bluuebunny/crossref_metadata_2025 huggingface dataset:
Domain structures and domain wall pinning in arrays of elliptical NiFe nanoelements
Authors: Scholz Werner , Suess Dieter , Schrefl Thomas , Fidler Josef
Published: July 2002

Stable domain patterns in a chain of NiFe nanoelements have been investigated using finite element micromagnetic simulations. The solution of the Landau-Lifshitz-Gilbert equation provides the hysteresis curves as well as the dynamic response of the system subject to an external field. We have studied elliptical elements with a long axis of 165 nm, a short axis of 55 nm and a thickness of 10 nm. Due to the shape anisotropy and magnetostatic interactions neighboring elements spontaneously align their magnetization antiparallel, if the magnetization of the elements was initially parallel to the short axis. If the initial magnetization is antiparallel, the magnetostatic stray field stabilizes this configuration. If the elements are in contact with each other, the effect of domain wall magnetoresistance can be used for applications. Chains of six elliptic elements with contact faces have been investigated. For sufficiently small contact faces, the antiparallel domain pattern is maintained, even though there is exchange coupling between neighboring elements. For small contact faces the stability of the antiparallel pattern is shown and a switching field of 48 kA/m has been obtained. The switching fields can be tailored by the shape and size of the elements.
Influence of write conditions on thermal stability
Authors: Brockie Richard M. , Richter Hans Jürgen
Published: July 2002

Thermal stability measurements of thin-film magnetic media are vital in determining the credibility of a medium design. Using our measurement technique, a series of thermal decay measurements have been performed to investigate the effect of various writing conditions. We find that as writing performance degrades, the measured thermal decay increases and that one must ensure that the medium is saturated at writing prior to a thermal stability measurement. It is therefore crucial that a decay test be carried out correctly, otherwise one runs the risk of rejecting stable medium designs.
Exchange anisotropy between single twin domain NiO and NiFe
Authors: Lai Chih-Huang , Ming Tai , Erwin R. , Borchers J.
Published: July 2002

Single-twin (T)-domain NiO films were obtained by stress induced during the growth, and the exchange anisotropy between NiO and NiFe films was studied by using micromagnetic modeling. The compressive strain along the [111] is −7.16×10−4, which forces spins of NiO to lie on the (111) plane parallel to the film surface. The neutron diffraction shows that the moments of NiO predominantly lie in the film plane with random orientations, that is, the NiO is of a single T-domain, and multi-spin-rotation-domains in the film plane. The effective crystalline anisotropy of (111) NiO, calculated from micromagnetic modeling, is 2.4×104 ergs/cm3. The majority of NiO moments at the interface switched along with NiFe moments due to the low effective anisotropy, leading to a large coercivity and a low exchange field.
Vibration-damping structure for an x-ray microprobe supporting system
Authors: Xu Shenglan , Lai Barry , Cai Zhonghou , Shu Deming
Published: July 2002

Focusing 8 keV x rays to a spot size of 150 and 90 nm full width at half maximum has been demonstrated at the first- and third-order foci, respectively, of a phase zone plate at the Advanced Photon Source 2-ID-D x-ray microprobe experiment station [Yun et al., Rev. Sci. Instrum. 70, 2238 (1999)]. In order to perform an x-ray microprobe experiment with such a high spatial resolution, vibration control of the x-ray microprobe supporting system becomes a critical issue. Recently, we have designed and constructed a vibration-damping structure for the APS 2-ID-D x-ray microprobe experiment station. In this article the vibration-damping structure design as well as the vibration test results for the x-ray microprobe supporting system are presented. This is an essential improvement toward future operation of the microprobe at sub-100-nm spatial resolution.
Control and acquisition systems for new scanning transmission x-ray microscopes at Advanced Light Source (abstract)
Authors: Tyliszczak T. , Hitchcock P. , Kilcoyne A. L. D. , Ade H. , Hitchcock A. P. , Fakra S. , Steele W. F. , Warwick T.
Published: July 2002

Two new scanning x-ray transmission microscopes are being built at beamline 5.3.2 and beamline 7.0 of the Advanced Light Source that have novel aspects in their control and acquisition systems. Both microscopes use multiaxis laser interferometry to improve the precision of pixel location during imaging and energy scans as well as to remove image distortions. Beam line 5.3.2 is a new beam line where the new microscope will be dedicated to studies of polymers in the 250–600 eV energy range. Since this is a bending magnet beam line with lower x-ray brightness than undulator beam lines, special attention is given to the design not only to minimize distortions and vibrations but also to optimize the controls and acquisition to improve data collection efficiency. 5.3.2 microscope control and acquisition is based on a PC computer running WINDOWS 2000. All mechanical stages are moved by stepper motors with rack mounted controllers. A dedicated counter board is used for counting and timing and a multi-input/output board is used for analog acquisition and control of the focusing mirror. A three axis differential laser interferometer is being used to improve stability and precision by careful tracking of the relative positions of the sample and zone plate. Each axis measures the relative distance between a mirror placed on the sample stage and a mirror attached to the zone plate holder. Agilent Technologies HP 10889A servo-axis interferometer boards are used. While they were designed to control servo motors, our tests show that they can be used to directly control the piezo stage. The use of the interferometer servo-axis boards provides excellent point stability for spectral measurements. The interferometric feedback also provides active vibration isolation which reduces deleterious impact of mechanical vibrations up to 20–30 Hz. It also can improve the speed and precision of image scans. Custom C++ software has been written to provide user friendly control of the microscope and integration with visual light microscopy indexing of the samples. The beam line 7.0 microscope upgrade is a new design which will replace the existing microscope. The design is similar to that of beam line 5.3.2, including interferometric position encoding. However the acquisition and control is based on VXI systems, a Sun computer, and LABVIEW™ software. The main objective of the BL 7.0 microscope upgrade is to achieve precise image scans at very high speed (pixel dwells as short as 10 μs) to take full advantage of the high brightness of the 7.0 undulator beamline. Results of tests and a discussion of the benefits of our scanning microscope designs will be presented.
Development of in situ x-ray tomography-diffraction technique (abstract)
Authors: Chu Y. S. , Carlo F. De , Almer J. D. , Mancini D. C.
Published: July 2002

We describe the experimental technique being developed at the bending magnet beamline 2-BM at the Advance Photon Source, which allows in situ measurement of x-ray microtomography and x-ray diffraction. With the combined use of a double crystal monochromator (dE/E∼10−4) and double multilayer monochromator (dE/E∼5×10−2), we have the capacity to make diffraction measurements with high energy resolution and tomography measurements with increased flux. The tomography-diffraction technique has application for studying deformation or fracture process in engineering materials where the measurements both in the real space (e.g., three dimensional mapping of cracks) and in the reciprocal space (e.g., peak shift and broadening due to strain) are extremely valuable. Combining these two complimentary techniques under the same experimental setup can bring the added advantage that both the propagation of cracks and the strains around the cracks can be measured while the specimen is under load. With the high-throughput data acquisition system and a cluster-based parallel computing system, topographic data acquisition and reconstruction of the 512×512×512 voxel sample with a few micron resolution can be obtained in less than 10 min. The x-ray diffraction measurement is performed using a charge coupled device (CCD) camera mounted on two orthogonal linear stages. This offers tracking capability of the diffracted beam to eliminate the systematic errors associated with the measurements of diffraction pattern at fixed distance from the sample. The CCD pixel size of 22.5 μm provides a strain resolution better than 10−4 for most engineering materials.
X-ray fluorescence beamline at the LNLS: Current instrumentation and future developments (abstract)
Authors: Pérez C. A. , Bueno M. I. S. , Neuenshwander R. T. , Sánchez H. J. , Tolentino H.
Published: July 2002

The x-ray fluorescence (XRF) beamline, constructed at the Brazilian National Synchrotron Radiation Laboratory (LNLS-http://www.lnls.br), has been operating for the external users since August of 1998 (C. A. Pérez et al., Proc. of the European Conference on Energy Dispersive X-Ray Spectrometry, Bologna, Italy, 1998, pp. 125–129). The synchrotron source for this beamline is the D09B (15°) dipole magnet of the LNLS storage ring. Two main experimental setups are mounted at the XRF beamline. One consists of a high vacuum chamber adapted to carry out experiments in grazing excitation conditions. This allows chemical trace and ultratrace element determination on several samples, mainly coming from environmental and biological sciences. Another setup consists of an experimental station, operated in air, in which x-ray fluorescence analysis with spatial resolution can be done. This station is equipped with a fine conical capillary, capable of achieving 20 μm spatial resolution, and with an optical microscope in order to select the region of interest on the sample surface. In this work, the main characteristic of the beamline, experimental stations as well as the description of some new experimental facilities will be given. Future development in the instrumentation focuses on an appropriate x-ray optic to be able to carry out chemical trace analysis of light elements using the total x-ray fluorescence technique. Also, chemical mapping below 10 μm spatial resolution, while keeping high flux of photon on the sample, will be achieved by using the Kirkpatrick–Baez x-ray microfocusing optic.
Quantitative microtomography of ultrastructure in bones with 1 μm volumetric resolution (abstract)
Authors: Lai Barry , Flynn M. J. , Seifert H. A. , Cai Z. , Irving T. C.
Published: July 2002

Refractive and diffractive artifacts can dominate the spatial resolution and affect quantitative measurements by microtomography at the micron level. We have recently developed x-ray computed tomography methods that demonstrated 1 μm resolution in three dimension in the final reconstruction. The first method used a Fresnel zone plate to produce a submicron focal spot which was then rastered across the sample see W. Yun et al., Rev. Sci. Instrum. 70, 2238 (1999). The second method used a collimated beam and a high resolution charge coupled device camera to capture the absorption image of the sample in close proximity see A. Koch et al., J. Opt. Soc. Am. A 15, 1940 (1998). Both methods were applied to study the mineral ultrastructure of individual trabeculae. Using volumetric viewing, the volume, shape, and orientation of osteocyte lacunae and major cannaliculae can be observed. Quantitative measurements and comparison between the two methods will be presented.
Self-amplified spontaneous emission saturation at the Advanced Photon Source free-electron laser (abstract) (invited)
Authors: Moog E. R. , Milton S. V. , Arnold N. D. , Benson C. , Berg W. , Biedron S. G. , Borland M. , Chae Y.-C. , Dejus R. J. , Den Hartog P. K. , Deriy B. , Erdmann M. , Gluskin E. , Huang Z. , Kim K.-J. , Lewellen J. W. , Li Y. , Lumpkin A. H. , Makarov O. , Nassiri A. , Sajaev V. , Soliday R. , Tieman B. J. , Trakhtenberg E. M. , Travish G. , Vasserman I. B. , Vinokurov N. A. , Wiemerslage G. , Yang B. X.
Published: July 2002

Today, many bright photon beams in the ultraviolet and x-ray wavelength range are produced by insertion devices installed in specially designed third-generation storage rings. There is the possibility of producing photon beams that are orders of magnitude brighter than presently achieved at synchrotron sources, by using self-amplified spontaneous emission (SASE). At the Advanced Photon Source (APS), the low-energy undulator test line (LEUTL) free-electron laser (FEL) project was built to explore the SASE process in the visible through vacuum ultraviolet wavelength range. While the understanding gained in these experiments will guide future work to extend SASE FELs to shorter wavelengths, the APS FEL itself will become a continuously tunable, bright light source. Measurements of the SASE process to saturation have been made at 530 and 385 nm. A number of quantities were measured to confirm our understanding of the SASE process and to verify that saturation was reached. The intensity of the FEL light was measured versus distance along the FEL, and was found to flatten out at saturation. The statistical variation of the light intensity was found to be wide in the exponential gain region where the intensity is expected to be noisy, and narrower once saturation was reached. Absolute power measurements compare well with GINGER simulations. The FEL light spectrum at different distances along the undulator line was measured with a high-resolution spectrometer, and the many sharp spectral spikes at the beginning of the SASE process coalesce into a single peak at saturation. The energy spread in the electron beam widens markedly after saturation due to the number of electrons that transfer a significant amount of energy to the photon beam. Coherent transition radiation measurements of the electron beam as it strikes a foil provide additional confirmation of the microbunching of the electron beam. The quantities measured confirm that saturation was indeed reached. Details are given in Milton et al., Science 292, 2037 (2001) (also online at www.sciencexpress.org as 10.1126/science. 1059955, 17 May 2001), and Lewellen et al., “Present Status and Recent Results from the APS SASE FEL,” to be published in the Proceedings of the 23rd International Free-Electron Laser Conference, Darmstadt, Germany, 20–24 August 2001.
An x-ray analog of the Michelson interferometer (abstract)
Authors: Fezzaa K. , Lee W.-K.
Published: July 2002

The first chromatic hard x-ray interferometer with a large and variable path length difference (PLD), based on a variation of the all-Bragg Graeff–Bonse x-ray interferometer has been built and successfully tested. See W. Graeff and U. Bonse, Z. Phys. B 27, 19 (1977). Such a capability can be used for better beam coherence measurements and also opens up the possibility of Fourier transform spectroscopy in the x-ray regime. Going through this interferometer, the beam is split in two and then recombined, by a set of four Bragg reflections in a (+−−+) geometry. In order to incorporate a large variable PLD between the two beams, a small angle (1°) was partly cut into the mirror faces (middle two reflections) in one of the arms of the interferometer. The PLD thus varies depending on the incident beam location on the beamsplitter surface. In this design, a very large PLD (millimeters) is easily achievable and is limited only by the available size of the silicon crystal. A larger angular cut in the mirror surfaces will also increase the achievable PLD. The test consisted of measuring the interference fringes visibility as a function of the PLD. The fringes are obtained by measuring the exit beam intensity through small slits (25 μm), while a plastic wedge is inserted in one of the beam paths and translated in the direction of its thickness gradient. Based on these measurements, fringe visibility analysis was performed to give the transmitted beam coherence lengths. The results agree very well with the expected coherence values based on the interferometer’s angular and spectral acceptances.
Diffraction dipole beamline and end station on the Center for the Advancement of Natural Discoveries using Light Emission synchrotron light source (abstract)
Authors: Abovyan E. S. , Grigoryan A. H. , Mikaelyan R. A.
Published: July 2002

The preliminary instrumentation for diffraction dipole beamline for a 3 GeV synchrotron light source called CANDLE—Center for the Advancement of Natural Discoveries using Light Emission, in the Republic of Armenia is given. The beamlines are expected to be used for both white and, monochromatic hard x-ray radiation ranging from 5 to 30 keV. One of the topics of study—material science research—that can be conducted at the end station is described, which includes the white topography, phase contrast imaging, interferometry, roentgenography, fluorescence analysis, and diffraction optics.
Bending-magnet beamline for x-ray microtechniques at the Advanced Photon Source (abstract)
Authors: Mancini Derrick C. , DeCarlo Francesco , S. Chu Yong , Lai Barry
Published: July 2002

Beamline 2-BM at the Advanced Photon Source has been fully commissioned for a range of x-ray microtechniques including micromachining, microtomography, and microcharacterization by scattering and fluorescence. The beamline has been designed and constructed to provide a highly collimated beam with great flexibility in tuning the energy bandpass. To achieve this, the beamline incorporates two mirrors, filters, and two monochromators allowing selection of energy in the range of 3–33 keV with a bandpass in the range of 1–1000 eV. The endstation includes precision instrumentation for deep x-ray lithography, x-ray microtomography, x-ray imaging, x-ray optics characterization, and the development of techniques for high-throughput x-ray microcharacterization of libraries of samples. The various experimental systems are controlled and integrated in the station to allow for the flexibility of techniques, while improving efficiency of use. We describe in detail the beamline design, capabilities, and endstation instrumentation.
Device for frequency chirp measurements of optical transmitters in real time
Authors: Niemi Tapio , Tammela Simo , Ludvigsen Hanne
Published: July 2002

We present a new automated device for measurement of time-resolved frequency chirp of optical transmitters. It measures chirp in real time by utilizing two temperature-tunable silicon-wafer etalons as frequency discriminators. The time-resolved frequency chirp in a range of ±12 GHz can be measured with a time resolution of approximately 40 ps.
Electron spin resonance experiments at ultralow temperatures by means of a dilution refrigerator
Authors: Sakon T. , Koyama K. , Motokawa M.
Published: July 2002

An electron spin resonance (ESR) apparatus with a high sensitivity vector network analyzer has been developed for use at ultralow temperatures. The microwave frequency can be varied between 45 and 110 GHz. The resonant cavity was installed in a He3–4He dilution refrigerator which provides ultralow temperatures down to 160 mK. In order to avoid the heating of the cavity and the sample by microwaves or heat leaks in the cryostat, attenuators and thermal anchors were placed at the most suitable positions. As the first experiment, the ESR was measured for a quantum spin chain Cu benzoate. The breather mode, which is a new ESR mode caused by a field induced gap, has been clearly observed at the lowest temperatures.
An oscilloscope trigger method for capturing elusive events
Authors: Gupta Deepak K. , John P. I.
Published: July 2002

A low cost trigger method, based on the change in slope of the signal, is used to capture infrequent and irregular events from a stream of regular events is described. This procedure is used to separate the modified corona current pulses, which arise due to dust interaction with corona discharge, from the regular corona current pulses. An electronic circuit, used in this procedure is described. The usefulness of this technique in studying glow to arc transition in plasma processing reactors and disruption phenomena in Tokamaks is also discussed.
Electronic-structure-based investigation of magnetism in the Fe8 molecular magnet
Authors: Pederson Mark R. , Kortus Jens , Khanna S. N.
Published: July 2002

We have performed density-functional-based electronic structure calculations on a single Fe8 molecular nanomagnet. Our calculated total moments and local moments are in excellent agreement with experiment. By including spin–orbit coupling we determine the easy, medium, and hard axes and find the ordering of the principle axes also agrees with experiment. From our calculated anisotropy Hamiltonian, we calculate the oscillations in the tunnel splittings and compare to the experimental results.
Single molecule magnets: High frequency electron paramagnetic resonance study of two isomeric forms of an Mn12 molecule
Authors: Aubin S. M. J. , Sun Z. , Rumberger E. M. , Hendrickson D. N. , Christou G.
Published: July 2002

Different crystallographic forms of the single molecule magnet [Mn12O12(O2CR)16(H2O)4] (complex 1) with a given R substituent have been isolated. The two different isomeric forms of the p-methylbenzoate complex crystallize as [Mn12O12(O2CC6H4-p-Me)16(H2O)4]⋅(HO2CC6H4-p-Me) (complex 2) and [Mn12O12(O2CC6H4-p-Me)16(H2O)4]⋅3H2O (complex 3). In complex 2, one MnIII ion has an abnormal Jahn–Teller distortion axis oriented at an oxide ion, and thus 2 and 3 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 2 compared with that of complex 3. Complex 2 likely has a larger tunneling matrix element and this explains why this complex shows an out-of-phase ac peak (χM″) in the signal in the 2–3 K region, whereas complex 3 has its χM″ peak in the 4–7 K range, i.e., the rate of tunneling of magnetization is greater in complex 2 than complex 3. High frequency electron paramagnetic resonance (HFEPR) experiments were performed on both isomers. Computed simulations of the experimental HFEPR data yield spin Hamiltonian parameters for both complexes.
Intrinsic spin valves in the layered manganite La1.4Sr1.6Mn2O7
Authors: Nachtrab T. , Heim S. , Mößle M. , Kleiner R. , Waldmann O. , Koch R. , Müller P. , Kimura T. , Tokura Y.
Published: July 2002

We report on transport measurements on micron-sized mesa structures fabricated on single crystals of the layered manganite La1.4Sr1.6Mn2O7, probing the magnetoresistance of a single magnetic domain for current transport perpendicular to the layers. In magnetic fields applied perpendicular to the layers switching of the magnetoresistance was observed in fields well below 0.4 T, while for parallel fields it decreased continuously. We interpret these observations within the framework of spin-polarized tunneling between adjacent MnO2 bilayers forming an intrinsic stack of spin valves.
Local properties at the boundaries of irradiated regions in La2/3Ca1/3MnO3 colossal magnetoresistance films
Authors: Vlasko-Vlasov V. K. , Welp U. , Miller D. J. , Lin Y. K. , Crabtree G. W.
Published: July 2002

Magnetic and magnetotransport properties and magnetization patterns in La2/3Ca1/3MnO3 films irradiated with heavy ions through a mask to yield a sharp boundary between irradiated and unirradiated areas were studied. It is found that this boundary enhances locally the resistance and magnetoresistance (MR) and shifts the maximum of MR to larger temperatures. Magneto-optical observations reveal a strong local anisotropy at the boundary tilting magnetic moments from the film plane and producing strong magnetic inhomogeneity responsible for the increase of magnetoresistance.
High-field magnetoresistance, giant magnetoresistance, and superparamagnetism in Co/Cu multilayers
Authors: Elefant D. , Tietjen D. , Schaefer R. , Eckert D. , Kaltofen R. , Mertig M. , Schneider C. M.
Published: July 2002

High-field magnetoresistance (HMR) in fields up to μ0H=16 T was measured on dc magnetron sputtered Co/Cu multilayers (MLs) with tCu=0.6–1.1 nm and tCo=0.3–4 nm, in the temperature range T=4.2–300 K. With decreasing single layer thickness the HMR at 300 K exceeds 10% and it is far from saturation up to μ0H=16 T. Independent of the HMR in the low field region μ0H⩽3 T a strongly Cu layer dependent GMR=0–60% is observed. Decreasing the temperature below 150 K down to 4.2 K the HMR changes into a GMR type field dependence with pronounced saturation. The superparamagnetic-like HMR dependence on H and T for high temperatures does not scale with H/T. Kerr microscopy on MLs, showing both HMR and GMR, reveals ferromagnetically and antiferromagnetically coupled domains, whereas in MLs showing HMR but not GMR only large ferromagnetic domains are visible. The results, including magnetization and roughness measurements, suggest the existence of very small Co clusters (almost in the atomic scale) with canted magnetic moments, causing the HMR to be independent of the antiferromagnetic and ferromagnetic coupling of the Co layers, respectively.
Field dependence of the switching field for nonellipsoidal single domain particles
Authors: Pardavi-Horvath Martha , Vertesy Gabor
Published: July 2002

Experimental data on a model system of a two-dimensional array of single domain garnet particles, switching by incoherent rotation, are presented to show that the switching field of individual particles, Hsw, and the coercivity of the major hysteresis loop for ∼1000 particles, Hc, depend on the previously applied saturating field. For the system measured the asymptotic, “true” value of Hc in large fields is 321 Oe, in contrast with Hc=225 Oe, measured in an applied field of Hsat=188 Oe, i.e., the smallest field adequate to close the major loop. Statistical data were collected on switching of a single particle, with an asymptotic value of Hsw=150 Oe. After the application of Hsat=160 Oe Hsw decreased to 111 Oe. Due to the nonellipsoidal shape of the particles, a significant canting of the magnetization near corners and edges persists up to very high fields. The torque, due to these canted magnetic moments, facilitates premature switching in lower fields. It is proposed that defects are responsible for the irreversibility of the process.
Magnetization reversal of individual nanowires with controlled defects
Authors: Sokolov A. , Sabirianov R. , Wernsdorfer W. , Doudin B.
Published: July 2002

Low temperature magnetization reversal measurements were performed on individual permalloy nanowires of diameters between 30 and 60 nm. During the electrochemical growth of the wire, a defect was induced by a short pulse in the deposition potential modifying locally the microstructure and composition. Magnetic measurements performed with micro-superconducting quantum interference devices were performed. The angular dependence of switching field revealed significant deviations from classical predictions. For specific angles, magnetization curves indicate a reversal occurring in two steps.
Ferromagnetic III–Mn–V semiconductor multilayers: Manipulation of magnetic properties by proximity effects and interface design (invited)
Authors: Furdyna J. K. , Liu X. , Sasaki Y. , Potashnik S. J. , Schiffer P.
Published: July 2002

We have investigated the degree to which magnetic properties of ferromagnetic (FM) III–Mn–V semiconductors can be modified by the proximity of other magnetic layers. This study was carried out on multilayers comprised of FM GaMnAs and either MnTe or ZnMnSe overlayers, both latter systems involving antiferromagnetic interactions between the Mn ions. The overlayers were grown directly adjacent to GaMnAs, or were separated from it by nonmagnetic layers (GaAs). We have observed that the presence of either MnTe or ZnMnSe overlayers leads to dramatic modifications of the coercive field of GaMnAs. In the case of GaMnAs/GaAs/MnTe multilayers the coercive field of GaMnAs could be modified by varying the thickness of the GaAs spacer. Additionally, in samples with both types of overlayers an increase in the Curie temperature was observed, as compared to “bare” GaMnAs.
Torque saturation and inherent temperature compensation of hybrid stepper motor used in space application
Authors: Rajagopal K. R. , Singh B. , Singh B. P. , Murthy S. S.
Published: July 2002

Hybrid stepper motors used in space applications will face varying supply voltage and a wider range of ambient temperatures. A higher reversible temperature coefficient of Br for SmCo5, Sm2Co17, or NdFeB magnets in these motors will cause undesirable changes in their performance. In this article, a method to achieve constant torque as an inherent feature in a hybrid stepper motor used as the drive motor of a solar array of a satellite is described. Holding torque specification of the motor in prime/redundant mode is 0.55 Nm±10% and in synchronized mode–a mode in which both prime and redundant windings are simultaneously energized—it is 0.75 Nm±10%: the supply voltage ranging from 28 to 42 V dc and ambient temperature varying from −55 °C to +75 °C. Based on finite element analysis (FEA) investigation on the operation of this motor, a “torque saturation region” has been incorporated in the design of the motor to permit constant torque despite voltage variations. Measured holding torques at the ambient temperature of +25 °C and at 28, 35 and 42 V are 0.49 Nm, 0.57 Nm, and 0.63 Nm in the prime/redundant mode and 0.76 Nm, 0.80 Nm, and 0.81 Nm in the synchronized mode against the FEA calculated values of 0.49, 0.55, 0.61, 0.73, 0.75, and 0.76 Nm, respectively: these values meet the requirements. The maximum voltage 42 V will be available in the raw bus during the time when the solar array is able to see the sun during which the ambient temperature will be maximum: +75 °C. Measured holding torque in the prime/redundant mode in this condition is the same as that at the nominal condition. That means the adverse effect caused by the increase in operating temperature is completely neutralized by the increase in voltage. Similar phenomenon occurs for lower temperatures also. Altogether, the torque variation is from 0.55 Nm (at −55 °C and 28 V) to 0.57 Nm (at +75 °C and 42 V) in prime/redundant mode. In synchronized mode, the corresponding values are 0.79 and 0.78 Nm. Thus, a combination of torque saturation phenomenon and the occurrence of simultaneous variation in voltage and temperature can achieve nearly constant torque from the hybrid stepper motor.
Design method of three-dimensional flux magnetic leakage profiles for high precise motor drive
Authors: Lin Tsung-Kun , Chang Yeong-Hwa , Huang Der-Ray
Published: July 2002

A design method of three-dimensional finite element method of magnetic circuit model is presented to calculate the flux magnetic leakage profiles, which are induced from permanent magnet of the rotor and applied as the sole input signal for commutation logic of circuit design in motor servo. In order to obtain the unique source to evaluate the efficiency for high precise servo motor drive in a simplified driver, the relative axial geometric configurations will be constructed to calculate flux magnetic leakage density. According to the proposed method, the relative axial geometric configurations will be applied as mechanical-electromagnetic design parameter to evaluate the dynamic performances for high precise spindle motor.
Domain structures during magnetization reversal in exchange-biased layers
Authors: Kirilyuk A. , Rasing Th. , Jaffrès H. , Lacour D. , Nguyen Van Dau F.
Published: July 2002

The magnetization reversal of an exchange-biased Co/NiO layer is studied with the help of magneto-optical microscopy, as a function of the angle between the applied magnetic field and the biasing direction. Based on domain patterns, a model of the magnetization reversal in these layers is presented. The drastic changes in the domain patterns indicate different domain nucleation conditions for different directions of the effective field.
Large in-plane anisotropies in Co/Re superlattices: What’s happening at the interface?
Authors: Burgei Wesley , Pechan Michael J. , Charlton Timothy , Lederman David
Published: July 2002

We have performed magnetization and 36 GHz ferromagnetic resonance measurements as a function of in-plane angle and Co layer thickness in order to determine the interface anisotropies in epitaxial hcp (101̄0) Co(x)/Re(1.2 nm) superlattices (1.0 nm<x<3.5 nm), whose Co c axis lies in the film plane. The linear dependence of magnetization upon inverse Co layer thickness indicates the bulk of the Co layer has a magnetization of 1390 emu/cm3, and nonmagnetic Co within 0.5 nm of each Re interface. The angular variation in the resonance position yields out-of-plane, and first and second order uniaxial in-plane anisotropies. These coefficients are plotted as a function of the inverse magnetic Co layer thickness (1/tCo), yeilding their interface and volume contributions. The volume out-of-plane term is 10.7×106 erg/cm3, which compares favorably with the 12.3×106 erg/cm3 expected from shape anisotropy. Despite the c axis being in plane, a significant out-of-plane interface energy of −0.074 erg/cm2 was detected, which favors out-of-plane magnetization. The first order uniaxial in-plane term is independent of tCo. Its average value of 1.2×106 erg/cm3 is a factor of 4 smaller than that observed in macroscopic Co. The second order in-plane uniaxial term does vary linearly with 1/tCo, yielding volume and interface contributions of 1.3×105 erg/cm3 (a factor of 10 smaller than that observed in macroscopic Co and −7×10−4 erg/cm2, respectively).
Low energy electron diffraction of the system In-[perylene-3,4,9, 10-tetracarboxylic dianhydride] on MoS2
Authors: Azuma Yasushi , Iwasawa Kazuaki , Kurihara Tsutomu , Okudaira Koji K. , Harada Yoshiya , Ueno Nobuo
Published: July 2002

The system In-[perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)] on MoS2, prepared by the sequential evaporation of PTCDA and In on a cleaved MoS2 surface, was studied by low energy electron diffraction. The result indicates that reaction products form an ordered structure on the MoS2 surface. From the analysis of the diffraction pattern, the presence of six symmetry-equivalent domains of an oblique unit cell of In-PTCDA species results with the dimensions of 9.5 Å, 16.3 Å, and an enclosed angle of 80.2°. In addition, splitting in two domains by a mirror plane exists with the rotation angle R=±10.8° with respect to each of the three equivalent surface crystal axes of the MoS2 substrate. The new structure is explained by assuming that four In atoms are chemically bonded to the four carbonyl groups of the PTCDA molecules. Furthermore, it is concluded that the In4PTCDA species become tilted after a chemical reaction between the PTCDA molecules and the In atoms, which is in agreement with results previously obtained by angle-resolved ultraviolet photoemission experiments.
Modelling of magnetotransport of hot electrons in a spin-valve transistor
Authors: Hong Jisang
Published: July 2002

This article explores the magnitude of spin dependent collector current in a spin-valve transistor varying the combination of ferromagnetic layers at finite temperatures. In these calculations, the spatial inhomogeneity of the Schottky barrier at the emitter side and spin dependent self-energy effect in ferromagnets have been taken into account. In addition, the magnetocurrent has been presented as well. It has been ascertained that the magnitude of spin dependent collector current strongly depends on the type of spin-valve base since the inelastic scattering strength is different in each material. These calculations may help find the best structural combination of ferromagnetic layers in the spin-valve base.
Molecular dynamics simulation of microstructure and counterion transport in dry ionic heteropolymers
Authors: Wong Chee , Clarke Julian H. R.
Published: July 2002

Structure and counterion diffusional mobility in simple models of solvent-free linear chain ionic heteropolymers has been investigated using molecular dynamics computer simulation. A generic coarse grain Kramers model was used in which a fraction f of charged monomers are distributed uniformly along the chains. Only excluded volume and Coulomb interactions are considered. Four compositions were studied with f=0.17, 0.25, 0.33, and 0.5. Computed partial pair distribution functions show that for all compositions the short range structure is consistent with essentially complete condensation of the counterions over the whole temperature range studied. For low values of f two glass transitions can be distinguished—a higher one, Tg′, which is invariant with composition and is associated with ionic clustering, and a lower Tg″ associated with neutral regions of the material. Partial structure factors confirm that the intermediate range structure is increasingly heterogeneous as the fraction of charged chain monomers is decreased and the associated order persists at temperatures up to at least 3.5Tg′. At greater than ∼1.7Tg′ the counterion mobility is clearly decoupled from the chain motions in all samples. Diffusion coefficients do not show a strong dependence on charge fraction in the range studied although there is evidence from plots of mean squared displacements against time that for intermediate compositions, e.g., f=0.33, large amplitude motions of neutral monomers may promote an increased mobility of counterions in the approach to Tg′.
Pivot-coupled grand canonical Monte Carlo method for ring simulations
Authors: Kindt James T.
Published: July 2002

A new method is presented for the simulation of an ensemble of polymer rings of variable size at fixed monomer chemical potential. Called pivot-coupled grand canonical Monte Carlo (PC-GCMC), it is based on the directed addition or removal of a monomer to or from a ring, coupled to the pivot of a section of the ring to maintain the ring’s continuity. Application of PC-GCMC to single, isolated rings yields the free energy of the ring polymer as a function of number of monomers, information useful in determining equilibrium constants for polymer cyclization. Ring closure probabilities (“J-factors”) for flexible and semiflexible polymers, both ideal and self-avoiding, in two and three dimensions are obtained in close agreement with available results from theory and other simulation methods. New results are obtained for two-dimensional semiflexible polygons. Potential applications of the method to simulations of ring-forming equilibrium polymers, disklike micelles, and self-assembling polymer loops are discussed.
Isobaric–isothermal fluctuation theorem
Authors: Mittag Emil , Searles Debra J. , Evans Denis J.
Published: July 2002

The fluctuation theorem (FT) gives an analytical expression for the probability that in a finite nonequilibrium system observed for a finite time, the Second Law of Thermodynamics is violated. Since FT deals with fluctuations, the precise form of the theorem is dependent on the particular statistical mechanical ensemble. In the present paper we describe the application of the FT to the isothermal–isobaric ensemble.
Collision-induced dissociation and photodetachment of singly and doubly charged anionic polynuclear transition metal carbonyl clusters: Ru3Co(CO)13−, Ru6C(CO)162−, and Ru6(CO)182−
Authors: Butcher Colin P. G. , Johnson Brian F. G. , McIndoe J. Scott , Yang Xin , Wang Xue-Bin , Wang Lai-Sheng
Published: July 2002

Three polynuclear transition metal carbonyl cluster anions, Ru3Co(CO)13−, Ru6C(CO)162−, and Ru6(CO)182− have been studied using energy-dependent electrospray ionization mass spectrometry (EDESI–MS) and photodetachment photoelectron spectroscopy (PES). EDESI–MS maps show a simple collision-induced dissociation (CID) process for Ru3Co(CO)n− by stripping CO down to the metal core. For the doubly charged species, two competing CID channels were observed, viz. loss of neutral CO and loss of CO+e−. It was found that the parent dianions first lose neutral CO down to n=9, producing a series of dianions, Ru6C(CO)n2− (n=9–16) and Ru6(CO)n2− (n=9–18). For n<9, the dianions become electronically unstable against autodetachment, and singly charged anions, Ru6C(CO)n− and Ru6(CO)n− (n=0–9), were observed. The PES spectra of the dianions show the electron binding energies decrease monotonically as n decreases and become ∼0.0 eV for n=9, in exact agreement with the CID patterns that reflect the electronic instability of the doubly charged metal complexes with n<9. All of the PES spectra show congested features, indicating very high density of low-lying electronic states for the transition metal carbonyl clusters. The electron binding energies of Ru3Co(CO)n− are similar for n=4–13, but decrease sharply from n=4 to 0. The intramolecular Coulomb repulsion in the dianions was observed to increase with loss of CO, ranging from ∼1.8 eV for Ru6C(CO)162− to ∼2.4 eV for Ru6C(CO)92−.
Thin-film nucleation through molecular cluster beam deposition: Comparison of tight-binding and many-body empirical potential molecular dynamics simulations
Authors: Hu Yanhong , Shen Sanguo , Liu Lei , Jayanthi Chakram S. , Wu Shi-Yu , Sinnott Susan B.
Published: July 2002

Molecular dynamics simulations are performed to investigate the chemical products of molecular ethylene-cluster beam deposition on diamond substrates at room temperature. The substrates are hydrogen-terminated diamond (111) surfaces of varying sizes. The computational approach is molecular dynamics simulations with two different methods for determining the forces on the atoms: an empirical reactive empirical bond-order hydrocarbon potential and an order-N nonorthogonal tight-binding method. The results of these two approaches to thin-film nucleation through ethylene-cluster beam deposition are compared and contrasted. The results are used to determine the similarities, differences, advantages, and limitations of these two approaches.
Ab initio study of lutetium dimer
Authors: Yang C. L. , Zhang Z. H. , Ren T. Q.
Published: July 2002

The ab initio quadratic CI calculation method and the effective-core-potential basis set have been used to calculate the equilibrium geometries, potential energy curves, dissociation energies of the ground state, and low-lying electronic states of Lu2 for the first time. The symmetries of these states are determined by analyzing the potential energy curves and optimization calculations at the same time. The analytical potential energy functions of these states have been fitted with the Murrell–Sorbie potential energy function from the calculated values with the ab initio method. The spectroscopic parameters of each state are calculated through the relationship equations between analytical potential energy function and spectroscopic parameters, and are compared with some other theoretical and experimental data available from the experiment at present. For the ground state, we obtain the symmetry is X 3Σg, ωe=113.3 cm−1 with ωeχe=0.16 cm−1, leading to a spectroscopic dissociation energy of 1.79 eV. They are in agreement with the recent experiments.
Coulomb blockade effects and conduction mechanism in extremely thin polycrystalline-silicon wires
Authors: Kawamura Kensaku , Kidera Toshirou , Nakajima Anri , Yokoyama Shin
Published: July 2002

Narrow (⩾95 nm) and extremely thin (∼7 nm) heavily phosphorous-doped polycrystalline-silicon (poly-Si) wires were fabricated by low-pressure chemical vapor deposition. The electrical conduction mechanism has been investigated at low temperatures (down to ∼5 K), and observation by transmission electron microscopy (TEM) was carried out. Single-electron effects such as Coulomb oscillations have been observed at temperatures up to 80 K. The size of the island in the poly-Si wires was estimated from the electrical properties, and it was in the same order as the grain size of the poly-Si measured by TEM. A maximum tunnel barrier height of ∼26 meV of the poly-Si grain boundary is obtained from the temperature dependence of the conductance of the sample. A model for the electronic conduction through multiple islands was proposed from the width dependence of their electrical properties.
Reactivity and deconstruction of the (1×2)-Rh(110) surface studied by scanning tunneling microscopy
Authors: Africh C. , Esch F. , Comelli G. , Rosei R.
Published: July 2002

The titration of a (2×2)p2mg–O/Rh(110) surface by hydrogen and the subsequent deconstruction of the nonequilibrium (1×2) reconstructed surface has been studied by scanning tunneling microscopy (STM) in a temperature range between 380 and 405 K. The titration reaction is seen to occur via reaction fronts, which start at steps, are elongated in the [11̄0] direction and leave behind a surface covered by diffusing oxygen atoms. Two deconstruction mechanisms are active: A long-range row pairing of adjacent (1×2) rows, which propagates over several hundreds of Å in the [11̄0] direction, and a local hole formation in which Rh atoms detach from the ends of broken [11̄0] rows, diffuse over up to 100 Å and condense into (1×1) islands. Both mechanisms lead to the formation of correlated up–down islands; at step edges, the second one leads to a step retraction. The deconstruction is strongly defect mediated and the STM results indicate that the (1×2) reconstructed surface is most probably metastable.
Analytic method for evaluation of the field of a charge traversing a geometric discontinuity
Authors: Banna S. , Schächter L.
Published: July 2002

An analytic time-domain method is developed for the electromagnetic field generated by a charge traversing a geometric discontinuity. The essence of the method employed here is to use the linear independence of the exponential functions, that control the temporal behavior of the field. As a result, we avoid the large (“infinite”) matrix inversion necessary for a frequency-domain solution. This method was utilized for the investigation of the wake generated by electrons in an optical accelerator as well as evaluation of the emittance growth and energy spread.
Tunable and coherent nanosecond radiation in the range of 2.7–28.7 μm based on difference-frequency generation in gallium selenide
Authors: Shi Wei , Ding Yujie J. , Mu Xiaodong , Fernelius Nils
Published: July 2002

Continuously tunable and coherent radiation in the mid-IR range of 2.7–28.7 μm has been achieved in GaSe through type-I difference-frequency generation (DFG). The two mixing beams used for the DFG consist of the idler beam (0.73–1.8 μm) of an optical parametric oscillator pumped by the nanosecond Nd:YAG laser and the residual Nd:YAG fundamental beam (1.064 μm). The maximum peak output power for type-I DFG is measured to be 2.2 kW at the wavelength of 4 μm. Moreover, the type-II DFG with the kilowatt peak power and output wavelength continuously tunable in the range of 4.14–28 μm has also been achieved.
Magnetic and electrical properties of Fe-B-N amorphous films (invited)
Authors: Karamon H. , Masumoto T. , Makino Y.
Published: July 2002

A new type of ferromagnetic amorphous alloy with a two-phase structure was found in cosputtered Fe-B-N films. The microstructure was identified to be composed of two amorphous phases with about 50 Å or less in size; one is a ferromagnetic amorphous phase of Fe-B-N alloy and the other is a nonmagnetic and high resistive amorphous phase of boron nitride. The homogeneous mixture of these two phases in the films resulted in unique features for magnetic and electrical properties. In comparison to sputtered Fe-B films, the Fe-B-N films exhibit higher saturation magnetizations and the electrical resistivity is about 3–103 times higher. The compositional dependence of Curie and crystallization temperatures also differs from those of the sputtered amorphous Fe-B films. Furthermore, the corrosion resistance of the films is improved in the presence of nitrogen.