bluuebunny/crossref_metadata_2025 huggingface dataset:
Longitudinal bias method using a long distance exchange coupling field in tunnel magnetoresistance junctions
Authors: Nakashio E. , Sugawara J. , Onoe S. , Kumagai S.
Published: July 2002

A longitudinal bias method using a long distance exchange coupling field, which was observed between an IrMn layer and a NiFe/CoFe bilayer across a Cu interlayer, has been investigated for magnetic read heads. The exchange coupling field could be changed in the layered film consisting of Ta/NiFe/Cu(0, 1.0, 1.5, 2.0 nm)/IrMn. In order to align the easy axes of the free and pinned ferromagnetic layers perpendicular to each other, a two-step annealing process was carried out to rotate the easy axis of the free layer in the tunnel magnetoresistive read heads. The angle between the two easy axes was 72° in the junction after an optimized two-step annealing. The tunnel magnetoresistive readers, which have a resistance×area product of about 2000 Ω μm2 and a tunnel magnetoresistance ratio of about 15%, showed an output voltage of 9080 μVp−p and 2270 μVp−p/μm, which was comparable to that of a typical giant magnetoresistive reader for a 7.5 Gbits/in.2 hard disk drive.
Mixing and available potential energy in stratified flows
Authors: Tseng Yu-heng , Ferziger Joel H.
Published: July 2002

Mixing plays an important role in atmospheric and oceanic flows. It occurs on the small scales, is due to molecular diffusion, and is irreversible. On the other hand, stirring is a kinematic process that enhances mixing but is reversible. Budgets of the available potential energy, which require that the reference potential energy be computed, are used to study these processes. We develop an approach for calculating the available potential energy from the probability density function that is more efficient than existing methods, especially in two and three dimensions. It is suitable for application to both numerical simulations and experiments. A new length scale is defined which quantifies stirring and provides a measure of the strength of overturns resulting from stirring as well as their size. Simulations of lid-driven cavity flow and stratified homogeneous turbulent shear flow provide illustrations of the method. The new length scale is similar to Thorpe scale in lid-driven cavity flow and closely related to the Ellison scale in homogeneous sheared turbulence.
Monodispersed, nonagglomerated silicon nanocrystallites
Authors: Suzuki Nobuyasu , Makino Toshiharu , Yamada Yuka , Yoshida Takehito , Seto Takafumi
Published: July 2002

We demonstrate the synthesis of monodispersed, nonagglomerated silicon (Si) nanocrystallites, using an integrated process system composed of a unit for the formation of nanocrystallites by pulsed-laser ablation in an inert background gas, a unit for classification using a differential mobility analyzer (DMA), and a unit for deposition onto a substrate using a nozzle jet. The DMA has been designed to operate under pressures less than 5.0 Torr. We have synthesized nonagglomerated Si nanocrystallites of 3.8 nm mean diameter and 1.2 geometrical standard deviation on carbon thin films using this integrated process system.
Temperature dependence of excitonic absorption spectra in ZnO/Zn0.88Mg0.12O multiquantum wells grown on lattice-matched substrates
Authors: Sun H. D. , Makino T. , Tuan N. T. , Segawa Y. , Kawasaki M. , Ohtomo A. , Tamura K. , Koinuma H.
Published: July 2002

The excitonic properties of high-quality ZnO/Zn0.88Mg0.12O multiquantum wells grown by laser-molecular-beam epitaxy were investigated using temperature-dependent optical absorption spectra from 5 K to room temperature. The strength of exciton-longitudinal-optical (LO) -phonon coupling was deduced from the temperature dependence of the linewidth of the fundamental excitonic peak. Effective reduction of the exciton-LO-phonon coupling with decreasing the well width was observed, which is consistent with the confinement-induced enhancement of the exciton binding energy. The thermal shift of the lowest excitonic energy is independent of well width, indicating that the strain effect is negligible for this material.
Equilibrium and relaxation of particulate charge in fluorocarbon plasmas
Authors: Ostrikov K. N. , Kumar S. , Sugai H.
Published: July 2002

Charging of micron-size particulates, often appearing in fluorocarbon plasma etching experiments, is considered. It is shown that in inductively coupled and microwave slot-excited plasmas of C4F8 and Ar gas mixtures, the equilibrium particle charge and charge relaxation processes are controlled by a combination of microscopic electron, atomic (Ar+ and F+), and molecular ion (CF3+, CF2+, and CF+) currents. The impact of molecular ion currents on the particulate charging and charge relaxation processes is analyzed. It is revealed that in low-power (<0.5 kW) microwave slot-excited plasmas, the impact of the combined molecular ion current to the total positive microscopic current on the particle can be as high as 40%. The particulate charge relaxation rate in fluorocarbon plasmas appears to exceed 108 s−1, which is almost one order of magnitude higher than that from purely argon plasmas. This can be attributed to the impact of positive currents of fluorocarbon molecular ions, as well as to the electron density fluctuations with particle charge, associated with electron capture and release by the particulates.
Excitonic transitions in ZnO/MgZnO quantum well heterostructures
Authors: Coli Giuliano , Bajaj K. K.
Published: July 2002

In this work we present the calculation of the excitonic transition energies in ZnO/MgZnO quantum well heterostructures, accounting for the effects of the exciton–phonon interaction. The results of our calculations clearly show that the description of the electron–hole interaction by means of the static screened Coulomb potential and the use of the polaron masses for the electron and the hole leads to a poor agreement with available experimental data. On the other hand, including the exciton–phonon interaction in the calculation of the exciton binding energies, leads to the values of the excitonic transitions which agree very well with the recently published experimental data. A critical discussion of the choice of the physical parameters used in ZnO is also presented, which leads us to suggest a value for the heavy-hole band mass of 0.78m0 and a conduction-valence band ratio in the range 60/40–70/30.
Voltage reduction in organic light-emitting diodes
Authors: Hung L. S. , Mason M. G.
Published: July 2002

For practical applications, it is important to operate organic light-emitting devices at low voltages and low power consumption. When both the cathode and anode are perfectly injecting, low electron mobility in electron-transport materials, such as tris-(8-hydroxyquinoline)aluminum (Alq), becomes a limiting factor on voltage reduction. In this letter copper phthalocyanine (CuPc) is replaced for Alq as an electron-transport layer, and interfacial modification is utilized to enhance electron injection from the CuPc electron-transport layer into the Alq emissive layer. The outcome of this structure significantly facilitates electron transport through the organic materials, thus resulting in substantial reduction in operating voltages and power consumption.
Ultrafast carrier relaxation in radiation-damaged silicon on sapphire studied by optical-pump–terahertz-probe experiments
Authors: Lui K. P. H. , Hegmann F. A.
Published: July 2002

We investigate the relaxation of photogenerated carriers in radiation-damaged silicon on sapphire using a collinear optical-pump–terahertz-probe arrangement. Carrier densities greater than 1020 cm−3 are injected using 400 nm, 100 fs pump pulses, and the change in transmission of the terahertz-probe pulse is measured as a function of pump–probe delay. The time-dependent carrier density is deduced using a thin-film Drude model. A carrier mobility of 422±17 cm2/V s is measured, and single-exponential carrier relaxation times of 4 ps at low fluence and 6 ps at high fluence are observed.
Investigation of indium tin oxide/zinc oxide multilayer ohmic contacts to n-type GaN isotype conjunction
Authors: Lee Ching-Ting , Yu Qing-Xuan , Tang Bang-Tai , Lee Hsin-Ying , Hwang Fu-Tsai
Published: July 2002

The ohmic performance of the ITO/ZnO multilayer deposited on n-type GaN layer was investigated. The best thermal annealing condition achieved for ohmic contact was 5 min at 500 °C, in hydrogen ambient. The measured specific contact resistance was 3×10−4 Ω cm2. Ohmic formation mechanisms would be attributed to the ITO/n-ZnO/n-GaN isotype conjunction and the reduction conduction band offset due to the quantum confinement effects in the thin ZnO layer.
Microstructural evidence on electrical properties of Ta/Ti/Al and Ti/Ta/Al ohmic contacts to n-AlGaN/GaN
Authors: Lim S.-H. , Washburn J. , Liliental-Weber Z. , Qiao D.
Published: July 2002

Electrical properties and microstructures of Ta/Ti/Al and Ti/Ta/Al contacts to n-AlGaN/GaN heterostructure field-effect transistor structures were investigated using the transmission line method and transmission electron microscopy. The specific resistivity (5.3×10−7 Ω cm2) of Ta/Ti/Al contacts is much lower than that (5.1×10−4 Ω cm2) of Ti/Ta/Al contacts, for the same heterostructure and similar metallization. The contact resistivity was found to depend on the thickness of the AlGaN layer, interfacial phase, and interface roughness. The formation of interfacial phases by solid-state reactions with the metal layer during annealing appears to be essential for ohmic behavior on n-III-nitrides suggesting a tunneling contact mechanism.
Direct determination of the spin-orbit reactivity in Cl(2P3/2,2P1/2)+H2/D2/HD reactions
Authors: Dong Feng , Lee Shih-Huang , Liu Kopin
Published: July 2002

By exploiting two different Cl-beam sources and concurrently monitoring the concentrations of the two reagents [Cl(2P3/2) and Cl*(2P1/2)] and the H- or D-atom product, the spin-orbit specific excitation functions of the title reactions were determined. The exceptionally large nonadiabatic reactivity for Cl*(2P1/2)+n-H2, inferred in our previous differential cross section investigation, is now confirmed and quantified. The isotope effects for both the spin-orbit ground and excited reagents are also elucidated.
In situ
time-resolved optical studies of Al oxidation for magnetic tunnel junctions
Authors: Knechten K. , LeClair P. , Kohlhepp J. T. , Swagten H. J. M. , Koopmans B. , de Jonge W. J. M.
Published: July 2002

Real-time in situ optical measurements were performed during the oxidation of ultrathin Al layers with a view to study oxidation kinetics and provide process control. The optical technique combines high temporal and spatial resolution with submonolayer sensitivity. In situ x-ray photoelectron spectroscopy measurements on the same samples are in good agreement, confirming the accuracy and sensitivity of the technique. For all thicknesses studied, the initial oxidation is extremely rapid, with quasilogarithmic behavior over the whole time scale. This technique could be utilized to study a wide variety of in situ reactive processes.
The transition from oxygen chemisorption to oxidation of ultra-thin Ni layers on Cu(111)
Authors: Domnick R. , Held G. , Witte P. , Steinrück H.-P.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) was used to study the initial stages of surface oxidation of pseudomorphic Ni monolayers on Cu(111). Oxygen was adsorbed at 150 K followed by annealing the sample to 300 K and 600 K, respectively. For oxygen coverages between 0.4 ML and 2.0 ML we find little change in the peak shapes of the O 1s XPS signal. The Ni 2p3/2 spectra change, however, drastically: the onset of the oxidation is marked by the appearance of a peak doublet shifted with respect to the peak of metallic Ni. Based on these spectra we find a minimum oxygen coverage of 0.7 ML necessary for the onset of oxidation. The oxidation is nearly complete after the adsorption of about 2.0 ML oxygen. The exposure of different Ni coverages (0.5–2.0 ML) to oxygen shows that oxidation takes place only in the top-most Ni layer.
Response properties and stability conditions in density matrix functional theory
Authors: Cioslowski Jerzy , Pernal Katarzyna
Published: July 2002

Expressions for the second-order energy variations in the density matrix functional theory (DMFT) are derived, resulting in a formalism for time-independent response properties (including absolute electronegativity and hardness) and stability conditions. A quadratically convergent scheme for a direct determination of natural spinorbitals and their occupancy numbers is developed and tested with the Goedecker–Umrigar and the exact two-electron functionals. The derivatives of the electronic energy with respect to the number of electrons are found to be very sensitive to the DMFT description of the exchange-correlation energy, providing a sensitive measure of accuracy that can be readily employed in testing and development of approximate functionals.
Low-frequency noise in Al0.4Ga0.6N-based Schottky barrier photodetectors
Authors: Rumyantsev S. L. , Pala N. , Shur M. S. , Gaska R. , Levinshtein M. E. , Adivarahan V. , Yang J. , Simin G. , Khan M. Asif
Published: July 2002

The low-frequency generation recombination and the 1/f noise in AlGaN Schottky barrier photodetectors with high (40%) Al fraction has been investigated under forward and reverse bias conditions. The activation energy of local level contributing to noise was found to be Ea≈1 eV. Depending on the forward current level, the noise from Schottky barrier or from the series resistance (contacts and/or base) predominates. The upper bound of the Hooge parameter in Al0.4Ga0.6N was estimated as α⩽10.
Static analysis of off-axis crystal film growth onto a lattice-mismatched substrate
Authors: Yamada A. , Fons P. J. , Hunger R. , Iwata K. , Matsubara K. , Niki S.
Published: July 2002

The interface construction of tilt growth that is sometimes observed for epilayer growth on a lattice mismatched substrate is modeled on the basis that tilt relieves misfit strain in the epilayer. In this model off-axis misfit accommodation is assumed to be due to only tilt dislocations or tilt dislocations combined with misfit dislocations. The average interval between successive dislocations which are formed along the interface can be estimated using the lattice units of both materials and that of the epilayer in another principal axis direction; the tilt angle can also be calculated geometrically. The tilt angle predicted by this model agrees well with experimental results for several examples of mismatched epilayer growth by molecular-beam epitaxy. The model suggests a method to grow a single domain of stress-free epilayer by using a substrate cut to an angle that allows off-axis fit to the lattice unit of the epilayer.
Deposition-order-dependent coercivity of CoNi/Gd bilayers
Authors: Altuncevahir B. , Koymen A. R.
Published: July 2002

CoNi films were grown on thin Gd layers at room temperature using dc magnetron sputtering. A large increase in the coercivity of CoNi/Gd bilayer, compared to the coercivity of CoNi film, was observed. The increase in coercivity was not significant when the order of film growth was reversed for the same thicknesses and identical deposition conditions. The large increase in coercivity is attributed to the pinning of the CoNi layer by the magnetically compensated ferrimagnetic alloy forming at the CoNi (top)/Gd (bottom) interface due to the diffusion of Co and Ni into Gd. Due to the large coercivity in this system, it can be used to pin soft ferromagnetic layers in spin valve structures.
Highly conformal ZrO2 deposition for dynamic random access memory application
Authors: Chang Jane P. , Lin You-Sheng
Published: July 2002

ZrO 2 is investigated in this work to replace SiO2 as the dielectric material in metal–oxide–metal capacitors in dynamic random access memory (DRAM) devices for its high dielectric constant, good thermal stability, excellent conformality, and large band gap. ZrO2 films were deposited on planar Si (100) wafers and patterned amorphous silicon cylinders by rapid thermal chemical vapor deposition process using a zirconium (IV) t-butoxide Zr(OC4H9)4 precursor and oxygen. At substrate temperature below 300 °C, no significant deposition was observed. At temperatures between 300 and 400 °C, the reaction is thermally activated with an activation energy of 29 kcal/mol, consistent with a β-hydride elimination mechanism leading to ZrO2 deposition. In this regime, one atomic layer of ZrO2 can be deposited after each alternating exposure to the precursor and oxygen, ideal for achieving conformal coverage of ZrO2 over high aspect ratio features. At temperatures above 400 °C, the deposition rate is less sensitive to temperature with an apparent activation energy of 4 kcal/mol due to decomposition and desorption of the precursors. Stoichiometric, uniform, and amorphous ZrO2 was obtained at all temperatures investigated. Highly conformal step coverage of the deposited ZrO2 was achieved on 300 nm×600 nm cylindrical features with an aspect ratio of 4. The dielectric constant of ZrO2 achieved in this work ranges from 15 to 18 depending upon process conditions and small capacitance–voltage hysteresis is observed, ideal for DRAM application.
Optical actinometry of Cl2, Cl, Cl+, and Ar+ densities in inductively coupled Cl2–Ar plasmas
Authors: Fuller N. C. M. , Herman Irving P. , Donnelly Vincent M.
Published: July 2002

Optical emission (OE) actinometry has been used to measure the absolute densities of Cl2, Cl, Cl+, and Ar+ in a high-density inductively coupled (ICP) Cl2–Ar plasma at 18 mTorr as a function of the 13.56 MHz radio frequency (rf) power and Ar fraction. The fractional dissociation of Cl2 to Cl increases with rf power, with the dissociated fraction increasing from 78% to 96% at 600 W (10.6 W cm−2) as the Ar fraction increases from 1% to 78% due to an increase in electron temperature. Emission from Cl+* and Ar+* originates primarily from electron excitation of Cl+ and Ar+ (and not excitation of Cl and Ar), making actinometric determination of Cl+ and Ar+ densities feasible. For powers exceeding 600 W, the neutral (Cl2 and Cl) to ion (Cl+ and Ar+) flux ratio is found to be strongly dependent on Ar fraction, decreasing by a factor of ∼3.0 as the latter is increased from 13% to 78%. This dependence can be attributed mostly to the decrease in Cl density and relatively little to the small decrease in the total positive ion density from 1.8×1011 to 1.4×1011 cm−3, over the same range. OE spectroscopy is also used to estimate the rate constant for the dissociative excitation of Cl2 to the Cl (4p2D0J′=3/2,5/2) excited state with emission at 822.2 nm, yielding ∼10−13 cm3 s−1.
Honeycomb domain structures in amorphous TbFe thin films
Authors: Sun Zhi-Gang , Zhang Zhen-Rong , Shen Bao-Gen , Han Bao-Shan , Shih Jhy-Chau , Chin Tsung-Shune
Published: July 2002

The magnetic properties and domain structure of amorphous TbFe thin films were studied by magnetic measurement and magnetic force microscope. Honeycomb domain structures were found in the as-deposited amorphous film. The domain structures were composed of many small white round dots embedded in the black matrix, which formed an irregular hexagonal pattern with some deformation. The average dot size was about 450 nm, with an average separation of 610 nm. The derived domain wall energy density γ and the exchange constant A were 0.95×10−2 and 1.79×10−11 J/m, respectively.
Nonlinear operation of GaInAs/InP-based three-terminal ballistic junctions
Authors: Shorubalko I. , Xu H. Q. , Maximov I. , Omling P. , Samuelson L. , Seifert W.
Published: July 2002

We report on nonlinear electrical properties of three-terminal ballistic junctions (TBJs) based on high-electron-mobility GaInAs/InP quantum-well structures. Nonlinear electrical transport behavior of the TBJs is found, and we show a correlation between this behavior and the linear regime of electron transmission in the devices. We also study device geometry effects on these electrical properties of the TBJs. Finally, we demonstrate room-temperature operation of the devices. The results obtained are compared with recent predictions by Xu [H. Q. Xu, Appl. Phys. Lett. 78, 2064 (2001)] and good agreement is found.
Well-width dependence of radiative and nonradiative recombination times in ZnO/Mg0.12Zn0.88O multiple quantum wells
Authors: Chia C. H. , Makino T. , Segawa Y. , Kawasaki M. , Ohtomo A. , Tamura K. , Koinuma H.
Published: July 2002

A set of ZnO/Mg0.12Zn0.88O multiple quantum wells (MQWs) with well widths, Lw, varying from 6.91 to 46.5 Å has been grown by laser molecular-beam epitaxy. We estimated the Lw dependence of the radiative and nonradiative recombination times of localized excitons at 5 K. Suppression of quantum efficiency can be avoided even in the MQWs having small Lws studied in this work. Effects of excitonic localization are discussed in order to explain the Lw dependence of radiative recombination time at 5 K.
GaN quantum-dot formation by self-assembling droplet epitaxy and application to single-electron transistors
Authors: Kawasaki Koji , Yamazaki Daisuke , Kinoshita Atsuhiro , Hirayama Hideki , Tsutsui Kazuo , Aoyagi Yoshinobu
Published: July 2002

GaN quantum dots were formed on an AlGaN/SiC substrate by gas-source molecular beam epitaxy using a self-assembling technique with Ga droplets. The photoluminescence properties of the quantum dots obtained at several crystallization temperatures were investigated. High photoluminescence intensity was observed from GaN quantum dots formed without a wetting layer at a crystallization temperature of 700 °C. Single-electron transistors formed using GaN quantum-dot islands exhibited Coulomb blockade phenomena with negative conductance at 2.7 K and a clear Coulomb staircase at 200 K.
Loading effect in SiGe layers grown by dichlorosilane- and silane-based epitaxy
Authors: Menon C. , Bentzen A. , Radamson H. H.
Published: July 2002

The evolution of the loading effect in Si1−xGex layers (0⩽x⩽20%) versus growth parameters has been investigated for selective and nonselective growth using silane- and dichlorosilane-based epitaxy. Various methods have been examined in order to reduce the loading effect, and their influence on the defect density will be further discussed. High-resolution x-ray diffraction and atomic force microscopy were applied as the main tools in these investigations. It is shown that SiGe epitaxy is strongly sensitive to the opening size on the patterned substrates. This dependence is affected by the chemistry of the deposition. This effect can be decreased by adding HCl to the gas mixture or decreasing the growth rate. Meanwhile, adding HCl during the growth of SiGe layers or using a low growth rate decreases the epitaxial quality of the layers. Depositing a Si seed layer prior to the growth of the SiGe layer reduces the loading effect without degrading the epitaxial quality of the layer.
A replica reference interaction site model theory for a polar molecular liquid sorbed in a disordered microporous material with polar chemical groups
Authors: Kovalenko Andriy , Hirata Fumio
Published: July 2002

We develop a replica generalization of the reference interaction site model (replica RISM) integral equation theory to describe the structure and thermodynamics of quenched-annealed systems comprising polar molecular species. It provides a successful approach to realistic models of molecular liquids, and properly allows for the effect of a quenched disordered matrix on the sorbed liquid. The description can be extended to an electrolyte solution in a disordered material containing charged chemical functionalities that determine its adsorption character. The replica reference interaction site model (RISM) equations are complemented with the hypernetted chain (HNC) closure and its partial linearization (PLHNC), adequate to ionic and polar molecular liquids. In these approximations, the excess chemical potentials are derived in a closed analytical form. We extend the description to a quenched-annealed system with soft-core interaction potentials between all species, in which the liquid and matrix equilibrium distributions are characterized in general by two different temperatures. The replica RISM/PLHNC-HNC theory is applied to water sorbed in a quenched disordered microporous network of atoms associated into interconnected branched chains, with activating polar groups grafted to matrix chains. The results are in qualitative agreement with experiment for water confined in disordered materials.
Nondispersive electron transport in Alq3
Authors: Malliaras George G. , Shen Yulong , Dunlap David H. , Murata Hideyuki , Kafafi Zakya H.
Published: July 2002

We have studied room temperature electron transport in amorphous films of tris (8-hydroxyquinolinolato) aluminum (III) (Alq3) with the time-of-flight technique. Nondispersive photocurrent transients indicate the absence of intrinsic traps in well-purified films. Exposure of the films to ambient atmosphere results in highly dispersive transport, indicating that oxygen is a likely candidate for a trapping site. The mobility was found to obey the Poole–Frenkel law. We use the correlated disorder model to determine an effective dipole moment for Alq3, and the corresponding meridional to facial isomeric ratio.
Tunneling criteria for magnetic-insulator-magnetic structures
Authors: Åkerman Johan J. , Slaughter J. M. , Dave Renu Whig , Schuller Ivan K.
Published: July 2002

The bias and temperature dependent resistance and magnetoresistance of magnetic tunnel junctions with and without intentional shorts through the insulating barrier were studied. Based on the experimental results, a set of quality criteria was formulated that enables the identification of barrier shorts. While the temperature and bias dependencies of the junction resistance and of the fitted barrier parameters are very sensitive to the presence of such shorts, the same dependencies of the magnetoresistance are surprisingly insensitive. Finally, junctions with a shorted barrier exhibit a dramatic increase in noise level and junction instability.
Influence of stress on nucleation field of CoCrPt perpendicular media
Authors: Platt C. L. , Howard J. K. , Roy A. G. , Laughlin D. E.
Published: July 2002

The influence of stress on the nucleation field was investigated for CoCrPt perpendicular media grown on a Ta/Hf buffer layer. The nucleation field increased to −1.5 kOe (second quadrant) following postdeposition annealing of a sample at 232 °C in air for 1 h. The residual in-plane stress for the annealed film was tensile, and in-plane x-ray diffraction showed that the a0 lattice parameter increased with increasing annealing time. It is believed that the change in stress was due to surface oxidation of the CoCrPt film. The increased stress anisotropy perpendicular to the plane of the film resulted in the increased nucleation field.
Accurate measurements of the pitch-angle scattering of beam ions
Authors: Heidbrink W. W.
Published: July 2002

The pitch-angle scattering rate of a dilute population of 75 keV deuterium ions is measured in a well-diagnosed, relatively quiet, magnetically-confined deuterium plasma. Neutral particle diagnostics detect the fast-ion density in velocity space following a short 10 ms pulse of injected beam ions. The data are compared to the classical theory of diffusion in velocity space caused by many, small-angle, Coulomb-scattering events. Within uncertainties of ≲15%, the data confirm the classical theory.
Selective growth of single InAs quantum dots using strain engineering
Authors: Lee B. C. , Lin S. D. , Lee C. P. , Lee H. M. , Wu J. C. , Sun K. W.
Published: July 2002

A method to achieve ordering and selective positioning of single InAs self-assembled quantum dots (QDs) has been developed. The selective growth was achieved by manipulating the strain distribution on the sample surface. The QDs are formed on predesigned mesas with added strain. Single dots were obtained on small mesas. Using this technique, two-dimensional single QD arrays have been achieved.
Combining IR spectroscopy with fluorescence imaging in a single microscope: Biomedical applications using a synchrotron infrared source (invited)
Authors: Miller Lisa M. , Dumas Paul , Jamin Nadege , Teillaud Jean-Luc , Miklossy Judit , Forro Laszlo
Published: July 2002

It has become increasingly clear that infrared microspectroscopy (IRMS) can be an extremely valuable analysis tool when determining the chemical composition of biological and biomedical samples. Frequently, fluorescence illumination is required for sample characterization, and is usually achieved on a separate and dedicated optical microscope. We report the development and use of a single microscope for concomitant fluorescence and synchrotron IRMS. This unique combination has been used to identify changes in the composition of newly remodeled bone after the onset of osteoporosis, misfolded protein structure in Alzheimer’s disease, and cellular changes in apoptosis.
Nanostructure of sol–gel films by x-ray specular reflectivity
Authors: Morelhão S. L. , Brito G. E. S. , Abramof E.
Published: July 2002

Recently, several studies have been carried out on sol–gel films for optical applications, mostly motivated by the quickness and low cost of the film preparation process. In order to preserve the coherence properties of the light, improvements in the current quality of such films are necessary as well as appropriated techniques for structural characterization and quality control. X-ray specular reflectivity could be one of such techniques, but it is limited by the complexity of the internal nanostructure of the films. In this work, we have developed a procedure to extract the exact density profile of sol–gel films, and applied it to analyze a sol–gel derived Er2O3 film.
Magnetic domain observations of freestanding single crystal patterned Ni2MnGa films
Authors: Pan Qi , Dong J. W. , Palmstrøm C. J. , Cui J. , James R. D.
Published: July 2002

Ni 2 MnGa thin films have been grown pseudomorphically on a 6-monolayer thick Sc0.3Er0.7As interlayer on GaAs(001) by molecular-beam epitaxy. They have a tetragonal structure (a=b=5.65 Å and c=6.18 Å) which is different from any of the known bulk phases. Magnetic measurements reveal Ni2MnGa to have an in-plane easy axis and a Curie temperature around 350 K. The magnetic properties of these films are given and compared to the corresponding measurements in bulk material. In contrast to bulk material, single crystal films have been predicted to exhibit exact austenite–martensite interfaces (without fine twinning of the martensite). Films have been patterned along the predicted interfaces using the conventional photolithography and reactive ion etching. The patterns are then released from the substrate by backside photolithography and selective wet chemical etching, to yield freestanding films. The martensitic transformation of the freestanding films has been observed slightly above the room temperature. Magnetic domain observations (by MFM) on the martensitic films are presented.
Effect of Zr substitution for rare earth on microstructure and magnetic properties of melt-spun (Nd0.75Pr0.25)12.5−xZrxFe82B5.5 (x=0–3) ribbons
Authors: Chen Z. , Smith B. R. , Brown D. N. , Ma B. M.
Published: July 2002

The effect of Zr substitution for rare earth on the microstructure and hard magnetic properties of melt-spun composition of (Nd0.75Pr0.25)12.5−xZrxFe82B5.5 (x=0–3) has been investigated. It has been found that the coercivity decreases monotonically with the increasing Zr content, from 12.0 kOe at x=0 to 10.1 kOe with x=3. Unlike the coercivity, the remanence first increased with Zr substitution, from 8.65 up to 8.93 kG at x=2, and then decreased with further Zr substitution. The energy product behaved similarly, increasing from 15.7 MGOe to a maximum of 16.5 MGOe at x=2. X-ray diffraction (XRD), transmission electron microscope (TEM), and thermogravimetric analyzer studies have shown that all the samples were primarily composed of the single R2Fe14B phase. No additional phase was observed with Zr substitution. The lattice constants and the Curie temperature of the 2:14:1 phase were found to decrease monotonically with increasing Zr content, suggesting that all the Zr is situated into the 2:14:1 matrix, occupying the rare earth sites. TEM and XRD studies also revealed a significant grain size reduction with Zr substitution. The average grain size was reduced from approximately 40 nm in Zr-free samples to about 20 nm in samples with x=2. The observed change in magnetic properties with Zr substitution is explained by the grain size reduction.
Phase separation induced by doping at Mn sites in Nd0.5Ca0.5Mn0.98Ga0.02O3
Authors: Li Run-Wei , Sun Ji-Rong , Li Qing-An , Wang Zhi-Hong , Zhang Shao-Ying , Cheng Zhao-Hua , Zhao Hong-Wu , Shen Bao-Gen
Published: July 2002

Magnetic properties of Nd0.5Ca0.5Mn0.98Ga0.02O3 were investigated systematically through magnetization and electron spin resonance (ESR) measurements. It was found that substituting 2% Mn with Ga reduces the charge ordering temperature and induces an additional peak in the magnetization–temperature curve at about 45 K. The magnetization and ESR measurements suggest that the spin-glass-like phase induced by doping Ga at Mn sites and antiferromagnetic charge-orbital-ordered phase coexist at low temperature, the paramagnetic phase and antiferromagnetic phase coexist when the temperature is between the Néel temperature and the charge-ordering temperature.
Effect of n-type modulation doping on the photoluminescence of GaN/Al0.07Ga0.93N multiple quantum wells
Authors: Haratizadeh H. , Paskov P. P. , Pozina G. , Holtz P. O. , Monemar B. , Kamiyama S. , Iwaya M. , Amano H. , Akasaki I.
Published: July 2002

The influence of Si doping on exciton localization in modulation-doped GaN/Al0.07Ga0.93N multiple-quantum-well structures has been studied by means of photoluminescence (PL) and time-resolved PL measurements. Surprisingly, the PL decay time is constant in the range 320–420 ps for all doping levels at 2 K in these samples, due to a strong localization of the holes. The temperature dependence of radiative as well as nonradiative lifetime has also been evaluated between 2 K and room temperature for different Si doping.
Magnetic entropy change and magnetoresistance in the LaFe11.375Al1.625 compound
Authors: Hu Feng-xia , Wang Guang-jun , Wang Jing , Sun Zhi-gang , Dong Cheng , Chen Hong , Zhang Xi-xiang , Sun Ji-rong , Cheng Zhao-hua , Shen Bao-gen
Published: July 2002

Large magnetic entropy change |ΔS| with a nearly temperature-independent magnitude over a wide temperature range (about 70 K span from ∼140 K to 210 K) was observed in an Fe-based NaZn13-type compound LaFe11.375Al1.625. Such a behavior of magnetic entropy change is ascribed to two closely spaced magnetic transitions. One at 181 K (TN) with second-order nature is from the paramagnetic to antiferromagnetic state; another at 140 K (TO) with first-order nature, temperature hysteresis ∼5 K, is from antiferromagnetic to ferromagnetic state. At the antiferromagnetic to ferromagnetic transition a large change of resistance was found, which can be induced as a function of temperature as well as field. The magnetoresistence under a field of 1 T reaches 3.4%.
Impurity effects on the two-phase isochoric heat capacity of fluids near the critical point
Authors: Kostrowicka Wyczalkowska A. , Anisimov M. A. , Sengers J. V. , Kim Y. C.
Published: July 2002

It has been generally assumed that the asymptotic critical behavior of real fluids can be characterized in terms of the same physical variables as that of the lattice gas. This assumption implies that, below Tc, the second derivative of the pressure with respect to temperature should asymptotically diverge like the isochoric heat capacity CV, while the second derivative of the chemical potential with respect to the temperature should remain finite at the critical point. The validity of this assumption has recently been questioned on the basis of an analysis of experimental two-phase CV data in terms of the so-called Yang–Yang relation. In this paper we show how such an analysis may be affected by the presence of a small amount of impurity as well as by other nonasymptotic deviations from lattice-gas symmetry. When corrections for a small amount of impurity are applied and allowance is made for the leading asymmetric Wegner correction, the experimental CV data are not inconsistent with previous treatments in which the second derivative of the chemical potential exhibits a cusplike singularity with a finite limiting value at the critical temperature.
Phase study of hydrogenation, disproportionation, desorption, and recombination treated Sm3(Fe,Co,V)29 alloy
Authors: Kwon H. W.
Published: July 2002

Sm 3 ( Fe,M ) 29 -type alloy with chemical composition of Sm9Fe65Co20V6 was hydrogenation, disproportionation, desorption, and recombination (HDDR)-treated, and the phases, in the recombined state, were analyzed by x-ray diffraction (XRD) and by thermomagnetic analysis (TMA). The alloy was disproportionated into a mixture of SmHx and α-Fe at high temperature under hydrogen gas. The disproportionated material was recombined into a mixture of Sm–(Fe,M) (M=Co and/or V) and α-Fe phases. The structure of the Sm–(Fe,M) phase was dependent upon the recombination conditions, and a detailed phase diagram showing the phase relationships in the HDDR-treated alloy has been established. The Sm–(Fe,M) phase in material recombined above 900 °C had the Sm2Fe17-type structure, and it exhibited the SmFe7-type structure when recombined at temperatures ranging from 700 to 850 °C. Recombination below 650 °C led to the SmFe3-type structure of the Sm–(Fe,M) phase. Curie temperatures of the Sm–(Fe,M) phases in the recombined material were significantly higher than those of the corresponding stoichiometric phases. It was suggested that the chemical composition of the Sm–(Fe,M) phases may be significantly different from that of the corresponding stoichiometric phases.
Neutron diffraction and Mössbauer studies on Fe1−xCr2S4(x=0.0, 0.04, 0.08)
Authors: Kim Sam Jin , Kim Woo Chul , Kim Chul Sung
Published: July 2002

Polycrystalline samples of Fe1−xCr2S4(x=0.0, 0.04, 0.08) have been studied with x-ray and neutron powder diffraction, Mössbauer spectroscopy, magnetization, and magnetoresitance (MR) measurements. Neutron diffraction patterns were obtained at various temperature ranges from 10 K to room temperature. Neutron diffraction on FeCr2S4 above 10 K shows that there is no crystallographic distortion and reveals antiferromagnetic ordering, with the magnetic moment of Fe+2 (−3.52 μB) aligned antiparallel to Cr3+ (2.72 μB). Mössbauer spectra shows asymmetric line broadening in the temperature range from 13 to 170 K and it is considered to be dynamic Jahn–Teller stabilization. The charge states of the iron ions are ferrous in character. With increasing Fe deficiency, the peak of maximum magnetoresistance of x=0.0, 0.04, and 0.08, occurred at 171, 174, and 186 K, respectively. The increasing temperature of the MR peak position is interpreted as due to an enhancement of activation energy.
Thickness and pore size dependence of coercivity for nanonetwork of iron produced by template synthesis
Authors: Tofail S. A. M. , Rahman I. Z. , Rahman M. A.
Published: July 2002

In this article we have studied thickness and pore size dependence of coercivity in iron films over the thickness range 1.875–45 nm at which Néel-type domain wall dominates. Nanonetwork of iron films were sputter deposited on commercial nanochannel alumina templates with nominal pore diameters of 20, 100, and 200 nm. Continuous films deposited on conventional glass substrates have also been studied in order to understand the effect of porosity on an otherwise nonporous film. While nanonetworks showed expected higher coercivity values than their continuous counterpart, it was found that the coercivity for nanonetworks was influenced by the thickness and the effective pore diameter of the deposited iron layer. Coercivity for conventional films on glass followed an inverse power law. For nanonetworks, thickness dependence of coercivity was complex and showed convolutions. An empirical thickness dependence of coercivity is proposed to explain the behavior of the thickness-varying coercivity for the nanonetworks.
Tunnel magnetoresistance in NiFe/TaOx/Al2O3/Co junctions with a thin TaOx layer
Authors: Dorneles L. S. , Sommer R. L. , Schelp L. F.
Published: July 2002

We have measured the magnetoresistance versus bias voltage (Vbias) in NiFe/TaOx/Al2O3/Co magnetic tunnel junctions. The TaOx layer was produced exposing a thin metallic Ta film (0.25 or 0.5 nm) to atmosphere before the deposition of Al2O3. The samples with 0.5 nm of Ta present a faster decrease of the magnetoresistance for increasing Vbias. They also present negative tunnel magnetoresistance values for Vbias larger than 250 mV. The results are discussed in terms of the oxidation of the Ta layers and may suggest quantum well state effects.
Readback model for double layer perpendicular recording media
Authors: Senanan K. , Xue Jianhua , Victora R. H.
Published: July 2002

A readback model is developed to study characteristics of perpendicular recording media with a soft underlayer. The model is applied to an experimental perpendicular recording media represented by Voronoi cells. The readback voltage from this model shows dependence both on the transition length and the fly-height separately. A roll-off curve produced from reading continuous, perfect arctangent transition media is compared to experimental results to obtain the transition length and the fly height. The calculated autocorrelation signal to noise ratio (SNR) from the model with Voronoi grain based media compared to those obtained from a particular example shows cluster forming with areas equal to about three times the grain size. This reduces the media SNR by about 4.7 dB. This readback model therefore allows SNR measurement to be converted to detailed understanding of the media structure.
High frequency dynamics of the soft underlayer in perpendicular recording system
Authors: Ju G. , van de Veerdonk R. J. M. , Tamaru S. , Crawford T. M. , Parker G. , Kubota Y. , Wu M. L. , Batra S. , Weller D. , Bain J. A.
Published: July 2002

In this study, we present time-resolved Kerr microscopy of the magnetization dynamics in response to the reversal of the write current in the soft magnetic underlayers (SULs) films used in dual layer perpendicular recording media. 20-nm-thick FeCoB films with a 5 nm Pd cap layer are positioned in contact with a single-pole-type high-speed perpendicular magnetic write head. The temporal responses of both in-plane and out-of-plane magnetization components of the SUL were simultaneously measured using the time-resolved Kerr effect. Precessional effects were observed in both magnetization components during the SUL switching, using a driving pulse current with rise-time of about 0.8 ns. The precessional frequencies range from 3 to 7 GHz, depending on the distance (x) away from the main pole. The present results highlight the importance of soft magnetic underlayer dynamics for high data rate perpendicular magnetic recording technology.
High moment FeRhN/NiFe laminated thin films for write head applications
Authors: Jiang Hai , Chen Yingjian , Chen Lifan , Huai Yiming
Published: July 2002

FeRhN(∼50 nm)/NiFe(∼5 nm) laminated thin films were deposited by dc magnetron sputtering. Upon optimizing the sputtering conditions, FeRhN/NiFe laminated thin films showed good soft magnetic properties, and a well defined uniaxial anisotropy: Bs∼20.3 kG, Hc<1.0 Oe, Hk∼5 Oe. It was shown that a NiFe underlayer improved the soft magnetic properties of a FeRhN layer more effectively than a NiFe top layer. The data obtained by x-ray diffraction study showed that the FeRhN film with a NiFe underlayer had a much stronger (110) texture, as compared to single layer FeRhN film. Columnar grain structure with size of about 10 nm was observed in FeRhN films by high resolution transmission electron microscope. It is believed that the modification of FeRhN microstructure by NiFe underlayer plays a more important role in improving the soft magnetic properties than the exchange coupling between the FeRhN and NiFe layers.
Remarkable improvement of coercivity in nanostructured TbMn6Sn6 powders produced by mechanical milling
Authors: Zhao Peng , Zhang Jian , Zhang Shao-ying , Zhang Hong-wei , Sun Zhi-gang , Shen Bao-gen
Published: July 2002

Nanostructured TbMn6Sn6 powders have been obtained by mechanical milling cast alloys and subsequent annealing at different temperatures. Powders were synthesized by milling for 1, 3, 5, and 8 h, and annealed from 573 to 773 K for 20 min. For the TbMn6Sn6 powders milled for 3 h and annealed at 723 K for 20 min, the highest coercivity, about 12 kOe is achieved at room temperature. The powder’s coercivities increase with decreasing temperature. The average grain size of the optimum powders is about 14 nm according to the x-ray diffraction patterns by using the Sherrer’s formula. The smooth and nearly square demagnetization curve suggests a very fine and uniform grain size, and the enhanced Mr/Ms (∼0.6) ratio indicates the existence of intergrain exchange interactions among the TbMn6Sn6 grains. The observed remarkable improvement of magnetic hardening in powders is believed to arise from the single-domain size in the powders.
Phase evolution, structure, and magnetic properties of Nd8.4Fe86Mo1.1B4.5 nanocomposite magnets
Authors: Cui B. Z. , Sui Y. C. , Sun X. K. , Xiong L. Y. , O’Shea M. J. , Zhang Z. D.
Published: July 2002

The relationship between the phase evolution, nanostructure, exchange coupling, and magnetic properties of Nd8.4Fe86Mo1.1B4.5 magnets prepared by melt spinning (MS) and mechanical milling (MM) are studied by x-ray diffraction, Mössbauer spectroscopy, positron lifetime measurements, high resolution transmission electron microscopy observations, and magnetization measurements. The maximum magnetic energy product (BH)max for MM magnets annealed at 650 °C or below are notably higher than those of the corresponding MS samples due to the larger amount of the Nd(Fe,Mo)7 present in the MS samples.
Distribution of nitrogen atoms inside epitaxial (110) FeTiN films
Authors: Ding Yunfei , Alexander Chester
Published: July 2002

Epitaxial (110) FeTiN films with varied nitrogen concentrations have been deposited, and the distribution of nitrogen atoms inside the lattices has been observed from the lattice distortion measured by both conventional and glancing-incidence x-ray diffraction. It was found that at low nitrogen concentration, nitrogen atoms tend to go to both (1) the interstitial positions between the (110) planes that are parallel to the film surface and to (2) the interstitial positions inside these (110) planes. At higher nitrogen concentrations (N>4 at. %), nitrogen atoms preferentially go to other sites. A relationship between the interstitial N positions and the magnetic anisotropy has been observed.
Effect of substrate bias and magnetic annealing on soft magnetic properties of rf-sputtered Fe49Co49V2 films
Authors: Du H. , Pan G. , Buckley R.
Published: July 2002

Fe 49 Co 49 V 2 films were deposited by rf sputtering with and without substrate bias. All the as-deposited films deposited with or without substrate bias exhibit high coercivity of ∼200 Oe. The effect of magnetic annealing on soft magnetic properties was pronounced for films deposited with a substrate bias of 300 V. Excellent uniaxial anisotropy films with easy axis coercivity of 10 Oe were obtained for bias-sputtered and magnetic annealed films. X-ray diffraction examination and energy dispersive analysis of x-ray suggest that the improvement of soft magnetic properties by bias sputtering and magnetic annealing is associated with the relaxation of film stress and the recrystallization during magnetic annealing and variation of the composition of film by substrate bias. The dependence of coercivity on grain size D of these films appears to follow the 1/D law.