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V. P. S. Awana

Publications and source records attributed to V. P. S. Awana.

At least 217 records · Page 12Linked to original sources
arXiv2008

Nonlinear magnetic response in ruthenocuprates

We have performed an investigation of the nonlinear magnetic response in ruthenocuprates. A negative, diverging-like peak at the main magnetic transition T_N in RuSr2RECu2O8 (RE = Gd, Y) indicates a possible canted antiferromagnetic order. Another well defined feature above T_N points to a blocking of superparamagnetic particles through the T^(-3) dependence of the third harmonic at higher temperatures. Below T_N a nondiverging peak appears, which is strongly affected by the addition of 10% of Cu ions in the RuO2 planes. In RuSr2RE(2-x)Ce(x)Cu2O10 the main magnetic transition T_M is accompanied by two characteristic temperatures in the third harmonic of the ac susceptibility, in agreement with recent studies from uSR and Mossbauer spectroscopy. We find that the spin-spin correlation temperature is the same in both families of ruthenocuprates.

cond-mat.str-el↗
arXiv2008

Superconductivity of bulk MgB2 + nano(n)-SiC composite system: A high field magnetization study

We study the effect of nano(n)-SiC addition on the crystal structure, critical temperature (Tc), critical current density (Jc) and flux pinning in MgB2 superconductor. X-ray diffraction patterns show that all the samples have MgB2 as the main phase with very small amount of MgO, further with n-SiC addition the presence of Mg2Si is also noted and confirmed by SEM & EDS. The Tc value for the pure MgB2 is 18.9K under 8 Tesla applied field, while is 20.8K for the 10-wt % n-SiC doped sample under the same field. This points towards the increment in upper-critical field value with n-SiC addition. The irreversibility field (Hirr) for the 5% n-SiC added sample reached 11.3, 10 and 5.8 Tesla, compared to 7.5, 6.5, and 4.2 Tesla for the pure MgB2 at 5, 10 and 20K respectively. The critical current density (Jc) for the 5-wt % n-SiC added sample is increased by a factor of 35 at 10K and 6.5 Tesla field and by a factor 20 at 20K and 4.2 Tesla field. These results are understood on the basis of superconducting condensate (sigma band) disorder and ensuing intrinsic pining due to B site C substitution clubbed with further external pinning due to available n-SiC/Mg2Si pins in the composite system.

cond-mat.supr-con↗
arXiv2008

Impact of silver addition on room temperature magneto-resistance in La0.7Ba0.3MnO3 (LBMO): Agx (x = 0.0, 0.1, 0.2, 0.3, 0.4)

La0.7Ba0.3MnO3 (LBMO):Agx (x = 0.0, 0.1, 0.2, 0.3, and 0.4) composites are synthesized by solid-state reaction route, the final sintering temperatures are varied from 1300 (LBMO1300Ag) to 1400 0C (LBMO1400Ag), and their physical properties are compared as a function of temperature and Ag content. All samples are crystallized in single phase accompanied by some distortion in main structural phase peaks at higher angles with increase in silver content. Though the lattice parameters (a, c) decrease, the b increases slightly with an increase in Ag content. The scanning electron micrographs (SEM) showed better grains morphology in terms of size and diffusion of grain boundaries with an increase in Ag content. In both LBMO1300Ag and LBMO1400Ag series the metal insulator transition (TMI) and accompanied paramagnetic-ferromagnetic transition (TC) temperatures are decreased with increase in Ag content. The sharpness of MI transition, defined by temperature coefficient of resistance (TCR), is improved for Ag added samples. At a particular content of Ag(0.3), the TMI and TC are tuned to 300K and maximum magneto-resistance at 7Tesla applied field (MR7T) of up to 55% is achieved at this temperature, which is more than double to that as observed for pure samples of the both 1300 and 1400 0C series at same temperature. The MR7T is further increased to above 60% for LBMOAg(0.4) samples, but is at 270K. The MR7T is measured at varying temperatures of 5, 100, 200, 300, and 400K in varying fields from +/- 7 Tesla, which exhibits U and V type shapes. Summarily, the addition of Ag in LBMO improves significantly the morphology of the grains and results in better physical properties of the parent manganite system.

cond-mat.str-el↗
arXiv2007

Effect of PVA doping on flux pinning in Bulk MgB2

The synthesis and characterization of PVA (Poly Vinyl Acetate) doped bulk MgB2 superconductor is reported here. PVA is used as a Carbon source. PVA doping effects made two distinguishable contributions: first enhancement of Jc field performance and second an increase in Hc2 value, both because of carbon incorporation into MgB2 crystal lattice. The susceptibility measurement reveals that Tc decreased from 37 to 36 K. Lattice parameter a decreased from 3.085 A to 3.081 A due to the partial substitution of Carbon at Boron site. PVA doped sample exhibited the Jc values greater than 10^5 A/cm2 at 5 & 10 K at low fields; which is almost 3 times higher than the pure one, while at high fields the Jc is increased by an order of magnitude in comparison to pure MgB2. From R(T)H measurements we found higher Tc values under magnetic field for doped sample; indicating an increase in Hc2. Also the magnetization measurements exhibited a significant enhancement in Hirr value. The improved performance of PVA doped MgB2 can be attributed to the substitution of carbon at boron site in parent MgB2 and the resulting impact on the carrier density and impurity scattering. The improved flux pinning behavior could easily be seen from reduced flux pinning force plots.

cond-mat.supr-con↗
arXiv2007

Inter comparison of the magneto transport of La2/3Ca1/3MnO3: Ag/In polycrystalline composites

In this article, we report the synthesis, magneto transport features, and magnetization of polycrystalline La2/3Ca1/3MnO3:Agx/Inx composites with x = 0, 0.1, 0.2, 0.3 and 0.4. In case of Ag the temperature coefficient of resistance (TCR) near ferromagnetic (FM) transition enhances significantly with addition of Ag. The FM transition temperature (TFM) is also increased slightly with Ag doping. Magneto-transport measurements revealed that magneto-resistance (MR) is found to be maximum near TFM. Very sharp TCR is seen near TFM with highest value of up to 15 % for Ag (0.4) sample, which is an order of magnitude higher than as for pristine sample and is the best value yet reported for any polycrystalline LCMO compound. Increased TCR, TFM and significant above room temperature MR of La2/3Ca1/3MnO3:Agx composites is explained on the basis of improved grains size. Interestingly the La2/3Ca1/3MnO3:Inx composites behaved exactly in opposite way resulting in decreased TFM, and TCR compared to pristine LCMO compound. In fact the grains morphology of LCMO:Inx composites is seen inferior to pristine LCMO which is opposite to the LCMO:Agx case.

cond-mat.str-el↗
arXiv2006

Impact of rare earth magnetic moment on ordering of Ru in Sr2RuREO6 (RE = Gd and Eu)

We report synthesis and magnetization of Sr2RuREO6 (RE/Ru-211O6) with RE = Gd and Eu. Both Gd,Eu/Ru-211O6 are formed in distorted perovskite-type single phase with monoclinic P21/n space group. Magnetization (M/T) measurements exhibited antiferromagnetic ordering of Ru moments for both Gd and Eu compounds at TN of around 30 K due to super-exchange Ru-O-O-Ru interaction. Interestingly enough, detailed M/H plots of Gd/Ru-211O6 exhibited, the development of a ferromagnetic (FM) component at 20 K. Further at below 10 K, the FM component disappears and rather an spin freezing field (HSF) of around 1000 Oe is seen at 5 K. In case of Eu/Ru-211O6, the basic antiferromagnetic ordering of Ru moments remains invariant down to 5 K, and the appearance of FM component is not seen. It seems that the high magnetic moment of 8mB for Gd3+ influences the AFM ordering of Ru in case of Gd/Ru-211O6.

cond-mat.str-el↗
arXiv2006

Magneto Transport of high TCR (temperature coefficient of resistance) La2/3Ca1/3MnO3: Ag Polycrystalline Composites

We report the synthesis, (micro)structural, magneto-transport and magnetization of polycrystalline La2/3Ca1/3MnO3:Agx composites with x = 0.0, 0.1, 0.2, 0.3 and 0.4. The temperature coefficient of resistance (TCR) near ferromagnetic (FM) transition is increased significantly with addition of Ag. The FM transition temperature (TFM) is also increased slightly with Ag addition. Magneto-transport measurements revealed that magneto-resistance MR is found to be maximum near TFM. Further the increased MR of up to 60% is seen above 300 K for higher silver added samples in an applied field of 7 Tesla. Sharp TCR is seen near TFM with highest value of up to 15 % for Ag (0.4) sample, which is an order of magnitude higher than as for present pristine sample and best value yet reported for any polycrystalline LCMO compound. Increased TCR, TFM and significant above room temperature MR of La2/3Ca1/3MnO3:Agx composites is explained on the basis of improved grains size and connectivity with silver addition in the matrix. Better coupled FM domains and nearly conducting grain boundaries give rise to improved physical properties of the La2/3Ca1/3MnO3 manganites.

cond-mat.str-el↗
arXiv2006

High pressure high temperature (HPHT) synthesis and magnetism of MSr2Y1.5Ce0.5Cu2O10 compounds with M = Ru, Mn, and Cr

The M-1222 compounds are synthesized in near single phase form. These compounds crystallise in I4/mmm Tetragonal structure. None of the studied compound is superconducting but they asre weakly magnetic. This is first report on synthesis of Cr and Mn -1222 compounds. These compounds are synthesized at ultra conditions of HPHT that is the 6 Gpa and 1400 C.

cond-mat.supr-con↗
arXiv2006

Physical property characterization of Fe-tube encapsulated and vacuum annealed bulk MgB2

We report phase formation, and detailed study of magnetization and resistivity under magnetic field of MgB2 polycrystalline bulk samples prepared by Fe-tube encapsulated and vacuum (10-5 torr) annealed (750 0C) route. Zero-field-cooled magnetic susceptibility (cZFC) measurements exhibited sharp transition to superconducting state with a sizeable diamagnetic signal at 39 K (Tc). The measured magnetization loops of the samples, despite the presence of flux jumps, exhibited a stable current density (Jc) of around 2.4 x 105 A/cm2 in up to 2 T (Tesla) field and at temperatures (T) up to 10 K. The upper critical field is estimated from resistivity measurements in various fields and shows a typical value of 8 T at 21 K. Further, cFC measurements at an applied field of 0.1 T reveal paramagnetic Meissner effect (PME) that is briefly discussed.

cond-mat.supr-con↗
arXiv2006

Anomalous lattice expansion of RuSr2Eu1.5Ce0.5Cu2O10(Ru-1222) magneto superconductor: A low temperature X-ray diffraction study

This is the first report of the observation of the onset of excess volume and also of the strain along the a-axis near the magnetic ordering temperature in Ru-1222 superconductor, and indicates a coupling between the lattice and the magnetism in this system. Magnetization, magneto transport and thermoelectric power measurements being carried out on the same sample are also reported.

cond-mat.supr-con↗
arXiv2006

Magnetism, Upper critical field and Thermoelectric power of Magneto-Superconductor RuSr2Eu1.5Ce0.5Cu2O10

Magnetic susceptibility, M-H plot, magnetoresistance and thermoelectric power of the RuSr2Eu1.5Ce0.5Cu2O10 superconductor are measured. Values of the magnetic transition temperature Tmag, superconductivity transition temperature Tc, upper critical field Hc2, chemical potential mu, and energy width for electric conduction W(sigma) are obtained from these measurements. It has been found that Tmag = 140 K, Tc = 25 K (33 K) from susceptibility (magnetoresistance) measurements, Hc2 (0) > 32 T, mu = 8 meV, and W(sigma) = 58.5 meV. These values are compared with other ruthenate superconductors, and resulting physical information is discussed.

cond-mat.supr-con↗
arXiv2006

Fluxiod jumps coupled high critical current density of nano-Co3O4 doped MgB2

Polycrystalline MgB2 samples with 0, 2, 4 and 6 percent added nano-Co3O4 being synthesized by vacuum (10-5 Torr) annealing at 750 0C for three hours each are found to be nearly single phase with presence of only a small quantity of Mg/MgO in pristine sample and in addition the Co2O3 in doped compounds. All the samples exhibited clear and sharp diamagnetic transitions at around 38 K, in Zero-field-cooled (ZFC) magnetic susceptibility measurements with sizeable signal. The Field cooled (FC) measurements though having sharp transitions, but showed a very small signal, indicating high level of pinning centers in these samples. Further some of the doped samples exhibited Paramagnetic-Meissner-Effect (PME) in applied field of 5 Oe. The critical current density (Jc), being estimated by invoking Bean model for the pristine compound increase by nearly an order of magnitude for 2 and 4 percent nano-Co3O4 doping and later the same decreases sharply for 6 percent sample at nearly all studied temperatures and applied fields. Further the increased Jc (~ 10^8 A/cm2) is coupled with fluxiod jumps (T = 20 K and H = 1 T). Fluxiod Jumps are not seen in relatively low Jc pristine or 6 percent sample. This means the fluxiod-jumps are intrinsic only to the high Jc samples.

cond-mat.supr-con↗
arXiv2006

Experimental study of magneto-superconductor RuSr2Eu1.5Ce0.5Cu2O10: Effect of Mo doping on magnetic behavior and Tc variation

Mo doped ruthenocuprates Ru1-xMoxSr2Eu1.5Ce0.5Cu2O10 are synthesized for x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, and their magnetic and superconducting properties are studied. It has been found that the magnetic transition temperature TZFCpeak, which corresponds to the appearance of weak ferromagnetic effect, decreases from its value of 75 K for x = 0.0 to 22 K, 25 K and 18 K, respectively for the x = 0.2, 0.4 and 0.6 samples. Another finding is that the magnetic susceptibility reduces at TZFCpeak by a factor of about 6, 85 and 413 for x = 0.2, 0.4, and 0.6 respectively. The samples of x = 0.8 and 1.0 are found to have no magnetic or superconducting effects. The values of the superconducting transition temperature are obtained from the resistivity versus temperature data. An important result is that Tc increases by 4.5 K and 7.0 K for x = 0.2 and 0.4 respectively, and then decreases by 17 K for x = 0.6. The observed variation of Tc with x has been explained in terms of a theory which combines the effects of weakening magnetic behavior and reducing carrier concentration in a phenomenological manner. The resulting theory is found to provide a good agreement with the observed value of Tc.

cond-mat.supr-con↗
arXiv2006

Phase formation and superconductivity of Fe-tube encapsulated and vacuum annealed MgB2

We report optimization of the synthesis parameters viz. heating temperature (TH), and hold time (thold) for vacuum (10-5 torr) annealed and LN2 (liquid nitrogen) quenched MgB2 compound. These are single-phase compounds crystallizing in the hexagonal structure (space group P6/mmm) at room temperature. Our XRD results indicated that for phase-pure MgB2, the TH for 10-5 torr annealed and LN2 quenched samples is 750 0C. The right stoichiometry i.e., MgB2 of the compound corresponding to 10-5 Torr and TH of 750 0C is found for the hold time (thold) of 2.30 hours. With varying thold from 1- 4 hours at fixed TH (750 0C) and vacuum (10-5 torr), the c-lattice parameter decreases first and later increases with thold (hours) before a near saturation, while the a-lattice parameter first increase and later decreases beyond thold of 2.30 hours. c/a ratio versus thold plot showed an inverted bell shape curve, touching the lowest value of 1.141 which is reported value for perfect stoichiometry of MgB2. The optimized stoichimetric MgB2 compound exhibited superconductivity at 39.2 K with transition width of 0.6 K. In conclusion, the synthesis parameters for phase pure stoichimetric vacuum annealed MgB2 compound are optimized and are compared with widely reported Ta tube encapsulated samples.

cond-mat.supr-con↗
arXiv2005

A comparison of the resistivity behavior of MgB2, AlB2 and AgB2 systems

Measurements have been performed of the resistivity of the samples of MgB2, AlB2 and AgB2. The samples show presence of impurities. Analyzing the data in terms of the impurity scattering, electron-phonon scattering, and weak localization it has been found that the AlB2 (AgB2) sample involves maximum (minimum) effect of the impurity, electron-phonon interaction and weak localization.

cond-mat.supr-con↗
arXiv2005

Impact of Zn substitution on phase formation and superconductivity of Bi1.6Pb0.4 Sr2Ca2Cu3-xZnxO10 with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12

Samples of series Bi1.6Pb0.4Sr2Ca2Cu3-xZnxO10 with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12 are synthesized by solid-state reaction route. All the samples crystallize in tetragonal structure with majority (> 90%) of Bi-2223 (Bi2Sr2Ca2Cu3O10) phase (c-lattice parameter ~ 36 A0). The proportion of Bi-2223 phase decreases slightly with an increase in x. The lattice parameters a and c of main phase (Bi-2223) do not change significantly with increasing x. Superconducting critical transition temperature (Tc) decreases with x as evidenced by both resistivity [(T)] and AC magnetic susceptibility [(T)] measurements. Interestingly the decrement of Tc is not monotonic and the same saturates at around 96 K for x > 0.06. In fact Tc decreases fast (~10K/at%) for x = 0.015 and 0.03 samples and later nearly saturates for higher x values. Present results of Zn doping in Bi-2223 system are compared with Zn doped other HTSC (High temperature superconducting) systems, namely the RE-123 (REBa2Cu3O7) and La-214 ((La,Sr)2CuO4).

cond-mat.supr-con↗
arXiv2005

Magnetic and superconducting properties of RuSr2 Gd1.5Ce0.5Cu2O10-x samples: dependence on the oxygen content and ageing effects

The magnetic and superconducting properties of RuSr2Gd1.5Ce0.5Cu2O10-x polycrystalline samples with different oxygen doping level are presented. A strong suppression of the superconducting temperature (Tc), as well as a reduction in the superconducting fraction, occurs as the oxygen content is reduced by annealing the samples in oxygen deprived atmospheres. Drastic changes in the electrical resistivity are observed above Tc, possibly associated with oxygen removal mainly from grain boundaries. However, the magnetic ordering is relatively less affected by the changes in oxygen content of the samples. The spin glass transition is enhanced and shifted to higher temperatures with the reduction in oxygen content. This could be correlated with an increase in the spins disorder and frustration for the oxygen depleted samples. Also, the same oxygen vacancy induced disorder could explain the reduction in the fraction of the sample showing antiferromagnetic order. We also report significant changes in the measured properties of the samples as a function of time.

cond-mat.supr-con↗