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  • 1
    Electronic Resource
    Electronic Resource
    Erratum: "Effects of local reactant concentration perturbations in oscillatory catalysis" [J. Chem. Phys. 108, 5565 (1998)] (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1619-1619 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480659
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  • 2
    Electronic Resource
    Electronic Resource
    Effects of local reactant concentration perturbations in oscillatory catalysis (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 5565-5570 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reaction-rate oscillations in CO oxidation over heterogeneous platinum thin-film catalysts were studied at atmospheric pressure where spatial coupling is due to gas-phase diffusion of reactant concentration gradients. The catalyst was perturbed locally by the introduction of CO directly above the surface, while the resulting spatiotemporal behavior was monitored using infrared imaging. The transient response of the system was studied in both the steady-state and oscillatory regimes, and comparisons are made between the observed behavior and predictions from models for the oscillations. Using these perturbations, global behavior was initiated using a localized effect. Similarities and differences between the induced oscillations and those occurring naturally are discussed, as are tests to vary the phase of the oscillatory cycle. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475946
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  • 3
    Electronic Resource
    Electronic Resource
    Shock compaction of SmCo5 particles (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 9236-9244 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Fine ferromagnetic particles (1–10 μm) of SmCo5 were compacted using shock pressures of 1.7–27 GPa (17–270 kbar). One-piece disks were produced with pressures in the range 2–9 GPa. X-ray-diffraction measurements revealed no changes in the composition of the compacted SmCo5 for shock pressures below 15 GPa. Magnetic properties of the shocked samples were measured with a superconducting quantum interference magnetometer. In the pressure range 2–9 GPa, the shocked specimens exhibit high intrinsic coercivity Hci. Below 2 GPa, lower values of coercivity were observed, presumably caused by incomplete bonding of particles. Above 10 GPa the intrinsic coercivity drops suddenly and levels off at higher pressures. The remnant magnetization Br, exhibits a broad peak at 6 GPa and then falls off rapidly at higher pressures. The saturation magnetization Ms, has a peak near 10 GPa that correlates with the drop in coercivity. The hardness coefficient a, derived from a fit of the phenomenological "law of approach'' formula to the hysteresis curve, shows a linear increase with pressure near 8 GPa. This suggests an increase of inclusions in the samples and correlates with the drop in Hci and the peak in Ms. There is a non-negligible linear contribution to the magnetization given by κH. The nonzero value of κ suggests the possibility of incomplete saturation or paramagnetic impurities. Maximum values in coercivity Hci, remnant magnetization Br, and saturation magnetization Ms are found at about 9 GPa. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362598
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  • 4
    Electronic Resource
    Electronic Resource
    Properties of polycrystalline samples of Nd2−xCexCuO4−y obtained from a sol-gel precursor : The 6th joint magnetism and magnetic materials (MMM)-intermag conference (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 7121-7123 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Polycrystalline samples of Nd2−xCexCuO4−y(0.0≤x≤0.17) obtained from a sol-gel precursor were sintered below the eutectic temperature and reduced under different conditions. From the results of x-ray powder diffraction, the tetragonal T' structure is preserved for Ce content up to x=0.17. A decrease in the lattice parameter c and a small increase in the lattice parameter a are observed with increasing Ce concentration. Electrical resistance R(T) measurements performed on reduced samples with x≥0.12 reveal superconducting behavior below T≈25 K. We also found that all reduced samples display a striking double resistive superconducting transition and that the zero resistance state is achieved through two distinct drops in R(T) at upper and lower transition temperatures Tci and Tcj. The upper transition temperature Tci has a maximum for Ce concentration x=0.14, which is independent of the reduction process. The lower transition temperature Tcj is observed in reduced samples with Ce content x≥0.14 and decreases monotonically with increasing Ce concentration. Magnetic susceptibility measurements confirm bulk superconductivity in several reduced samples, and a significant diamagnetic contribution is only observed for temperatures below Tcj. The results are discussed within the framework of a granular superconductor model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358050
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic properties of polycrystalline Sm2−xCexCuO4−y at high magnetic fields : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 6564-6566 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have performed measurements of ac magnetic susceptibility χac as a function of temperature and magnetic field on polycrystalline specimens of Sm2−xCexCuO4−y (0.15≤x≤0.18) prepared from a sol-gel precursor. All the samples studied show a striking double resistive superconducting transition. One of the resistive transitions occurs at a higher temperature Tci and the other one at a lower Josephson-coupling temperature Tcj. Magnetic measurements reveal the appearance of superconductivity below Tcj which is evident from the resistive component χ″ which peaks only at Tcj. Measurements made on pellets and crushed samples with average grain size ∼1 μm show essentially the same features. This suggests that Josephson coupling occurs inside physical grains with dimensions close to 5 μm. We have also utilized the behavior of the peak in the χ″ component as a function of applied magnetic fields to build a H vs T phase diagram for these compounds. Some interesting features were observed in this H vs T phase diagram: (1) a Hα, α=2/3, dependence at low applied magnetic fields (H≤2 kOe) and high temperatures, and (2) an abrupt increase in the magnitude of the exponent at high fields and low temperatures. The behavior at low fields and high temperatures suggests that the magnetic response of the system is mainly controlled by a disordered Josephson network. Possible connections between these two different behaviors are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361947
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  • 6
    Electronic Resource
    Electronic Resource
    Non-Fermi liquid ground states in strongly correlated f-electron materials (invited) (abstract) : The 6th joint magnetism and magnetic materials (MMM)-intermag conference (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 6137-6137 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Strongly correlated electron materials, especially the high transition temperature (Tc) cuprate superconductors and heavy fermion rare earth and actinide compounds, have been the focus of intense investigation in recent years. The unusual normal state properties of the high Tc cuprates have been ascribed by some researchers to a non-Fermi liquid (NFL) metallic state, while the anomalous properties of certain heavy fermion U-based superconductors such as UBe13 have been attributed to NFL behavior due to a two-channel quadrupolar Kondo effect. Although NFL behavior has not been established in any of the U-based heavy fermion superconductors, evidence for its occurrence in an f-electron material has been found in the U alloy system Y1−xUxPd3. Subsequently, NFL behavior was observed in a number of other f-electron alloy systems, including Sc1−xUxPd3, UCu3.5Pd1.5, Th0.1U0.9Be13, Th1−xUxRu2Si2 (x≤0.07), Th1−xUxPd2Al3, La0.9Ce0.1Cu2Si2, and CeCu5.9Au0.1. Evidence for NFL behavior in Y1−xUxPd3 and related systems is reviewed and discussed within the context of possible microscopic mechanisms. Some systematics of the NFL low temperature behavior observed in several f-electron materials include a linear temperature dependence of the electrical resistivity ρ∼1−aT with the coefficient a either (approximately-greater-than)0 or 〈0, a logarithmically diverging specific heat C/T∼−ln T, and T1/2 asymptotic behavior of the magnetic susceptibility χ∼1−T1/2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358331
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  • 7
    Electronic Resource
    Electronic Resource
    Positron annihilation measurements of Pr-containing superconductor compounds (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 5534-5539 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron annihilation lifetime and Doppler broadening energy spectra have been measured in Pr-containing YBa2Cu3O7−δ as a function of temperature between 18 and 295 K. The defect-related positron lifetime component τ2 was independent of temperature for the PrBa2Cu3O7−δ compound and was very close to the value τ1 associated with bulk or defect-free material. These results indicate that the structure of this compound is nearly free of defects which trap positrons and that there is no electronic structure change during cooling. There was a slight temperature dependence below Tc for the Y0.5Pr0.5Ba2Cu3O7−δ compound. It was also found that a change in the positron Doppler line-shape parameter S occurred at the superconducting transition temperature for 0.5 Pr-containing compound but not for the 1.0 Pr-containing compound. These results show that the average electron momentum at the annihilation sites increases as temperature is lowered across the superconducting transition range for the 0.5 Pr-containing compound but not for the PrBa2Cu3O7−δ compound. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.359672
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  • 8
    Electronic Resource
    Electronic Resource
    Low-temperature magnetic relaxation of organic coated NiFe2O4 particles : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 5639-5641 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report measurements of relaxation rate of the remanent magnetization versus temperature for NiFe2O4 particles coated with the surfactant oleic acid. It has been reported that the organic coating induces a strong surface anisotropy resulting in surface spin canting and anomalous hysteresis properties at low temperature as compared to the bulk material. We have measured, at temperatures down to 400 mK, relaxation of remanent magnetization which extrapolates to a nonzero value at zero temperature. Macroscopic quantum tunneling at these temperatures has been predicted and observed only for materials with high magnetocrystalline anisotropy, so it is particularly interesting to observe it in NiFe2O4, which has a relatively low bulk anisotropy. It is suggested that the strong surface anisotropy induced by the organic coating may be responsible for tunneling in these particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355619
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  • 9
    Electronic Resource
    Electronic Resource
    Ultrahigh vacuum SQUID magnetometry study of the magnetic properties of Co/Co-oxide thin films : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 4926-4928 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Exchange coupling between Co and Co-oxide in bilayer and trilayer thin-film structures has been studied using a recently developed ultrahigh vacuum SQUID magnetometer (UHVSQM) system. Using this novel technique, newly deposited Co films from a separate molecular beam epitaxy facility were transported into the UHVSQM without breaking vacuum and magnetically characterized before and after in situ room temperature oxidation. Field-cooled hysteresis measurements performed at low temperatures indicate the presence of a complex antiferromagnetic Co-oxide domain structure as a result of oxidation. The stability of the magnetic structure is determined by competition between frustration generated at the interface when the ferromagnetic Co moments are reversed and stabilizing antiferromagnetic anisotropy which becomes large only at low temperatures (〈150 K). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361590
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  • 10
    Electronic Resource
    Electronic Resource
    Spatial coupling in heterogeneous catalysis (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 8209-8215 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Spatial coupling mechanisms are studied in the heterogeneous catalytic oxidation of carbon monoxide over platinum at atmospheric pressure under oscillatory conditions. Experiments are conducted in a continuous flow reactor, and the reaction rate is monitored using both infrared imaging and thermocouples. The catalysts are in the form of platinum annular thin films on washer-shaped quartz substrates, and they provide highly repeatable oscillatory behavior. Oscillations are typically spatially synchronized with the entire catalyst "flashing'' on and off uniformly. Spatial coupling is investigated by introducing various barriers which split the annular ring in half. Infrared images show that coupling through the gas phase dominates coupling via the diffusion of CO on the surface or heat diffusion through the substrate. The introduction of a localized heat perturbation to the catalyst surface does not induce a transition in the reaction rate. Thus, it is likely that the primary mode of communication is through the gas-phase diffusion of reactants. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470185
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