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991.
Multiple metamagnetic transitions in the magnetic refrigerant La(Fe,Si)13Hx   总被引:1,自引:0,他引:1  
The effect of hydrostatic pressure on thermally and field-induced first-order magnetic phase transitions is studied in the La(Fe,Si)_(13)-type compounds. A peculiar series of consecutive field-induced transitions is realized using a distinct combination of hydrostatic pressure and negative pressure created by the interstitial insertion of hydrogen. The pressure-induced discontinuous magnetization jumps result in an enhanced cooling power, thus opening up the possibility to exploit in full the magnetocaloric potential of this compound class.  相似文献   
992.
The Hoffman-Wielandt inequality for the distance between the eigenvalues of two normal matrices is extended to Hilbert-Schmidt operators. Analogues for other norms are obtained in a special case.  相似文献   
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995.
The dynamical behavior of a Si(III) and a Ge(III) surface is studied. The formation of a superstructure, i.e. the surface reconstruction, is shown to be a consequence of the softening of surface phonons. Within our model we derive a (7×7) reconstruction for Si and an (8×8) reconstruction for Ge. A buckled model for the superstructure on Si(III) surface as well as on the Ge(III) surface is proposed.  相似文献   
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A computer simulation has been used to predict crystallization kinetics and crystalline morphology in composite materials based on thermally nucleated crystallizable matrices. As demonstrated for athermally nucleated composites, the presence of reinforcing fibers increases the complexity of the system. Fibers are shown to have a dual effect on the spherulitic crystallization process. The influence that fibers have depends on the interplay between the enhancing effects that fibers have on nucleation and the depressing effects that fibers have on spherulitic growth. Fibers that do not provide additional nuclei to the system depress the rate of crystallization relative to an unreinforced polymer, while fibers that add nuclei to the system increase the rate of crystallization. The transcrystalline morphologies that develop in thermally nucleated fiber-reinforced polymers are controlled primarily by the relative numbers of bulk and fiber nuclei. The extent of transcrystalline regions can be suppressed either by increasing the rate of bulk nucleation, or by decreasing the rate of fiber nucleation. Finally, the qualitative appearance of the morphology in the transcrystalline region was found to be indicative of the mode of fiber nucleation. © 1995 John Wiley & Sons, Inc.  相似文献   
998.
Concentration and fluence dependent tendency of FeNiCrMoSiB metallic glass towards disordering as well as a decrease of the Curie temperature were revealed by Mössbauer and electron-positron annihilation spectroscopies and initial permeability measurements, respectively. The observed changes can be assigned to irradiation-induced defects resulting in fluctuations in chemical and/or topological short-range order.  相似文献   
999.
The pressure effect on Tc of polycrystalline and single crystalline YBa2Cu3Ox investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x> 6.93 the single crystals show a negative dTc/d p, as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find dTc/d P = 0.4 K/GPa which points to pressure effects on Tc aside from charge transfer. In the underdoped region x < 6.93 the dTc/d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, dTc/d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the dTc/d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa2Cu3Ox structure and can be explained by considering the anisotropic structure of YBa2Cu3Ox. The decrease of the c-axis lattice parameter results in a charge transfer to the CuO2-planes mainly [1], whereas the compression of the a- and b-axis lattice parameter is known to produce different pressure effects which are responsible for the peak in dTc/d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of Tc to the equilibrium value Tc(p) at this pressure with a time constant depending on the oxygen content x. A decrease x results in a peak effect in dTc/d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, dTc/d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x < 6.42. These giant pressure effects at low oxygen contents are mainly caused by a reversible Tc increase (dTc/d p)O due to pressure induced oxygen ordering via oxygen motion between unit cells.  相似文献   
1000.
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