bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical characterization of Ca1−xErxF2+x luminescent thin films
Authors: Hirsch L. , Barrière A. S. , Salardenne J. , Réau J. M.
Published: July 2002

Ca 1−x Er x F 2+x thin films, epitaxially grown on silicon substrates, present a 1.53 μm infrared luminescence line, which gives them an evident interest for optical communications. At room temperature, using an argon laser as the excitation source, the maximum of this emission is obtained in thin films for x=0.16, when it is observed for very low erbium contents in bulk CaF2:Er3+ single crystals. Thus, we can think that the Er3+ ion environment, which governs the self-quenching phenomena, differs in thin films from that in bulk single crystals. In this paper, the nature of the erbium doping centers and their spatial distribution into the host material are studied versus x by using complex admittance and thermally stimulated depolarization techniques. In thin films, it is shown that for x⩽0.01, the luminescent centers correspond to isolated Er3+–Fi− ion pairs in nn sites, which behave like dipoles, D1. Their activation energy is 0.68 eV. From x=0.01 to x=0.05 clustering begins to be observed and leads when x>0.05, to the formation of extended clusters, usually called D2a in the literature. So, the clustering process in thin films appears only for concentrations two orders of magnitude higher than in single crystals. Moreover, it is shown that, whatever x varying from 0.001 to 0.20, charges can be transferred between pairs or clusters. Then, the activation energy depends on the mean distance between these defects, and thus, on the erbium content: the dielectric response corresponds to a homogeneous distribution of the pairs or clusters into the bulk of the host matrix.
Reflection electro-optical measurements on electroluminescent polymer films: A good tool for investigating charge injection and space charge effects
Authors: Michelotti F. , Taggi V. , Bertolotti M. , Gabler T. , Hörhold H. H. , Bräuer A.
Published: July 2002

We present experimental results on the electro-optic response of electroluminescent polymer films, pertaining to the family of the poly-phenylene-vinylenes. The samples used in the experiments were obtained by sandwiching polymer films between chromium-gold and indium tin oxide electrodes. The technique that was adopted is based on the Teng and Man setup, that is usually adopted for the determination of the electro-optic Pockels coefficient in poled polymers. As the structure of the polymer used is not provided with polar groups and, moreover, is completely disordered and therefore centrosymmetric, as expected, only quadratic electro-optical effects were observed (Kerr), which are connected to the presence of third-order optical nonlinearities (χ(2)≡0, χ(3)≠0). The existence of charge injection and trapping in the samples, giving rise to built-in static electric fields in the polymer layers, is put into evidence. The experimental data show the presence of a third-order susceptibility for Kerr electro-optic effect which is comparable to values reported in literature, namely χ(3)(−ω;ω,0,0)=7.9×10−21 m2/V2 at the wavelength λ=0.6328 μm. The possibility that the measured signals are due to a mechanical compression of the films is taken into account and discarded on the basis of the experimental results. The technique can be usefully applied in order to perform charge injection and trapping measurements in electroluminescent devices. Moreover, the results show that in the case of measurements performed on poled polymers for χ(2) applications, the measured values for the electro-optic coefficient can be offset by a third-order contribution, even in those cases when usually one assumes it may be neglected. In the article, the main models reported in literature, describing the effects contributing to the electro-optic response, are critically reviewed. The measurement on the centrosymmetric polymer films were used in order to check those models and permit us to evaluate.
Planar semiconductor lasers using the photoelastic effect
Authors: Liu Q. Z. , Chen W. X. , Li N. Y. , Yu L. S. , Tu C. W. , Yu P. K. L. , Lau S. S. , Zappe H. P.
Published: July 2002

Planar separate-confinement, double-heterostructure, single-quantum-well photoelastic GaAs/AlGaAs lasers have been fabricated using a novel yet practical processing technique involving thin-film surface WNi stressors for waveguiding and ion implantation for isolation. A p++-GaAs contact layer regrown by chemical beam epitaxy has been used to improve the WNi ohmic contacts to the lasers. Even without bonding on heat sinks, these planar photoelastic lasers operate at continuous wave at room temperature. The lowest threshold is 29 mA for a cavity length of 178 μm and a stressor width of 5 μm. The internal quantum efficiency above threshold is 75%. The characteristic temperature is 114 K. The main waveguiding mechanism of the photoelastic lasers is determined to be weak index guiding with the beam waist in the junction plane measured 10 μm behind the end facet.
The angular dependence of the planar Hall effect in Co-Ni films
Authors: Jen S. U. , Huang K. P. , Lee J. C.
Published: July 2002

Co 100−x Ni x  (x=70, 75, 80, 90 at. % Ni) alloy films were made by the vapor deposition method. An easy axis could be defined by a deposition field parallel to either the length (L film) or width (T film) of the sample. The anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) hysteresis loops were simultaneously measured with a slowly sweeping field H. The angle θ between the hard axis and H could be varied. Domain patterns were also recorded at each stage of the magnetization or demagnetization process. Besides the usual AMR and PHE behaviors, we have observed the forms of a voltage “mountain,” like ΔVmr and ΔVph, superimposed on the normal curves at the switching field Hsw. ΔVmr is an even function of θ and ΔVph is an odd function. The well known magneto-transport theory based on the single-domain model is modified to adopt the real situation of the multidomain structure. We have been successful in explaining all the ΔVph data observed experimentally. In addition, from the angular dependence of ΔVph, we not only check the direction of the mean easy axis consistently, but also tell the degree of easy-axis dispersion in the film. It is found that the Co30Ni70 T film has the least dispersion and a sizable AMR ratio Δρ/ρ⊥=3.6% at room temperature.
The influence of a viscous fluid on the vibration dynamics of scanning near-field optical microscopy fiber probes and atomic force microscopy cantilevers
Authors: Kirstein Stefan , Mertesdorf Michael , Schönhoff Monika
Published: July 2002

The influence of a viscous fluid on the dynamic behavior of a vibrating scanning near-field optical microscopy fiber tip is investigated both theoretically and experimentally. A continuum mechanical description of a cylindric cantilever is used to calculate the resonance frequencies and the widths of the resonance bands. The linearized Naviers–Stokes equations are analytically solved and describe the interaction of the beam with the viscous fluid. The contribution of the liquid to the shift and the broadening of the resonance lines is summarized by two constants that can be derived from a master function and the kinetic Reynolds number. The theoretical values are compared with experimental data collected from an optical fiber which is used as a probe in a scanning near-field microscope. Agreement, with a relative error of less than 1%, is achieved. The theory is further developed for the application to atomic force microscopy cantilevers with a rectangular cross section. Experimental data taken from literature are in good agreement with the theory.
Information from probe characteristics in negative ion containing plasma
Authors: Vucelić M. , Mijović S.
Published: July 2002

The effect of negative ions on the probe characteristic is studied theoretically. The reliability of results obtained from probe characteristics is analyzed. The methods of determining the energy distribution functions of charged particles is analyzed as an inverse ill-posed problem. A Tikhonov’s regularization procedure for detection of negative ions from a probe characteristic is described. This method parallels the Dryvestein method with the key difference stemming from the fact that experimental determination of the second derivative of the probe characteristic is avoided. For a small ratio of negative ion to electron densities and quite plasma, the energy distribution, temperature and density for the electrons and negative ions can be estimated only from the probe characteristic and the errors in the experimental data. The model functions, simulating experimental energy distribution for the electrons and negative ions, are introduced to test applicability limitations of the method. In addition, the comparative advantages of the method are presented as well.
Grain refinement and growth instability in undercooled alloys at low undercooling
Authors: Mullis A. M. , Cochrane R. F.
Published: July 2002

In a previous paper [J. Appl. Phys. 82, 3783 (1997)] we presented an analysis of the stability of a dendrite against a small perturbation to the tip velocity. This model demonstrated that dendritic growth in pure metals and alloys would become unstable above some upper critical undercooling ΔT2*, while in alloys above a critical concentration, dendritic growth may also become unstable below a lower critical undercooling, ΔT1*. However, quantitative agreement between the example system studied, Ni–Cu, and the available data was poor. In this article we present an improved computational scheme which allows us to relax the assumption that dendrites propagate as isothermal paraboloids of revolution. This results in much improved agreement with the experimental data.
Density of band-gap traps in polycrystalline films from photoconductivity transients using an improved Laplace transform method
Authors: Studenikin S. A. , Golego Nickolay , Cocivera Michael
Published: July 2002

Slow relaxation of the photoconductivity over a period of days was studied in polycrystalline ZnO and TiO2 films prepared by spray pyrolysis. The phenomenon is described by a model involving deep sensitizing hole traps in the forbidden gap. The trap state distribution based on this model was calculated using an improved Laplace transform method. For ZnO and TiO2 films, the density of states was shown to have a peak-like distribution with a maximum near the lower third of the energy gap.
The annealing effects on ZnCdSe/ZnSe quantum wells and ZnSe/GaAs interfaces
Authors: Tu R. C. , Su Y. K. , Huang Y. S. , Chou S. T.
Published: July 2002

This study investigated the thermally induced interdiffusion in ZnCdSe/ZnSe quantum wells and the outdiffusion of Ga atoms from the GaAs substrate by photoluminescence (PL), secondary ion mass spectrometry (SIMS), and contactless electroreflectance (CER) spectroscopy. The quantum well structures grown by molecular beam epitaxy were thermally annealed at temperatures between 250 and 700 °C. According to our results, the 15 K PL main peak disappeared when the annealing temperature reached 550 °C and above for samples grown with a 0.1 μm ZnSe buffer (sample A). In contrast, the PL main peak disappeared when the annealing temperature reached 600 °C and above for samples grown with 0.85 μm ZnSe buffer (sample C). In addition, for sample A, two extra PL peaks around 2.0 and 2.3 eV were observed when the annealing temperature reached 500 °C and above; those peaks were observed only when the annealing temperature reached 700 °C for sample C. SIMS results indicated the interdiffusion of Cd in the ZnCdSe/ZnSe quantum well regions in both samples. However, only sample A revealed a strong outdiffusion of Ga atoms from the substrate into the epilayer side, but a weaker one in sample C. The main peaks in PL spectra and the intersubband transitions in CER spectra disappeared owing to the Cd interdiffusion in the wells and the defect-related transitions introduced by the Ga outdiffusion onto the epilayer side. Based on the SIMS, PL, and CER spectra results, a thicker ZnSe buffer layer can increase the thermal stability of ZnCdSe/ZnSe quantum wells grown on GaAs substrates because of its effectiveness in mitigating the outdiffusion of Ga atoms into buffer layers and the interdiffusion of quantum well regions.
High density plasma processing of nanostructured diamond films on metals
Authors: Catledge Shane A. , Vohra Yogesh K.
Published: July 2002

A new regime is reported for synthesis of nanostructured diamond films on Ti–6Al–4V alloy and molybdenum substrates using microwave plasma chemical vapor deposition (CVD) at high pressure (125 Torr) and high methane feed-gas concentration (5%–15% in a balance of hydrogen). These films are shown to be predominantly comprised of diamond nanocrystallites in a matrix of tetrahedral amorphous carbon. In contrast to standard CVD conditions, the high density plasma results in adhered films on the metal substrates even at substrate temperatures of 850 °C. These films are also significantly smoother than conventionally processed CVD diamond films and have a rms surface roughness of about 50 nm. The average grain size of these nanocrystalline diamond films is 13 nm as determined from x-ray diffraction data and the hardness of the films as determined from nanoindentation data is 90% that of natural diamond. Micro-Raman studies using 458 and 647.1 nm excitation indicate an increasing tetrahedral amorphous carbon content with increasing methane feed-gas concentration. The adhesion and mechanical properties of these films makes them attractive for a variety of tribological applications.
Thermal reversal of magnetization as “random walk” dynamics of a nonlinear oscillator
Authors: Safonov Vladimir L.
Published: July 2002

Coherent rotation of magnetization in a grain is represented as the dynamics of a nonlinear oscillator excited by random force. A stochastic differential equation is introduced to study the role of thermal fluctuations and the corresponding Fokker–Planck equation is obtained. It is shown that conventional Landau–Lifshitz–Gilbert damping is consistent with thermodynamics only for small deviations of the magnetization from the equilibrium state. An exact formula for the first passage time from the initial potential well is obtained.
Electronic noise in magnetic tunnel junctions
Authors: Ingvarsson S. , Xiao Gang , Wanner R. A. , Trouilloud P. , Lu Yu , Gallagher W. J. , Marley A. , Roche K. P. , Parkin S. S. P.
Published: July 2002

We have studied bias and magnetic field dependence of voltage noise in metallic magnetic tunnel junctions with areal dimensions on the order of 1 μm. We generally observe noise with Gaussian amplitude distribution and pure 1/f power spectra at low frequencies. The 1/f noise scales with bias voltage as V2. Two kinds of deviations from this low frequency behavior have been observed. One is at fixed magnetic field when the junction bias reaches above a critical value, the other occurs at a fixed bias when the external magnetic field brings the sample to certain magnetic configurations. In both cases the noise spectra become dominated by Lorentzian noise and in both cases we have observed two level fluctuators in the time domain. We attribute the bias dependent noise to charge traps in the tunnel barrier. The field dependent noise is associated with the switching of the magnetization direction of portions of the top electrode, which we believe to be reversible.
The effect of Pt interlayers on the magnetic and structural properties of perpendicularly oriented barium ferrite media
Authors: Zhuang Zailong , Rao Maithri , Laughlin David E. , Kryder Mark H.
Published: July 2002

Perpendicular barium ferrite (BaM) films with Pt interlayers were successfully fabricated. The magnetic and structural properties of BaM films were compared with BaM films without Pt interlayers. It was found that perpendicular c-axis orientation was greatly improved for films with Pt interlayers. Perpendicular remanent squareness is increased, and the c-axis dispersion angle Δθ50 is decreased with Pt interlayers in BaM films. Also the coercivity was found to increase for films with Pt interlayers. A relative increase in perpendicular nucleation sites over in-plane and/or random nucleation sites contributes to the improvement in perpendicular c-axis orientation. The increase in coercivity may be explained by a weakening of magnetostatic interactions among BaM grains, due to magnetostatic isolation by the nonmagnetic Pt interlayers and the interdiffusion between Pt layers and BaM layers during high temperature ex situ annealing.
Giant magnetoelastic response in MnAs
Authors: Chernenko V. A. , Wee L. , McCormick P. G. , Street R.
Published: July 2002

MnAs has been prepared from elemental powders using a modified ball-milling technique, followed by hot pressing and subsequent annealing. Giant magnetic field-induced uniaxial strains of up to 0.7% were observed in a temperature range above the Curie temperature (TC=305 K). Dilatometric and magnetization measurements showed that the strains were associated with the volume change accompanying a field-induced magnetostructural paramagnetic to ferromagnetic transition above TC. The occurrence of a tricritical point (TCP) in temperature-field space, where the two low temperature phases and the high temperature NiAs-type phase are identical was shown to be responsible for the decrease of the magnitude of both the maximum strain and magnetization change with increasing temperature while approaching the TCP along the line of the first-order transitions. The coordinates of the TCP are approximately Ttr=405 K and Htr=165 kOe.
Growth mechanism of hydrogenated amorphous silicon studied by in situ scanning tunneling microscopy
Authors: Flewitt A. J. , Robertson J. , Milne W. I.
Published: July 2002

In situ scanning tunneling microscopy has been used to study the evolution of the surface topography of the growing surface of hydrogenated amorphous silicon (a-Si:H) in order to understand its growth mechanism. The surface is found to possess an island-like structure and the island diameter is found to increase with increasing growth temperature. A Fourier analysis of the surface roughness has an exponent of i=1.17. A comparison of the roughness of films of different thickness gives a dynamic scaling exponent of β=0.28, but the films are not particularly self-affine in character. It is argued that the exponent i is not evidence of a viscous flow regime, but that nonstochastic growth of a random network occurs, caused by a preferential hydrogen abstraction at kink-like and step-like surface sites. A simple simulation of the topography is used to support this conclusion.
Energy-dispersive x-ray diffraction and Raman scattering of Zn1−xMnxSe bulk crystals at high pressure
Authors: Yang C. S. , Ro C. S. , Chou W. C. , Lin C. M. , Chuu D. S. , Hu J. , Huang E. , Xu J.
Published: July 2002

Energy-dispersive x-ray diffraction experiments were carried out to investigate the structure of phase transitions under high pressure. It was found that the zinc blende (B3) to rock salt (B1) phase transition pressures of Zn0.93Mn0.07Se and Zn0.76Mn0.24Se bulk crystals are found 11.8±1.5 and 9.9±0.5 GPa, respectively. The respective bulk moduli are 61.8±0.8 and 60.5±0.8 GPa. The pressure-induced zinc blende (ZB) to rock salt (RS) structure phase transition is interpreted as a signature of the semiconductor to metal transition for Zn1−xMnxSe. The above interpretation is further corroborated by the observation of the disappearance of the longitudinal optical phonon at the pressure where the ZB to RS structure transition occurs.
Tracer diffusion and mechanism of non-Arrhenius diffusion behavior of Zr and Nb in body-centered cubic Zr–Nb alloys
Authors: Herzig Chr. , Köhler U. , Divinski S. V.
Published: July 2002

Tracer diffusion of Zr95 and Nb95 has been measured in a set of β-Zr-Nb alloys (Zr-5.5 at. % Nb, Zr-16.3 at. % Nb, and Zr-28.1 at. % Nb) in a wide temperature range from 1035 to 1871 K. All experimental concentration profiles revealed a Gaussian-like behavior over two to three decades in logarithm of concentrations. Experimental data were analyzed in terms of the Manning’s random alloy model and the partial correlation factors were calculated as functions of temperature. The partial correlation factors vary smoothly over the whole temperature range, whereas the diffusivity shows a pronounced curvature of the Arrhenius plots. A correlation was found between the degree of deviation from the Arrhenius behavior of the diffusion coefficients of both components and the lattice dynamics features of the measured phonon dispersions in their dependence on the Nb content in these alloys. The temperature dependence of the isotope effect parameter E of Zr diffusion was measured in the Zr-5.5 at.% Nb alloy. E essentially increases with temperature and the energy transfer factor ΔK deduced for the alloy was found to be somewhat larger than the corresponding values for pure β-Zr, especially at low temperatures. The whole set of experimental data leads to the conclusion that only one diffusion mechanism—the vacancy mechanism—is operating in the whole investigated temperature range.
Grain oriented crystallization, piezoelectric, and pyroelectric properties of (BaxSr2−x)TiSi2O8 glass ceramics
Authors: Zhang Jianping , Lee Burtrand I. , Schwartz Robert W. , Ding Zhenya
Published: July 2002

Polar, nonferroelectric (BaxSr2−x)TiSi2O8 glass ceramics with highly oriented crystallites were prepared by a gradient temperature heat treatment technique. The crystallization mechanism and microstructures of (BaxSr2−x)TiSi2O8 glass ceramics were investigated by means of differential thermal analysis, x-ray diffraction and scanning electron microscopy, and the dielectric, piezoelectric and pyroelectric properties were investigated for various compositions. The results show that polar (BaxSr2−x)TiSi2O8 glass ceramics have a low dielectric constant and a high hydrostatic figure of merit dh×gh=∼2500. This high hydrostatic figure of merit, along with other unique characteristics, such as no aging or depoling problems and good stability at high temperatures, high pressure, and in harsh environments, makes (BaxSr2−x)TiSi2O8 glass ceramics attractive for use as hydrophones and high temperature infrared detectors.
Isothermal transient ionic current as a characterization technique for ion transport in Ta2O5
Authors: Mattsson M. Stro/mme , Niklasson G. A.
Published: July 2002

The Isothermal Transient Ionic Current (ITIC) technique was applied to β-Ta2O5 samples made by chemical vapor deposition. The mobility, conductivity, and number of protons in the material could be extracted from the measurements. Li ions were intercalated electrochemically into the β-Ta2O5 and it was found that the ITIC method could separate the contribution from proton and Li ion conduction in such a way that the mobilities of both species could be extracted. Furthermore, the lattice-gas model [A. J. Berlinsky, W. G. Unruh, W. R. McKinnon, and R. R. Haering, Solid State Commun. 31, 135 (1979)] was employed to describe the Li interaction process. This model showed that the Li ions tended to distribute uniformly, rather then to attract each other and form clusters in the material.
Dependence of growth and nanomechanical properties of ultrathin amorphous carbon films on radio frequency sputtering conditions
Authors: Lu W. , Komvopoulos K.
Published: July 2002

Ultrathin films of amorphous carbon (a-C) were deposited on Si(100) substrates by radio frequency (rf) sputtering using pure Ar as sputtering gas, rf power of 80–1000 W, and substrate bias voltage between 0 and −300 V. The films possessed a thickness of 6–95 nm, nanohardness of 12–40 GPa, and root-mean-square surface roughness of 0.15–32 nm, depending on the deposition conditions. Plasma parameters of the film growth environment were correlated to the deposition conditions to obtain insight into the phenomena responsible for changing the growth characteristics and nanomechanical properties of the a-C films. The surface binding energies of carbon atoms in the films were interpreted in terms of measured sputter etching rates due to energetic Ar ion bombardment at a kinetic energy of 850 eV. Higher etching rates were found for a-C films with higher growth rates and lower hardness. Ultrathin (10 nm) a-C films of maximum nanohardness (∼39 GPa) were synthesized at 3 mTorr working pressure, 750 W rf power, −200 V substrate bias, and 5 min deposition time. Results are presented to elucidate the effects of rf power, working pressure, and substrate bias on the quality of a-C films deposited by controlling the ion-current density, mean free path, and sheath voltages in the rf discharges. The latter are important parameters affecting the ratio of ion to atom fluxes and the intensity (power density) of ion bombardment on the growing film surface.
Visible electroluminescence from native SiO2 on n-type Si substrates
Authors: Yuan J. , Haneman D.
Published: July 2002

Visible electroluminescence (EL) has been observed from Ag (film or paste)/native SiO2/n-type Si structures. The diodes show good rectification behavior and the EL occurs only under reverse bias conditions when the top Ag electrode is negative. For p-type samples, a forward bias is required. The intensity of the EL is proportional to the diode current and its spectrum peaks at 620–640 nm. A model based on oxygen-related luminescent centers in the native oxide is proposed.
Spectroscopic polar magneto-optical Kerr rotation on Co/Pt interface
Authors: Train C. , Beauvillain P. , Mathet V. , Pénissard G. , Veillet P.
Published: July 2002

Spectroscopic magneto-optic (MO) Kerr rotation experiments have been performed on Pt/Co interfaces. This very sensitive experimental technique allows a precise chemical analysis of this interface. From these measurements, we deduced that the Co/Pt interface is formed of a disordered CoPt alloy layer and a spin polarized platinum atomic monolayer whose MO indices were calculated. In situ spectroscopic MO experiments have been performed during the Pt growth on the hcp Co (0001) layer. In situ postdeposition annealing were done on Pt/Co/Pt trilayers. In the 400–500 °C temperature range, the MO spectra show an important increase of the Kerr rotation, in particular around 370 nm, while the magnetic anisotropy goes through a maximum at 500 °C. These results are related to the formation of a CoPt alloy ordered along the (111) direction.
Characterization of all-chromium tunnel junctions and single-electron tunneling devices fabricated by direct-writing multilayer technique
Authors: Scherer H. , Weimann Th. , Hinze P. , Samwer B. W. , Zorin A. B. , Niemeyer J.
Published: July 2002

We report about the fabrication and analysis of the properties of Cr/CrOx/Cr tunnel junctions and single-electron tunneling transistors, prepared by different variants of direct-writing multilayer technique. In all cases, the chromium oxide tunnel barriers were formed in air under ambient conditions. From the experiments on single junctions, values for the effective barrier height and thickness were derived. For the all-chromium single-electron tunneling transistors, we achieved minimal junction areas of 17×60 nm2 using a scanning transmission electron microscope for the e-beam exposure on Si3N4 membrane substrate. We discuss the electrical performance of the transistor samples as well as their noise behavior.
Inverse magnetic force microscopy of superconductors in half-space geometry
Authors: Coffey Mark W.
Published: July 2002

A key step in an inversion algorithm to recover the penetration depth λ for a superconductor in the Meissner state from one-dimensional magnetic force microscopy (MFM) data is demonstrated numerically. The profile λ(z) is found as a function of distance z into a superconducting half space. The algorithm reduces the inversion to the solution of a nonlinear system of ordinary differential equations of first order. A precursor inverse Laplace transformation of the MFM force data as a function of height gives a kernel function K which provides the initial conditions for integrating the nonlinear system. The results show that the penetration depth can be reconstructed to increasing distances within the sample as the size of the truncation of the nonlinear system is increased. Analytic results for the force function for the examples are also presented.
Thickness dependent binary behavior of elongated single-domain cobalt nanostructures
Authors: Evoy S. , Carr D. W. , Sekaric L. , Suzuki Y. , Parpia J. M. , Craighead H. G.
Published: July 2002

We have studied the switching behavior of single-domain cobalt nanostructures using a combination of magnetometry and magnetic force microscopy. The elongated nanostructures are 80×140 nm wide, and range in thickness from 14 to 30 nm. Structures thinner than 20 nm form a single-domain binary system featuring two preferred orientations of the magnetization. An additional double-domain configuration becomes increasingly favorable as the thickness approaches 30 nm. The onset of this double-domain state agrees with previously reported numerical calculations. We also present a quantitative study of interparticle coupling in tightly packed arrays. The local dipolar field increases the squareness of the hysteresis loop as the interparticle separation decreases below 400 nm.
Microstructures and magnetic properties of Co–Al–O granular thin films
Authors: Ohnuma M. , Hono K. , Onodera H. , Ohnuma S. , Fujimori H. , Pedersen J. S.
Published: July 2002

The microstructures of Co–Al–O thin films of wide varieties of compositions are studied by transmission electron microscopy and small angle x-ray scattering (SAXS). In the superparamagnetic specimens, high resolution electron microscope images reveal that isolated spherical Co particles are surrounded by an amorphous aluminum oxide matrix. However, in the soft ferromagnetic films, the shape of the Co particles is prolate ellipsoidal. SAXS intensities from the soft magnetic specimens decrease inversely with the wave vector, q, in a low wave-vector region, while an interparticle interference peak is observed for the superparamagnetic specimens. The scattering profiles of the soft magnetic films imply that the Co particles have a cylindrical shape and are randomly oriented. The correlation between the magnetic properties and the microstructures is discussed.
High-pressure high-temperature annealing of ion-implanted GaN films monitored by visible and ultraviolet micro-Raman scattering
Authors: Kuball M. , Hayes J. M. , Suski T. , Jun J. , Leszczynski M. , Domagala J. , Tan H. H. , Williams J. S. , Jagadish C.
Published: July 2002

Visible and ultraviolet micro-Raman scattering was employed to monitor the high-pressure high-temperature annealing of Mg/P-implanted GaN films. The results illustrate the use of Raman scattering to monitor processing of GaN where fast feedback is required. Temperatures up to 1500 °C with nitrogen overpressures of 1–1.5 GPa were used during the annealing. The crystalline quality, the strain, and the free carrier concentration in the ion-implanted GaN films was monitored, averaged over the layer thickness and in a 40-nm-thin surface layer of the sample. Annealing temperatures of 1400–1500 °C were found to result in the nearly full recovery of the crystalline quality of ion-implanted GaN. No significant surface degradation occurred during the annealing. High nitrogen overpressures proved very effective in preventing the nitrogen out-diffusion from the GaN surface at high temperatures. Strain was introduced during the annealing. Changes in the free carrier concentration were studied.
Electromigration-induced drift in damascene and plasma-etched Al(Cu). II. Mass transport mechanisms in bamboo interconnects
Authors: Proost Joris , Maex Karen , Delacy Luc
Published: July 2002

We have discussed electromigration (EM)-induced drift in polycrystalline damascene versus reactive ion etched (RIE) Al(Cu) in part I. For polycrystalline Al(Cu), mass transport is well documented to occur through sequential stages : an incubation period (attributed to Cu depletion beyond a critical length) followed by the Al drift stage. In this work, the drift behavior of bamboo RIE and damascene Al(Cu) is analyzed. Using Blech-type test structures, mass transport in RIE lines was shown to proceed both by lattice and interfacial diffusion. The dominating mechanism depends on the Cu distribution in the line, as was evidenced by comparing as-patterned (lattice EM) and RTP-annealed (interface EM) samples. The interfacial EM only occurs at metallic interfaces. In that case, Cu alloying was observed to retard Al interfacial mass transport, giving rise to an incubation time. Although the activation energy for the incubation time was found similar to the one controlling Al lattice drift, for which no incubation time was observed, lattice EM is preferred over interfacial EM because it is insensitive to enhancing geometrical effects upon scaling. When comparing interfacial electromigration in RIE with bamboo damascene Al(Cu), with the incubation time rate controlling for both, the higher EM threshold observed for damascene was shown to be insufficient to compensate for its significantly increased Cu depletion rate, contrary to the case of polycrystalline Al(Cu) interconnects. Two factors were demonstrated to contribute. First, there are more metallic interfaces, intrinsically related to the use of wetting or barrier layers in recessed features. Second, specific to this study, the additional formation of TiAl3 at the trench sidewalls further enhanced the Cu depletion rate, and reduced the rate-controlling incubation time. A separate drift study on RIE via-type test structures indicated that it is very difficult to suppress interfacial mass transport in favor of lattice EM upon TiAl3 formation.
Magneto-transport and exchange biasing in La–Ca–Mn–O compositionally modulated ferromagnetic/antiferromagnetic multilayers
Authors: Panagiotopoulos I. , Christides C. , Niarchos D. , Pissas M.
Published: July 2002

Compositionally modulated structures consisting of La2/3Ca1/3MnO3 ferromagnetic (FM) layers and La1/3Ca2/3MnO3 antiferromagnetic (AF) layers, with layer thickness 1.5 nm ⩽tFM,tAF⩽6 nm, were grown on (100)LaAlO3 by pulsed laser deposition. Thermomagnetic and isothermal magnetic measurements reveal an exchange-biasing mechanism with the same blocking temperature TB≈70 K for all the examined combinations of tFM and tAF. The ratio of colossal magnetoresistance increases in multilayers with larger tFM, whereas the exchange-biasing field increases in samples with larger tAF. The independence of TB on tFM and tAF shows that the artificially induced La1−xCaxMnO3 (x≈0.5) compositional modulation at the interfaces is of critical importance for exchange biasing in La–Ca–Mn–O based multilayers.
A new estimation of the axial shielding factors for multishell cylindrical shields
Authors: Paperno Eugene , Koide Hiroyuki , Sasada Ichiro
Published: July 2002

A new, more accurate formula describing double and multiple-shell axial magnetic shielding is obtained as a result of numerical verification of existing estimations. A standard ANSYS® software package was used. Parameters of the numerical model are as follows. Two concentric, closed cylinders of equal thickness and constant permeability are considered. The thickness-to-diameter ratio of the outer cylinder is t/D2=1/100, its length-to-diameter ratio varies as L2/D2=3, 4, and 5, the ratio of the cylinders’ outer diameters varies as D1/D2=0.5, 0.6, 0.7, 0.8, and 0.9, a range of the ratio of the cylinders’ lengths is L1/L2=0.1–0.9, and a range of the relative permeability is μ =103–106. A significant disagreement between the existing estimations and between each of them and the numerical model is found. One of the examined algorithms is modified to improve its precision. A remarkable improvement in the accuracy of the new algorithm compared to both existing methods is achieved. On a basis of the new algorithm, a new formula describing multishell axial magnetic shielding is suggested.
Flash temperature induced magnetic degradation in high density magnetic recording
Authors: Yuan Zhi-min , Liu Bo , Wang Jiwen , Ding Jun
Published: July 2002

Slider disk impact, especially the impact with particles in between, generates flash temperatures at the contact area and creates thermal stress in the magnetic layer of disk media. By modeling the effective magnetic fields of the thermal stress and the thermal agitation, the flash temperature induced magnetic degradation is studied via micro-magnetic simulation. It is noticed that the recorded bit pattern can be fully erased near the area of the maximum thermal stress if the flash temperature approaches 673 K (grain size: 12 nm). The bit patterns with higher bit density corresponds to higher likelihood of data erasure if other conditions are the same.
Imaging mechanisms of force detected FMR microscopy
Authors: Midzor M. M. , Wigen P. E. , Pelekhov D. , Chen W. , Hammel P. C. , Roukes M. L.
Published: July 2002

We demonstrate spatial resolution of ferromagnetic resonance in a microscopic sample of YIG using ferromagnetic resonance force microscopy (FMRFM). Measurements were performed on a small single crystal YIG film grown on a GGG substrate, roughly rectangular in shape 20 μm×∼150 μm and 3 μm thick. The perpendicular and parallel force geometries of FMRFM, in conjunction with an external bias field both parallel and perpendicular to the film, were used to scan the sample. This enabled the detection of strong signals, even at atmospheric pressure and room temperature. The fundamental and higher-order magnetostatic modes were observed to have 26–29 Gauss separation. The intensity of these modes exhibited spatial variation as the magnetic tip was scanned over the sample, and this behavior is qualitatively explained by DE theory. An improved fabrication method for magnet on cantilever was employed, which yielded a spatial resolution of 15 μm. These results demonstrate the potential of FMRFM for investigating the spatial dependence of ferromagnetic resonance, and for studying the anisotropy fields and exchange coupling effects within multilayer films and small magnetic systems.
Biaxial exchange anisotropy in PtMn/Ni80Fe20(110) bicrystal films
Authors: Huang J. C. A. , Yu C. C. , Lee C. H.
Published: July 2002

By using molecular beam epitaxy PtMn/Ni80Fe20 bilayers were prepared on Mo(100) seeding layer on MgO(100) substrate. X-ray diffraction showed that the PtMn/Ni80Fe20 bilayers were mainly grown as (110) bicrystal films with 90° twin domains. The exchange anisotropy was studied by angular dependent magneto-optical Kerr effect. The results indicate that the PtMn/Ni80Fe20(110) bicrystal films possess biaxial exchange anisotropy with exchange field up to 130 Oe.
Pressure effects on antiferromagnetism in UNiAl
Authors: Mikulina O. , Kamarád J. , Lacerda A. , Syshchenko O. , Fujita T. , Prokeš K. , Sechovský V. , Nakotte H. , Beyerman W. , Menovsky A. A.
Published: July 2002

The temperature dependencies of the electrical resistivity ρ for current along and perpendicular to the c axis were measured on single crystalline UNiAl under various hydrostatic pressures. The ρ(T) curves at ambient pressure exhibit a Cr anomaly around the Neél temperature TN. Application of pressure causes a reduction of TN value. Linear extrapolation of low-pressure TN vs p data yields an estimate of a critical pressure for antiferromagnetism pc≈10 GPa. Measurements above 1 GPa, however, reveal a collapse of the antiferromagnetic (AF) ordering already below 3 GPa, although AF correlations seem to affect resistivity behavior ρ(T) in pressures up to 8 GPa. A sudden change of the ρ(T) curve character for i⊥c and T<TN indicates a pressure-induced change of magnetic ordering or fluctuations within the basal plane. The results are discussed in terms of the instability of the itinerant 5f-electron antiferromagnetism in UNiAl.
Intrinsic mechanism of nonlinear damping in magnetization reversal
Authors: Safonov Vladimir L. , Bertram H. Neal
Published: July 2002

The process of magnetization reversal in a fine ferromagnetic grain with a strong uniaxial anisotropy has been simulated for the case of an instantaneously applied reversal magnetic field. The “quasi” single-domain grain was considered as a system of 64 or 1000 subcubes coupled by exchange and dipole–dipole interactions. The system of Landau–Lifshitz equations was numerically solved without any phenomenological damping terms. The dynamic process is characterized by a rapid magnetization reversal accompanied by a decrease in the average magnetization. The system exhibits a nonlinear excitation of nonuniform magnetic oscillations (spin waves) driven by the uniform mode. We found limitations on magnetization reversal for small grain size and large applied field magnitude. The damping parameter has been obtained for small magnetization oscillations.
Behavior of μMAG standard problem No. 2 in the small particle limit
Authors: Donahue M. J. , Porter D. G. , McMichael R. D. , Eicke J.
Published: July 2002

For a uniformly magnetized rectangular particle with dimensions in the ratio 5 : 1 : 0.1, the coercive and switching fields in the (1,1,1) direction are determined to be Hc/Ms=0.057069478 and Hs/Ms=0.057142805. Previous micromagnetic computations of coercive and switching fields that did not approach these values for small particles are analyzed. It is shown that the disagreement was primarily due to a disparity in the method of calculating demagnetization energy. Corrected simulations are shown to agree with analytically determined values.
Optoacoustic infrared spectroscopy of soft tissue
Authors: Köstli Kornel P. , Frenz Martin , Weber Heinz P. , Paltauf Günther , Schmidt-Kloiber Heinz
Published: July 2002

Optical properties of soft tissue in the near infrared are determined using optoacoustic spectroscopy. The acoustic signals are generated with an optical parametric oscillator (OPO) having a tuning range from 1500 to 3500 nm. In order to record the acoustic wave on the same side as the exciting laser pulse (backward mode), an infrared transparent pressure transducer was developed. The effective attenuation coefficients of cartilage and chicken breast were determined in a range between 1860 and 1940 nm. The minimum absorption or effective attenuation coefficient that could be measured with the presented method was 10 cm−1, limited by the detector sensitivity of 1.68% signal change per bar. The maximum measurable coefficient was about 1000 cm−1, limited by temporal broadening of the acoustic signals due to the finite pulse duration (6 ns) of the OPO. Optoacoustic infrared spectroscopy is shown to be suitable for on-line noninvasive, in vivo tissue characterization.
Quasicontinuum effects in molecular excitation
Authors: Kyrölä E. , Eberly J. H.
Published: July 2002

We study a model where two discrete levels are coupled to a band of closely spaced levels forming a quasicontinuum (QC). This model describes how the initial-stage coherence in multiphoton excitation of polyatomic molecules interferes with the recurrent time dynamics due to the QC coupling. The time evolution of the system is solved exactly assuming the QC to be infinite, flat, and uniform; and a detailed investigation of different special cases is carried out. Weak QC excitation is found to exhibit features which are connected to few level dynamics rather than to an irreversible decay, and we study how this modifies the absorption cross section and the Rabi oscillations. The conditions for coherent trapping are discussed.
A phenomenological approach to the calculation of the diffusion coefficient for Si on Si(111) using classical trajectories
Authors: NoorBatcha I. , Raff L. M. , Thompson Donald L.
Published: July 2002

A general method to calculate a lower bound and an estimated upper bound for the surface diffusion coefficient from jump frequencies of an adatom from one absorption site to another has been formulated. This method has been applied to the surface diffusion of Si on Si(111). Keating’s potential has been used for the Si(111) lattice. The interaction potential between the adatom and the lattice is a pairwise sum of 60 Morse potentials involving the Si atoms in the first and second layers of the crystal. This potential formulation predicts the existence of two different types of adsorption sites on the Si(111) surface. The jump frequencies from these adsorption sites have been calculated by classical trajectory methods. Using these jump frequencies, a lower bound for the diffusion coefficient is calculated by solving a set of coupled phenomenological kinetic equations describing the jumping of adatoms between adjacent adsorption sites. The results at 800, 1000, 1200, and 1500 K yield a lower bound for the diffusion coefficient of D>(8.53±1.11)×exp{−(2430±270)/RT} cm2/s. At 1500 K, the computed mean-square displacement and velocity autocorrelation function give diffusion coefficients of 7.11×10−4 and 8.69×10−4 cm2/s, respectively, which is in excess of the calculated lower bound at 1500 K by about a factor of 2. This suggests that diffusion of Si on Si(111) involves highly correlated motion. An estimate for the upper bound for the diffusion coefficient is obtained by removing from the set of coupled kinetic equations all terms involving adatom motion which leads back toward the original adsorption site. The upper bound calculated in this manner at 1500 K is 1.41×10−3 cm2/s, which is a factor of 2 greater than the computed diffusion coefficient. The calculated activation energy for surface diffusion (2.43 kcal/mol) suggests that the experimental value for this quantity obtained from the direct deposition of Si on Si(111) in ultra high vacuum most accurately represents the true zero-coverage limit on a Si(111) crystal free of kinks and steps.
The effect of intramolecular processes on the polarizabilities and hyperpolarizabilities of some amines
Authors: Waite J. , Papadopoulos M. G.
Published: July 2002

We demonstrate that small, properly extended and optimized basis sets, within CNDO, are appropriate for the calculation of the electric polarizabilities and hyperpolarizabilities of amines. Employing such a wave function we show that the intramolecular processes (inversion and rotation) in some aromatic amines [C6H5NH2, C6H5N(CH3)2, and 1-C10H7NH2] have a small effect on the average polarizability and second hyperpolarizability while large changes are observed in the anisotropies of the charge cloud and the first hyperpolarizability. These observations are discussed and rationalized.
Rotational relaxation in the H2CO Ã 1A2 state by transient gain spectroscopy
Authors: Vaccaro P. H. , Redington R. L. , Schmidt J. , Kinsey James L. , Field R. W.
Published: July 2002

A new, pulsed pump–cw probe technique, transient gain spectroscopy, has been used to measure the zero pressure lifetime (279±10 ns) and self-collision depopulation rate (110.5±3.1 μs−1 Torr−1) for the H2CO Ã 1A2 41 JKa,Kc =132,12 level as well as the 122,11←132,12(31±8 μs−1 Torr−1) and 142,13←132,12(40±6 μs−1 Torr−1) rotational relaxation rates. The dominant collisional process is rotational energy transfer, which, for the H2CO(Ã)+H2CO(X̃) process examined here, appears to follow dipole–dipole scaling and propensity rules. The 110.5 μs−1 Torr−1 depopulation rate in the Ã state agrees with the vibrationless X̃ state rate obtained from microwave power broadening scaled by the 1.46 D/2.33 D electric dipole moment ratio. The two strongest a-dipole rotational state-to-state rates account for more than 50% of the total removal rate from H2CO Ã 41 132,12.