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  • 11
    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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  • 12
    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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  • 13
    Electronic Resource
    Electronic Resource
    Spatio-temporal dynamics of oscillatory heterogeneous catalysis: CO oxidation on platinum (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8614-8625 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reaction-rate oscillations in the oxidation of carbon monoxide on the surface of platinum catalysts are studied in a continuous flow reactor at atmospheric pressure using infrared imaging. Small-amplitude temperature oscillations (0.2–8 K) result in approximately isothermal conditions, where changes in rate constants, for typical activation energies and temperatures, are small. The catalysts are in the form of platinum thin films on quartz substrates and provide highly repeatable oscillatory behavior. The platinum films are fabricated in the form of annular rings which provide a quasi-one-dimensional geometry in order to simplify comparison to theoretical models. Time-series measurements by means of thermocouples are used to characterize the oscillations. The infrared images show that most oscillations are spatially synchronized to within the 0.25 s time resolution of the experiment. The images also show that "fine structure'' oscillations (i.e., small-amplitude, high frequency oscillations superimposed on larger-amplitude waveforms) are associated with spatially desynchronized patterns. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468963
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  • 14
    Electronic Resource
    Electronic Resource
    EPR, magnetization, and resistivity studies in doped (4-f or 3-d ions) and undoped RBa2Cu3Oy high TC superconductors (R=Y,Pr,Nd,Eu,Gd,Ho,Er, or Yb) (abstract) (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 4202-4202 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have measured electron paramagnetic resonance (EPR), resistivity, and dc susceptibility from 2 to 300 K for the oxide high Tc superconductors (R)Ba2Cu3Oy (R=Y,Pr,Nd,Eu,Gd,Ho,Er,Tm, or Yb). Selected systems were doped with 3-d ions (Cr,Mn,Fe,Ni,Co, or Zn) or 4-f ions (Gd or Er) which presumably substitute for the Cu or R site, respectively. In the systems studied we have observed an EPR line at low temperatures (T〈40 K), which exhibits an increase in intensity and decrease in field for resonance as the temperature is lowered. The ESR linewidth is also temperature dependent and exhibits a minimum at about 15 K. An additional EPR line that can be associated with a Gd3+, Mn2+ or Er3+ ion was observed for those samples where these ions were present as dilute impurities. In some of the samples another EPR signal is observed with properties that depend on sample preparation conditions. The behavior and origin of all lines will be discussed. The variation of Tc with concentration of the added impurities over the range (1%–15%) will also be presented, and compared with previous studies in other superconducting systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.340224
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  • 15
    Electronic Resource
    Electronic Resource
    Thermoelectric properties of chemically substituted skutterudites YbyCo4SnxSb12−x (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 1948-1951 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report the results of a study of thermoelectric properties of chemically substituted quasiternary materials related to the recently discovered filled skutterudite compound YbFe4Sb12. The study explored partial filling at the Yb site as well as chemical doping with Sn at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system YbyCo4SnxSb12−x. Our measurements of these physical quantities from room temperature down to T=10 K indicate that, in our study, only the alloy Yb0.44Co4Sb12 possessed thermoelectric properties that are improved over the parent compound YbFe4Sb12, attaining a value of ZT=0.1 at T=300 K. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1305837
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  • 16
    Electronic Resource
    Electronic Resource
    Properties of alloy precursors and the resulting superconducting YBa2Cu3O7−δ-Ag composites (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 1327-1335 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A significant obstacle to the widespread application of high-temperature superconductors are the poor mechanical properties of these oxides. A novel processing technique was investigated that attempts to circumvent this problem using ductile alloy precursors to superconducting composites. Alloys of composition Y-2Ba-3Cu-Ag were prepared by arc- and induction melting the metals. A variety of mechanical tests show that alloys with 90 wt % Ag are ductile, and differential thermal analysis indicates a solidus temperature of 703 °C. Using thermogravimetric analysis, the oxidation rate of Y-2Ba-3Cu-90 wt % Ag alloys was studied. This rate does not follow that which is expected based upon the theory of internal oxidation, nor is there a transition to external oxidation upon lowering the partial pressure of oxygen. The superconducting properties of the oxide in the resulting composite are similar to those of polycrystalline samples, with a Tc of 90 K. Superconductivity is achieved only when the alloy is oxidized above the solidus temperature, where the presence of a liquid phase enables the oxides to be concentrated and connected.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351251
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  • 17
    Electronic Resource
    Electronic Resource
    Pressure and magnetic field dependence of the antiferromagnetism of PrFe4P12 (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 4818-4820 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The compound PrFe4P12, which has the cubic LaFe4P12 modified CoAs3 -type structure, orders antiferromagnetically, with TN=6.2 K and with several field-induced magnetic transitions for T〈TN, up to 5 T for T(very-much-less-than)TN. Heat capacity measurements show the transition is a bulk effect, with magnetic entropy consistent with a magnetic doublet or triplet crystal-field ground state. dc magnetization measurements, M(H), at T=4.2 K up to 23 T reveal no further field-induced transitions, and M(H) attains only 54% of the Pr3+ free-ion moment at highest fields. Pulsed field measurements of dM/dH at 4.2 K up to 45 T fail to resolve further transitions. The transition to the antiferromagnetic state is accompanied by a very sharp Suezaki–Mori-type electrical resistivity transition due to critical scattering of electrons by spin fluctuations. Hydrostatic pressure decreases TN, (dTN/dP=−0.11 K/kbar), in sharp contrast to the behavior for the isomorphic semiconducting ferromagnet, UFe4P12 (TC=3.15 K; dTC/dP=+0.26 K/kbar). The field-induced magnetic transitions are also sharply reduced with pressure. The pressure dependence of the electrical resistivity showed a decrease in TN, with a concomitant decrease in the size of the Suezaki–Mori anomaly. The results are discussed in terms of crystal-field splittings of the Pr ion and possible hybridization effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.344773
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  • 18
    Electronic Resource
    Electronic Resource
    Magnetic phase diagrams of UCu2Si2 and UNi2Si2 (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 5218-5220 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetization measurements performed on UCu2Si2 and UNi2Si2 are reported. The compound UCu2Si2 appears to undergo both an antiferromagnetic (AFM) transition at T≈107 K and a ferromagnetic (FM) transition at Tc≈103 K. While the second transition at Tc has already been reported, the AFM phase was apparently overlooked in earlier studies. The compound UNi2Si2 undergoes a transition at T≈97 K, which was previously shown to be AFM by neutron scattering; we present evidence for the occurrence of a FM state below the AFM transition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.344615
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  • 19
    Electronic Resource
    Electronic Resource
    Transport and magnetic properties of polycrystalline Sm2−xCexCuO4−y : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6720-6722 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A systematic study of transport and magnetic properties of polycrystalline Sm2−xCexCuO4−y (0.15≤x(very-much-less-than)0.18) specimens obtained from a sol-gel precursor reveals a striking double resistive superconducting transition. One of the resistive transitions takes place at a temperature Tci and is almost insensitive to Ce concentration, applied magnetic field, and current density, while the other transition occurs at a lower temperature Tcj and is very sensitive to Ce content, applied magnetic field, and current density. Alternating-current magnetic susceptibility χac measurements reveal two interesting features. There is only one contribution to the diamagnetic signal below ∼20 K, which is particularly evident in the resistive component χ‘ of χac where a peak at Tcj can be identified. The application of an applied magnetic field shifts the χ‘ peak to lower temperatures and reveals a spin-glass-like behavior. All the results can be satisfactorily explained in terms of weakly linked Josephson junctions between small superconducting islands. The small islands have a superconducting transition at a higher temperature Tci, and the long-range phase coherence is achieved through Josephson coupling at lower temperatures Tcj. All the results suggest a superconducting glass state in a disordered Josephson network.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356849
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  • 20
    Electronic Resource
    Electronic Resource
    Flux pinning in Y1−xPrxBa2Cu3O7−δ high Tc superconductors (1991)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 1792-1794 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetic relaxation measurements have been made on two sets of samples of Y1−xPrxBa2Cu3O7−δ high Tc superconductors with 0≤x≤0.10. The samples were prepared as fibers using a sol-gel process and as polycrystalline pellets using a standard solid-state reaction technique. In an applied field of 5 kOe at 30 K, the fluxoid pinning potential U was observed to increase approximately linearly with Pr concentration x at the rate dU/dx≈1 eV. A mechanism for the enhancement of U may be the suppression of the superconducting order parameter in the vicinity of the Pr ions due to magnetic pair breaking.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.105093
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