A systematic investigation has been carried out using core level X-ray photoelectron spectroscopy (XPS) in the Fe- and Ni-doped YBa2Cu3O7—y (YBCO) system by varying the concentration of Cu. The experimental spectra of Ba-3d, Ba-4d and Y-3d have been utilized to probe the dopant sites. This study indicates that Fe goes to Cu(1) site and Ni to both Cu(1) and Cu(2) sites. It appears that each metal substituent has a preference for a specific Cu site or combination of sites, which is consistent with the reports available in literature. The present work emphasizes that XPS can be a suitable tool to probe the dopant sites in two inequivalent local sites.
Bulk YBa2(Cu1-y57Fey)4O8 samples with y=0.00-0.05 were synthesized by an aqueous sol-gel method under 1 atm oxygen pressure. The samples were characterized by XRD for phase purity and lattice parameters, by thermal analysis for oxygen stability as well as by magnetic susceptibility and SQUID measurements for magnetic and superconducting properties. Iron was found to both destabilize the YBa2Cu4O8 phase and have a strong effect on its properties even at very low concentrations. The site assignment proposed for the iron dopant is based on the results of 57Fe Mossbauer spectroscopy studies. The results are compared with the observed iron doping effects in the YBa2Cu3O7- delta phase.
The superconducting and ground state samples of PrFeAsO0.8F0.2 and PrFeAsO have been synthesised via easy and versatile single step solid state reaction route. X-ray & Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic like step in resistivity below 150K followed by another step at 12K. The former is associated with the spin density wave (SDW) like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of superconducting PrFeAsO0.8F0.2 sample. Detailed high field (up to 12Tesla) electrical and magnetization measurements are carried out for superconducting PrFeAsO0.8F0.2 sample. The PrFeAsO0.8F0.2 exhibited superconducting onset (Tconset) at around 47K with Tc({\rho} =0) at 38K. Though the Tconset remains nearly invariant, the Tc({\rho} =0) is decreased with applied field, and the same is around 23K under applied field of 12Tesla. The upper critical field (Hc2) is estimated from the Ginzburg Landau equation (GL) fitting, which is found to be ~ 182Tesla. Critical current density (Jc) being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 103 A/cm2. Summarily, the superconductivity characterization of single step synthesised PrFeAsO0.8F0.2 superconductor is presented.
We report a comparative study of crystal structure, electrical, and magnetic properties of the pure and F-doped superconducting REFeAsO (Re = Pr, Nd, and Sm) samples. The presence of superconductivity in these iso-structural compounds provided an opportunity to understand the doping mechanism in oxy-pnictide superconductor. Bulk polycrystalline samples are synthesized by solid state reaction route in an evacuated sealed quartz tube. The Rietveld analysis of room temperature X-ray diffraction (XRD) data show that all the studied samples crystallize in single phase in a tetragonal structure with space group P4/nmm. The lattice parameters of the studied samples follow the well-known rare earth contraction. The decrease in c-parameter and the volume is indicative of successful substitution of F−1(RF = 1.33 Ǻ) at O2-(RO = 1.40 Ǻ) site. The ground state REFeAsO compounds shows a metallic step in resistivity measurements below say 150–130 K. This metallic step is attributed to structural and SDW transition. The superconductivity in the F-doped sample is confirmed by resistivity measurements as well as magnetic measurements. Superconducting transition temperature (Tc) is found to be at 51K, 48K and 38 K respectively for RE = Sm, Nd and Pr. The superconducting transition temperature of the F-doped samples increase with deceasing the ionic radii of the rare earth. The temperature dependent upper critical field Hc2(0) is calculated from detailed up to 14 Tesla R(T)H measurements using the extrapolation method employing Ginzburg- Landau (GL) theory. Thus calculated Hc2(0) is found to be above 200 Tesla, which is second best to the High Tc cuprates.
It is well known that the effective exponent γ(T) for the paramagnetic susceptibility in amorphous ferromagnets exhibits an unusual temperature dependence over a wide range. In sharp contrast to the monotonic behavior observed for ‘‘pure’’ magnetic systems, with increasing temperatures γ(T) for amorphous ferromagnetic materials increases from the critical Heisenberg value of 1.38 near TC to much larger values (≥2) and after a maximum eventually approaches the Curie-Weiss behavior. Such a crossover behavior in χ(T) has been explained as being due to a noncritical interplay between the magnetic correlations and the spatial fluctuations of the exchange interactions. With this point of view a recent Monte Carlo simulation1 of the phase transition in random ferromagnets predicts that site disorder has a much stronger influence on χ(T) than bond disorder. In order to study the consequences of site, and bond disorder and also the possible role of competing exchange interactions on γ(T) behavior we have measured the low-field ac and dc susceptibilities for melt-spun amorphous Fe-rich (Fe-Ni)90Zr10 alloys over an extended range of temperature. In these ‘‘ferroglasses’’ substitution of Fe with Ni is found to suppress the low-temperature ‘‘spin-glass phase’’ completely for Ni conc. above 8 at. %, while at the same time increasing TC. Thus Ni substitution in these alloys results in a material where γ(T) behavior exhibits progressive change from a nonmonotonic to conventional behavior. We find that the dynamics of correlated spins and its sensitivity to local inhomogeneities in an amorphous material can thus be well resolved near the percolation threshold for the loss of long-range ferromagnetic order.
We report high field magneto transport of Sm/PrFeAsO. Below spin density wave transition (TSDW), the magneto-resistance (MR) of Sm/PrFeAsO is positive and increasing with decreasing temperature. The MR of SmFeAsO is found to be 16%, whereas it is 21.5% in the case of PrFeAsO, at 2.5 K under applied magnetic field of 14 Tesla (T). In the case of SmFeAsO, the variation of isothermal MR with field below 20 K is nonlinear at lower magnetic fields (<2 T) and it is linear at moderately higher magnetic fields (H ≥ 3.5 T). On the other hand, PrFeAsO shows almost linear MR at all temperatures below 20 K. The anomalous behavior of MR being exhibited in PrFeAsO is originated from Dirac cone states. The stronger interplay of Fe and Pr ordered moments is responsible for this distinct behavior. PrFeAsO also shows a hump in resistivity (R-T) with a possible conduction band (FeAs) mediated ordering of Pr moments at around 12 K. However, the same is absent in SmFeAsO even down to 2 K. Our results of high field magneto-transport of up to 14 T brings about clear distinction between ground states of SmFeAsO and PrFeAsO.
The electronic structures of hole-doped La0.7Ca0.3MnO3 and electron-doped La0.7Ce0.3MnO3 manganites are investigated by x-ray absorption near-edge structure spectroscopy at the O and Mn K-, and Mn L3,2-edges. The Mn K- and L3,2-edge results show that Ce dopants increase the occupation of the Mn 4p and majority-spin eg orbitals and reduce the positive effective charge of some Mn ions. However, Ce doping also induces holes in O 2p derived states. As for La0.7Ca0.3MnO3, in contrast to previous understanding that Ca doping converts some Mn ions into the Mn4+ state, we find that Ca dopants actually increase the number of majority-spin eg electrons. We find instead that the holes created by Ca dopants are in the O 2p derived states.
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