Volume 87, Issue 9, 01 May 2000
- lasers, optics, and optoelectronics (pacs 42)
- plasmas and electrical discharges (pacs 51-52)
- structural, mechanical thermodynamic, and optical properties of condensed matter (pacs 61-68, 78)
- electronic structure and transport (pacs 71-73)
- magnetism and superconductivity (pacs 74-76)
- dielectrics and ferroelectricity (pacs 77)
- device physics (pacs 85)
- interdisciplinary and general physics (pacs 1-41, 43-47, 79, 81-84, 89-99)
- 44th annual conference on magnetism and magnetic materials
- semiconductors, tunneling, and micromagnetics
- symposium on magneto-elastic and shape memory alloys
- self-assembly in nanostructures
- modeling and analysis of hard magnets
- thermal relaxations and hysteresis modeling
- giant magnetoimpedance
- gmr, experimental
- electronic structure i
- electronic structure ii
- exchange biasing i: experiment and theory
- microwave materials
- recording: systems, theory
- cmr fundamentals and layered systems
- exchange biasing ii: domains and coupling
- spin dynamics i
- nanoparticle array characterization
- strongly correlated d- and f-electron systems
- spin electronics and gmr theory
- magnetic tunnel junction materials
- novel effects and models of tunneling
- soft magnetic materials
- novel intermetallic compounds
- r2tm17 and related compounds
- permanent magnet processing
- magnetic sensors and memory
- tutorial on challenges in magnetic recording
- recording head and materials i
- symposium on neutron scattering methods for the study of magnetism
- ultra thin films and alloys
- spin dynamics and relaxation
- micromagnetic modeling
- cmr: films, nanosystems, and other materials
- cmr in bulk perovskites
- nanoparticles and patterned films i
- nanoparticles and patterned films ii
- particulate media and metals
- media dynamics and modeling
- advanced recording media characterization
- read sensor materials
- multilayers i: coupling and anisotropy
- magneto-elastic and giant magnetoelastic materials i
- cmr: spin dynamics and magnetic transitions
- soft magnetic thin films
- electronic structure iii
- critical phenomena and frustration
- ultrathin films and surface effects
- instrumentation and measurement techniques
- novel magnetic materials including organic i
- novel magnetic materials including organic ii
- tunnel junction materials and physics
- symposium on kondo effect in restricted geometry and the heavy fermion state
- thin films: anisotropy and surface effects
- hard magnet particles and films
- head-medium interface and tribology
- bio/chemical magnetism
- ferrites i
- ferrites ii
- spin dynamics
- magneto-elastic and giant magneto-elastic materials
- new applications
- nanoparticle characterization
- recording media i
- magnetic memory
- exchange biasing iii: dynamics and relaxation
- magnetic semiconductors
- magnetic microscopy and imaging
- crystalline soft magnets and domains
- spin glasses and frustration
- numerical methods
- nanostructured hard magnets
- multilayers and thin films: metallic systems
- recording heads and materials ii
- symposium on imaging and magnetic reversal
- exchange biasing ii: structural effects
- symposium on magnetic technology for a single chip computer
- multilayers iii: films
- hard magnet structures
- cmr: thin films
- magneto-optical materials
- strongly correlated electron systems and superconductivity
- magnetic imaging and modeling
- multilayers and thin films
- recording media ii
- magneto-optic media
- microwave and millimeter wave devices
- power magnetics and magnetic fluids
- nanocomposite and high anisotropy media
- spin valve, amr, and hall effect
- novel nanoscale structures
- critical phenomena
- magnetic memory elements and sensors
- amorphous and nanocrystalline soft magnetic materials
- electronic structure iv
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
87(2000); http://dx.doi.org/10.1063/1.373043View Description Hide Description
Cerium-doped lithium niobate crystals are tested for holographic recording. A photochromic effect is observed in crystals doped with cerium and manganese. But two-center recording in the sample is not as effective as in iron and manganese doubly doped crystals. Photocurrent measurements in cerium and iron singly doped crystals indicate that the photovoltaic constant in the cerium-doped crystal is only one third of that of the iron-doped one. This is the main reason accounting for the low sensitivity of cerium-doped lithium niobate crystals. However, in the diffusion dominated case, i.e., for reflection geometry, cerium-doped lithium niobate may give a strong effect.
87(2000); http://dx.doi.org/10.1063/1.373030View Description Hide Description
The laser performance of the nonlinear crystal is assessed under Ti:sapphire laser pumping.Spectral properties related to infrared laser action such as the emission and absorption cross sections have been evaluated. Different crystallographic configurations have been used. Laser oscillation in the near infrared region has been observed along the crystallographic axis and along the type I phase matching direction for infrared to green generation. High slope efficiency in the infrared region (62%) and the possibility of low laser thresholds (14.7 mW) are demonstrated. By using a birefringent filter, broadband tuning from 1030 to 1070 nm is obtained in this system. In addition, 58 mW of green power by self-frequency doubling of infrared laser emission is achieved, revealing the potential of as a self-frequency-doubling material.
87(2000); http://dx.doi.org/10.1063/1.373031View Description Hide Description
We report on the laser oscillation of -doped yttriumscandiumgalliumgarnet around 1.2 and 3 μm under laser diode excitation. At 1208 nm output powers of up to 135 mW and slope efficiencies up to 7.6% with respect to the incident pump power were achieved at room temperature under continuous wave excitation. Quasicontinuous wave excitation led to laser emission at 2844 nm with pulse energies up to 10 m.J. The laser relevant spectroscopic data, i.e., lifetimes, absorption, and emission cross sections are given and the excited state absorptionspectrum is presented.
Interdependence of the electrical and optical properties of liquid crystals for phase modulation applications87(2000); http://dx.doi.org/10.1063/1.373032View Description Hide Description
The electrical capacitance and conductance of nematic liquid crystal(LC) cells were measured in combination with their optical properties as functions of applied voltage magnitude and frequency. A single experimental system was used in order to determine the correlation between these characteristics. These parameters are crucial for understanding and optimizing the performance of modal LC devices. Both ordinary and dual frequency LCs were investigated. For the latter type, Cole-Cole diagrams show a Debye type frequency dispersion in a limited range of 4–9 kHz. Also, a phenomenological theory of the measured parameters was developed and is in good agreement with experimental data. Two examples are discussed, and they illustrate the importance of taking into account the equivalent conductance of LC cells.
87(2000); http://dx.doi.org/10.1063/1.373033View Description Hide Description
A model, which generalizes the concept of keyhole formation due to sideways melt displacement, allows us to study the keyhole geometry as a function of the main operating parameters such as welding speed, laser incident intensity, or sample material. It is based on a drilling velocity whose combination with the welding velocity causes the inclination of the front keyhole wall. This front inclination is shown to be stationary and stable all along the front side. The penetration depth results from the product of this drilling velocity and a characteristic time defined as the beam diameter divided by the welding speed. By using a ray-tracing procedure, the dynamics and the complete keyhole geometry can be determined by taking into account the possible multiple reflections inside the keyhole and a simplified description of the closure process of the rear keyhole wall. It is shown that for usual conditions of laser irradiation, this rear keyhole wall cannot be stationary all along its surface and only an adequate laser intensity distribution can make it stationary. The interest of elongated focal spots or twin spots is then demonstrated. At high welding velocity the front wall is inclined and is composed of several layers resulting from the successive reflections. The rear wall fluctuates around an apparent equilibrium, and corresponding fluctuations occur at maximum penetration depth. At low welding speeds, the keyhole appears to be more symmetric as a consequence of the multiple reflections between the rear and the front keyhole walls.
87(2000); http://dx.doi.org/10.1063/1.373034View Description Hide Description
The polarization properties of highly directional electromagnetic beams generated by planar sources of different states of spatial coherence are investigated. Cross-spectral density matrix based on modifications of Gaussian Schell-model beams is used. Azimuthally symmetric beams and dipolar beams are considered. The electromagnetic fields are assumed to be completely polarized in the source plane. The variations of the degree of polarization in the transverse directions and in the axial direction are examined. The dependence of these variations on the degree of coherence of the source distribution is discussed. The depolarization effects of the fluctuations in the source distribution are found to be less severe for the dipolar beam as compared to that for the azimuthally symmetric beam.
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
87(2000); http://dx.doi.org/10.1063/1.373035View Description Hide Description
A novel plasmaimplantation technique performed in a low pressure steady state dc mode utilizing a grounded conducting grid on top of the wafer stage is presented. By numerically simulating the ion paths by the particle-in-cell method, it is observed that the ion paths are optimized for certain implant geometry. In the optimal configuration, the directional angle of the acceleration vector does not depend on the mass and charge state of the ions, and the ratio of the partial differential of the scalar potential φ along the radial and longitudinal directions remains constant for varying applied voltages. The retained dose and impact energy uniformity are totally determined by the ratio of the radius of the wafer stage radius of the vacuum chamber distance between the wafer stage and the grid and thickness of the wafer stage The optimal ratio is that is, suggesting a disk shape vacuum chamber, which is quite different from that of a conventional plasma immersion ion implanter. In addition to retaining the large area and parallel processing advantages of plasma immersion ion implantation(PIII), the implantation energy can be extended far beyond the limit of PIII as the technique obviates the use of the power modulator, which not only limits the implantation energy but also is the most expensive and technologically complex hardware component in a PIII system.
Measurements of the electron energy distribution function in molecular gases in an inductively coupled plasma87(2000); http://dx.doi.org/10.1063/1.373036View Description Hide Description
A tuned, cylindrical Langmuir probe has been used to measure the electron energy distribution function(EEDF) in atomic and molecular gases in an inductively coupled plasma. We have discussed the precautions necessary for making Langmuir probemeasurements in fluorocarbon plasmas. The ionic and neutral composition of the plasma is measured using mass spectrometry. While the EEDFs in argon are non-Maxwellian, the EEDFs in molecular gases are found to be approximately Maxwellian at low pressures (<20 mTorr) in the gases studied The EEDFs in argon–molecular gas mixtures change from Maxwellian to two-temperature distributions, as the fraction of argon is increased in the plasma. At higher pressures, the molecular gases exhibit EEDFs reflecting the electron collision cross sections of these gases. In particular, plasmas show a “hole” in the EEDF near 3 eV due to the resonant vibrational collisions. plasmas show a three-temperature structure, with a low-energy high-temperature electron group, a low-temperature intermediate-energy electron group, and a high-temperature high-energy tail. The fractional degree of dissociation in the and plasmas is below 0.1, with the parent molecules and molecular ions being the dominant species. The spatial variation of the EEDF in an oxygen plasma at low pressures (10–20 mTorr) is found to be consistent with the nonlocal theory.
87(2000); http://dx.doi.org/10.1063/1.373037View Description Hide Description
Color temperature change in a mercury-rare gas low pressure discharge has been investigated. Different pulse waveforms have been employed to increase the ratio of mercury upper level transitions with respect to the resonant 254 nm radiation. Low pressure fluorescent light sources were made with coatings consisting of a blue phosphor, sensitive to 365 nm ultraviolet radiation, blended with the standard 254 nm excited red/green phosphors. With a fast rise excitation waveform, a color temperature rise of as much as 1700 K was realized although at a cost of 26% in relative luminous efficacy. An improved scheme for greater color temperature change is proposed based on a phosphor that is excitable by the mercury 185 nm ultraviolet radiation but which does not absorb 254 nm radiation.
87(2000); http://dx.doi.org/10.1063/1.373038View Description Hide Description
Striations observed in a coplanar or a wedge-shaped planar ac-type discharges are reported in this article. The observed data indicate that the striations are related to ion charge waves of the wall generated by the self-sustained perturbations that originate from the force balance between the ion and the electron wall-charge clouds accumulated on the dielectric layers over the metal electrodes.
- STRUCTURAL, MECHANICAL THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER (PACS 61-68, 78)
87(2000); http://dx.doi.org/10.1063/1.373039View Description Hide Description
Depth distributions and species of defects were determined from measurements of Doppler broadening spectra of annihilation radiation and lifetime spectra of positrons for 6H-SiC implanted with 200 keV at a dose of The annealing behavior of an amorphous layer was divided into four stages. Stages I (100–500 °C) and II (500–1100 °C) were identified as the relaxation of amorphous networks and the agglomeration of open spaces owing to rearrangements of atoms, respectively. In states III (1100–1500 °C) and IV (1500–1700 °C), corresponding to the recrystallization of the amorphous layer, the mean size of the open volume of defects decreased with increasing annealing temperature; these defects were identified as open spaces adjacent to extended defects. Vacancy-type defects were found in the subsurface region (<100 nm) at high concentration even subsequent to an annealing at 1700 °C. The annealing behavior of defects in the specimens irradiated at elevated temperatures is also discussed.
87(2000); http://dx.doi.org/10.1063/1.373040View Description Hide Description
Acceptor and donor dopants affect the critical ratio for the change-over from interstitial to vacancy incorporation is the growth rate and G is the near-interface temperature gradient) in growingsilicon crystals. The boron effect (an increase in the critical is nicely accounted for by a simple mechanism of electronic shift in the equilibrium concentrations of the charged point defects at the melting point By fitting the theoretical curve to the experimental data, the ratio of the equilibrium concentrations of vacancy and self-interstitial is defined to be 1.3 at Given this the interstitial diffusivity and the two equilibrium concentrations at can then be specified. Beside the electronic shift, alternative mechanisms (interstitial impurity component, impurity pairing to vacancy) are discussed to provide a general formula for the impurity-induced shift in the critical
87(2000); http://dx.doi.org/10.1063/1.373041View Description Hide Description
Temperature dependent measurements of the electrical resistance have been employed to study structural changes in sputteredfilms. The pronounced changes of filmresistance due to structural changes enable a precise determination of transition temperatures and activation energies. Furthermore the technique is sensitive enough to measure the influence of ultrathin capping layers on the transformation kinetics. With increasing temperature the films undergo a structural change from an amorphous to rock salt structure around 140 °C and finally a hexagonal structure around 310 °C. Both structural changes are accompanied by a major drop of resistance. Applying the Kissinger method [Anal. Chem. 29, 1702 (1957)] the activation energy for crystallization to the rock salt structure is determined to be eV, and for the phase transformation to the hexagonal phase to be eV, respectively. A thin capping layer of leads to an increase of the first transition temperature as well as of the corresponding activation energy eV).
87(2000); http://dx.doi.org/10.1063/1.373042View Description Hide Description
We have examined the crystallographic structure of GaAs/InP interfaces obtained by wafer fusion following different procedures. Plan-view and cross-sectional transmission electron microscopy reveal that the interface is not only composed of a regular array of two sets of edge dislocations and is more complex than generally supposed. If a twist is created due to misalignment of the two substrates, the dislocations are not edge dislocations but also have a screw component. Dislocations for which the Burgers vectors have a component normal to the interface are also present. Those dislocations probably result from steps and some of them accommodate the tilt between the two substrates. Inclusions and voids as well as a low number of volume dislocations are present in all the samples. The observed volume dislocation density near the interface lies in the range and these volume dislocations may be associated with thermal mismatch. The origin of all these defects is discussed.
Glass-forming range of the Ni–Mo system derived from molecular dynamics simulation and generalized Lindemann criterion87(2000); http://dx.doi.org/10.1063/1.373044View Description Hide Description
By using molecular dynamics simulation with an n-body Ni–Mo potential, the relative stability of the Ni- and Mo-based solid solutions versus their amorphous counterparts is studied as a function of solute concentrations at 300 K. It is observed that the supersaturated terminal solid solutions have the maximum or critical solute concentrations of 21 at. % of Mo in Ni and 25 at. % of Ni in Mo, respectively, beyond which the solid solutions transform into an amorphous phase. As such, the glass-forming range of the Ni–Mo system is derived to be 21–75 at. % of Mo. The computed critical solute concentrations are compared with those predicted by the generalized Lindemann melting criterion as well as with those revealed by ion-beam mixing/solid-state interdiffusion reaction experiments of Ni–Mo multilayered films.
87(2000); http://dx.doi.org/10.1063/1.373045View Description Hide Description
An in situx-ray study up to 137 GPa using a diamond anvil cell combined with synchrotron x-rayradiation has revealed three new phase transitions in MnO. The rhombohedral distortion from a structure starts at about 30 GPa. The volume compression curve of the distorted phase is in good agreement with recent shock compression experiments, suggesting the possibility of a paramagnetic–antiferromagnetic transition associated with the increase of the Néel temperature A drastic change in the x-ray pattern was observed at about 90 and 120 GPa. The transition pressure of 90 GPa is also consistent with shock compression data. The phases above 90 GPa were expected to be metallic based on the highly reflective nature of the sample. The crystal structure of the high pressure phase above 120 GPa was successfully explained by a (NiAs) structure as expected based on recent first principles calculations.
87(2000); http://dx.doi.org/10.1063/1.373046View Description Hide Description
The generation of Frenkel defects (a self-interstitial and a vacancy) in heavily As doped Si is investigated theoretically based on first-principles total energy calculations. We find that it is much easier to generate a self-interstitial and a vacancy close to substitutional As atoms than in pure Si, due to the lower energy cost. The As atom binds strongly with the vacancy, but does not bind with Si self-interstitial and other As atoms. We have considered several different reactions such as and The theoretical results are in good agreement with experimental observations.
87(2000); http://dx.doi.org/10.1063/1.373047View Description Hide Description
The room-temperature dc conductivity is used to monitor the damage and structural modifications induced by swift heavy ion irradiations in yttriumirongarnet or YIG) epitaxial layers doped with calcium (CaYIG) or silicon (SiYIG), with a variable conductivity due to a variable degree of compensation, and amorphous YIG layers. Irradiations are performed with heavy ions in the 0.8–6 MeV amu−1 energy range, in the electronic slowing down regime, with an electronic stopping power ranging between 7 and 41 MeV μm−1 above the amorphous track formation threshold (4.5 MeV μm−1) in this low-ion velocity range. A conductivity decrease versus ion fluence is found in the case of the high-conductivity uncompensated epilayers whereas an increase occurs for the low-conductivity compensated ones, either p-type (CaYIG) or n-type (SiYIG). These results are discussed by considering the competing effects of disorder on the carrier density and mobility in the case of compensated and uncompensated semiconductors. In both cases, the low-fluence data display a plateau at around the same conductivity value corresponding to the amorphous YIG above an amorphous fraction around 50% regardless of the ions. All the high-fluence data exhibit a power-law behavior without saturation, above a threshold fluence decreasing with increasing amorphization cross section (A). These results are interpreted by the formation of amorphous tracks and of a more conducting nanophase after recrystallization of the tracks under ion impacts. All the data are rescaled versus the product of A times fluence (φ) where amorphization dominates for whereas recrystallization dominates for However, significantly larger A values than the ones previously determined from the RBS-channeling data are derived from a mean-field analysis of the low-fluence conductivity data with a 2D Bruggeman model. These deviations are ascribed to a contribution of the crystalline track halos where internal stresses are accumulated due to the atomic density difference between the crystal and amorphous phase. A simple phenomenological approach of the amorphization and recrystallization processes is proposed on the basis of two kinetic rate equations with a recrystallization cross section (S) at least one order of magnitude smaller than A. These S values are in agreement with a thermal spike model assuming vaporization of the amorphous YIG phase along the ion path. At such high temperatures in the ion tracks, the garnet phase may decompose into a more conducting nanocrystalline phase. Finally, an law for the thermal dependence of conductivity at low temperature is found in the nanophase like in the amorphous one, most probably because of the strong contribution of the disordered grain boundary cores in the conduction process.
Reduction mechanisms for defect densities in GaN using one- or two-step epitaxial lateral overgrowth methods87(2000); http://dx.doi.org/10.1063/1.373048View Description Hide Description
A transmission electron microscopy study of the reduction mechanisms for defect densities in epitaxial lateral overgrown (ELO)GaN films is presented. In the standard one step ELO, the propagation of defects under the mask is blocked, whereas the defects in the window regions thread up to the surface. We propose an alternative two step ELO method. In a first step, dislocations close to the edge of the (0001) top facet bend at thereby producing a drastic reduction in the density of defects above the window. After the coalescence, induced by lateral growth in a second step, dislocations are mainly observed in the coalescence boundaries. The density of defects is decreased to over the entire surface and areas nearly 5 μm wide with dislocations between the center of the windows and the coalescence boundaries are obtained.
Shear moduli of polystyrene thin films determined with quartz crystal resonators in the sandwich configuration87(2000); http://dx.doi.org/10.1063/1.373049View Description Hide Description
We report on the determination of the shear compliance of spin-cast polystyrene films with quartz crystal resonators. The films with a thickness between 10 and 93 nm were sandwiched between the quartz blank and an overlayer of aluminum, which had been evaporated on top of the polymer film in order to enhance the shear stress. The shear compliance was inferred from the frequency shifts on the various overtones. For ease of analysis, the film was placed directly onto the quartz blank with no electrodes underneath, the oscillation being excited externally across an air gap. The derived shear moduli are consistent with the literature values for the bulk material determined from ultrasonic spectroscopy. However, they depend on thickness. For films thicker than 10 nm the apparent compliance decreases with film thickness, which presumably is a consequence of surface roughness. The 10 nm specimen had a much increased compliance, which we attribute to a less dense packing of the polymer chains caused by geometric confinement.