bluuebunny/crossref_metadata_2025 huggingface dataset:
Fourier transform infrared photoacoustic spectroscopy of polymeric laminates
Authors: Wahls M. W. C. , Leyte J. C.
Published: July 2002

Photoacoustic (PA) magnitude and phase spectra of, respectively, polyethyleneterephthalate/polyethyleneterephthalate (PET/PET), polypropylene/polyethyleneterephthalate (PP/PET), and polyethyleneterephthalate/polypropylene (PET/PP) laminates are presented. The extension of the Rosencwaig–Gersho theory to a double-layered system introduced by N. C. Fernelius [J. Opt. Soc. Am. 70, 480 (1980) and J. Appl. Phys. 51, 650 (1980)] is applied to compare the experimental data with simulated PA magnitudes and phases. The simulations for the double-layered systems indicate that the absorption coefficient β at the optically transparent region of the single layers—estimated from photoacoustic data—strongly influences the phase spectra of the polymeric laminate. In particular, the relative magnitude of these residual absorptions in the two layers is important. The optical absorption spectra of the polypropylene and polyethyleneterephthalate single layers (accessible from infrared absorbance and infrared photoacoustic spectra) were used to simulate the photoacoustic magnitude and phase spectra of the two possible double layers, i.e., PP/PET and PET/PP. A reasonable agreement of experimental and simulated photoacoustic data is observed.
Direct observation of magnetization reversal processes in micron-sized elements of spin-valve material
Authors: Chapman J. N. , Aitchison P. R. , Kirk K. J. , McVitie S. , Kools J. C. S. , Gillies M. F.
Published: July 2002

Simple calculations suggest that when continuous films of spin-valve material are patterned into micron-sized elements the magnetic properties should change markedly, depending on the element shape and size. We have used the differential phase contrast imaging mode of transmission electron microscopy to study directly the magnetization distributions supported by such elements in zero field and when subjected to an applied field in the pinning direction. For elements whose long axis is parallel to the pinning direction a parallel alignment of the free and pinned layers is favored. When subjected to a field a complex domain structure evolves and different irreversible paths are followed as the element is taken from negative to positive saturation and back again. By contrast, when the pinning direction is parallel to the short axis an antiparallel arrangement, where the magnetostatic contribution to the energy is effectively suppressed, can be preferred and simpler reversal mechanisms, with a higher degree of reversibility, are frequently seen.
Mixed electron emission from lead zirconate–titanate ceramics
Authors: Zhang Weiming , Huebner Wayne , Sampayan Stephen E. , Krogh Mike L.
Published: July 2002

Simultaneous ferroelectric and plasma emission from Pb(Zr,Ti)O3 was observed with only a negative driving pulse applied to the sample, and without an extraction potential on the electron collector. Plasma emission was a strong, inconsistent, and self-destructive process. In addition, a positive ion current was detected. Comparatively, ferroelectric emission was a relatively stable self-emission process, exhibiting no apparent delay time, and no positive ion current. The relationship between the switching and emission current of ferroelectric samples measured simultaneously cannot only be used to determine the existence of ferroelectric emission, but can also give direction to choosing suitable ferroelectric materials for emitter applications.
Magnetic measurements on the III-VI diluted magnetic semiconductor Ga1−xMnxSe
Authors: Pekarek T. M. , Crooker B. C. , Miotkowski I. , Ramdas A. K.
Published: July 2002

We have investigated the magnetic properties of Ga1−xMnxSe, which represent a new class of diluted magnetic semiconductors based on a III-VI semiconductor. These are layered materials; however the local environment is tetrahedral as in the II-VI materials. In contrast to the II-VI semiconductors, the Mn substitutional atoms have direct bonds to three Se atoms and to either a Ga or Mn atom. This leads to a complex temperature dependent magnetization. In fields of 100 G and below, a broad peak is observed in the magnetization centered at 160 K. In addition, a sharp change in magnetization is observed at 119 K. In a field of 100 G, the peak has a magnitude of 3×10−5 emu/g above the background of 7×10−5 emu/g. With increasing magnetic fields, these features are broadened which is suggestive of some type of short-range antiferromagnetic ordering. At 5 K we observe a magnetization which increases linearly with field up to 6 T similar to the Van Vleck paramagnetic behavior observed in the Fe substituted II-VI semiconductors.
An approach using a subamorphizing threshold dose silicon implant of optimal energy to achieve shallower junctions
Authors: Sultan Akif , Banerjee Sanjay , List Scott , McNeil Vincent
Published: July 2002

Significant channeling during the B+/BF2+ implant and enhanced diffusion during the subsequent anneal step limit the ability of forming ultrashallow p+-type junctions. The problem is becoming even more serious as device dimensions are shrinking and shallower junctions are needed to prevent short channel effects. An approach using a subamorphous threshold dose Si+ preimplant of optimal energy is proposed and demonstrated, and it reduces both channeling during the subsequent dopant implant and enhanced B diffusion during the anneal step. In this approach, the Si+ implant is performed prior to the dopant implant (B+/BF2+); thus it is referred to as a Si+ preimplant. The energy of the Si+ preimplant determines the spatial distribution of the vacancies and interstitials formed by it. The vacancy excess created by the Si+ preimplant creates strain in the lattice by disrupting the periodic potential of the lattice, which in turn helps in reducing channeling during the subsequent dopant implant. The vacancy distribution created by the Si+ preimplant also recombines with the interstitials produced by the dopant implant resulting in fewer interstitials that can contribute to the enhanced diffusion of B, thereby reducing the enhanced B diffusion and thus resulting in shallower junctions. The energy of the Si+ preimplant is chosen such that the excess interstitials produced by it lie beyond the depth to which the dopants would diffuse in the absence of a Si+ preimplant, thus the interstitials produced by the Si+ preimplant do not contribute to enhanced B diffusion. Junction depth reduction was studied using unpatterned n-type wafers by secondary ion mass spectrometry analysis. A Si+ preimplant of the desired energy (ranging from 50 to 100 keV) was then performed through a thin screen oxide. The control samples were not implanted with Si+. Subsequent B+or BF2+ implants were carried out at 5 keV. The samples were annealed at 1000 °C. The junction depth was reduced by 200 Å for 5 keV, 6×1014 cm−2 BF2+ implants and 450 Å for 5 keV, 2×1014 cm−2 B+ implants using this approach of preimplanting with a neutral species (Si+ was used in this study) of optimal energy. Low leakage devices have been fabricated using this approach to demonstrate that the Si+ preimplant does not create any unwanted defects.
Microstructural origin of soft magnetic properties of sendust films prepared by N2 reactive sputtering
Authors: Chen Yingjian , Ryan Patrick J. , Dolejsi James F. , Rao Maithri , Laughlin David E. , Kryder Mark H. , Al-Jumaily Ghanim , Yang Zhijun
Published: July 2002

Sendust films were deposited using dc magnetron sputtering at room temperature in a mixture of Ar and N2 gas. The soft magnetic properties of the sendust films were, in general, improved with the addition of N2. The optimum soft magnetic properties were achieved when 6% N2 gas was used. Transmission electron microscopy studies showed that the sendust film without N2 contains average grain sizes of 200 nm; whereas the films with increasing N2 content contain increasing percentages of much smaller grains of 20 nm. The smaller grains have a random crystalline orientation with respect to each other. The cross-sectional morphological structures were studied using a scanning electron microscope. In the film without N2, power-law cone structures were observed. In the N2 containing films, needlelike morphological structures were formed. X-ray photoelectron spectroscopy studies on these films indicate that N2 included in the films reacts preferentially with Al to form AlN. Excess N2 may then react with Si, possibly to form Si3N4. The atomic concentrations of Al and N become equal in the films when 6% N2 was used, which also gives rise to the optimum soft magnetic properties. To understand the advent of soft magnetic properties, a random magnetic anisotropy model and a static wall coercive force model were considered.
Effective conductivities of aligned spheroid dispersions estimated by an equivalent inclusion model
Authors: Lu Shih-Yuan
Published: July 2002

A simple equivalent inclusion model is proposed to estimate the effective conductivities of dispersions containing aligned spheroids. The spheroids are either in perfect contact with the matrix, possessive of certain contact resistance, or coated with a confocal layer of a third material. For the perfect contact case, our results coincide with Willis’ bounds [J. Mech. Phys. Solids 25, 185 (1977)] and, for the coating case, they compare extremely well with those of Hatta and Taya [J. Appl. Phys. 59, 1851 (1986)]. New results are readily obtained for the contact resistance case through use of the proposed model. For the present systems, there exist two independent effective conductivities, one defined in the direction parallel with the symmetric axis of the spheroid and the other defined in the perpendicular direction. Interestingly, the inclusion effect for both the contact resistance and coating cases may be enhancing in one direction, but impairing in the other. But for the perfect contact case, the inclusion effect is always consistent in both directions. It is found that, for the perfect contact case, the reduced effective conductivity (σeff) is a function of the spheroid volume fraction, spheroid aspect ratio, and reduced spheroid conductivity. One more parameter, the Biot number, is needed for the contact resistance case, while two more parameters, the reduced coating layer conductivity and the relative coating thickness, appear in the coating problem. Effects of these parameters on σeff are thoroughly investigated.
Charge injection from polyaniline-poly (methylmethacrylate) blends into poly (p-phenylene vinylene)
Authors: de Lima J. R. , Schreiner C. , Hümmelgen I. A. , Fornari C. C. M. , Ferreira C. A. , Nart F. C.
Published: July 2002

Blends of polyaniline doped with camphorsulfonic acid, (PAni–CSA), and poly(methylmethacrylate) (PMMA) are important candidates for hole injection electrode material in organic light emitting devices. We present a quantitative analysis of the potential barrier heights at interfaces between the PAni–CSA/PMMA blend and poly-(p-phenylene vinylene) (PPV). The barrier height determination, which is of crucial importance for radiant efficiency control of organic light emitting diodes, is based on temperature dependent tunneling current measurements. The value of the [PAni–CSA/PMMA]/PPV potential barrier height is found to be independent of the PAni–CSA concentration in the blend, except near the percolation threshold, where an increase in the barrier height value is observed.
Gap states caused by oxygen precipitation in Czochralski silicon crystals
Authors: Koizuka M. , Yamada-Kaneta H.
Published: July 2002

Deep level transient spectroscopy measurements have been made on both n- and p-type silicon crystals containing various types of the oxygen precipitates to investigate the associated gap states. A new signal in the gap-state density was observed at about Ec−0.25 eV, as well as the previously reported peak at about Ev+0.3 eV. Both peaks are attributed to a defect generated by oxygen precipitation. The observed distribution of the gap-state density is very similar to that of the well-known Pb center due to a dangling bond of a silicon atom at the interface between the silicon and silicon dioxide at the surface of the sample. The measured gap states could be passivated by hydrogen, as found for the Pb center.
Impact ionization in AlxGa1−xAs/GaAs single heterostructures
Authors: Chia C. K. , David J. P. R. , Rees G. J. , Plimmer S. A. , Grey R. , Robson P. N.
Published: July 2002

Impact ionization involving transport across a single heterojunction has been studied by measuring the electron and hole initiated multiplication, Me and Mh, in a series of p+in+ AlxGa1−xAs(500 Å)/GaAs(500 Å) heterostructures with x ranging from 0.3 to 0.6. At low electric fields, because of dead space effects, Me and Mh in these devices are very different and are primarily determined by the ionization properties of the material in the latter half of the structure. As the electric field increases, feedback from the opposite carrier type causes Me and Mh to converge to the values measured in the equivalent alloy. The effects of the band-edge discontinuities at the heterojunction interface on Me and Mh in these heterostructures are compensated by the different energy-loss rates in AlxGa1−xAs and GaAs. A simple Monte Carlo model using effective conduction and valence bands is used to interpret the experimental results.
Rubbing strength dependence of surface interaction potential and surface-induced order above the nematic–isotropic transition
Authors: Kim Jong-Hyun , Rosenblatt Charles
Published: July 2002

Optical retardation measurements as a function of rubbing strength are reported for both rubbed polyimide-coated substrates and cells containing the liquid crystal hexylcyanobiphenyl. Our analysis assumes a Sheng surface interaction potential of the form −GSsδ(z), where Ss is the order parameter of the liquid crystal at the interface and G is related to the rubbing strength. Moreover, we assume that there can be a distribution ΔG in the parameter G. Several features are noted: There appears to be a minimum rubbing strength before the onset of substantial surface-induced order in the liquid crystal; the parameter that characterizes the interaction potential between the polyimide and liquid crystal is proportional to the optical retardation of the polyimide-coated substrate; and, in light of our analysis, there are substantial inhomogeneities in the liquid crystal interaction potential and surface order parameter, perhaps even on length scales comparable to the nematic correlation length.
Growth kinetics of amorphous interlayers by solid-state diffusion in ultrahigh vacuum deposited polycrystalline Gd thin films on (001)Si
Authors: Chen J. C. , Shen G. H. , Chen L. J.
Published: July 2002

The growth kinetics of amorphous interlayer (a-interlayer) formed by solid-state diffusion in ultrahigh vacuum deposited polycrystalline Gd thin films on (001)Si have been investigated by cross-sectional transmission electron microscopy. The growth of the a-interlayer was found to follow a linear growth law initially. The growth then slows down and deviates from a linear growth law. The result clearly indicated that the growth at early stage is consistent with an interface-reaction-controlled growth model rather than diffusion limited. The activation energy of the linear growth and maximum thickness of the a-interlayer were measured to be about 0.37 eV and 6.6 nm, respectively. Following the growth of amorphous interlayer, epitaxial hexagonal GdSi2−x was found to form at the a-interlayer/(001)Si interface at 225 °C. The relatively small lattice mismatch between GdSi2−x and (001)Si compared to other RESi2−x (RE=rare-earth metal) and (001)Si systems facilitates the epitaxial growth of GdSi2−x on (001)Si, which in turn hampers the further growth of a-interlayer.
What is the role of spin–orbit interaction in magneto-optical effects?
Authors: Xu You , Yang Jiehui , Guillot Maurice
Published: July 2002

The effect of the spin–orbit interactions of both the ground and excited configurations of Pr3+ ions on the magneto-optical (MO) effect induced by the Pr3+ ions in two crystals (Pr-substituted yttrium iron garnet and PrF3) has been studied based on the quantum theory. It is found that, in both Pr:YIG and PrF3, the influence of the spin–orbit coupling strength of the ground configuration on Faraday rotation (FR) is large; however the FR does not scale linearly with the spin–orbit coupling. It is surprising that the influence of the spin–orbit coupling strength of the excited configuration on FR is small except when the light frequency is near a MO resonance frequency. It is demonstrated that, contrary to widespread ideas, spin–orbit interactions of both the ground and excited configurations are not always needed to produce a FR. For PrF3, when spin–orbit interactions of both the ground and excited configurations are absent, the FR will be even larger than that under normal conditions. It is also shown that certain aspects of the effect of spin–orbit coupling on FR are related to the symmetry of the crystal field acting on magnetic ions.
Noise analysis of a 1 MHz–3 GHz magnetic thin film permeance meter
Authors: Yabukami S. , Yamaguchi M. , Arai K. I. , Watanabe M. , Itagaki A. , Ando H.
Published: July 2002

We analyzed the permeability measurement error of a low permeance thin film. We clarified that the noise voltage was excited by a current loop which is composed of the coaxial cable and the ground plane. The current loop should be removed for high sensitivity of the permeameter. The permeability of a high electrical resistivity film (CoFeHfO) has been demonstrated 1 MHz–3.5 GHz.
A magnetic phase transition temperature calibration device
Authors: Garcia F. , Cernicchiaro G. R. C. , Takeuchi A. Y.
Published: July 2002

We have devised a technique of manufacturing temperature sensor calibration devices based on the magnetic properties of the pseudobinary compounds of formula (RxR1−x′)Co2, where R and R′ are heavy rare earth elements. The device is a solid sensor which provides an easily detectable first order magnetic phase transition at fixed temperature points. It is known that a first order magnetic phase transition from ferrimagnetism to paramagnetism is observed in compounds as TmCo2 (3.70 K), ErCo2 (32.05 K), HoCo2 (77.12 K), and DyCo2 (136.55 K). These transitions correspond to a large anomaly in the characteristic properties as function of temperature. In this work, we present the electrical resistivity and magnetization measurements of (Er, Ho)Co2 series and DyCo2 compounds showing the viability of the proposed devices from 32.05 up to 136.55 K. This range can be extended below and above by substitution of the chemical components and the stoichiometric composition. The number of transitions can be fixed by a convenient arrangement of several elements.
Single layer and multilayer tip coatings in magnetic force microscopy
Authors: Casey S. M. , Lord D. G. , Grundy P. J. , Slade M. , Lambrick D.
Published: July 2002

Interactions between the imaging tip and the sample in magnetic force microscopy (MFM) have been investigated by studying the magnetic microstructure of a range of epitaxial garnet films. Etched silicon cantilever probes, coated with CoPt alloy films and Co/Pt multilayers, provided a range of MFM probes for this study. Resonant torque magnetometry was used to characterize their magnetic properties. Phase change images were found to vary considerably in terms of relative “domain volumes” at the surface depending on which probe was used. Decreasing the moment of the alloy coated tips by using thinner layers reduces the “magnetizing” interaction of the tip field but also reduced the signal to noise ratio. By coating the tip with a multilayer a good signal to noise ratio could be obtained with very little interaction. Force–distance curves were used to study the response of the tips at various lift heights. The tips coated with alloy films gave a significant decrease in signal to noise ratio as the lift height increased whereas the multilayer tips maintained a signal which varied little with lift height.
Computation of distortions in magnetic field and spectrum for nuclear magnetic resonance instruments
Authors: Growney Eric , Friedman Gary , Gerald Rex
Published: July 2002

A method for highly accurate modeling of induced field distortions in nuclear magnetic resonance (NMR) is developed based on the partial field formulation of the magnetostatic problem and use of the surface integral equation technique. A bound for the error in the NMR spectrum dispersion computation is derived in terms of the energy norm error associated with the static field calculations. It is shown that distortion fields can be computed with sufficient accuracy in many NMR applications using only the equivalent sources induced by the applied uniform static field.
Selectively observed spatially indirect Stark-ladder transitions in asymmetric double-well GaAs/AlAs superlattices
Authors: Takeuchi Misaichi , Kawashima Kenji , Fujiwara Kenzo
Published: July 2002

The Wannier-Stark localization of asymmetric double well superlattices (ADW-SLs) has been investigated by low-temperature photocurrent spectroscopy. The ADW-SLs consist of arrays of asymmetric double quantum wells formed by coupling of two different size wells. Reflecting the difference in the eigenenergies of the two wells, the occurrence of the Stark-ladder transitions is, in principle, characterized by the superposition of two fan diagrams. Hence, four ±1st-order spatially indirect transitions are expected to exist. However, they are experimentally observed only partially. This selectivity is interpreted by considering the coupling pattern between two wells in the ADW-SLs. The control of the coupling pattern is a new concept to design SL structures and its Wannier-Stark localization.
Picosecond infrared laser stimulation of luminescence in CaS:Eu,Sm
Authors: Fan Wenhui , Wang Yongchang , Hou Xun , Du Li , Zhao Wei , Yang Binzhou , Niu Lihong
Published: July 2002

The response and decay time of the optically stimulated luminescence of electron trapping material CaS:Eu,Sm to ultrashort infrared pulse has been measured by projecting two 1.064 μm laser pulses on the photocathode of a streak camera, one illuminates ETM screen on the photocathode, another illuminates the photocathode after a frequency doubling. The result indicates that the response and decay time is less than 14.3 ps. The decay law of the infrared upconversion luminescence followed by CaS:Eu,Sm is also presented. The work shows that this kind of electron trapping materials could be widely used in the rapid infrared upconversion and detection.
Temperature dependence of the transfer characteristics of polysilicon thin film transistors fabricated by excimer laser crystallization
Authors: Foglietti V. , Mariucci L. , Fortunato G.
Published: July 2002

The transfer characteristics of polysilicon thin film transistors have been measured in the temperature range from 400 to 80 K. The active layer has been made by excimer laser crystallization of amorphous silicon. The devices show high field-effect mobility values (>200 cm2/V s), even at low temperature. The electrical characteristics have been analyzed using a uniformly distributed density of states (DOS) model. The DOS has been derived using the values of the conductance at various temperatures. Using the DOS derived from the “temperature method,” we have calculated the transfer characteristics and the threshold voltage versus temperature, obtaining a very good agreement with experimental data.
Study of frequency dependent ε*(ω) in amorphous ferroelectrics: Modified generalized Langevin equation analysis
Authors: Kim Seong Hyun , Hwang Yoon-Hwae , Kim Jung-Ae , Yang Yong Suk , Chae Byung Gyu , Jang Min Su
Published: July 2002

We studied the frequency dependent dielectric constants of amorphous LiNbO3, KNbO3, and PbTiO3 at temperatures below the crystallization. We analyzed the real and imaginary part of dielectric constants ε′(ω) and ε″(ω) using the generalized Langevin equation modified by the space charge effect and the phenomenological frequency scaling. With this model, we can explain the low frequency dispersion and the high frequency relaxation behavior of ε′(ω) and ε″(ω) simultaneously. Our approach is new and different from our previous equivalent circuit model.
Effect of annealing of Ag ultrathin films on Co/Pt(111) surface
Authors: Shern C. S. , Tsay J. S. , Chen S. L. , Wu Y. E.
Published: July 2002

Low-energy electron diffraction was used to study the annealing effects of Ag ultrathin films on the surface of one monolayer Co/Pt(111). The intensity of the specular beam versus temperature has an unusual minimum point and maximum point. Further studies by Auger electron spectroscopy indicate that the adatoms of Ag become a best ordered state after the formation of Co–Pt alloy is complete. The new order state comes from the structure change of Co/Pt(111) and the smaller lattice mismatch between Ag and Pt. The electron density of states of d band shifts to a lower binding energy when the system starts to form the Co–Pt alloy measured by ultraviolet photoelectron spectroscopy. The chemical compositions at interfaces of the thin film were investigated by a depth profile before and after the annealing. The sputtering rates of Ag on the topmost layer of the unannealed film and the annealed film were calculated and comparatively studied. A shadow effect causes the different sputtering rates of these two ultrathin films.
Identification of materials in integrated circuit interconnects using x-ray absorption near-edge spectroscopy
Authors: Levine Zachary H. , Ravel Bruce
Published: July 2002

Most integrated circuit interconnects are principally composed of a few metals, including aluminum alloyed with copper, tungsten, titanium, Al3Ti, and Al2Cu, in a silica matrix. Integrated circuit interconnects have recently been proposed as a candidate system for visualization by computerized microtomography using absorption in the soft x-ray region. In this work, we demonstrate the feasibility of materials identification using volume-resolved x-ray absorption near edge spectra (XANES) obtained by tomographic reconstruction. A similar experiment could be performed with an energy-resolved high-voltage transmission electron microscope. We calculate the XANES for interconnect materials near the Al K edge, the Cu LI, LII, and LIII edges, and the Ti LII and LIII, and compare them to experiment when possible. The signal-to-noise ratio required to distinguish among the aluminum compounds from their Al K edge spectra is shown to be about one order of magnitude higher than that needed to detect elemental aluminum.
Hierarchical design of quantum-dot cellular automata devices
Authors: Gin Aaron , Williams Sara , Meng Haoyu , Tougaw P. Douglas
Published: July 2002

We use hierarchical layout methods to combine fundamental quantum-dot cellular automata (QCA) devices into more complex devices, including a 2-to-1 multiplexer, a 4-to-1 multiplexer, and an eight-function bit-slice arithmetic logic unit (ALU). The ALU unit is implemented using 1245 cells, making it the largest QCA device ever designed and simulated. This hierarchical design method is possible because the Coulombic interaction between QCA cells is very local in nature. Since distant cells have very little effect on each other, the state of a cell can be very well approximated by considering only those cells that are near it. In a QCA device, nearby cells are usually part of the same functional unit, so we can design each functional unit separately, connect them with appropriate binary wires, and then simulate the entire device to verify its correct operation.
Spin dynamics in Co/Pd multilayers by varying the number of repeats
Authors: Choe Sug-Bong , Jang Hyuk-Jae , Shin Sung-Chul
Published: July 2002

The spin reversal of nanostructured Co/Pd multilayers by varying the number of repeats has been investigated quantitatively. The magnetization viscosity measurement and the direct observation of the time-dependent spin reversal patterns showed that their reversal behavior gradually changed from wall-motion dominant to nucleation dominant by increasing the number of repeats. Both the wall-motion speed V and the nucleation rate R of the samples were determined by a quantitative analysis of the time-dependent domain patterns. It was found that V was significantly decreased and R was increased by increasing the number of repeats, and the ratio of V/R was an important parameter of different spin reversal behavior.
The dependence of hard transition shift on medium velocity
Authors: Taratorin A. , Cheng D. , Arnett P. , Olson R. , Diola T.
Published: July 2002

The experimental measurements of hard transition shift (HTS), obtained for a set of different velocities of the magnetic medium, indicate that HTS increases at higher medium velocity. The experimental data can be explained by the expansion dynamics of the writing zone. Numerical simulation of the writing zone dynamics indicates a time delay in the writing of the hard transition compared with the easy one. The dependence of HTS on medium velocity explains the degradation of overwrite at high recording data rates.
Magnetism of nanometer-scale iron particles arrays (invited)
Authors: Wirth S. , von Molnár S. , Field M. , Awschalom D. D.
Published: July 2002

The magnetization behavior in arrays of small ferromagnetic iron particles is investigated. Arrays were fabricated by a combination of chemical vapor deposition and scanning tunneling microscopy. This method allows a variety of particle arrays to be grown differing in particle height, diameter, or arrangement. Moreover, the arrays can be grown directly onto different materials such as Au or permalloy. Magnetic measurements were conducted by Hall magnetometery up to 100 K and compared to switching field measurements by means of magnetic force microscopy at room temperature. The magnetization reversal mechanisms were studied from magnetization curves measured for an arbitrary angle ϑ of the applied field with respect to the long axis of the particles. By analyzing the reversible rotation, the particles’ magnetic core diameter and the shape anisotropy could be determined. A phenomenological model based on thermally activated magnetization reversal was introduced and compared to experimental switching field dependences on temperature as well as on ϑ. Thermal effects may govern the magnetization reversal in particles of 10 nm diameter at all temperatures. For 14 nm particles and temperatures below 30 K, curling (for ϑ⩽30°) and homogeneous reversal modes appear to dominate.
Spin dependent tunnel/spin-valve devices with different pinning structures made by photolithography
Authors: Wang D. , Tondra M. , Daughton J. M. , Nordman C. , Pohm A. V.
Published: July 2002

Spin dependent tunnel and spin-valve devices were made using rf diode sputtering, with patterning done using standard semiconductor photolithography techniques. In order to tailor the pinning strength of the hard magnetic layers, three types of structures were tried: (1) NiFeCo/spacer/CoFe; (2) NiFeCo/spacer/CoFe/IrMn; and (3) NiFeCo/spacer/CoFe/Ru/CoFe/FeMn, with Al2O3 or Cu as spacers. The magnetoresistance of the spin dependent tunnel devices is up to 24% with a switching field of a few Oe for the free layer of NiFeCo. The saturation fields of the hard layers are a few tens, a few hundreds, and a few thousands of Oe for the three structures, respectively. The first structure is suitable for magnetic memory applications with the hard layer storing the information. The second structure is suitable for magnetic field sensors which must function after relatively high magnetic field excursions. The third structure makes use of the synthetic antiferromagnet of CoFe/Ru/CoFe in addition to the antiferromagnet (FeMn) to achieve the highest pinning field. It also reduces the fringing field to the free layer caused by the pinned layer, due to the flux closure of the two ferromagnetic layers in the synthetic antiferromagnet. This third structure is especially suitable for field sensor applications in environments with excursions of very high magnetic fields between sensing operations.
Exchange-biased magnetic tunnel junctions fabricated with in situ natural oxidation
Authors: Matsuda K. , Kamijo A. , Mitsuzuka T. , Tsuge H.
Published: July 2002

Exchange-biased magnetic tunnel junctions with a Ta/NiFe/FeMn/NiFe/Al–oxide/NiFe/Ta structure have been fabricated. The tunnel barrier was formed by the in situ natural oxidation of an Al metal layer under controlled oxygen pressure. Photolithography and ion milling were used to pattern the multilayer into junction structures of 2×2 μm2–20×20 μm2 dimensions. Magnetoresistance (MR) curves show spin-valve-like characteristics, in which an antiparallel configuration of magnetizations in both ferromagnetic layers is observed between 50 and 240 Oe, and the hysteresis loops for both the free and pinned layers exhibit sufficient separation. An evaluation of the MR curves shows the exchange-bias field to be 340 Oe and coercivity levels in the free layer to become as low as 13 Oe. At room temperature normalized junction resistance is 2×10−5 Ω cm2, with MR ratios still being maintained at 13%. This resistance value is much lower than previously reported values for junctions produced either with plasma oxidation or thermal oxidation in air. Maximum variation in junction resistance is only ±5% for 10×10 μm2 junctions over a 2 in. wafer. The MR ratio decreases by half when the bias voltage is raised from 0 to 440 mV, approximately the same ratio of decrease as has been previously reported for other successful junctions.
Influence of growth temperature on the spin reorientation of Ni/Cu(100) ultrathin films
Authors: Zheng M. , Shen J. , Ohresser P. , Mohan Ch. V. , Klaua M. , Barthel J. , Kirschner J.
Published: July 2002

Ni/Cu(100) films were prepared by thermal deposition at room temperature (RT) and 170 K low temperature (LT) separately to study the influence of substrate temperature on the spin reorientation. The critical thickness of the LT grown films is observed to be about 1 ML smaller than that of the RT films. Though both types of films show similar tetragonal distortion and chemical composition, their morphology differs dramatically: the island density of the LT films is significantly higher than that of the RT films. We use this to interpret the different magnetic behavior between the RT and LT films.
Magnetization reversal as a nonlinear multimode process
Authors: Safonov Vladimir L. , Bertram H. Neal
Published: July 2002

Dynamic magnetization reversal has been simulated in a single grain for an instantaneously applied reversal magnetic field. The grain was considered as a system of n(=2,4,8) subgrains with uniaxial anisotropy coupled by exchange interaction. The gyromagnetic equations of motion were solved without phenomenological damping terms. The evolution of the magnetization was found to exhibit nonlinear spin wave excitation on a short timescale. The average magnetization significantly decreases and demonstrates irregular oscillations at a well defined mean value. The final stage of reversal is the spin-lattice damping on a longer time scale. The magnetic moment is, thereby, reassembled in a new equilibrium direction.
Writing on perpendicular patterned media at high density and data rate
Authors: Boerner Eric D. , Bertram H. Neal , Hughes Gordon F.
Published: July 2002

The feasibility of reversing single bit per particle media at a 1 GHz data rate with a density of 100 Gb/in.2 is examined. The media particles are cubic with 40 nm side length, 80 nm center to center spacing and perpendicular uniaxial anisotropy. Micromagnetic simulations are employed, and writing of data is done via an elongated probe to provide localized fields at the particle being reversed. The reversal of individual elongated particles is also examined with fields from a moving probe. A 1 GHz data rate appears to be more attainable with cubic particles than with elongated particles.
Magnetic ordering in strongly correlated-electron uranium systems: Consequences of two kinds of f-electron-band–electron states
Authors: Cooper Bernard R. , Lin Yeong-Lieh , Sheng Qing-Guang
Published: July 2002

Magnetic ordering involves the electronic behavior globally; and for uranium-based systems, the hybridization-induced effects dominate over the Coulomb exchange effects in determining the magnetic ordering. Therefore, as long as the hybridization is treated as acting between properly exchange-symmetrized two-electron wave functions, the effects of exchange can be incorporated in the one-electron exchange-correlation potential. As a consequence of the necessary exchange symmetrization, there are essentially two kinds of f electrons, localized magnetic and itinerant nonmagnetic. This has enabled us to make absolute material-specific predictions of alloying or high-pressure effects on magnetic ordering in uranium strongly correlated-electron (SCE) systems using local-density approximation input into many-electron dynamics. Experimentally, the alloying effects can be dramatic, e.g., in UxLa1−xS the magnetic ordering abruptly disappears at about 55% uranium. The theory is quite successful in its detailed absolute predictions, and this has important implications for the overall understanding of electronic behavior in SCE systems including heavy fermion systems. The key conclusion is that strengthening the hybridization, as kinematically restricted by exchange symmetry, leads to a chemical-environment-dependent sharp phase transition in SCE systems with dramatic observable consequences. This phase transition is associated with the elimination of the localized-magnetic transition-shell electrons (f electrons for light actinide and cerium-based SCE materials, d electrons for transition-metal–oxide-based SCE materials).
Dependence of exchange anisotropy and coercivity on the Fe–oxide structure in oxygen-passivated Fe nanoparticles
Authors: Prados C. , Multigner M. , Hernando A. , Sánchez J. C. , Fernández A. , Conde C. F. , Conde A.
Published: July 2002

Ultrafine Fe particles have been prepared by the inert gas condensation method and subsequently oxygen passivated. The as-obtained particles consist in an Fe core surrounded by an amorphous Fe-oxide surface layer. The antiferromagnetic character of the Fe-oxide surface induces an exchange anisotropy in the ferromagnetic Fe core when the system is field cooled. Samples have been heat treated in vacuum at different temperatures. Structural changes of the Fe–O layer have been monitored by x-ray diffraction and transmission electron microscopy. Magnetic properties as coercivity, hysteresis loop shift, and evolution of magnetization with temperature have been analyzed for different oxide crystallization stages. A decrease of the exchange anisotropy strength is reported as the structural disorder of the surface oxide layer is decreased with thermal treatment.
Theory of the electron mobility in n-type 6H–SiC
Authors: Kinoshita T. , Itoh K. M. , Schadt M. , Pensl G.
Published: July 2002

We report on calculations of the anisotropy of the electron Hall mobility and its temperature dependence in n-type 6H–SiC. The model is based on the conduction band structure determined recently by a first-principle calculation. It provides explicit and easy to use analytical expressions for both drift and Hall mobilities. The calculation of the Hall mobility based on our model agrees very well with experimentally determined anisotropic Hall mobility in 6H–SiC.
Removing optical artifacts in near-field scanning optical microscopy by using a three-dimensional scanning mode
Authors: Jordan Claire E. , Stranick Stephan J. , Richter Lee J. , Cavanagh Richard R.
Published: July 2002

We demonstrate a method of acquiring near-field scanning optical microscopy data that allow for the construction of three different types of images from one data set: topographic, constantgap, and constant-height. This data set includes the topographic features of the surface and the optical response at various heights above the sample surface. Comparisons are made between the images recorded in this format and both conventional, constant-gap mode images, and pseudoconstant-height mode images constructed using a single retraction curve. Zmotion artifacts are identified by analyzing the optical intensity for a given image as a function of the sample topography. Using this procedure it is shown that significant z-motion artifacts exist in the constant-gap images of gold particles immobilized on a glass slide. These artifacts are avoided by constructing constant-height images.
Relaxation of thermal stresses in double-coated optical fibers
Authors: Shiue Sham-Tsong , Tu Yuan-Kuang
Published: July 2002

The thermal stresses in double-coated optical fibers are analyzed by the viscoelastic theory. A closed form solution of the thermal stresses is obtained. The thermal stresses are proportional to the temperature change, and are a function of the material’s properties of the polymeric coatings and their thicknesses. The material’s properties of the polymeric coatings include the Young’s modulus, thermal expansion coefficient, Poisson’s ratio, and relaxation time. The relaxation of thermal stresses is strongly dependent on the relaxation time of the polymeric coating. If the relaxation time of the polymeric coating is very long, the viscous behavior of the polymeric coatings will not appear, and the thermal stresses solved by the viscoelastic theory are the same as those solved by the elastic theory. On the other hand, if the relaxation time of the polymeric coating is very short, the relaxation of thermal stresses is very apparent. A compressive radial stress at the interface of the glass fiber and primary coating will result in an increase of the microbending losses, and a tensile interfacial radial stress will possibly produce the debond at the interface of the glass fiber and primary coating. To minimize this interfacial radial stress, the radii, Young’s moduli, thermal expansion coefficients, and Poisson’s ratios of polymeric coatings should be appropriately selected, and the relaxation time of the primary coating should be decreased. Finally, the thermal stresses in single- and double-coated optical fibers are discussed.
A noninvasive rf probe for the study of ionization and dissociation processes in technological plasmas
Authors: Law V. J. , Kenyon A. J. , Clary D. C. , Batty I.
Published: July 2002

A swept frequency absorbance plasma diagnostic technique for measurement of self-resonance frequency, intrinsic plasma-tool distributed capacitance, radiative energy loss, and effective plasma capacitance is described. The ex situ probe measures the plasma properties independently of all contributions from the plasma-tool and transmission line connection to the rf supply. The technique employs a swept frequency source and a balanced equal ratio arm bridge to measure the frequency response of the plasma tool after the plasma has been extinguished under plasma conjugate matching conditions. The resonant frequency of the combination of capacitances due to plasma-tool geometry (intrinsic capacitance, Ci) and the matching network (Cm) exhibits a shift from the excitation frequency (13.56 MHz) that is dependent on the effective plasma capacitance. Resonance frequency shift data are given for He, Ne, Ar, O2, N2, and N2O as a function of both pressure (0.02–0.8 mbar) and incident power (50 and 100 W). This technique allows the differentiation between dissociation and ionization processes within the plasma through a simple noninvasive rf measurement.
Nondestructive determination of beryllium outdiffusion in AlGaAs/GaAs heterojunction bipolar transistor structures by low-temperature photoluminescence
Authors: Wang Hong , Ing Ng Geok , Zheng Haiqun , Zhang Penghua
Published: July 2002

In this work, low-temperature photoluminescence (PL) is used for the investigation of beryllium (Be) dopant outdiffusion in AlGaAs/GaAs single-heterojunction bipolar transistors (HBTs). Near the typical emission peak at ∼1.92 eV which is due to the band-to-band transition from AlGaAs emitter, an additional emission peak is found in the PL spectrum. This peak is found to be caused by the Be-related transition from AlGaAs emitter due to the Be outdiffusion from the GaAs base to the AlGaAs emitter. The concentration of Be outdiffused into the emitter can be estimated from the energy separation between these two emission peaks in AlGaAs range based on the band gap narrowing effect. The measured dc current gain and the emitter-base turn-on voltage of the HBTs fabricated on different wafers with different growth conditions were found to correlate well with the PL results. Our results demonstrate that low temperature PL technique is an efficient method in identifying Be outdiffusion in HBTs and is useful in HBT growth and device structure optimization.
Interaction between guided optical waves and magnetostatic waves in bismuth-doped yttrium–iron–garnet waveguide under inclined magnetic fields
Authors: Bao-Jian Wu , Gong-Qiang Liu
Published: July 2002

The noncollinear interaction between guided optical waves and magnetostatic waves under inclined bias magnetic field is studied in detail. The diffraction efficiency (DE) and the mode-conversion efficiency (MCE) of guided optical waves with magnetostatic forward volume waves in the bismuth-doped yttrium–iron–garnet (Bi-doped YIG) waveguide are calculated numerically. The calculation results indicate that the DE and the MCE are different in physical conception and the DE is not equal to the MCE in magnitude under inclined bias magnetic field. The DE may be greatly increased by using an appropriately inclined bias magnetic field, which has been experimentally confirmed in a pure YIG waveguide. By using an inclined bias magnetic field and the Bi-doped YIG waveguide, under the phase-matching conditions, the maximum DE can come up to 79.5% or more.
Preparation and characterization of Pb(Nb,Ti)O3 thin films by metalorganic chemical vapor deposition
Authors: Matsuzaki Tomokazu , Funakubo Hiroshi
Published: July 2002

Lead niobium titanate, Pb(Nb,Ti)O3, thin films were prepared on various substrates by metalorganic chemical vapor deposition from the Pb(C11H19O2)2–Nb(O⋅C2H5)5–Ti(O⋅i-C3H7)4–O2 system. Polycrystalline films with (111) orientation and epitaxially grown films with c-axis orientation were deposited on the (111)Pt/Ti/SiO2(100)Si and PbTiO3/(111)Pt/Ti/SiO2/(100)Si substrates, and (100)SrRuO3//(100)LaAlO3 substrate, respectively, at 620 °C. Nb content in the film can be controlled only by changing the input gas flow rate of Nb(O⋅C2H5)5 under the excess gas flow rate of Pb(C11H19O2)2. The composition of the film was independent of the deposition temperature from 400 to 620 °C. The single phase of Pb(Nb,Ti)O3 was deposited up to the Nb content of about 5 at % and pyrochlore phase was codeposited above this Nb content. Leakage current density decreased with the increasing Nb content in the film up to 6.5 at %. Coercive field (Ec) of the film was about 130 kV/cm and was independent of the Nb content and the kinds of substrates. On the other hand, the remanent polarization (Pr) of the film on the (111)Pt/Ti/SiO2/(100)Si substrate increased from 9 to 29 μC/cm2 with the increasing Nb content from 2.1 to 4.8 at %. Pr of the film on the (100)SrRuO3//(100)LaAlO3 substrate was smaller than those of the (111)Pt/Ti/SiO2/Si and PbTiO3/(111)Pt/Ti/SiO2/Si substrates.