature for various wait times tw for studying the aging effect, (5) magnetization memory effect and (6) exchange bias as a function of cooling field. The detailed magnetic measurement analysis indicates that the pure Fe3O4 nanoparticles sample behaves like a superparamagnet and on incorporation of gold (Au) nanoparticles the nanocomposite system slowly evolves from superparamagnetic to superspin glass state. The memory and aging effect enhances with the increase of the Au nanoparticle size/content. The most important observation in this study is the enhancement of magnetization with the incorporation of Au nanoparticles. The enhancement increases with the increase in the Au content in the nanocomposite. We have explained the cause of this enhancement of magnetization as due to large orbital magnetic moment formation at the Au/magnetic particle interface., Comment: 12 figures, 33 pages total

This paper has been withdrawn., Comment: This paper has been withdrawn due to further modification in the model

The relation between the contact angle of a liquid drop and the morphological parameters of self-affine solid surfaces have been investigated. We show experimentally that the wetting property of a solid surface crucially depends on the surface morphological parameters such as: (1) root mean square (rms) roughness $\sigma$, (2) in-plane roughness correlation length $\xi$ and (3) roughness exponent $\alpha$ of the self-affine surface. We have shown that the contact angle monotonically decreases with the increase in the rms local surface slope $\rho$ ($\propto \sigma/\xi^\alpha$) for the cases where the liquid wets the crevices of the surface upon contact. We have shown that the same solid surface can be made hydrophobic or hydrophilic by merely tuning these self-affine surface morphological parameters., Comment: 4 page in double column format, 5 figures

In this letter we propose a model that demonstrates the effect of free surface on the lattice resistance experienced by a moving dislocation in nanodimensional systems. This effect manifests in an enhanced velocity of dislocation due to the proximity of the dislocation line to the surface. To verify this finding, molecular dynamics simulations for an edge dislocation in bcc molybdenum are performed and the results are found to be in agreement with the numerical implementations of this model. The reduction in this effect at higher stresses and temperatures, as revealed by the simulations, confirms the role of lattice resistance behind the observed change in the dislocation velocity., Comment: 4 Figures

The Portevin-Le Chatelier(PLC) effect has been investigated by deforming Al-2.5%Mg alloy in the strain rate regime where simultaneously two types (type B and type A) of serrations appear in the stress strain curve. Our analysis reveal that in this strain rate regime the entire PLC dynamics for a particular strain rate experiment is governed by a single band which changes its character during the deformation., Comment: 13 pages, 4 figures

Tensile tests were carried out by deforming polycrystalline samples of substitutional Al-2.5%Mg alloy at room temperature for a range of strain rates. The Portevin-Le Chatelier (PLC) effect was observed throughout the strain rate regime. The deformation bands in this region are found to be of type A in nature. From the analysis of the experimental stress time series data we could infer that the dynamics of type A dislocation band propagation is a Markov process., Comment: 17 pages, 8 figures, to appear in Physics Letters A

We present in this paper the changes in the room temperature magnetic property of ZnO on Mn doping prepared using solvo-thermal process. The zero field cooled (ZFC) and field cooled (FC) magnetisation of undoped ZnO showed bifurcation and magnetic hysteresis at room temperature. Upon Mn doping the magnetic hysteresis at room temperature and the bifurcation in ZFC-FC magnetization vanishes. The results seem to indicate that undoped ZnO is ferromagnetic while on the other hand the Mn doped ZnO is not a ferromagnetic system. We observe that on addition of Mn atoms the system shows antiferromagnetism with very giant magnetic moments., Comment: 5 figures

We report here enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field upto 300K and a finite thermoremanent magnetization upto 340K for the annealed sample. We propose that magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers, or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like behaviour below 10K., Comment: 4 figures

In this letter we report observation of room temperature ferromagnetism in bulk Ga$_2$O$_3$ . We also observe large (10-60%) increase/decrease in photoluminescence. In the red(700 nm wavelength)/blue(500 nm), with the application of small magnetic field(0.4 Tesla). We argue, that ferromagnetism occurs entirely due to chains of oxygen(O(3) sites, Fig. 5) vacancies. We propose a simple model to explain, origin and location of moment, formation of ferromagnetic dislocation needles, and strong increase/decrease ofred/blue photoluminescence intensity with magnetic field., Comment: 1 LaTex file and 7 figure gziped ps files

Thermopower measurements have been carried out on the Ni substituted samples of Na0.75CoO2 in the temperature range 4.2K to 300K. The room temperature TEP increases by 20microV/K even with 1% Ni substitution and systematically increases with increasing Ni content upto 5%. The increase in TEP is accompanied by a decrease in rho, thus increasing the ratio of S^(2)/rho on Ni substitution. At low T, the TEP shows an anomaly in the substituted samples, showing a peak at T ~ 20K., Comment: To appear in Sol. St. Phys.India Vol 50 (2005)

We report here that atomic force microscope (AFM) in frictional force mode can be used to detect onset of chain scission and crosslinking in polymeric and macromolecular samples upon irradiation. A systematic investigation to detect chain scission and crosslinking of two elastomers: (1) Ethylene-propylene-diene monomer rubber (EPDM) and (2) Fluorocarbon rubber (FKM) upon gamma-ray irradiation has been carried out using frictional force microscopy (FFM). From the AFM results we observed that both the elastomers show a systematic smoothening of its surfaces, as the gamma-ray dose rate increases. However, the frictional property studied using FFM of the sample surfaces show an initial increase and then a decrease as a function of dose rate. This behavior of increase in its frictional property has been attributed to the onset of chain scission and the subsequent decrease in friction has been attributed to the onset of crosslinking of the polymer chains. The evaluated qualitative and semi-quantitative changes observed in the overall frictional property as a function of gamma-ray dose rate for the two elastomers are presented in this paper., Comment: 18 pages, 7 figs included

We report on the effect of Ni substitution at the Co site on the physical properties of NaxCoO2 system by investigating the series NaxCo1-yNiyO2 (x=0.75, 0

Room temperature Raman scattering measurements have been carried out on well characterized samples of MgB2-xCx. The Raman line corresponding to the E2g phonon mode shows progressive hardening from 620 cm-1 in pristine MgB2 to 775 cm-1 in the sample with carbon fraction x=0.2. The corresponding line width on the other hand, increases from a value of about 220 cm-1 to 286 cm-1 in samples with x = 0.1, beyond which it decreases to a value 167 cm-1 for x=0.2. From the average mode frequency and the line width obtained from Raman measurements and taking the values of N(0) obtained from the calculated variation in s hole density of states in MgB2-xCx, the electron phonon coupling strength to the E2g phonon, l2g, is evaluated using Allen’s formula. This remains large for low C fraction, but shows rapid decrease for x > 0.10. Using this value of l2g appropriately weighted, TC is obtained from McMillan’s equation. These values are in good agreement with the experimentally measured TC variation in MgB2-xCx., Comment: PDF format; containing text and figures. accepted for publication in Journal of Physics Condensed Matter

We present an extensive study on electrical spectroscopy of graphene ribbons and edges of highly oriented pyrolytic graphite (HOPG) using atomic force microscope (AFM). We have addressed in the present study two main issues, (1) How does the electrical property of the graphite (graphene) sheet change when the graphite layer is displaced by shear forces? and (2) How does the electrical property of the graphite sheet change across a step edge? While addressing these two issues we observed, (1) variation of conductance among the graphite ribbons on the surface of HOPG. The top layer always exhibits more conductance than the lower layers, (2) two different monolayer ribbons on the same sheet of graphite shows different conductance, (3) certain ribbon/sheet edges show sharp rise in current, (4) certain ribbons/sheets on the same edge shows both presence and absense of the sharp rise in the current, (5) some lower layers at the interface near a step edge shows a strange dip in the current/conductance (depletion of charge). We discuss possible reasons for such rich conducting landscape on the surface of graphite., Comment: 13 pages, 9 figures. For better quality figures please contact author

We report a systematic study to determine local elastic properties of surfaces combining atomic force microscope (AFM) with acoustic waves which is known as atomic force acoustic microscopy - AFAM. We describe the methodology of AFAM in detail and interpret the measurement using simple arguments and other complementary measurements using AFM.We have used a few selected samples to elucidate the capability of the AFAM technique to map the local elastic constant of the sample surface. Using the force-distance measurement and the change in the frequency of resonance peak at various regions we quantitatively determine the change in the local stiffness (elastic constant) of the sample surface. We have also shown that AFAM technique can be used to get a better surface image contrast where contact mode AFM fails., Comment: High resolution figures available on request from author

In this paper we report the results of electrical resistivity (1.5K < T <300K) and point contact spectroscopy (PCS) measurements on single crystals of metallic sodium tungsten bronze with varying sodium content. We have shown that the electron-phonon coupling function as measured through PCS can explain quantitatively the large temperature dependence of resistivity $\rho$ seen in these materials over the entire temperature range. The electron-phonon coupling function shows predominatly large peaks for phonon frequency range of 30meV < $\omega$ 100meV which match well with the calculated optical phonon modes for WO$_6$ octahedra. The integrated electron-phonon coupling constant $\lambda$ from this data is $\approx$ 0.25-0.45., Comment: RevTEX format with 8 figures

We report a study of the magnetoresistance (MR) of the metallic perovskite oxide LaNiO$_{3-\delta}$ as a function of the oxygen stoichiometry $\delta$ ($\delta \leq$ 0.14), magnetic field (H $\leq 6T$) and temperature (1.5K $\leq $ T $\leq $ 25K). We find a strong dependence of the nature of MR on the oxygen stoichiometry. The MR at low temperatures change from positive to negative as the sample becomes more oxygen deficient (i.e, $\delta$ increases). Some of the samples which are more resistive, show a resistivity minima at $T_{min}$ $\approx$ 20K. We find that in these samples the MR is positive at T > $T_{min}$ and negative for T < $T_{min}$. We conclude that in the absence of strong magnetic interaction, the negative MR in these oxides can arise from weak localisation effects., Comment: 10 pages in REVTeX format, 4 eps figs