Search

Studies the hydrogen passivation of nitrogen in 6H-SiC. Definition on the complexation of nitrogen with hydrogen.

Examines the effect of hydrogen passivation on donors and acceptors in silicon carbide crystals. Conduction of electron spin resonance and transport measurement; Extraction of hydrogen passivation by annealing in hydrogen ion atmosphere; Determination of free-electron concentration and mobility of SiC before and after hydrogenation.

Presents a study that investigated the vibrational absorption of semi-insulating oxygen-containing gallium arsenide using high-resolution spectroscopy. Detection of the presence of interstitial oxygen in the samples; Effect of near-infrared illumination on the observation of the (gallium, oxygen)-related modes; Evaluation of the localized vibrational modes of the samples.

A theoretical determination of the Jahn–Teller coupling constants of d5 ions in tetrahedral clusters is performed in the framework of the angular overlap model and the quasimolecular approximation. It is shown that the angular model correctly predicts the trend of the preponderant coupling of the 4T1(G) and 4T2(G) levels of Mn2+ in MnS4 and MnSe4 to the E-vibrational effective mode. Finally, the limitations of this model are discussed. In particular, it is shown that more elaborated covalent models are needed to interpret the Jahn–Teller coupling coefficients of the 4E and 4A1 levels of d5 configurations and to determine the influence of the crystal on the clusters. [ABSTRACT FROM AUTHOR]

A very detailed analysis of the fine structure of the 4E(G) state of Mn2+ in cubic and nearly cubic clusters is presented to account for the unusual fine structure observed in ZnSe:Mn. First, experimental results are reported which show that the spin-orbit interaction in the 4E(G) state of Mn2+ in ZnSe is about one half of that previously observed in cubic or nearly cubic clusters as MnS4, MnBr4, and MnCl4 and that the state Γ8 is neatly shifted with respect to the Kramers doublets Γ6 and Γ7. Second, a model is elaborated which shows that second-order spin-orbit interactions reduced by a Jahn–Teller coupling to E vibrational modes accounts well for the observed overall splitting of the 4E(G) state in the studied clusters and that fourth-order spin-orbit interactions correctly describe the shifts of the Γ8 state. The validity of the perturbation schemes is discussed and the results are compared to those obtained from a complete diagonalization in the d5 configuration. [ABSTRACT FROM AUTHOR]

The temperature dependence of the interlayer exchange coupling has been investigated in epitaxial tunnel junctions Fe1/MgO/Fe2/Co/V with thin MgO layers using X-band ferromagnetic resonance (FMR) in the range 2–300 K. Variations of FMR parameters allow concluding that the coupling strength increases with temperature. This is in agreement with predictions of the theories considering pure tunneling mechanisms and contradicts the model of a resonant assisted tunneling related to defects in the insulator. The temperature dependence of the FMR linewidth shows the line narrowing under the sample heating. This may be due to the additional mechanism associated with the coupling. [ABSTRACT FROM AUTHOR]

The structure and magnetic properties of lithium iron pyrophosphate, i.e., Li2Fe3(P2O7)2 or LiFe1.5P2O7, synthesized using a facile metal acetate approach for application in lithium-ion batteries, are investigated in detail. The high-resolution transmission electron microscopy, selected area electron diffraction, and x-ray diffraction measurements indicate that Li2Fe3(P2O7)2 is crystallized in the monoclinic structure, without any indication of crystallographic defects such as dislocations or misfits, and exhibit smooth surface morphology. The evaluated lattice parameters are a=0.698 76 nm, b=0.812 36 nm, c=0.964 22 nm, and β=111.83° (P21/c space group). Infrared spectroscopic measurements indicate the presence of P2O7 groups, which are formed by the two PO4 tetrahedral groups connected together. The magnetic measurements indicate that Li2Fe3(P2O7)2 is a weak antiferromagnetic material with TN=20 K exhibiting a Curie constant Cp=3.38 emu K/mol per Fe ion and a negative value of the Weiss temperature (Θp=-15 K). The absence of higher valence state Fe impurities and antiferromagnetic interactions due to the greater distance between two equivalent magnetic ions, which vanishes the Fe–O–Fe superexchange interactions, is confirmed. [ABSTRACT FROM AUTHOR]

While nanosized ferromagnetic particles could poison the performance of the Li batteries containing phospho-olivine, the carbon-film coating the LiFePO4 particles has a beneficial effect on cycling life of the cells. In this paper, we present the properties of the carbon layer deposited at the surface of the LiFePO4 grains. Characteristics of the carbon layer are analyzed using scanning electron microscopy, high-resolution transmission scanning electron microscopy, Fourier transform infrared, and Raman scattering (RS) spectroscopy. The carbon deposit characterized by RS spectroscopy is hydrogenated with very small hydrogen/carbon ratio, so that it belongs to the family of the amorphous graphitic carbon. The carbon deposit is similar to that obtained by pyrolysis technique at high temperature. It is expected to have the same properties (small hardness, high electronic conductivity) that favor both the Li diffusion from the LiFePO4 bulk and the charge-discharge rate of the cell. A model for the Li-ion transport throughout the coating is given. [ABSTRACT FROM AUTHOR]

We report on a study of thin (500–4000 Å) and ultrathin (100–500 Å) yttrium–iron–garnet (YIG) films deposited onto amorphous quartz substrates by the pulsed laser deposition technique. The growth conditions of well-formed polycrystalline films have been determined. The crystalline structure and the magnetic behavior of the films are strongly influenced by the processes occurring on the film–substrate interface. Primarily, a strain due to the difference in thermal expansion coefficients of the film and substrate induces a uniaxial anisotropy. Another source of strain are lattice distortions due to oxygen vacancies. The results obtained from x-ray diffraction analysis, magneto-optical, superconducting quantum interference device, vibrating sample magnetometer, and ferromagnetic resonance (FMR) measurements indicate that films thicker than 200 Å can be approximated by a two-layer model. One of the layers with a highly distorted structure and low magnetization is located near the surface. The other one, the upper layer, can be estimated as an almost perfectly formed YIG. Their relative thickness determines the magnetic and FMR behavior of the samples. An equilibrium direction of the magnetization along the film normal has been found to occur in the ultrathin films with the lowest magnetization where the perpendicular anisotropy energy exceeds the dipolar energy. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the magnetic properties at room temperature of Fe1-xGax (FeGa) epitaxial thin films grown on ZnSe/GaAs(100) for 0.14≤x≤0.29 range concentration, and film thicknesses, d = 36 and 72 nm. The study was performed by means of magnetometric measurements and magnetic force microscopy scans. Increasing x promotes the loss of the four-fold magnetic-crystalline anisotropy associated to an Fe-like behavior, which is lost completely above x = 0.20. Stripe domains with rotatable anisotropy are observed even in samples in which the theoretical conditions for stripe appearance are not completely fulfilled. An unexpected 'saw-tooth' stripe structure has been found under certain conditions. [ABSTRACT FROM AUTHOR]

The results of optical absorption experiments on proton- and deuteron-implanted indium phosphide are reported. It is shown that these implantations produce two types of hydrogen- (or deuterium-) related defects. These defects also exist in as-grown material not intentionally doped with hydrogen. The nature of these two defects is discussed. [ABSTRACT FROM AUTHOR]