N. C. Mishra

Dependence on cation distribution of particle size, lattice parameter, and magnetic properties in nanosize Mn–Zn ferrite

In Mn1−xZnxFe2O4 (x=0 to 1) nanosize particles prepared through hydrothermal precipitation we observe a decrease in particle size from 13 to 4 nm with increasing Zn concentration from 0 to 1. The lattice constant, a, for all Mn/Zn concentrations is found to be less than that for the corresponding bulk values. At specific compositions within x=0.35 and 0.5, the temperature dependence of the magnetization exhibits a cusp-like behavior below the temperature at which the nanoparticles undergo a ferri- to para-magnetic transition (Tc). The Curie temperatures, Tc, of the nanoparticles are in the range of 175–500 °C, which are much higher than their corresponding bulk values. To explain these unusual features, the strong preferential occupancy of cations in chemically inequivalent A and B sites and the metastable cation distribution in nanoparticles are invoked.

OXYGEN VACANCY INDUCED STRUCTURAL PHASE TRANSFORMATION IN TIO2 NANOPARTICLES

Nanosize TiO2 powders prepared by the sol–gel technique at pH of precipitation 4.5 and 6.5 show the anatase phase after calcining at 500 °C. Anatase to rutile phase transformation (ART), however, occurs at 650 °C in the case of pH 6.5 while 850 °C is found to be the ART temperature for the lower pH sample. pH of precipitation dependent ART temperature has not been reported so far to the best of our knowledge. It is known that the smaller the particle size, the lower the ART temperature, and vice versa. The observation of higher crystallite size and lower ART temperature in the case of the higher pH sample contradicts the reported result. We realized from x-ray photoelectron spectroscopic studies that oxygen vacancy concentration drives the ART temperature to lower values in the higher pH sample compared with the sample synthesized at lower pH; even the particle size is found to be higher in the former one.

Appearance of superparamagnetism on heating nanosize Mn0.65Zn0.35Fe2O4

Nanosize particles (average size ∼12 nm) of mixed ferrite Mn0.65Zn0.35Fe2O4 were prepared by the hydrothermal precipitation route and studied using x-ray diffraction, transmission electron microscopy, differential scanning calorimetry, magnetization measurements, and Mossbauer spectroscopy. The as-prepared sample was largely ferrimagnetic and, as the sample was annealed at temperatures above 250 °C, it gradually became superparamagnetic. This unexpected behavior is explained by assuming that the cation distribution in the nanosize as-prepared sample is in a metastable state and, as the sample is heated, this distribution changes to a more stable state while the grain size remains nearly the same.

Structural evolution of TiO2 nanocrystalline thin films by thermal annealing and swift heavy ion irradiation

The present study probes into the transition from anatase to rutile phase of TiO2 in 100 nm thick nanocrystalline thin films under thermal annealing and swift heavy ion (SHI) irradiation. The films were prepared using sol-gel and spin coating techniques on silicon (100) substrates. The as-deposited films are found to be amorphous by glancing angle x-ray diffraction and Raman spectroscopy. Though thermal annealing is known to cause transformation from anatase to rutile phase of TiO2 in a temperature interval of 700–900 °C, in nanoparticle thin films, we found that a sizable volume fraction of anatase still remains even after annealing at 1000 °C. Irradiations by 200 MeV Ag ions on the other hand suppressed the anatase phase and almost phase pure rutile TiO2 could be obtained at a fluence of 3×1012 ions cm−2. A mechanism based on the competing effect of grain growth and conversion of anatase to rutile at the grain boundary of the anatase on annealing and conversion of anatase to rutile in the grains of the ...

Oxygen vacancy induced phase formation and room temperature ferromagnetism in undoped and Co-doped TiO2 thin films

TiO2 and Co-doped TiO2 (CTO) thin films deposited at various oxygen partial pressures by pulsed laser deposition exhibit room temperature ferromagnetism (RTFM) independent of their phase. Films deposited at 0.1?mTorr oxygen partial pressure show a complete rutile phase confirmed from glancing angle x-ray diffraction and Raman spectroscopy. At the highest oxygen partial pressure, i.e. 300?mTorr, although the TiO2 film shows a complete anatase phase, a small peak corresponding to the rutile phase along with the anatase phase is identified in the case of CTO film. An increase in O to Ti/(Ti+Co) ratio with increase in oxygen partial pressure is observed from Rutherford backscattering spectroscopy. It is revealed from x-ray photoelectron spectroscopy (XPS) that oxygen vacancies are found to be higher in the CTO film than TiO2, while the valency of cobalt remains in the +2 state. Therefore, the CTO film deposited at 300?mTorr does not show a complete anatase phase unlike the TiO2 film deposited at the same partial pressure. We conclude that RTFM in both films is not due to impurities/contaminants, as confirmed from XPS depth profiling and cross-sectional transmission electron microscopy (TEM), but due to oxygen vacancies. The magnitude of moment, however, depends not only on the phase of TiO2 but also on the crystallinity of the films.

Temperature-dependent roughness of electronically excited InP surfaces

Topographical evolution of 100 MeV Au8+-ion-irradiated InP surfaces was studied using atomic-force microscopy (AFM). The surfaces were roughened under dense electronic excitations. Root-mean-square roughness measured from AFM studies showed an exponential saturation behavior with fluence. Sample temperature during irradiation was found as a parameter to control the amount of roughness on the surface and the evolution of irradiated surface topography is discussed in terms of thermal spike model.

Transport of oxygen atoms mediated by electronic excitation

Abstract Diffusion of oxygen from substrate to the film was observed under the influence of large electronic excitation. CuO thin film ∼210 nm on float glass was irradiated with 210 MeV I ions. We noticed the transport of oxygen from the substrate to the CuO film through the interface. The amount of oxygen transport from glass to the film was found to be fluence dependent. The loss of oxygen from the films was also observed. The erosion of Cu atoms was also observed beyond a fluence of 9.6×10 13 ions/cm2. The measurements were performed by on-line elastic recoil detection using a large area position sensitive detector. Since the electronic energy loss dominates in the present case of 210 MeV I ions, the observed changes at the interface and surface are attributed to inelastic collisions of the incident ions with the atomic electrons in the sample.

200 MeV silver ion irradiation induced structural modification in YBa2Cu3O7-y thin films at 89 K: An in situ x-ray diffraction study

We report in situ x-ray diffraction (XRD) study of 200 MeV Ag ion irradiation induced structural modification in c-axis oriented YBa2Cu3O7−y (YBCO) thin films at 89 K. The films remained c-axis oriented up to a fluence of 2×1013 ionscm−2, where complete amorphization sets in. The amorphous ion tracks, the strained region around these tracks, and irradiation induced point defects are shown to control the evolution of the structure with ion fluence. Secondary electrons emanating from the ion paths are shown to create point defects in a cylindrical region of 97 nm radius, which corresponds to their maximum range in the YBCO medium. The point defects are created exclusively in the CuO basal planes of fully oxygenated YBCO, which has not been possible, by other techniques including low energy ion irradiation and thermal quenching. The point defects led to a faster decrease in the integral intensity of XRD peaks at very low fluences of irradiation (Φ≤3×1010 ionscm−2) than what can be expected from amorphous tra...

Fluctuation conductivity and inhomogeneity in granular YBa2Cu3O7−y/Ag composite thick films

We analyse the effect of interplay of the different types of inhomogeneities and the thermal fluctuation of the superconducting order parameter on the excess conductivity in a set of YBa2Cu3O7−y/Ag composite thick films. We show that the mesoscopic inhomogeneities arising due to most of the Ag residing at the grain boundaries strongly influence the tailing and the critical regions below and above the mean field transition temperature, Tc, respectively. A small fraction of Ag diffusing into the grains also produces microscopic inhomogeneities. Though these inhomogeneities are not expected to influence the SCOPF, we found that TLD, corresponding to the transition from the two to three dimensional fluctuation of the order parameter in the mean field region, as well as the inter-planar coupling strength, strongly depend on the Ag content in the composites. The effect of Ag induced inhomogeneities on the temperature window corresponding to different phases in the mean field and the critical regions are depicted in the form of a phase diagram. The large variation of TLD, and near invariance of Tc with Ag content in the films as seen in the phase diagram, are explained by invoking the role of Ag in modifying the overall electronic structure of the grains.

Swift heavy ion-based materials science research at NSC

Abstract Effects of large electronic excitation in different types of materials, viz. metals, semiconductors, superconductors, polymers and organic crystals are being investigated using the swift heavy ions (SHI) of energies from 10 to 270 MeV available from 15 UD NSC Pelletron. Defect production in semiconductors, structural strain on electronic transport properties of materials having colossal magneto resistance (CMR), flux pinning in high T c superconductors (HTS), desorption of hydrogen from polymers, ion beam mixing in metal/Si interface and optical waveguide formation are some of our research activities. Resistivity and 1/ f noise measurements from room temperature down to 77 K are used for the studies of SHI-induced defects into the materials. Quadrupole mass spectrometer (QMS) in the materials science beam line is used for the studies of radiolysis of polymers induced by SHI. For polymers, ion track diameters have been estimated by on-line monitoring of the loss of hydrogen (using ERD and QMS) during ion irradiation. An ultra high vacuum scanning tunneling microscope (UHV STM) is being installed at the materials science beam line to study the individual damage created by SHI on the solid surfaces. A three-axes goniometer has been installed for RBS-channeling studies of various materials. Experimental facilities and the research work carried out at Nuclear Science Centre are briefed.