H. Kishan

Magnetotransport of La0.70Ca0.3−xSrxMnO3(Ag): A potential room temperature bolometer and magnetic sensor

Here we report the optimized magnetotransport properties of polycrystalline La0.70Ca0.3−xSrxMnO3 and their composites with Ag. The optimization was carried out by varying the Sr and Ag contents simultaneously to achieve large temperature coefficient of resistance (TCR) as well as low field magnetoresistance (MR) at room temperature. Sharpest paramagnetic (PM)-ferromagnetic (FM) and insulator-metal (IM) transition is observed in the vicinity of the room temperature (TC≈300u2002K≈TIM) for the composition La0.70Ca0.20Sr0.10MnO3:Ag0.20. Partial substitution of larger Sr2+ ions at the Ca2+ ions sites controls the magnitude of the FM and IM transition temperatures, while the Ag induces the desired sharpness in these transitions. For the optimized composition, maximum TCR and MR are tuned to room temperature (300 K) with the former being as high as 9% and the later being ∼20% and ∼30% at 5 and 10 kOe magnetic fields respectively. Such sharp single peak (TCR∼9%) at room temperature can be used for the bolometric and ...

Induction of superconductivity by sulphur doping in FeTe system and post effect of low temperature oxygen annealing

The AC susceptibility at zero DC magnetic field of a polycrystalline sample of LaFeAsO_{0.94}F_{0.06} (T_c = 24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range Hac = 0.003 - 4 Oe) and the frequency (in the range f = 10 kHz - 100 kHz). The temperature dependence of the AC susceptibility exhibits the typical two-step transition arising from the combined response of superconduncting grains and intergranular weak-coupled medium. The intergranular part of the susceptibility strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T_c down to T = 4.2 K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe1-xSx; (x = 0-30 %) system and study the impact of low temperature oxygen (O2) annealing as well. Rietveld analysis of room temperature x-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at around 78 K, which corresponds to a structural phase transition. Heat capacity Cp(T) measurement also confirms the structural phase transition of FeTe compound. Superconductivity appears by S substitution; the onset of superconducting transition temperature is about 8 K for FeTe0.75S0.25 sample. Thermoelectric power measurements S(T) also shows the superconducting transition at around 7 K for FeTe0.75S0.25 sample. The upper critical fields Hc2(10%), Hc2(50%) and Hc2(90%) are estimated to be 400, 650 and 900 kOe respectively at 0 K by applying Ginzburg Landau (GL) equation. Interestingly, superconducting volume fraction is increased with low temperature (200 oC) O2 annealing at normal pressure. Detailed investigations related to structural (XRD), transport [S(T), R(T)H], magnetization (AC and DC susceptibility) and thermal [Cp(T)] measurements for FeTe1-xS:O2 system are presented and discussed.

Superconductivity of Fe based pnictides and chalcogenides: Material aspects, doping routes, future prospects and challenges

I review here constant efforts of my superconducting materials group on synthesis and physical properties of title compounds from very beginning (May 2008) itself. Both direct (Fe/Co, Ni) and indirect O/F carrier doping routes are applied and superconductivity is achieved up to 14K and 50K respectively. It infers that in F doped samples the Tc is collective effect of electron doping and short range ordering of Fe spins. Whereas in Co/Ni doped samples occurrence of Tc is mainly due to electron doping. Seemingly magnetic excitations play an important role in achieving superconductivity. This along with drastic depression of Tc with Fe site Zn doping in superconducting LaFeAsO0.8 indicates towards s± coupling. Magneto transport measurements showed that these superconductors are quite robust against magnetic field having upper critical field [Hc2(0)] of up to 400 Tesla. It is found that though FeSe/Te superconductor possesses lower Tc, the same is even more robust against magnetic field than the REFeAsO. Seve...

NANO-VANADIUM DOPING-DRIVEN LOW TEMPERATURE STRUCTURAL PHASE TRANSFORMATION IN TITANIA

Synthesis, X-ray diffraction (XRD), morphological, and magnetization studies of nano-vanadium doped TiO2 are reported here. Commercially available anatase nano-TiO2 and nano-vanadium powders were mixed and heat-treated in air at 600°C. X-ray diffraction revealed a complete anatase to rutile phase transformation at low vanadium content of 5%. Samples annealed under different environments such as Ar and O2 also exhibited the same trend. Such a phase transformation at relatively low temperature (600°C) has been explained based on enhanced reactivity between vanadium and TiO2 powders. Scanning electron micrographs elucidated the tunable microstructures from irregular-shaped, in the pristine TiO2 (x=0.0), to well-faceted polyhedra V-doped (x=0.05 to 0.15) samples. Room temperature magnetization results revealed that upto 10% vanadium doping the samples are diamagnetic in nature and exhibit a paramagnetic transition for higher V content which presumably can be attributed to the presence of vanadium pentaoxide observed in the XRD.