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An extensive Monte Carlo (MC) simulation is performed to investigate exchange bias in a novel model of a nanoparticle consisting of a ferromagnetic (FM) core and an amorphous magnetic (AM) shell. The magnetic glassy ordering of the AM shell governed by random magnetic anisotropy (RMA) is shown to be responsible for the unidirectional anisotropy to the FM core. Our MC results show that, while the bias field and coercivity exhibit a strong dependence on RMA strength, which looks like that on the antiferromagnetic (AFM) thickness in traditional FM/AFM bilayers, they are independent of AM thickness. However, the bias field and coercivity abruptly change with increasing small core size and get saturated for a large enough core size. The cooling field and temperature dependencies show the peculiar sign inversion of the bias field. Our study supports the argument that the exchange bias observed in some magnetic nanoparticles or nanocrystallined alloys is not necessarily due to the presence of an AFM phase. [ABSTRACT FROM AUTHOR]

Using cross-section transmission electron microscopy and grazing incidence x-ray diffraction measurements, this work investigates the defect reduction in a wurtize GaN thin film with a multiple composite layer grown by atmospheric-pressure organometallic vapor phase epitaxy on sapphire substrate. According to those results, the sequential composite layer can terminate most threading dislocations and only a few dislocations can be deeply propagated into the next epitaxial layer. Moreover, the multiple composite layer structure significantly reduces the density of threading dislocation generated from the GaN/Al[sub 2]O[sub 3] interface. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Attainment of perpendicular coercivity higher than 900 Oe and accurate calculation of the degree of long-range order in an FePd alloy thin film is essential in the realization of the perpendicular magnetic recording media. Here, we prepared the off-stoichiometric FePd films through the multilayer growth of [Fe(2 nm)/Pd(2 nm)] × 30 deposited on MgO(001) substrates by magnetron sputtering. The effects of post-annealing these films at 700 °C for 1 and 10 h on their surface morphologies, and crystallographic and magnetic characteristics were investigated. The results show that, with increasing annealing time, the FePd thin films are predominantly (001)-oriented with a progressive increase of the grain size and surface roughness, as well as the improved crystallinity and reduced micro-strain. Accordingly, the perpendicular coercivity is significantly increased to the value of 1.5 kOe, and Pd loses its 4d electrons upon alloying. Furthermore, analysis of X-ray absorption data elucidates some structural details of the Fe scattering phase in addition to the dominant tetragonal FePd phase. We also present a way of determining the long-range order parameter for the FePd alloy thin film, using the combined information of the tetragonality ratio extracted from extended x-ray absorption fine-structure spectroscopy and the stoichiometric deviation of the thin film. [ABSTRACT FROM AUTHOR]

Using x-ray spectromicroscopy, we studied the interface of pentacene (Pn)-cobalt (Co) heterojunction fabricated by thermal evaporation in an ultra-high vacuum environment. Through element-specific images and absorption spectroscopic analysis, we found evidences suggesting that part of the Co top layer penetrates into Pn film and hybridizes with molecules. As the Curie temperature of infiltrated Co clusters is dimension sensitive, the Co penetration would create an ill-defined interfacial region whose magnetization depends on the temperature and depth of cobalt penetration. The magnetic complexity at Pn/Co interface was found reduced after inserting a thin Cu layer between Pn and Co. [ABSTRACT FROM AUTHOR]

Explores the capability of energy dispersive small angle x-ray scattering with synchrotron radiation. Use of a detector mask with three acceptance rings of different ring radius; Signal to background ratio for a standard cross-linked polyethylene sample; Limitation of the application at low energy by sample transmission and detector response.

Examines the design parameters and performance of double-crystal monochromator (DCM) for an x-ray beamline. Factors affecting the stability of motions and overall size of the monochromator; Development of a software program to control the motion of DCM; Absorption spectrum of the device.

The effect of Fe doping on the crystalline phase transformation and on the local environment around Fe dopant ions is investigated for BaTi1-xFexO3-δ (0.0 ≤ x ≤ 0.5) polycrystalline samples, using x-ray diffraction, x-ray absorption spectroscopy, and Raman scattering spectroscopy. Our experimental results show that the tetragonal-to-hexagonal transformation is gradually taken place when increasing the Fe content in the range 0.02 ≤ x ≤ 0.12. Although both hexagonal and tetragonal polymorphs coexist in this doping range, Fe ions preferably substitute for Ti sites in the hexagonal lattice and exist in both Fe3+ and Fe4+ forms. Our work is of paramount importance to provide a direct evidence to the preferable substitution of transition metal ions for Ti ions. [ABSTRACT FROM AUTHOR]

The effect of Fe dopant content, x, on the magnetism of polycrystalline BaTi1-xFexO3-δ (BTFO) multiferroic samples was investigated using magnetization measurements in correlation with the crystallographic and local structures. The changes of coercivity and magnetization as functions of x are closely related to the variation of oxygen vacancy content, δ, which can be deduced from the Fe oxidation number estimation using Fe K-edge x-ray absorption spectroscopy. It is shown that although the samples exhibit ferromagnetic (FM) hysteresis at room temperature, a dominant paramagnetic phase coexisting with the FM phase makes it difficult to identify the FM contribution to the total magnetization. Spin-glass and/or superparamagnetic behavior may be ruled out due to the fact that both zero-field-cooled and field-cooled thermomagnetization curves almost totally overlap together over the whole range of measured temperatures. [ABSTRACT FROM AUTHOR]

Studies a segmented cascade ion chamber for measuring the intensities and energies of synchrotron radiation x-rays. Cathode cells comprising the chamber; Measurement of ionized current collected in each cell; Schematic view of the chamber.

Uniform single-crystal lead telluride (PbTe) nanowires (NWs) were synthesized using a stress-induced method, which is an alternative technique for synthesizing PbTe NWs without a catalyst. The thermoelectrical transport measurement of a NW with a diameter of 217 nm exhibited a notable enhancement over the room-temperature thermopower of -342 μV K-1, which was approximately 95% larger than the bulk of PbTe. The power factor of 104 μW m-1 K-2 is also higher than any previously reported power factor in PbTe NWs. [ABSTRACT FROM AUTHOR]

The net modal gain of the InGaN/GaN multiple quantum well has been measured by the variable excitation stripe length method for an optically pumped cavity along each crystal orientation on the (0001) plane. These results demonstrated the theoretical prediction of the fact that the maximum optical gain can be obtained at a [1¯21¯0]-oriented edge-emitting laser cavity, which has been reported in the literature. “Crystal orientation” is confirmed to be a related parameter to the optical gain for a GaN-based strained structure. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]