arXiv subjects

A. K. Tyagi

Publications and source records attributed to A. K. Tyagi.

44 records · Page 3Linked to original sources

arXiv2009

Superconducting Fe(1+delta)Se(1-x)Te(x) thin films: Growth, characterization and properties

Thin films of Fe(1+delta)Se(1-x)Te(x) (delta ~ 0.18 & x ~ 0.5) have been successfully grown on (100) oriented single crystalline SrTiO3 and LaAlO3 substrates by pulsed laser deposition. The crystal structure was characterized by x-ray diffraction, and the superconducting properties by electrical resistivity measurements. X-ray diffraction analysis establishes the epitaxial growth of the films with c-axis orientation. Atomic force microscopy showed a smooth surface morphology for the films grown on SrTiO3 substrates. All the films are observed to be superconducting with a Tc of ~ 8-14 K depending on the deposition conditions. The deposition parameters were optimized to obtain good quality films with Tc comparable to that of the target material.

cond-mat.supr-con↗

arXiv2008

Fe and N self-diffusion in amorphous FeN: A SIMS and neutron reflectivity study

Simultaneous measurement of self-diffusion of iron and nitrogen in amorphous iron nitride (Fe86N14) using secondary ion mass spectroscopy (SIMS) technique has been done. In addition neutron reflectivity (NR) technique was employed to study the Fe diffusion in the same compound. The broadening of a tracer layer of 57Fe8615N14 sandwiched between Fe86N14 layers was observed after isothermal vacuum annealing of the films at different temperatures in SIMS measurements. And a decay of the Bragg peak intensity after isothermal annealing was observed in [Fe86N14/57Fe86N14]10 multilayers in NR. Strong structural relaxation of diffusion coefficient was observed below the crystallization temperature of the amorphous phase in both measurements. It was observed from the SIMS measurements that Fe diffusion was about 2 orders of magnitude smaller compared to nitrogen at a given temperature. The NR measurements reveal that the mechanism of Fe self-diffusion is very similar to that in metal-metal type metallic glasses. The structural relaxation time for Fe and N diffusion was found comparable indicating that the obtained relaxation time essentially pertain to the structural relaxation of the amorphous phase.

cond-mat.mtrl-sci↗

arXiv2007

Multi-phonon Raman scattering in GaN nanowires

UV Raman scattering studies show longitudinal optical (LO) mode up to 4th order in wurtzite GaN nanowire system. Frohlich interaction of electron with the long range electrostatic field of ionic bonded GaN gives rise to enhancement in LO phonon modes. Good crystalline quality, as indicated by the crystallographic as well as luminescence studies, is thought to be responsible for this significant observation. Calculated size dependence, incorporating size corrected dielectric constants, of electron-phonon interaction energy agrees well with measured values and also predict stronger interaction energy than that of the bulk for diameter below ~3 nm.

cond-mat.mtrl-sci↗

arXiv2007

Room temperature Ferromagnetism in Th1-xFexO2-d (x = 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25) nanoparticles

Nanocrystalline (Th1-xFex)O2-d particles with different Fe concentrations (x = 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25) have been prepared by a gel combustion method. Rietveld refinement analyses of X-ray diffraction data revealed the formation of an impurity free cubic type Th1-xFexO2-d structure up to x = 0.20. This observation is further confirmed from the detailed studies conducted on 10 at. percent Fe doped ThO2 using high-resolution transmission electron microscopy (HRTEM) imaging and indexing of the selected-area electron diffraction (SAED) patterns. DC Magnetization studies as a function field indicate that they are ferromagnetic with Curie temperature (Tc) well above room temperature.

cond-mat.mtrl-sci↗

arXiv2006

Particle size dependence of magnetization and phase transition near T_N in multiferroic BiFeO3

We report results of a comprehensive study of the phase transition at T_N (~643 K) as a function of particle size in multiferroic BiFeO3 system. We employed electrical, thermal, and temperature dependent X-ray diffraction (XRD) studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime 2-300 K under a magnetic field 100-10000 Oe both on bulk and nano-crystalline systems. While in the bulk system a sharp endothermic peak at T_N together with a broad feature, ranging over nearly ~150 K (Delta_T), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at T_N, is found to occur both at T_O and T_N across Delta_T in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metastability increases in the nanoscale BiFeO3 with divergence between zero-field cooled (ZFC) and field cooled (FC) magnetization below ~100 K and faster magnetic relaxation. Interestingly, in nanoscale BiFeO3, one also observes finite coercivity at lower temperature which points out that suitable design of particle size and shape may induce ferromagnetism. The inhomogeneous distribution of Bi/Fe-ions and/or oxygen non-stoichiometry seems to be giving rise to broad features in thermal, magnetic as well as in electrical responses.

cond-mat.mtrl-sci↗

arXiv2005

Detecting onset of chain scission and crosslinking of gamma-ray irradiated elastomer surfaces using frictional force microscopy

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.

cond-mat.mtrl-sci↗

arXiv2005

Conductivity landscape of highly oriented pyrolytic graphite surface containing ribbons and edges

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.

cond-mat.other↗

arXiv2004

Mapping of surface local elastic constant using Atomic Force Acoustic Microscopy

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.

cond-mat.mtrl-sci↗