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  • 1
    Publication Date: 1993-01-01
    Print ISSN: 0256-307X
    Electronic ISSN: 1741-3540
    Topics: Physics
    Published by Institute of Physics
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Until now, the origin of hard magnetic properties of rapidly quenched Nd-Fe-B alloys with lower Nd concentration is not clear. In this paper, the phase compositions of rapidly quenched Nd4Fe77.5B18.5 alloys annealed under different conditions have been studied by using zero-field spin-echo nuclear magnetic resonance (NMR) and Mössbauer effect (ME) techniques. It is found that there exists a certain amount of Nd2Fe14B phase in the samples annealed at 960 °C and ingot alloy, which have poor hard magnetic properties; while, the sample annealed under optimal condition consists only of bct-Fe3B as the main phase and a small amount of a-Fe. However, the ME result indicates that about 5 at. % Fe atoms in FeIII (8 g) site of bct-Fe3B have been replaced by Nd atoms; the NMR result demonstrates that 11B NMR spectrum is the characteristic peak of bct-Fe3B, but it broadens asymmetrically to the high frequency side, which is due to the bct-Fe3B influenced by Nd atoms. The amplitude of radio frequency (rf) excitation field required to get the maximum 11B spin-echo signal from bct-Fe3B in the sample annealed at 839 °C is only about one third as much as that required to excite the 11B in the bct-Fe3B influenced by Nd atoms in the sample annealed at 670 °C for a short time, which implies the latter has a larger coercivity field than the former. It is concluded that the origin of hard magnetic properties of Nd4Fe77.5B18.5 alloy is not related to the 2:14:1 phase, but to the change of bct-Fe3B itself.
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  • 3
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Fe–B amorphous alloys powders with high boron concentration can be prepared by borohydride reduction (BHR). By adjusting technological factors, the Fe100−xBx amorphous powders with compositions ranging from x=17 to 40 can be easily obtained, but it is difficult to make the amorphous ribbons with such a high B concentration by using rapidly quenched technique, which can usually obtain the amorphous ribbons with the compositions ranging from x=12 to 25. Up to now, there are different points of view about if the short range order (SRO) in the Fe–B amorphous powders prepared by the BHR around room temperature is the same with that in the amorphous ribbons made by rapidly quenched and vapor depositing techniques. In this paper, the SRO in Fe100−xBx amorphous powders with various boron concentration prepared by the BHR have been studied by the zero field spin echo nuclear magnetic resonance (NMR) technique. It was shown that the types of SRO in these samples vary with the B concentration. There exist Fe3B- and Fe2B-like SRO and a small amount of α-Fe in the samples with lower B concentration, while there are Fe3B- and FeB-like SRO in the alloys with higher B concentration. Fe3B-like SRO, however, only exists in the amorphous ribbons obtained by rapidly quenched method. It can be concluded that there are different types of SRO in the amorphous alloys prepared by different methods. This conclusion can explain the reason why the average hyperfine field at Fe sites deceases with the B concentration increasing.
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 265-267 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on poling of a low dielectric constant preimidized fluorinated fully aromatic guest-host polyimide nonlinear optical material including thermal stability of optical nonlinearities and waveguiding properties. We measured a second-harmonic coefficient (d33=4.9±0.5 pm/V at 1217 nm fundamental wavelength) which is accurately predicted by a thermodynamic model of poled polymers. The optical nonlinearity of a poled sample was thermally stable at 80 °C for over 300 h. Films were observed to have negative birefringence. Optical losses for slab waveguides in lowest order TE and TM modes were ≥7.7 dB/cm for doped waveguides at 800 nm wavelength and increased after poling.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 2277-2278 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Giant magnetoresistance was observed in a disordered two-dimensional electron system in a variable-range hopping regime. The possibility of utilizing the phenomenon as a magnetic field sensor was demonstrated by the detection of trapped vortices in a superconducting Pb film. The minimum size of such a device is discussed.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 1777-1779 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have studied the photovoltaic effects in Si doping superlattices (nipi) under different excitation conditions with and without additional cw optical biasing using a He-Ne laser. On the basis of the photovoltaic theory of carrier spatial separation in superlattices, we propose the concept of spatial fixity of the photovoltage polarity in type-II superlattices and examine the experimental results. The photovoltaic effect in Si nipi is found mainly from the direct transitions related with shallow impurities in real space, not the electron-hole band-to-band process as in GaAs nipi.
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  • 7
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 46 (1990), S. 111-117 
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 49 (1993), S. 877-880 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method is proposed to correct for the dynamical electron diffraction effect in crystal structure analysis. A rough structure model is first obtained by conventional structure-analysis methods neglecting the dynamical diffraction effect. From the rough structure model, multislice calculations are used to estimate the crystal thickness through the observed dynamical diffraction wave amplitudes. With this estimated thickness, the observed diffraction wave amplitudes are calibrated to give a set of fictitious observed kinematic structure-factor magnitudes. Based on such a set of magnitudes, a traditional least-squares procedure is used to refine structural parameters. The reliability of the result is checked by the consistency between the observed dynamical diffraction wave amplitudes and those found from the multislice calculation. The process can be made iterative. Tests were performed with two known structures, Bi-2212 and Pb-doped Bi-2223 high-Tc superconductors, and satisfactory results were obtained.
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 46 (1990), S. 459-463 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An X-ray crystallographic method has been introduced into the image processing of high-resolution electron microscopy. This enables the deconvolution of single electron micrographs of a crystalline sample. For this purpose the chemical composition of the sample should be known approximately, the image should be taken near the optimum defocus condition, but no preliminary knowledge of the crystal structure is needed. The method has been proved to be efficient with a high-resolution electron micrograph of chlorinated copper phthalocyanine taken on the Kyoto 500 kV electron microscope.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 46 (1990), S. 473-478 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A possible approach to direct phasing of quasicrystal diffraction data is described. The data are first converted to a set of structure-factor amplitudes of a multidimensional crystal. The Patterson function of the quasicrystal is used to derive the converting factor. A direct method is then used to solve the phase problem in multi-dimensional space. The method has been tested with a hypothetical one-dimensional quasicrystal yielding a satisfactory result.
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