G. D. Varma

Effect of graphene oxide doping on superconducting properties of bulk MgB2

In the present paper we report the effect of graphene oxide (GO) doping on the structural and superconducting properties of MgB2. Bulk polycrystalline samples have been synthesized via a solid state reaction route with compositions MgB2?+?x?wt% of GO (x?=?0, 1, 2, 3, 5, 7 and 10) by sintering at ?850?? C in a reducing atmosphere of Ar/H2 (9:1). The x-ray diffraction results confirm the formation of the MgB2 phase in all samples, together with traces of a MgO impurity phase. The XRD data results also show substitution of carbon for boron, but in the present case the actual amount of carbon substituting for boron is very small as compared to other carbon sources. A substantial improvement in the critical current density, Jc(H), has been observed in the entire magnetic field range (0?8?T) for samples x?=?1, 2 and 3 as compared to the undoped sample. In addition to Jc(H), marginal improvements in the upper critical field (Hc2) and the irreversibility field (Hirr) have been observed for the doped samples x?=?1, 2 and 3 with respect to pristine MgB2. Furthermore, a curious result of the present investigation is that there is no change in the superconducting transition temperature (Tc) up to a doping level of 10?wt%. The possible mechanisms of flux pinning and correlations between the observed superconducting properties and structural characteristics of the samples have been described and discussed in this paper.

Sintering temperature effect on electrical transport and magnetoresistance of nanophasic La0.7Sr0.3MnO3

Nanophasic La0.7Sr0.3MnO3 (LSMO) samples were prepared by the sol–gel method. The samples were sintered at different temperatures ranging from 600 to 1000 °C. It is shown that the transport and magnetoresistive properties of LSMO samples strongly depend on the sintering temperature (Ts). A substantial decrease in the insulator–metal transition temperature (TIM) and an enhancement in resistivity are found on lowering the sintering temperature. Furthermore, a reduction in magnetization and a slight decrease in paramagnetic–ferromagnetic (PM–FM) transition temperatures (Tc) have been observed as the sintering temperature decreases. The magnetoresistance (MR) at T<Tc increases on decreasing the sintering temperature as well as increasing the applied magnetic field. The enhancement in MR on decreasing the sintering temperature is explained by enhanced spin-polarized tunnelling by assuming an increase of the grain boundary contribution as the sintering temperature decreases.

Effect of Sb and Si doping on the superconducting properties of FeSe0.9

Spin excitations are one of the top candidates for mediating electron pairing in unconventional superconductors. Their coupling to superconductivity is evident in a large number of systems, by the observation of an abrupt redistribution of magnetic spectral weight at the superconducting transition temperature, T(c), for energies comparable to the superconducting gap. Here we report inelastic neutron scattering measurements on Fe-based superconductors, Fe(1+y-x)(Ni/Cu)(x)Te(0.5)Se(0.5) that emphasize an additional signature. The overall shape of the low energy magnetic dispersion changes from two incommensurate vertical columns at T≫T(c) to a distinctly different U-shaped dispersion at low temperature. Importantly, this spectral reconstruction is apparent for temperatures up to ~3T(c). If the magnetic excitations are involved in the pairing mechanism, their surprising modification on the approach to T(c) demonstrates that strong interactions are involved.

Macroscopic phase segregation in superconducting K0.73Fe1.67Se2as seen by muon spin rotation and infrared spectroscopy

Using muon spin rotation and infrared spectroscopy, we investigated the recently discovered superconductor K0.73Fe1.67Se2 with T-c approximate to 32 K. We show that the combined data can be consistently described in terms of a macroscopically phase-segregated state with a matrix of similar to 88% volume fraction that is insulating and strongly magnetic and inclusions with an similar to 12% volume fraction, which are metallic, superconducting, and nonmagnetic. The electronic properties of the latter, in terms of the normal state plasma frequency and the superconducting condensate density, appear to be similar as in other iron selenide or arsenide superconductors.

Effect of Fe composition on the superconducting properties (Tc, Hc2 and Hirr) of FexSe1/2Te1/2 (x = 0.95, 1.00, 1.05 and 1.10)

In the present work, we have studied the effect of Fe composition on the superconducting properties, such as transition temperature (Tc), upper critical field (Hc2), and irreversibility field (Hirr) of FeSe1/2Te1/2. The polycrystalline samples have been prepared via solid state reaction route with nominal compositions FexSe1/2Te1/2 (x = 0.95, 1.00, 1.05 and 1.10). The x-ray diffraction results show the presence of tetragonal α-FeSe phase with the p4/nmm space group symmetry in all the samples. The zero resistance temperatures, Tczero, measured in zero magnetic field, have been found to be 10.0, 12.4, 12.3, and 11.7 K for x = 0.95, 1.00, 1.05, and 1.10, respectively. The temperature dependence of Hc2(T) and Hirr(T) have been calculated from the magnetoresistance data using the criteria of 90% and 10% of normal state resistivity (ρn) values, respectively. The values of Hc2(0) are 121.3 T, 142.8 T, 82.7 T, and 79.3 T for x = 0.95, 1.00, 1.05, and 1.10, respectively. The possible reasons for the variation of ...

Microwave absorption properties of nanostructured nickel ferrite

The present paper discusses the development, characterization and microwave absorption properties of nanostructured nickel ferrite and the influence of loading of the frequency selective surfaces (FSS) on microwave absorption. Synthesis of the material was done by a sol-gel mediated auto-combustion technique and phase purity was determined by X-ray diffraction studies. As prepared nickel ferrite of crystallite size of 12.0 nm and the calcined sample of 36.0 nm were used for further studies. The complex permittivity, permeability and the reflection loss of the materials were measured in X-band (8.2 GHz to 12.4 GHz) and the properties are compared with their crystallite size. Simulations have been carried out using Ansoft HFSS (High Frequency Structure Simulator) microwave software of single and double square geometry for nickel ferrite to obtain the maximum absorption with large band width. Single layer nickel ferrite absorber of thickness 3 mm, loaded with double-square FSS has given the reflection loss (RL) of -12.5 dB at 9.6 GHz and -10 dB with a bandwidth of 2.0 GHz. The loading of FSS increased the reflection loss of nickel ferrite with 12.0 nm crystallite size whereas a broadening in the bandwidth of absorption was observed for nickel ferrite of larger crystallite size.

Critical analysis of Frequency Selective Surfaces for dual band GSM-900 & 1800 MHz transmission

A critical analysis of various Frequency Selective Surface (FSS) structures like square, hexagonal and circular has been presented in this paper, which can be used to obtain a dual pass band for GSM 900 & 1800 MHz frequency. This analysis can be used to construct the FSSs for the better performance of mobile communication where energy efficient panes are used to isolate the area by restricting free passage of radiating waves, working as carriers of energy. The aperture type FSS allows the transmission of selected band frequencies. The simulation results provide an insight about the performance of these FSSs. The substrate used is teflon (εr =2.1) and it is being considered as per the need of lesser thickness (1 mm) and providing flexibility so as to be incorporated over any individual surface.

Spin canting and magnetic transition in NixZn1-xFe2O4 (x=0.0, 0.5 and 1.0) nanoparticles

Nanoparticles of NixZn1-xFe2O4(x=0.0, 0.5 and 1.0) have been synthesized via co-precipitation method and studied thestructural and magnetic properties. Rietveld refinement of X ray diffraction data of as synthesized samples revealthat the samples have mixed spinel structure with space group Fd-3m. The lattice parameter of the samples decreases as doping concentration of Ni ions increases. Magnetic measurements show paramagnetic to ferrimagnetic transition at room temperature on Ni doping in ZnFe2O4 nanoparticles. The magnetic measurements also show spin canting in samples possibly due to their nanocrystalline nature. The spin canting angles have been calculated with the help of Yafet-Kittel (Y-K) model. Furthermore, the Law of approach (LA) fitting of M-H curves indicates that the samples are highly anisotropicin nature. The Arrot plots of as synthesized samples also indicate the paramagnetic to ferrimagnetic transition. The correlation between the structural and observed magnetic properties of NixZn1-xFe2O4(x=0.0, 0.5 and 1.0) nanocrystals will be described and discussed in this paper.Nanoparticles of NixZn1-xFe2O4(x=0.0, 0.5 and 1.0) have been synthesized via co-precipitation method and studied thestructural and magnetic properties. Rietveld refinement of X ray diffraction data of as synthesized samples revealthat the samples have mixed spinel structure with space group Fd-3m. The lattice parameter of the samples decreases as doping concentration of Ni ions increases. Magnetic measurements show paramagnetic to ferrimagnetic transition at room temperature on Ni doping in ZnFe2O4 nanoparticles. The magnetic measurements also show spin canting in samples possibly due to their nanocrystalline nature. The spin canting angles have been calculated with the help of Yafet-Kittel (Y-K) model. Furthermore, the Law of approach (LA) fitting of M-H curves indicates that the samples are highly anisotropicin nature. The Arrot plots of as synthesized samples also indicate the paramagnetic to ferrimagnetic transition. The correlation between the structural and observed magnetic properties of NixZn1-xFe...

Highly selective room temperature NO2 gas sensor based on rGO-ZnO composite

Blending metal oxide nanoparticles with graphene or its derivatives can greatly enhance gas sensing characteristics. In the present work, ZnO nanoparticles have been synthesized via reflux method. ...

Tuning of structural and magnetic properties of Mn-doped CoFe2O4 nanoparticles

In the present manuscript, MnxCo1-xFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoparticles have been synthesized with co-precipitation method by annealing at 900 °C and studied the structural and magnetic properties. X ray diffraction pattern reveals the formation of single phase cubic spinel structure with the space group Fd-3m. An impurity peak of α-Fe2O3 has been observed in the XRD patterns of samples with Mn composition, x ≥ 0.4. However, this impurity phase of α-Fe2O3 is not observed in the MnFe2O4 sample synthesized by annealing at 500 °C. FESEM results indicate the formation of nano-sized particles. Magnetic measurements show ferrimagnetic interaction in MnxCo1-xFe2O4 (x≥0) nanoparticles at room temperature. However, MnFe2O4 sample synthesized by annealing at 500 °C shows superparamagnetic behavior. We have fitted the M-H curve of MnFe2O4 synthesized by annealing at 500 °C with modified Langvenin function to confirm superparamagnetism. The correlation between the structural and observed magnetic properties of as synthesized samples nanoparticles will be described and discussed in this paper.In the present manuscript, MnxCo1-xFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoparticles have been synthesized with co-precipitation method by annealing at 900 °C and studied the structural and magnetic properties. X ray diffraction pattern reveals the formation of single phase cubic spinel structure with the space group Fd-3m. An impurity peak of α-Fe2O3 has been observed in the XRD patterns of samples with Mn composition, x ≥ 0.4. However, this impurity phase of α-Fe2O3 is not observed in the MnFe2O4 sample synthesized by annealing at 500 °C. FESEM results indicate the formation of nano-sized particles. Magnetic measurements show ferrimagnetic interaction in MnxCo1-xFe2O4 (x≥0) nanoparticles at room temperature. However, MnFe2O4 sample synthesized by annealing at 500 °C shows superparamagnetic behavior. We have fitted the M-H curve of MnFe2O4 synthesized by annealing at 500 °C with modified Langvenin function to confirm superparamagnetism. The correlation between the structural and observed magnetic p...