摘要:

Small (30 A˚ diameter) spinel iron oxide particles which form the protective layer on acicular iron metal particle recording media are found to exhibit no observable magnetization. The typical experimental decrease in sample magnetization for several media specimens is about 3% from 5 to 300 K. This change is the same as the thermal decrease in metal core magnetization, determined by Mo¨ssbauer spectroscopy. The predicted decrease in sample magnetization assuming normal spinel oxide behavior is typically about 20%. Two intraparticle models for reduced magnetization in oxide nanoparticles are found to be incompatible with the magnetization data. An interparticle ‘‘super‐spin‐glass’’ model is compatible with the magnetization data. The alignment between adjacent oxide nanoparticle moments is hypothesized to depend on the particular sublattices in contact, since the sublattice exchange interactions are all antiferromagnetic. An inverse particle size dependence is thus expected, in agreement with spinel oxide magnetization reduction and other phenomena seen in a wide range of studies on larger oxide particles and films. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359166

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Two charge states are observed for Ru in doped PbTiO3crystals using electron paramagnetic resonance (EPR). The two paramagnetic charge states are +3 and +5. Thegtensors for the Ru3+and Ru5+defects were found to be near axial:g∥&bartil;1.155 andg⊥&bartil;2.188 for the Ru3+center andge∥&bartil;2.0045 andge⊥&bartil;3.959 for the Ru5+center. Analysis of the EPR spectra by crystal field theory indicates that the Ru3+ions are in tetragonally distorted (elongated) octahedral sites, strongly suggesting that they substitute for the central Ti4+ions in the perovskite lattice. Analysis of the Ru5+EPR spectra indicate that this defect is also in an octahedral site with a very large zero field splitting parameter. The results suggest that both Ru3+and Ru5+can substitute for Ti4+in the perovskite lattice with self‐compensation. Last, it is found that optical illumination of the crystals with band‐gap light at low temperatures decreases the density of both Ru centers. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359167

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A study of the dielectric‐loss curves of cubic and octahedral AgBr emulsions is presented. Special attention is paid to the occurrence of multiple loss peaks. Several analytical Debye‐like forms were used to fit the experimental curves. The effect of several experimental parameters such as crystal habit, edge length, temperature,pAg, triazaindolizine addition, etc. were examined. The meaning of the calculatedFmaxfrequencies and resulting activation energies is critically discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359168

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The optical‐absorption spectrum of Ho3+:Y3Al5O12(Ho:YAG) has been analyzed between 220 and 2160 nm under variable temperature conditions between 4 and 60 K. Measurements between 220 and 470 nm were obtained from a Ho:YAG crystal that contained 15 at. % holmium. A crystal containing 0.5 at. % holmium was used to obtain the spectrum between 470 and 2160 nm. Within the temperature and total spectral range, more than 1500 transitions due to Ho3+(4f10) were observed and attributed to transitions between individual crystal‐field (Stark) levels of the ground‐state multiplet5I8, and 49 different excited multiplet manifolds, 4f10(2S+1LJ), of trivalent holmium. From an analysis of these transitions it was possible to establish 419 of the 486 Stark levels predicted below 45 000 cm−1. This large number of experimentally characterized energy levels provided a suitable basis for a detailed analysis of the 4f10(Ho3+) electronic state structure in Ho:YAG. The analysis was carried out using a model Hamiltonian that assumesD2site symmetry for the Ho3+ions and incorporates all of the interactions known to have a significant influence on the 4fNelectronic state structure of trivalent lanthanide ions in crystalline hosts. The radial‐dependent parts of the interaction terms in the model Hamiltonian are represented in parametric form, and the resulting parameters are used as variables in fitting calculated‐to‐experimental energy‐level data.The parametric fits of energy‐level data yield a rms deviation of ∼11 cm−1between the calculated and observed locations of the 419 experimentally assigned levels (between 0 and 45 000 cm−1), and both the isotropic and anisotropic (crystal‐field) interaction parameters of the model Hamiltonian are well characterized by these data fits. Among the 50 4f10[SL]Jmultiplet manifolds represented in the energy‐level analysis, only three show major discrepancies between calculated and observed crystal‐field splitting energies. The three problematic multiplet regions are: (5G6,5F1), centered at 22 258 cm−1,5G5, centered at ∼24 040 cm−1; and (5G3,3L9), centered close to 28 950 cm−1. These same multiplet regions have posed problems in energy‐level analyses of Ho3+(4f10) in other systems. The possible influence of correlation crystal‐field (CCF) interactions on these multiplet manifolds is explored in the present study, but CCF effects appear to be small. Overall, the model Hamiltonian derived gives an excellent representation of 4f10(Ho3+) energy‐level structure in Ho:YAG, and the eigenvectors of this Hamiltonian should provide a satisfactory basis for future calculations and modeling studies of Ho:YAG optical and magneto‐optical properties. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359169

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The results of a low‐temperature (4.5 K) photoluminescence study of Te‐doped GaSb layers grown by liquid phase electroepitaxy are reported. A doubly ionizable native residual acceptor (A/A−) with shallow and deep levels is observed at 34 and 97 meV, respectively, another native acceptor level for GaSb (B) is seen at 54 meV, and two Te‐related acceptor levels (CandD) are found at 68 and 83 meV, respectively. In addition, a few Te‐related deep levels are also seen between 114–129 meV at higher Te concentrations. The relative dominance of each of these transitions depends on the degree of Te compensation and the incident excitation intensity. At low excitation intensities, the spectra are dominated by deep impurity levels and with increasing intensity the transitions associated with the shallow acceptors become more prominent. The limited data on the PL integrated intensity dependance on excitation intensity further confirms the nature of these transitions. And finally, we also present preliminary results of our PL studies on Te‐doped GaInAsSb alloys. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359170

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The surface layer of crystalline silicon implanted with 1.5 MeV Si ions with doses ranging from 1×1011to 1×1015Si+/cm2has been studied by Raman scattering. Raman line intensities, shapes and shifts have been used to investigate the defects in the near‐surface layer. Above doses of 1×1013Si+/cm2, Raman provides evidence for the presence of amorphous silicon islands within the crystalline structure. The phonon‐confinement model (PCM) which is based on the breakdown in wavevector selection rules due to scattering from finite domain sizes has been used to estimate the effective average distance between defects. The PCM has also been modified by introducing a term attributable to residual stress in the near‐surface layer. By fitting experimental Raman spectra with the modified PCM, the Raman line shifts due to stress effects are decoupled from those due to phonon confinement. The mechanisms for the stress creation are also discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359171

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The Raman spectra of diamond and chemical‐vapor‐deposition (CVD) diamond films in the UV have been excited within the diamond band gap at 228.9 nm for the first time. The lack of fluorescence in the UV‐excited Raman spectrum of diamond and CVD diamond films allows Raman spectroscopy to monitor the carbon‐hydrogen (C‐H) stretching vibrations of the nondiamond components of the CVD film as well as the third‐order phonon bands of diamond. The relative intensity of the C‐H stretching bands at ∼2930 cm−1to the diamond first‐order phonon band at 1332 cm−1is proportional to the atomic fraction of covalently bound hydrogen in the CVD diamond film. The third‐order phonon band intensity and frequency maxima are very sensitive to the size of the diamond crystallite. Its intensity decreases, and the maximum shifts to lower frequency as the size of the diamond crystallite decreases. It is shown here that UV Raman diamond measurements have significantly greater information content than visible Raman measurements. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359172

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We have studied band‐to‐band optical magnetoabsorption in a semiconductor quantum wire subjected to a transverse magnetic field. The magnetic field induces a blueshift in the absorption peaks and makes the linewidth narrower. Furthermore, it quenches photoluminescence and absorption in much the same way as an electric field in the quantum confined Franz–Keldysh effect (QCFKE). We call this the quantum confined Lorentz effect (QCLE), since it is the Lorentz force skewing the electron and hole wavefunctions in the quantum wire that causes the quenching. The QCLE has an advantage over the QCFKE in that it may be observed even in quantum wires with relatively leaky barriers. The other important difference is that while the QCFKE is accompanied by a redshift in the absorption or photoluminescence peak, the QCLE is accompanied by a blueshift. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359173

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A systematic investigation is presented of the polarized optical absorption of the Nd3+ion in LiNbO3and LiNbO3(MgO) crystals. Energy levels of Nd3+are identified and labeled by appropriate crystal quantum numbers &mgr;=1/2 or &mgr;=3/2, in agreement withC3local symmetry. The anisotropic Judd–Ofelt theory is applied to estimate spectroscopic parameters relevant for laser applications (radiative lifetimes and branching ratios from the4F3/2laser state). Fluorescence lifetimes are also determined to estimate the quantum efficiency from the4F3/2state: &eegr;=0.6 for LiNbO3:Nd and &eegr;=0.5 for LiNbO3(MgO):Nd. The effect of MgO codoping in the energy levels as well as in the spectroscopic parameters of the Nd3+ion is evaluated and discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359174

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this work, the influence of surrounding ambient atmosphere on the stability of electroluminescent (EL) porous Si (PS) diodes is examined. We have fabricated electroluminescent porous Si layers from anodic oxidation of (1) epitaxially grownp‐type layers onn‐type Si substrates; (2)n‐type substrates with Au/Pd contacts; (3)p‐type substrates with Au/Pd contacts. These structures are characterized using photoluminescence (PL), EL, and infrared (IR) spectroscopies, as well as scanning electron microscopy (SEM). In the case of the porous Si structures fabricated fromp‐njunctions, such structures yield orange emission with maxima near 620 nm upon the application of moderate applied voltages (3–7 V). For each type of diode, it is found that in strong oxidizing environments, EL intensity degrades completely within 30 min; in contrast, the integrated intensity remains essentially unchanged in the same time frame in the presence of a vigorous flow of inert gases such as nitrogen and argon. Infrared spectroscopic studies strongly suggest that electroluminescence degradation is related to porous silicon surface oxidation. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359550

出版商:AIP    

年代:1995

数据来源: AIP