P. M. Aswathy

Transport and magnetic properties of yttrium doped NdFeAs(O,F) superconductor

A.L. Solovjov, V. N. Svetlov, V. B. Stepanov, S. L. Sidorov, V.Yu. Tarenkov, A. I. D’yachenko, and A.B.Agafonov B.Verkin Institute for Low Temperature Physics & Engineering National Academy of Sciences of Ukraine, Lenin Ave. 47, 61103 Kharkov, Ukraine A.Galkin Institute for Physics & Engineering National Academy of Sciences of Ukraine, R. Luxemburg 72, 83114 Donetzk, Ukraine and Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, D-30167 Hannover, Germany (Dated: December 7, 2010)

Rare Earth (RE - Ce, Gd) Modified Nd1-xRExFeAsO0.7F0.3 Superconductor with Enhanced Magneto-transport Properties

The influence of rare earth (RE – Ce, Gd) doping at the Nd site in the NdFeAsO0.7F0.3 superconductor wherein Ce and Gd have ionic radii in the order Ce > Nd > Gd is investigated. The structural and superconductivity characterization of the pure and doped samples show that Ce doping enhances the TC of Nd1−xRExFeAsO0.7F0.3 to a maximum of 53.6 K at x = 0.1 while Gd doping attains a TC of 55.1 K at x = 0.15. Interestingly, both Ce and Gd doping create neither secondary phases nor precipitates within the detection limit of XRD. However, the lattice defects due to Ce and Gd doping modifies NdFeAsO0.7F0.3 and assists in pinning the flux lines on these defects thereby exhibiting an enhanced JC(H) performance especially at high fields. It is also observed that the relatively small ionic size of Gd is more effective in TC enhancement, while the lattice defects due to larger ionic size of Ce favor the remarkable enhancement of JC(H).

Structural and Transport Properties of the

The effects of Lu-doping on phase evolution, microstructure, electrical, and superconducting properties of Bi-2212 have been studied. The results show that Lu+3 ions enter into the crystal structure in place of Sr+2 ions and, hence, reduce the hole concentration in the electronic structure of the system. Lu-doping results in the reduction of c-axis length and increases the Josephson coupling between the Cu-O2 layers at lower Lu-contents. The critical temperature Tc and the critical current density Jc are significantly enhanced for optimum Lu concentration. The variations in the electrical and superconducting properties are due to the structural and electronic inhomogeneities and changes in charge carrier concentration due to Lu-doping in the (Bi, Pb)-2212 superconductor.

( \hbox{Bi}_{1.6}\hbox{Pb}_{0.5})(\hbox{Sr}_{2-x}\hbox{Lu}_{x})\hbox{Ca}_{1.1}\hbox{Cu}_{2.1}\hbox{O}_{8 + \delta}

The effects of rare earth site doping on the structural, superconducting and magnetic properties of SmFeAsO0.7F0.3 iron pnictide are investigated. Gd3+ and Ce3+ ions are chosen by virtue of their position being on either side of Sm3+. Doping of both smaller (Gd3+) and larger (Ce3+) ions at the Sm3+ site increases the TC up to 55 K. Doping with the smaller Gd3+ results in lattice contraction and thereby enhances TC. It is interesting to observe that though Ce3+ doping in SmFeAsO0.7F0.3 exhibits an increase in lattice parameters, a substantial enhancement of TC occurs. The enhancement in TC due to increased charge carrier concentration is also confirmed using Hall Effect measurement. Apart from TC enhancement, the simultaneous doping of both at oxygen and rare earth sites prominently increases the superconducting properties such as JC, HC1 and HC2. The co-doped samples also exhibit better magnetic field dependence of JC over the entire field of study. It is also observed that the Ce3+ doped sample shows higher JC in the high field region due to its enhanced flux pinning properties.

Superconductor

The transport and magnetic properties of Gd doped NdFeAsO0.7F0.3 (Nd1111) samples synthesized at ambient pressures were investigated. The sample with x = 0.15 shows a maximum TC of 55.1 K and a magnetic JC of 3.4 × 103 A/cm2 at 5 K. A TC enhancement of 6.3 K and a JC enhancement by a factor of 2, is observed in the 15 % Gd doped sample as compared to the pure sample.

Influence of rare earth doping on the structural and electro-magnetic properties of SmFeAsO0.7F0.3 iron pnictide

The iron-pnictide Sm1−xCaxFeAsO1−2xF2x superconductor was prepared and the combined effect of electron and hole doping was studied in detail. It is observed that the binary doping using CaF2 improves the microstructure tremendously with a preferred orientation of the (00l) planes. Moreover, a maximum TC of 53.8 K and a transport JC of 880 A/cm2 (12 K), which is double to that of the F-doped sample, are achieved. The dopant CaF2 seems to be a potential candidate for solving the grain-connectivity concerns in iron-pnictides paving the way towards conductor development.

Enhanced transport and magnetic properties in gadolinium doped NdFeAsO0.7F0.3 superconductors

We report the synthesis of Nd1−xCaxFeAsO1−2xF2x superconductors using CaF2 as a binary dopant that provides both holes and electrons by simultaneous substitution of Ca2+ ions and F− ions at Nd3+ and O2− sites, respectively. The sample with x = 0.2 exhibits a maximum critical temperature (TC) of 52.3 K and transport critical current density (JC) of 1240 A/cm2 at 12 K. An interesting feature observed for binary doped samples is the preferential alignment of (00l) planes and the refinement of microstructure both in terms of grain size and grain connectivity. NdFeAsO based superconductors are already known for their very high critical fields and field independent JC(H) performance. However, the development of the bulk superconductors with oriented large grains and good grain connectivity has not been reported so far and hence the present work will be definitely a step towards the progress in NdFeAsO based conductor development.

Microstructure refinement and enhanced critical current density in binary doped SmFeAsO superconductor

A small amount of nano Cu addition (2.5 wt %) was found to dramatically decrease the reaction temperature of magnesium and boron, forming MgB2 without any degradation in the transport critical current. The presence of reacted Mg2Cu liquid phase could accelerate the reaction between Mg and B and finally results in the formation of MgB2 phase even at low temperatures. All Cu doped samples heat treated below 650 °C exhibited highly enhanced transport JC at 30 K and the sample heat treated at 550 °C recorded 30‐fold increase in transport JC compared to the pure sample.