350 research outputs found
Magnetic Properties of Pr0.7Ca0.3MnO3/SrRuO3 Superlattices
High-quality Pr0.7Ca0.3MnO3/SrRuO3 superlattices were fabricated by pulsed
laser deposition and were investigated by high-resolution transmission electron
microscopy and SQUID magnetometry. Superlattices with orthorhombic and
tetragonal SrRuO3 layers were investigated. The superlattices grew coherently;
in the growth direction Pr0.7Ca0.3MnO3 layers were terminated by MnO2- and
SrRuO3 layers by RuO2-planes. All superlattices showed antiferromagnetic
interlayer coupling in low magnetic fields. The coupling strength was
significantly higher for orthorhombic than for tetragonal symmetry of the
SrRuO3 layers. The strong interlayer exchange coupling in the superlattice with
orthorhombic SrRuO3 layers led to a magnetization reversal mechanism with a
partially inverted hysteresis loop.Comment: 12 pages, 4 figure
Operationally Efficient Propulsion System Study (OEPSS) data book. Volume 4: OEPSS design concepts
This study was initiated to identify operations problems and cost drivers for current propulsion systems and to identify technology and design approaches to increase the operational efficiency and reduce operations costs for future propulsion systems. To provide readily usable data for the Advanced Launch System (ALS) program, the results of the OEPSS study have been organized into a series of OEPSS Data Books. This volume describes three propulsion concepts that will simplify the propulsion system design and significantly reduce operational requirements. The concepts include: (1) a fully integrated, booster propulsion module concept for the ALS that avoids the complex system created by using autonomous engines with numerous artificial interfaces; (2) an LOX tank aft concept which avoids potentially dangerous geysering in long LOX propellant lines; and (3) an air augmented, rocket engine nozzle afterburning propulsion concept that will significantly reduce LOX propellant requirements, reduce vehicle size and simplify ground operations and ground support equipment and facilities
Operationally efficient propulsion system study (OEPSS) data book
A design tool of figure of merit was developed that allows the operability of a propulsion system design to be measured. This Launch Operations Index (LOI) relates Operations Efficiency to System Complexity. The figure of Merit can be used by conceptual designers to compare different propulsion system designs based on their impact on launch operations. The LOI will improve the design process by making sure direct launch operations experience is a necessary feedback to the design process
Evidence of defect-induced ferromagnetism in ZnFeO thin films
X-ray absorption near-edge and grazing incidence X-ray fluorescence
spectroscopy are employed to investigate the electronic structure of
ZnFeO thin films. The spectroscopy techniques are used to determine
the non-equilibrium cation site occupancy as a function of depth and oxygen
pressure during deposition and its effects on the magnetic properties. It is
found that low deposition pressures below 10 mbar cause iron
superoccupation of tetrahedral sites without Zn inversion, resulting in
an ordered magnetic phase with high room temperature magnetic moment.Comment: Accepted for publication in Phys. Rev.
Direct correlation between 1/f magnetonoise and magnetoresistance in La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>MnO<SUB>3</SUB> and (La<SUB>0.5</SUB>Pr<SUB>0.2</SUB>)Ba<SUB>0.3</SUB>MnO<SUB>3</SUB> manganites
Temperature- and magnetic-field-dependent electrical noise and electrical resistivity measurements were carried out on epitaxial thin films of a large bandwidth La0.7Sr0.3MnO3 and a disordered intermediate bandwidth (La0.5Pr0.2)Ba0.3MnO3 manganite system. The power spectral density was dominated by 1/f noise. This 1/f noise was observed to follow the overall temperature dependence of the resistivity. Moreover, in these compounds the magnetonoise effect was found to be of nearly the same magnitude as the magnetoresistance. This direct correlation between magnetonoise and magnetoresistance suggests that the enhanced 1/f noise has its origin in intrinsic charge-carrier density fluctuations
Orthorhombic to tetragonal transition of SrRuO3 layers in Pr0.7Ca0.3MnO3/SrRuO3 superlattices
High-quality Pr0.7Ca0.3MnO3/SrRuO3 superlattices with ultrathin layers were
fabricated by pulsed laser deposition on SrTiO3 substrates. The superlattices
were studied by atomically resolved scanning transmission electron microscopy,
high-resolution transmission electron microscopy, resistivity and
magnetoresistance measurements. The superlattices grew coherently without
growth defects. Viewed along the growth direction, SrRuO3 and Pr0.7Ca0.3MnO3
layers were terminated by RuO2 and MnO2, respectively, which imposes a unique
structure to their interfaces. Superlattices with a constant thickness of the
SrRuO3 layers, but varying thickness of the Pr0.7Ca0.3MnO3 layers showed a
change of crystalline symmetry of the SrRuO3 layers. At a low Pr0.7Ca0.3MnO3
layer thickness of 1.5 nm transmission electron microscopy proved the SrRuO3
layers to be orthorhombic, whereas these were non-orthorhombic for a
Pr0.7Ca0.3MnO3 layer thickness of 4.0 nm. Angular magnetoresistance
measurements showed orthorhombic (with small monoclinic distortion) symmetry in
the first case and tetragonal symmetry of the SrRuO3 layers in the second case.
Mechanisms driving this orthorhombic to tetragonal transition are briefly
discussed.Comment: 23 pages, 12 figure
Calibration of LACIS as a CCN detector and its use in measuring activation and hygroscopic growth of atmospheric aerosol particles
A calibration for LACIS (Leipzig Aerosol Cloud Interaction Simulator) for its use as a CCN (cloud condensation nuclei) detector has been developed. For this purpose, sodium chloride and ammonium sulfate particles of known sizes were generated and their grown sizes were detected at the LACIS outlet. From these signals, the effective critical super-saturation was derived as a function of the LACIS wall temperature. With this, LACIS is calibrated for its use as a CCN detector. The applicability of LACIS for measurements of the droplet activation, and also of the hygroscopic growth of atmospheric aerosol particles was tested. The activation of the urban aerosol particles used in the measurements was found to occur at a critical super-saturation of 0.46% for particles with a dry diameter of 75 nm, and at 0.42% for 85 nm, respectively. Hygroscopic growth was measured for atmospheric aerosol particles with dry diameters of 150, 300 and 350 nm at relative humidities of 98 and 99%, and it was found that the larger dry particles contained a larger soluble volume fraction of about 0.85, compared to about 0.6 for the 150 nm particles
Size-dependent electronic-transport mechanism and sign reversal of magnetoresistance in Nd0.5Sr0.5CoO3
A detailed investigation of electronic-transport properties of Nd0.5Sr0.5CoO3
has been carried out as a function of grain size ranging from micrometer order
down to an average size of 28 nm. Interestingly, we observe a size induced
metal-insulator transition in the lowest grain size sample while the bulk-like
sample is metallic in the whole measured temperature regime. An analysis of the
temperature dependent resistivity in the metallic regime reveals that the
electron-electron interaction is the dominating mechanism while other processes
like electron-magnon and electron-phonon scatterings are also likely to be
present. The fascinating observation of enhanced low temperature upturn and
minimum in resistivity on reduction of grain size is found due to
electron-electron interaction (quantum interference effect). This effect is
attributed to enhanced disorder on reduction of grain size. Interestingly, we
observed a cross over from positive to negative magnetoresistance in the low
temperature regime as the grain size is reduced. This observed sign reversal is
attributed to enhanced phase separation on decreasing the grain size of the
cobaltite
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