Dr. Muhammad Mumtaz

Condensed Matter Physics

PhD, Postdoc
34.07

Publications

  • F. Zeb · A.R. Qureshi · K. Nadeem · M. Mumtaz · H. Krenn
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    ABSTRACT: Surface effects in uncoated and amorphous silica (SiO2) coated cobalt ferrite nanoparticles have been studied by using dc magnetization. Both uncoated and SiO2 coated nanoparticle samples were synthesized by using sol–gel method. SiO2 coated nanoparticles exhibit smaller average crystallite size as compared to uncoated nanoparticles. Saturation magnetization (Ms) revealed decreasing trend at low temperatures for uncoated nanoparticles as compared to coated nanoparticles which may be due to magnetically dead nanoparticle's surface layer for uncoated nanoparticles. A step-like behavior near remanent field was observed for both the samples but more pronounced for SiO2 coated nanoparticles. This fast magnetic relaxation near remanent field in coated nanoparticles is due to lesser dipolar (interparticle) interactions among nanoparticles. Bloch's law shows a good fit for coated nanoparticles. SiO2 coated nanoparticles showed larger values of coercivity at low temperatures due to their enhanced surface anisotropy as compared to uncoated nanoparticles. Kneller's law for temperature dependent coercivity shows a good fit for SiO2 coated nanoparticles and is attributed to lesser dipolar interactions among coated nanoparticles as compared to uncoated nanoparticles. The exchange bias phenomenon was also observed for both the samples, which signify the presence of core–shell interactions. However, SiO2 coated nanoparticles exhibit larger values of exchange bias at low temperatures due to their strong surface anisotropy as compared to uncoated nanoparticles. The presence of exchange bias in uncoated nanoparticles indicates that their surface layer is not totally magnetically dead. In summary, (i) the SiO2 coated nanoparticles have lesser dipolar interactions due to SiO2 coating and (ii) surface layer in uncoated nanoparticles is not magnetically dead but may have different preferred ordering at low temperatures as compared to SiO2 coated nanoparticles.
    No preview · Article · Mar 2016 · Journal of Non-Crystalline Solids
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    ABSTRACT: Surface effects in cobalt ferrite (CoFe2O4) nanoparticles dispersed in a silica (SiO2) matrix were studied by using AC and DC magnetization. Nanoparticles with different concentration of SiO2 were synthesized by using sol-gel method. Average crystallite size lies in the range 25 nm – 34 nm for different SiO2 concentration. TEM image showed that particles are spherical and elongated in shape. Nanoparticles with higher concentration of SiO2 exhibit two peaks in the out-of-phase ac-susceptibility. First peak lies in the high temperature regime and corresponds to average blocking temperature of the nanoparticles. Second peak lies in the low temperature regime and is attributed to surface spin-glass freezing in these nanoparticles. Low temperature peak showed SiO2 concentration dependence and was vanished for large uncoated nanoparticles. The frequency dependence of the AC-susceptibility of low temperature peak was fitted with dynamic scaling law which ensures the presence of spin-glass behavior. With increasing applied DC field, the low temperature peak showed less shift as compared to blocking peak, broaden, and decreased in magnitude which also signifies its identity as spin-glass peak for smaller nanoparticles. M-H loops showed the presence of more surface disorder in nanoparticles dispersed in 60% SiO2 matrix. All these measurements revealed that surface effects become strengthen with increasing SiO2 matrix concentration and surface spins freeze in to spin-glass state at low temperatures.
    No preview · Article · Jan 2016 · Journal of Magnetism and Magnetic Materials
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    ABSTRACT: Nickel ferrite (NiFe2O4) nanoparticles and Cu0.5Tl0.5Ba2Ca2Cu3O10−δ (CuTl-1223) superconductor were prepared separately and then mixed in an appropriate ratios at the final stage to obtain (NiFe2O4)x /CuTl-1223 (x = 0, 0.5, and 1.0 wt%) nano-superconductor composites. There was no significant change observed in crystal structure of the host CuTl-1223 superconducting matrix after the addition of NiFe2O4 nanoparticles. The value of zero-resistivity critical temperature { T c(R = 0) (K)} was decreased with increasing content of these nanoparticles in these composites. Maximum values of dielectric loss tangent (tanδ) at lowest possible frequency of 40 Hz were increased with the increase of operating temperature, while its values were decreased and become almost zero at higher frequencies for all these samples at all operating temperatures. A peak in dielectric loss tangent was shifted towards lower frequency values with the addition of these nanoparticles in CuTl-1223 superconducting matrix. The dielectric loss tangent peak was also shifted towards lower frequency values in all these samples with increasing operating temperature, which shows the relaxator-like behavior in these samples. The dielectric parameters of these composites can be tuned by frequency, operating temperatures, and nature and content of these nanoparticles.
    Full-text · Article · Jan 2016 · Journal of Superconductivity and Novel Magnetism
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    ABSTRACT: We investigated infield response of nickel (Ni) nanoparticles added Cu0.5Tl0.5Ba2Ca2Cu3O10-δ (CuTl-1223) superconductor. Significant superconducting transition broadening (ΔT) was observed under external applied magnetic field. Variation in activation energy Uo(H, T) and flux pinning strength were explored in external applied magnetic field from 0 to 7 T. Infield experimental dc-resistivity data was explained by thermally activated flux flow (TAFF) model. External applied magnetic field gave rise to vortex motion and caused dissipation in (Ni)x-(CuTl-1223) (0≤x≥1.0 wt%) nanoparticles-superconductor composites. Flux pinning strength was reduced with increased Ni nanoparticles content in CuTl-1223 matrix. The systematic increase in ΔT and reduction in Uo (H, T) with increasing magnetic field showed the motion of vortices, which diminished the flux pinning strength.
    Full-text · Article · Dec 2015 · Solid State Communications
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    ABSTRACT: Infield superconducting properties of (Cu0.5Tl0.5)Ba2Ca2Cu3O10-δ (CuTl-1223) superconducting phase after addition of cobalt (Co) nanoparticles were reported. Crystal structure of host CuTl-1223 phase was not affected after inclusion of Co nanoparticles. Superconducting resistive transition broadening was observed with applied magnetic field from 0 to 7 T, which was attributed to dissipation mechanism caused by thermally activated flux flow (TAFF) in the presence of small current (10 μA). Reduction in activation energy Uo(H, T) showed suppression of flux pinning strength with increasing content of Co nanoparticles.
    Full-text · Article · Dec 2015 · Cryogenics
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    ABSTRACT: (Ni)(x)/CuTl-1223 (x=0, 0.25, 0.75, and 1.0 wt%) nanoparticles-superconductor composites were synthesized by the addition of ferromagnetic nickel (Ni) nanoparticles in appropriate ratio to Cu0.5Tl0.5Ba2Ca2Cu3O10-delta (CuTl-1223) superconducting matrix. Structural, morphological, compositional and superconducting transport properties of these composites were studied by different experimental techniques. It was observed that the addition of Ni nanoparticles had not altered the crystal structure of host CuTl-1223 phase, which is somehow an evidence of the occupancy of these nanoparticles at the grain-boundaries. Suppression of superconducting properties was attributed to pair-breaking due to spin scattering across these ferromagnetic Ni nanoparticles. The enhanced magnetization of ferromagnetic Ni nanoparticles at reduced temperatures plays a significant role to reduce the diamagnetism of (Ni)(x)/CuTl-1223 composites. Fluctuation induced conductivity (FIC) analysis of resistivity versus temperature data has explained very well the effects of Ni nanoparticles on superconductivity of CuTl -1223 phase.
    No preview · Article · Dec 2015 · Journal of Magnetism and Magnetic Materials
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    ABSTRACT: (CoFe2O4)x/Cu0.5Tl0.5Ba2Ca2Cu3O10−δ {(CoFe2O4)x/CuTl-1223}; (0 ≤ x ≤ 2) nanoparticles-superconductor composites were synthesized by solid–state reaction technique and dissipative mechanism was investigated by infield measurements and analysis. Activation energy {Uo(H)} was calculated from Arrhenius plots of infield resistivity measurements. The infield resistive properties of superconductors depend upon the fluxoid motion, which causes resistive transition broadening by shifting Tc (0) towards lower temperature values. The enhancement of transition broadening is attributed to spread of upper critical field in vortex state and dissipation process with applied magnetic field. Addition of magnetic CoFe2O4 nanoparticles reduces the fluxoid motion by introducing nano-sized defects in the host CuTl-1223 superconducting matrix, which act as effective flux pinning centers. An overall increase in the activation energy Uo(H) has been observed with increasing contents of magnetic CoFe2O4 nanoparticles, which elucidate the enhanced flux pinning with increasing CoFe2O4 magnetic nanoparticles content up to x = 1.5 wt. % in CuTl-1223 superconducting matrix.
    No preview · Article · Oct 2015 · Journal of Alloys and Compounds
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    ABSTRACT: We investigated the infield superconducting properties of carbon nanotubes-Cu0.5Tl0.5Ba2Ca2Cu3O10−δ superconductor {(CNTs) x-(CuTl-1223)}; x = 0∼7 wt.% composites. The zero resistivity critical temperature {Tc(0)}, critical superconducting onset transition temperature {Tc onset(K)} and glass transition temperature (Tg) have been shifted towards lower values after the addition of carbon nanotubes (CNTs). The double transitions for all concentration of CNTs have indicated the existence of vortex-glass phase. The significant resistive broadening {ΔT = Tc onset(K)-Tc(0)} has been observed with increasing values of external applied magnetic field. The more increasing trend in resistive broadening has also been observed after the addition of CNTs in CuTl-1223 matrix. The magnetic field dependent activation energy Uo (H) has been calculated according to thermally activated flux flow (TAFF) model. The overall decreasing rate in pinning energy could be due to diffusion of carbon across the grain-boundaries.
    Full-text · Article · Oct 2015 · AIP Advances

  • No preview · Article · Sep 2015 · Ceramics International
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    ABSTRACT: Cobalt (Co) nanoparticles were added in Cu0.5Tl0.5Ba2Ca2Cu3O10-δ(CuTl-1223) superconducting matrix to obtain (Co)x/CuTl-1223 (x=0, 0.25, 0.50, 0.75, and 1.0 wt%) nanoparticles-superconductor composites. The crystal structure of host CuTl-1223 phase was not affected but superconducting transport properties were suppressed after the addition of these nanoparticles. Suppression of superconductivity was attributed to ferromagnetic nature of Co nanoparticles due to enhanced scattering cross-section of the carriers. The average crystallite size of Co nanoparticles was 50 nm, which were usually found in blocked state along their anisotropy axis below room temperature. The sharp suppression of diamagnetic signal could be due to two possible reasons (i) decrease in superconducting volume fraction and (ii) pair-breaking across ferromagnetic Co nanoparticles in blocked state. © 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
    Full-text · Article · Sep 2015 · Ceramics International
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    ABSTRACT: Anti-ferromagnetic chromium (Cr) nanoparticles were added to Cu 0.5 Tl 0.5 Ba 2 Ca 2 Cu 3 O 10Àd (CuTl-1223) superconducting matrix to synthesize (Cr) x /CuTl-1223 (x ¼ 0, 0.5, 0.75, and 1.0 wt.%) nano-superconductor composites. Different experimental techniques were used to study the structural, morphological, compositional and superconducting transport properties of these composites. It was observed that the addition of Cr nanoparticles had not altered the crystal structure of the host CuTl-1223 superconducting matrix, which indicates the presence of these nanoparticles at the grain-boundaries. Suppression of superconducting transport properties can be attributed to two factors (i) spins scattering by net magnetization due to uncompensated spins on the surface of these anti-ferromagnetic Cr nanoparticles and (ii) oxygen imbalance due to formation of Cr 2 O 3 surface layers on these nanoparticles. The prominent reduction in the diamagnetic signal was also observed, which had indicated the decrease of superconducting volume fraction after the inclusion of Cr nanoparticles in the composites. Super-conducting microscopic parameters such as coherence length {x c (0)}, inter-layer coupling (J), Fermi velocity (V F) and Fermi energy (E F) of the carriers were determined as a function of Cr content in (Cr) x /CuTl-1223 nano-superconductor composites by excess conductivity analyses in three dimensional (3D), two dimensional (2D) and short wave dimensional (0D) fluctuation regions.
    Full-text · Article · Aug 2015 · Journal of Alloys and Compounds
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    ABSTRACT: We have investingated the effects of Al 2 O 3 nanoparticles addition on infield superconducting transport properties of (Cu 0.5 Tl 0.5)Ba 2 Ca 2 Cu 3 O 10–δ (CuTl-1223) matrix. The addition of Al 2 O 3 nanoparticles has not affected the crystal structure of host CuTl-1223 phase, which has confirmed the occupancy of these nanoparticles across the inter-granular spaces. A significant resistive transition broadening was observed with externally applied magnetic field from 0–7 T. The enhancement of transition broadening was attributed to vortex–antivortex pair-breaking by applied magnetic field and temperature due to increase of dissipation process. Dissipative behavior of resistivity and activation energy {U o (H,T)} was explained by thermally activated flux flow (TAFF) model. Suppression of U o (H,T) showed the reduction of flux pinning strength with increasing content of Al 2 O 3 nanoparticles.
    Full-text · Article · Aug 2015 · Physica B Condensed Matter
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    K. Nadeem · G. Hussain · M. Mumtaz · A. Haider · S. Ahmed
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    ABSTRACT: Nickel ferrite (NiFe2O4) nanoparticles were added in to (Cu0.5Tl0.5)Ba2Ca2Cu3O10-� (CuTl-1223) superconducting matrix to get (NiFe2O4)x/CuTl-1223 composites with x = 0, 0.25, 0.50, and 1 wt. %. The inverse spinel (cubic) structure of NiFe2O4 nanoparticles and tetragonal structure of CuTl-1223 superconducting matrix were confirmed by using x-ray diffraction (XRD). There were no significant variations observed in the lattice parameters of the tetragonal structure of CuTl-1223 superconducting matrix with increasing nanoparticle concentration. The zero resistivity critical temperature (Tc) was decreased, while normal state resistivity (�300K) was increased with increasing nanoparticle concentration, which is most probably due to Cooper pair-breaking and localization of mobile free carriers across these magnetic nanoparticles. The suppression of activation energy (U) was also observed with increasing nanoparticle concentration, which is attributed due to inhomogeneous distribution and agglomeration of these magnetic nanoparticles. The microscopic superconductivity parameters as deduced from fluctuation induced conductivity (FIC) analysis of (NiFe2O4)x/CuTl-1223 composites are found to be in accordance with the experimental findings.
    Full-text · Article · Aug 2015 · Ceramics International
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    ABSTRACT: Superconducting Cu0.5Tl0.5Ba2(Ca2−x Bex ) (Cu2 Ti)O10−δ (x = 0, 0.5, and 1.0) samples were synthesized using a two-step solid-state reaction method. Superconducting properties were suppressed after Be substitution at Ca sites in these samples. Variation of oxygen phonon modes after Be doping was according to the masses of Ti (47.90 amu) and Cu (63.54 amu)atoms in conducting planes. Decrease in microscopic superconducting parameters deduced from excess conductivity analysis was attributed to an-harmonic oscillations, which have reduced Cooper pairs density from a desired level for optimum superconductivity.
    Full-text · Article · Aug 2015 · Journal of Superconductivity and Novel Magnetism
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    Abdul Jabbar · Irfan Qasim · M. Mumtaz · K. Nadeem
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    ABSTRACT: Series of (Ag)x/(Cu0.5Tl0.5Ba2Ca2Cu3O10-δ) {(Ag)x/CuTl-1223} nano-superconductor composites were synthesized with different concentrations (i.e. x=0~4.0 wt%) of silver (Ag) nanoparticles. Low anisotropic CuTl-1223 superconducting matrix was prepared by solid-state reaction and Ag nanoparticles were prepared by a sol–gel method separately. The required (Ag)x/CuTl-1223 composition was obtained by the inclusion of Ag nanoparticles in CuTl-1223 superconducting matrix. Structural, morphological, compositional and superconducting transport properties of these composites were investigated in detail by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-rays (EDX) spectroscopy and four-point probe electrical resistivity (ρ) measurements. The inclusion of Ag nanoparticles enhanced the superconducting properties without affecting the tetragonal structure of the host CuTl-1223 matrix. The improvement in superconducting properties of (Ag)x/CuTl-1223 composites is most likely due to enhanced inter-grains coupling and increased superconducting volume fraction after the addition of metallic Ag nanoparticles at the inter-crystallite sites in the samples. The presence of Ag nanoparticles at the grain-boundaries may increase the number of flux pinning centers, which were present in the form of weak-links in the pure CuTl-1223 superconducting matrix.
    Full-text · Article · Jul 2015 · Progress in Natural Science
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    ABSTRACT: Superconducting Mg-doped TlBa2(Ca2-yMgy)Cu3O10-δ (y=0, 0.5, 1.0, 1.5) samples were irradiated with gold (Au+4) ions. The zero resistivity critical temperature {Tc (0)} was decreased and residual resistivity was increased after irradiation. The irradiation-induced suppression of Tc (0) can be related with several factors, such as reduction of superconducting volume fraction caused by damaging and deterioration of samples quality after Au+4 ions irradiation. A pronounced fishtail was observed in all the samples with the applied magnetic field and all these features qualitatively agreed with the predictions of the order-disorder theory of vortex matter. The significant broadening in transition widths as function of external applied field was observed after ions irradiation. The activation energy was increased with the increasing Mg contents and Au+4 ions irradiation. But the activation energy of irradiated samples remained higher as compared to un-irradiated samples for all the values of external applied magnetic field. Almost similar decreasing trend in activation energy was observed with increasing magnitude of external applied magnetic field in all the samples. The ions irradiation hardening was observed in Mg-doped TlBa2(Ca2-yMgy)Cu3O10-δ samples.
    Full-text · Article · Jun 2015 · Radiation Physics and Chemistry
  • Abdul Jabbar · Muhammad Mumtaz · Kashif Nadeem
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    ABSTRACT: Low anisotropic (Cu0.5Tl0.5) Ba2Ca2Cu3O10-δ (CuTl-1223) high temperature superconducting phase was synthesized by solid-state reaction, silver (Ag) nanoparticles were prepared by sol-gel method and gold (Au) nanoparticles were extracted from colloidal solution. We added Ag and Au nanoparticles in CuTl-1223 matrix separately with same concentration during the final sintering process to get (M)x/CuTl-1223; M = Ag nanoparticles or Au nanoparticles (x = 0 and 1.0 wt.%) nano-superconductor composites. We investigated and compared the effects of these noble metals nanoparticles addition on structural, morphological and superconducting transport properties of CuTl-1223 phase. The crystal structure of the host CuTl-1223 superconducting phase was not affected significantly after the addition of these nanoparticles. The enhancement of superconducting properties was observed after the addition of both Ag and Au nanoparticles, which is most probably due to improved inter-grains weak-links and reduction of defects such as oxygen deficiencies, etc. The reduction of normal state room temperature resistivity is the finger prints of the reduction of barriers and facilitation to the carriers transport across the inter-crystallite sites due to improved inter-grains weak-links. The greater improvement of superconducting properties in Ag nanoparticles added samples is attributed to the higher conductivity of silver as compared to gold, which also suits for practical applications due to lower cost and easy synthesis of Ag nanoparticles as compared to Au nanoparticles.
    No preview · Article · Mar 2015 · The European Physical Journal Applied Physics
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    ABSTRACT: We explored the effects of highly coercive cobalt ferrite (CoFe2O4) nanoparticles addition on structural, morphological, and superconducting properties of Cu0.5Tl0.5Ba2Ca2Cu3O10-delta(CuTl-1223} matrix. Series of (CoFe2O4)(x)/CuTl-1223 (x=0 similar to 2.0 wt%) composites samples were synthesized and were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) absorption spectroscopy, and dc-resistivity versus temperature measurements. The magnetic behavior of CoFe2O4 nanoparticles was determined by MH-loops with the help of superconducting quantum interference device (SQUID). MH-loops analysis showed that these nanoparticles exhibit high saturation magnetization (86 emu/g) and high coercivity (3350 Oe) at 50 K. The tetragonal structure of host CuTl-1223 superconducting matrix was not altered after the addition of CoFe2O4 nanoparticles, which gave us a clue that these nanoparticles had occupied the inter granular sites (grain-boundaries) and had filled the pores. The increase of mass density with increasing content of these nanoparticles in composites can also be an evidence of filling up the voids in the matrix. The resistivity versus temperature measurements showed an increase in zero resistivity critical {T-c(0)}, which could be most probably due to improvement of weak-links by the addition of these nanoparticles. But the addition of these nanoparticles beyond an optimum level caused the agglomeration and produced additional stresses in material and suppressed the superconductivity.
    Full-text · Article · Mar 2015 · Journal of Magnetism and Magnetic Materials
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    ABSTRACT: Pristine carbon nanotube (CNT)/elastomer composites were fabricated using pristine multi-walled carbon nanotubes and a thermoplastic elastomer. These composites exhibited a unique phenomenon of two electrical percolation thresholds that invoked very high dielectric values for the resulting composites. The first percolation was associated with a relatively low dielectric constant value of about 100, while in the vicinity of the second percolation threshold a very high dielectric constant value of 8,000 was achieved. The presence of two percolation thresholds was attributed to the unique distribution patterns of CNTs that ensued in a CNT/elastomer composite system with unique electrical properties.
    Full-text · Article · Jan 2015
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    ABSTRACT: The excess conductivity analysis of resistivity versus temperature data of TlBa2(Ca2−yMgy)Cu3O10−δ (y=0.5, 1.0, 1.5) superconductor samples have been performed in the light of Aslamasov-Larkin and Lawrence-Doniach theories before and after irradiation with a beam of gold (Au4+) ions of energy 20 MeV having a fluence of 1×1015 ions/cm2. The microscopic parameters such as zero temperature coherence length {ξc(0)}, inter-plane coupling constant (α), and Fermi velocity (VF) have been calculated from the ln-ln plot of excess conductivity (Δσ) versus reduced temperature (ε) using the Lawrence and Doniach model. The cross-over temperatures separating different fluctuation regions in the ln-ln plot are also determined. It is observed from the excess conductivity analysis that all the microscopic parameters have been decreased after the irradiations. Whereas, calculations have shown that critical magnetic fields (Bc) and critical current density (Jc) are increased after the ion irradiation, which, confirms the validity of the results because Bc and Jc are increased by the formation of artificial pinning centers after bombardment of energetic heavy ion beam.
    Full-text · Article · Jan 2015 · Physica B Condensed Matter

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