bluuebunny/crossref_metadata_2025 huggingface dataset:
Initial results on logarithmic spiral bent Laue analyzer for fluorescence x-ray absorption spectroscopy (abstract)
Authors: Karanfil C. , Chapman D. , Bunker G. , Segre C.
Published: July 2002

We have previously described [Z. Zhong, D. Chapman, B. A. Bunker, G. B. Bunker, R. Fischetti, and C. U. Segre, J. Synchrotron Radiat. 6, 212 (1999); C. Karanfil et al., AIP Conf. Proc. 521, 178 (2000)] a novel concept of using highly bent single crystals in Laue geometry for wide bandwidth monochromatization of fluorescence x rays emerging from a point source, such as the sample in fluorescence x-ray absorption spectroscopy (XAFS) experiments. In this article we describe the first measurements (summer 2000) of the fluorescence XAFS of millimolar solutions of several elements. The potential of this type of analyzer is clearly demonstrated.
Application of polarization modulation spectroscopy to the study of magnetic materials (abstract) (invited)
Authors: Sanchez-Hanke C. , Lott D. , Stohr J. , Kao C.-C.
Published: July 2002

Over the last decade, x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic scattering (XRMS) have become important experimental techniques in the study of magnetic materials. In order to reduce systematic errors in these measurements, typical XMCD and XRMS experiments are carried out using magnetization switching, although there are many situations where polarization switching is clearly desirable. Recently, fast polarization modulation techniques have been developed using either polarization conversion optical elements, such as phase plate, or special insertion devices. At the National Synchrotron Light Source (NSLS), an elliptically polarized wiggler (EPW), jointly developed by NSLS, Advanced Photon Source, and BINP of Novosibirsk, for this purpose. The EPW consists of a permanent magnet vertical wiggler and an electromagnet horizontal wiggler. The polarization of the device can be switched up to 100 Hz by switching the electromagnet. To take advantage of the fast switching capability, a phase sensitive detection system was also implemented. The sensitivity of using fast polarization modulation is demonstrated by measuring the element specific hysteresis loop and MCD spectrum of Cu induced moment at the interface of a Co/Cu multilayer. By comparing with results obtained using conventional measurements from similar samples, it clearly shows the advantages of using polarization modulation for small MCD effects. The sensitivity of this technique and the possibility of performing magnetic field dependent measurements of using polarization modulation have been applied to a number of magnetic systems. First, the spatial distribution of Cr induced moment in an ideal exchange-biased Fe/Cr multilayer was measured using soft-x-ray XRMS. Specular reflectivity was measured as a function of both angle and energy near Cr and Fe L3 edges. The Cr induced moment was clearly observed. Moreover, the spatial distribution of the induced Cr moment was determined by combining the measurements with detailed simulation of the energy and angle dependent specular reflectivity. Second, the magnetic behavior and spatial distribution of the interfacial Ni spins in an exchange-biased Co/NiO bilayer were studied using angle and energy dependent specular reflectivity as well as total electron yield. These results will be discussed in relation to several existing models of interface spin structures for exchange bias. Other applications of polarization modulation will also be discussed.
Coherent inelastic Mössbauer scattering of synchrotron radiation (abstract)
Authors: Belyakov V. A.
Published: July 2002

Recent success of coherent elastic [Nuclear Resonant Scattering of Synchrotron Radiation, Part A edited by E. Gerdau and H. de Woard (Baltzer Science, 2000), Hyperfine Interact. 123/124, Chap. 4] and incoherent inelastic (Hyperfine Interact. 123/124, Chap. 5) Mössbauer scattering of synchrotron radiation (SR) in investigations of very delicate properties of the condensed matter also makes it urgent to perform experiments on coherent inelastic Mössbauer scattering (CIMS) of synchrotron radiation (the common meaning of the term CIMS is coherent inelastic Mössbauer scattering accompanied by creation or annihilation of phonons in the crystal lattice, i.e., by very low energy losses of SR quanta). However up to now there were no publications on experimental observation of CIMS so there is a need in theoretical investigations to reveal the most favorable conditions for CIMS observation. The theory of CIMS is presented below and applied to specific processes of CIMS such as forward scattering, scattering at grazing incidence angles, and scattering via a cascade of Mössbauer transitions. It is shown that the phase matching (between the incident and scattered beam) is very important for the angular and frequency distribution in CIMS and processes where phase matching can be reached, which the best candidates for CIMS experimental investigations. The performed analysis shows that because of the phase matching demands the forward CIMS is suppressed significantly in comparison with the coherent elastic Mössbauer scattering [V. A. Belyakov, JETP Lett. 67, 8 (1998)] and more favorable for observation is CIMS at a nonzero scattering angle. Some examples of CIMS specific geometries are discussed. In particular, it is shown that for the grazing CIMS at isotope interface (a plane interface between regions with different abundance of the Mössbauer isotope) there is enhancement of CIMS at the critical angle of total reflection and suppression of CIMS at angles below the critical one [V. A. Belyakov, JETP Lett. 68, 287 (1998); V. A. Belyakov and S. V. Semenov, JETP 90, 290 (2000)]. Another possibility of CIMS in a more general meaning of the term is Mössbauer scattering of SR via a cascade of Mössbauer transitions (CIMC) which is connected with huge losses of energy by SR quanta in the process [V. A. Belyakov and Yu. M. Aivazian, Nucl. Instrum. Methods, Phys. Res. A 359, 190 (1995); 448, 222 (2000)]. Analysis of CIMC for the two transitions cascade in Fe57 and the corresponding calculations are presented. Optimal conditions of experimental observation for various cases of CIMS are discussed.
Submicron stabilization of the x-ray beam position on long beamlines (abstract)
Authors: Fischetti R. F. , Heurich R. , Perry D. , Stepanov S. , Kondrashkina E. , Rosenbaum G.
Published: July 2002

The stability of the particle beam position and angle on third generation synchrotron sources such as the Advanced Photon Source (APS) at Argonne National Laboratory is extremely good. However, because of the long length of beamlines (50–70 m) at such sources, small perturbations of the particle beam angle can result in undesirable beam movement in the experimental end station. Additionally, instabilities in the beamline optics may also result in beam movement. At the BioCAT undulator source beamline at the APS, we have employed an x-ray beam position monitor (BPM) [R. Alkire, G. Rosenbaum, and G. Evans, J. Synchrotron Radiat. 7, 61 (2000)] in the experimental station at 61 m from the source to record the real time beam position in both the horizontal and vertical directions. A comparison of the rf-BPM in the storage ring and the x-ray BPM in the beamline confirm that positional changes of ±20 μm in the experimental end station can be traced to angular changes of ±0.3 μrad in the particle beam angle. Motion on this scale is particularly undesirable for small angle and solution scattering experiments that require small focal sizes on the order of 50 μm and positional stability on the order of 5 μm. In this article we demonstrate that the vertical beam position can be stabilizing to better than ±1 μm at distances of 60–70 m from the source. This was accomplished by using the position output in a closed loop feedback system to drive a piezo translator effecting the Bragg angle of the second crystal of the double crystal monochromator.
Software feedback for monochromator tuning at UNICAT (abstract)
Authors: Jemian Pete R.
Published: July 2002

Automatic tuning of double-crystal monochromators presents an interesting challenge in software. The goal is to either maximize, or hold constant, the throughput of the monochromator. An additional goal of the software feedback is to disable itself when there is no beam and then, at the user’s discretion, re-enable itself when the beam returns. These and other routine goals, such as adherence to limits of travel for positioners, are maintained by software controls. Many solutions exist to lock in and maintain a fixed throughput. Among these include a hardware solution involving a wave form generator, and a lock-in amplifier to autocorrelate the movement of a piezoelectric transducer (PZT) providing fine adjustment of the second crystal Bragg angle. This solution does not work when the positioner is a slow acting device such as a stepping motor. Proportional integral differential (PID) loops have been used to provide feedback through software but additional controls must be provided to maximize the monochromator throughput. Presented here is a software variation of the PID loop which meets the above goals. By using two floating point variables as inputs, representing the intensity of x rays measured before and after the monochromator, it attempts to maximize (or hold constant) the ratio of these two inputs by adjusting an output floating point variable. These floating point variables are connected to hardware channels corresponding to detectors and positioners. When the inputs go out of range, the software will stop making adjustments to the control output. Not limited to monochromator feedback, the software could be used, with beam steering positioners, to maintain a measure of beam position. Advantages of this software feedback are the flexibility of its various components. It has been used with stepping motors and PZTs as positioners. Various devices such as ion chambers, scintillation counters, photodiodes, and photoelectron collectors have been used as detectors. The software provides significant cost savings over hardware feedback methods. Presently implemented in EPICS, the software is sufficiently general to any automated instrument control system.
A synchronized rotating crystal beam chopper (abstract)
Authors: McPherson Armon , Lee Wah-Keat , Mills Dennis M.
Published: July 2002

A rotating crystal beam chopper that can be synchronized to the synchrotron storage ring frequency has been successfully fabricated and tested. The crystal chopper uses the storage ring frequency master clock as input and operates at 1/500 times its input frequency. It is phase locked to the input frequency. At the Advanced Photon Source, using the ring frequency clock (∼271 kHz), the crystal beam chopper rotates at about 542 Hz, with less than 3 ns jitter in the period. With its Si(111) reflecting surfaces, this corresponds to a transmission window of about 10 ns for 10 keV x rays, assuming the upstream monochromator is also Si(111). By dividing its input frequency, it is possible to operate the crystal chopper at 271 and 135.5 Hz for wider transmission windows. To date, this is the only device that has demonstrated a ns range transmission window and operates phase locked to the synchrotron storage ring frequency.
Thermal hysteresis of microwave loss in (La 1−xPrx)0.7Ca 0.3MnO3 films
Authors: Kale Sangeeta , Lofland S. E. , Bhagat S. M. , Garcia-Miquel H. , Ogale S. B. , Shinde S. R. , Venkatesan T.
Published: July 2002

We have measured the temperature (T) dependencies of the dc resistances (Rdc) and the microwave loss (Rμw) in a variety of samples of (La1−xPrx)0.7Ca0.3MnO3 while varying x from 0 to 0.4. Whereas both the sets of data exhibit maxima, the ac loss peak is much flatter and, during cooling, appears at a much lower temperature than the peak temperature in Rdc. The discrepancy, which vanishes for x=0, increases with lowering tolerance factor (t) (or increasing x). Also Rμw vs T exhibits large thermal hysteresis for x=0.4 indicating that the transition is first order. Cooling in a magnetic field of 9 kOe causes an upward shift of about 20 K in the Rμwpeak, in some of the x=0.4 films, yielding a large magnetoimpedance. Further, once these films are exposed to a magnetic field at low T, they fail to recover their virginal behavior on subsequent cooling from room T. These films could be brought to their original state by annealing at high T. The discrepancy between Rdc and Rμwimplies that the system is inhomogeneous at low T, providing, for the first time, microwave absorption evidence that manganites exhibit multiphase behavior. Presumably, disorder and strain (increasing with x) combine to stabilize a mixed phase.
Microwave response of a high electron mobility transistor in the presence of a Dyakonov–Shur instability
Authors: Crowne Frank J.
Published: July 2002

The plasma-wave response of the two-dimensional electron gas that forms the active layer of a high-electron mobility transistor (HEMTs) makes a contribution to the high-frequency behavior of these devices that is distinct from their adiabatic response, i.e., unrelated to low-frequency parameters such as dc transconductance, capacitances, and channel resistance, which are usually derived from dc IV curves. Since the plasma-wave response has the potential to make the HEMT active at very high (terahertz) frequencies, it is important to frame its description within the standard language of microwave device engineering, i.e., as admittance or S-parameters of the device. In this paper a full set of microwave admittance parameters is derived for a real HEMT, based on recent work by the author [J. Appl. Phys. 87, 8056–8064 (2000)].
A small volume, rapid translation cryostat insert constructed from commercial components for the detection of ultrafast optical signals
Authors: Larsen Delmar S.
Published: July 2002

A linear motion device was designed and built to move small sample cells within the confined space of a liquid nitrogen Dewar cryostat. Instead of the often-used methods of flowing or rotating samples to generate motion, this cryostat insert simply translates the sample cell while maintaining atmospheric isolation. Accurate, repeatable and rapid translation over a 5 cm range with peak linear velocities of 1 m/s is attainable. The insert is constructed mainly from commercially available components and can be built with minimal effort. Another benefit is that the cryostat insert uses commercially available sample cells, allowing for maximal flexibility in satisfying pathlength and volume requirements and the cells can be easily replaced during the duration of the experiment. The motion device system can be easily installed into existing experimental apparatuses with confined volumes with minor modifications and can be used with liquid helium cryostats or for use in high vacuum applications. This motion is demonstrated by collecting temperature dependent ultrafast signals from the laser dye Rhodamine 640 in viscous glycerol at 170 and 293 K.
Generation of intense beams of Pb+4 to Pb+10 ions in a laser ion source
Authors: Balabaev A. , Kondrashev S. , Konukov K. , Logkin A. , Mescheryakov N. , Sharkov B. , Shumshurov A. , Khomenko S. , Makarov K. , Nischuk S. , Roerich V. , Satov Yu. , Smakovskii Yu. , Stepanov A.
Published: July 2002

Experiments have been carried out to optimize the yield of Pb+4 ions from the plasma produced by a 100 J CO2 laser. The laser power density on the target surface was varied between 1010 W/cm2 and 9×1010 W/cm2 by changing the focal spot size. Data on the production of Pb+4 to Pb+10 ions are presented for the 15 and 40 ns laser pulse durations. The 10 mA/80 μs pulses of Pb+4 ions (about 1012 ions per pulse) were directed into an extraction aperture of 3.4 cm in diameter for optimal irradiation conditions. This is comparable with the parameters of the MEVVA ion source. Laser ion sources can be especially attractive due to the absence of the “noise” problem, typical for the MEVVA ion source, and because of the possibility of generating higher charge states. The emittance of the lead ion beam extracted from the laser-produced plasma was measured by using a 5 J laser, which provided plasma parameters (the electron temperature, ion velocities, and the charge state distribution) close to those of the plasma generated by the 100 J CO2 laser under optimized (for the yield of Pb+4 ions) conditions. The emittance was measured for 30 and 50 kV extraction voltages with a 10 μs temporal resolution. The value of about 400 π mm mrad was obtained for the lead ion beam with the total current of 10 mA, and a pulse length of 75 μs for about 75% of the beam ions.
Differential microcalorimeter for isothermal measurements of heat effects in two-phase systems and examples of its application
Authors: Ostrovskii Victor E.
Published: July 2002

A differential heat-conducting double microcalorimeter intended for heat measurements at 300–700 K is described. Platinum resistance thermometers are applied as the heat-flux sensors. The calorimeter is a low-noise instrument with the baseline fluctuations ranging up to 0.02×10−6 V (5×10−6 W); it allows measurements of the heat effects with an error of 1%–1.5% at a recorded scale of about 1×10−2 J/cm2. The design features and the method of the heat-flux transformation to the electric signals, the calorimetric ampoules for measurements and for calibration and the calibrating device, the heat-balance equation and the equation for the temperature dependence of the calorimetric sensitivity, the results of the calibration and the verification of the calorimeter, and a number of illustrative examples of calorimetric measurements are presented. The calorimeters are applicable for studying solid–gas, solid–liquid, and liquid–gas interactions and thermophysical characteristics of solids and liquids.
Role of alloying on the shape memory effect in Ni2MnGa
Authors: MacLaren J. M.
Published: July 2002

Ni 2 MnGa undergoes a reversible structural phase transition between a cubic phase and a martensitic phase both of which are ferromagnetic. The large, controlled, shape change associated with the transition is technologically useful in a variety of applications such as actuators and sensors. The transition temperature for the structural transformation is influenced by the stoichiometry of the alloy. In this work we have calculated properties of Ni2MnGa along with several related substitutional alloys. These including substitutions that do not change the electron to atom ratio (e/a) (Al for Ga), as well as additions such as Cu for Ni and Ni for Mn that increase the e/a ratio, and Co for Ni which decreases the ratio. The transition is associated with a soft c11–c12 elastic constant which we have calculated using first principles electronic structure calculations. Changes in structural properties are correlated with changes in the electronic density of states.
Field sensing using the magnetoresistance of IrMn exchange-biased tunnel junctions
Authors: Lacour D. , Jaffrès H. , Nguyen Van Dau F. , Petroff F. , Vaurès A. , Humbert J.
Published: July 2002

An original concept of high sensitivity magnetic field sensor using the spin-dependent tunneling effect has been investigated. The required crossed-biased configuration is obtained by combining both shape energy originating from vicinal step bunched Si substrates and unidirectional exchange anisotropy supplied by an Ir20Mn80 film in the “top-biased” geometry. We demonstrate a linear and reversible signal at room temperature and above. The smooth loss of sensitivity at higher temperature is shown to be correlated to the thermal dependence of the exchange bias property when IrMn is deposited above the insulating Al2O3 barrier.
Ultrathin Co films on flat and stepped Cu(111) surfaces: Determination of per atom orbital and spin moments
Authors: Hahlin Anders , Karis Olof , Hunter Dunn Jonathan , Arvanitis Dimitri
Published: July 2002

We have used the x-ray magnetic circular dichroism technique to characterize ultrathin Co films deposited on flat and vicinal Cu(111) single crystal surfaces. All films were prepared and characterized in situ, in ultrahigh vacuum. Similar to other investigators we report a 40% increase of the ratio ml/ms (orbital to spin moment) of ultrathin Co on both flat and vicinal Cu(111). In addition, an apparently correlated enhancement of both the orbital and spin moments is observed for Co on vicinal Cu(111). This finding is in contrast to what has earlier been reported for a similar system, Co on vicinal Cu(100).
Power and initial pulse width dependent soliton velocity in ferrite films
Authors: Slavin A. N. , Zaspel C. E.
Published: July 2002

Magnetostatic backward volume wave propagation in yttrium–iron–garnet thin films supports soliton formation and propagation, which is modeled by the nonlinear Schroedinger equation. Previous experimental data have shown that the propagation velocity depends both on the initial pulse power and the duration; however, in the nonlinear Schroedinger solution the soliton velocity, or wave number deviation, is a free parameter that is independent of the initial pulse characteristics. It is shown that these dependences on the initial pulse parameters arise simply from the nonlinear nature of the magnetostatic backward volume wave dispersion relation. Moreover, this same dependence is predicted in the nonlinear Schroedinger model, but the deviation in wave number is determined by the initial pulse characteristics and is no longer a free parameter.
Ferromagnetic resonance of Fe–Sm–O thin films
Authors: Baek Jong-Sung , Min Seong-Gi , Yu Seong-Cho , Lim Woo-Young
Published: July 2002

In order to investigate the temperature and the annealing conditions dependence of magnetic properties in Fe72.1Sm5.2O22.7(at. %) thin films, such as the effective magnetization Meff, the spectroscopic splitting factor g, and the exchange stiffness constant A, we carried out ferromagnetic resonance (FMR) experiments. Fe–Sm–O thin films were deposited in an argon and oxygen mixed atmosphere using a rf magnetron sputtering apparatus at room temperature. The g is almost constant in the temperature range from 298 to 238 K, but increases with decreasing temperature in the range below 238 K. The A increases slightly with decreasing temperature to 238 K, and then increases linearly below 238 K. When the annealing time increases up to 6 h the Meff is almost constant, but the A decreases slightly with increasing annealing time.
Model of Fe nanostripes on Cu(111)
Authors: Brown G. , Lee H. K. , Schulthess T. C. , Ujfalussy B. , Stocks G. M. , Butler W. H. , Landau D. P. , Pierce J. P. , Shen J. , Kirschner J.
Published: July 2002

Magnetization as a function of temperature calculated with Monte Carlo techniques is compared to experimental results of Fe stripes grown on vicinal Cu(111) surfaces. The stripes are step decorations grown with molecular beam epitaxy (MBE), are 1–2 monolayers thick, and display perpendicular magnetization. The atomic parameters are determined from fully relativistic electronic structure calculations. The moments are found to be 2.57 μB, with some variation due to film thickness, and uniaxial anisotropy of 40 μRy/atom for Fe atoms facing vacuum. The Heisenberg model extended to include crystalline anisotropy as well as dipole–dipole interactions is considered for two different values of the exchange constant: J=20 and 2 meV. Under a large applied field (4000 G), the calculated saturation magnetization falls slowly from 507 emu/cm3 with an increase in temperature until it falls rapidly around 600 K, after which a more modest falloff with an increase in temperature is observed. For larger J the rapid change occurs for higher temperatures. The importance of disorder in the height and width of the stripes is investigated by generating stripe geometries with a model that incorporates nucleation and growth of Fe particles at step edges under the constraint of constant deposition from MBE. The primary effect of disorder in the stripes is to reduce the saturated magnetization at lower temperatures.
High magnetic field measurements on the layered III–VI diluted magnetic semiconductor Ga1−xMnxS
Authors: Pekarek T. M. , Maymi C. , Garner J. , Hall D. , Miotkowski I. , Ramdas A. K.
Published: July 2002

Magnetic measurements on Ga1−xMnxS in fields up to 25 T have been made at the National High Magnetic Field Laboratory using the cantilever technique. Ga1−xMnxS is in the new class of III–VI diluted magnetic semiconductors that are nominally two-dimensional (similar to mica). At temperatures below 30 K and in fields above 15 T the magnetization deviates from the previously reported linear behavior in lower field measurement. The diamagnetic constant for the host GaS has been measured to be M/H=−3.7×10−7 emu/g G. The magnetization of Ga1−xMnxS exhibits a small anisotropy with the easy axis along the c axis. Theoretical calculations for noninteracting moments were made and show reasonable agreement with the experimental data above 40 K.
Dynamic simulation of nanoscale lubricant films
Authors: Phillips David M. , Jhon Myung S.
Published: July 2002

The nanoscale lubricant film on the disk surface of hard disk drive systems is crucial to the durability of the head–disk interface. In an effort to better understand the performance of these lubricant films, the dynamic behavior of nanoscale, perfluoropolyether lubricant films is analyzed via kinetic Monte Carlo techniques. The lattice-based model, which represents lubricant molecules as simple reactive spheres, specifically includes long-range van der Waals, molecule–surface; interlayer, polar endgroup–endgroup coupling; and polar endgroup–surface coupling interactions. The full-blown three dimensional capabilities of the model allow for a detailed nanostructure analysis of the lubricant films. The model is used to investigate the steady-state surface structure of lubricant films, which describes the dewetting phenomenon and is examined via a function of the interaction parameters, surface loading, and initial surface structure. The evolution of the surface structures is monitored to better understand the mechanism for dewetting or “mogul” dynamics. This modeling effort initializes the groundwork for future research with heterogeneous, polar films. Critical endgroup–endgroup and endgroup–surface interaction energies are identified in terms of film dewetting. A layer-by-layer endgroup orientation analysis provides insight into the surface structure and dewetting.
Temperature-induced magnetization reorientation in arrays of magnetic dots on a magnetic substrate
Authors: Santamaria C. , Diep H. T.
Published: July 2002

We studied magnetic properties of regular arrays of magnetic dots on a magnetic layer. We took into account the exchange interaction, the dipolar interaction of amplitude D between spins, and the perpendicular anisotropy A. The spins are of the classical Heisenberg model. Due to the competition between D and A, the ground state (GS) can be perpendicular or parallel to the substrate. We have studied the body-centered-cubic and hexagonal-close-packed dot lattices. We have determined the GS magnetic structure as a function of A and D by a steepest descent method. The results show that below (above) a critical value of D/A, the GS spin configuration is perpendicular (parallel). We have studied finite-temperature properties of our system of dots in the vicinity of the critical D/A by Monte Carlo simulations. The obtained results show spectacular features, in particular on the spin reorientation. A phase diagram in the vinicity of the critical value of D/A is presented.
Modeling of temperature-dependent hysteresis within the framework of dynamical systems
Authors: Freidlin M. I. , Mayergoyz I. D.
Published: July 2002

It is demonstrated that temperature sensitive rate-independent deterministic hysteresis can be modeled within the framework of randomly perturbed fast dynamical systems with multiple attractors. By using the theory of large deviations, the mathematical machinery of identifying the attractors, through which such systems evolve in time, is presented.
Resonant activation in a driven magnetic system
Authors: Klik I. , Yao Y. D.
Published: July 2002

High accuracy solutions of Brown’s Fokker-Planck equation are used here to analyze the relaxation of a superparamagnetic particle driven by a high frequency ac field with a time independent dc component. The time τr required to reach the stationary periodic state is computed, and it is found that at every temperature there exists a range of ac field amplitudes and dc field components in which the relaxation time τr is nonmonotonic, with a local minimum, as a function of the ac field frequency. This phenomenon is known as resonant activation in a bistable system.
New electromagnetic methods for the evaluation of prosthetic heart valves (invited)
Authors: Udpa Satish
Published: July 2002

Prosthetic devices are being implanted at record levels as the nation “ages” and advances in prosthetic science are made. Devices that are implanted range from artificial limbs and hips to devices such as heart valves. Periodic evaluation of the state of the devices is of significant interest particularly in the case of prosthetics whose failure can be fatal. An example of such a device is the artificial heart valve. Heart valves are usually replaced when stenosis or incompetence is indicated. This article presents a selection of some new techniques that are being developed for the detection of outlet strut failures in Bjork-Shiley heart valves. Methods that show particular promise include a noninvasive electromagnetic method that relies on the excitation and measurement of the resonant vibration modes of the strut. An alternate approach involves the induction of currents in the outlet strut using a pair of external excitation coils. The field generated by the current induced in the strut perturbs the field generated by the excitation coils. The field perturbations are measured using a catheter-mounted gradiometer. Test results obtained using an experimental rig designed to demonstrate the proof-of-concept are presented.
The magnetic anisotropy and its distribution in CoCrPtTa/CrV thin film recording media
Authors: Bottoni G. , Candolfo D. , Cecchetti A.
Published: July 2002

CoCrPtTa/CrV thin films for longitudinal media, with high coercivity and small thickness, are studied. Particularly, the magnetic anisotropy is analyzed with determination of the distribution of anisotropy fields and measurement of rotational hysteresis energy losses. The results are correlated to the basic processes of magnetization and to the thermal stability of the magnetization. The role of the grain segregation state of the films is enlightened. The analyzed media have different thickness of the magnetic layer. The films with smaller thickness have a narrower distribution of anisotropy fields, as a result of the better segregation and morphological regularity of the grains, which results in a more ordered switching of the magnetization, with a minor incidence of domain wall motion.
Thermal stability investigations on laminated antiferromagnetically coupled media
Authors: Pang S. I. , Piramanayagam S. N. , Wang J. P.
Published: July 2002

The effect of the antiferromagnetic coupling strength (J), on the thermal stability factors (SF =KuV/kBT) obtained by different techniques is studied. For this purpose, laminated antiferromagnetically coupled (LAC) media were prepared with different values of J. The results indicated that values of SF are almost the same, when the value of J is smaller than J=0.1 erg/cm2. However, for larger values of J (0.3 erg/cm2), it is very clear that the different methods give different values of SF. It was explained that the difference in the values of SF comes mainly from the pinning of the bottom layer and the different time scales of application of the reverse field. In general, the method that uses larger time scales gave larger values of stability factor. The studies also indicate a possibility that the stability factors obtained from Sharrock's equation may not be accurate for LAC media with very high values of J.
Effects of plasma etching on orientation ratio for longitudinal recording media
Authors: Huang L. , Wang J. P.
Published: July 2002

Studies on the effects of rf plasma etching treatment on textured NiP/Al substrate on the properties of CoCrTaPt/CrMo longitudinal recording media were carried out. The dependence of coercivity (Hc), coercivity orientation ratio (OR), and disk surface roughness (Ra) on etching parameters, i.e., etching time, pressure, and power, were presented. It was found that by optimizing the etching process parameters, an increase of Hc and OR and a decrease of Ra could be achieved. It was observed that good preferred orientation is necessary for obtaining high OR. The increase of OR could be attributed to the cleaning effect or surface modification by the rf plasma etching treatment on the textured substrate.
Thermal stability dependence on Tb content in TbFeCo recording media
Authors: Mochida Mitsunori , Suzuki Takao
Published: July 2002

Thermal stability of magnetic domain size in magneto-optical recording media TbFeCo is discussed. Shrinkage in domain size under accelerated conditions using irradiating laser light is found more significant for rare earth (RE) rich disk than transition metal (TM) rich disk. The reason for this difference is because the coercivity Hc of the RE-rich type is higher than the TM-rich type at the time of laser irradiation, which is effective against a wall field for minimizing domain size shrinkage.
Effects of cross grooving on a domain wall in magnetic bubble garnet
Authors: Morrissey R. P. , Redjdal M. , Ruane M. F. , Humphrey F. B.
Published: July 2002

The effects of a cross-groove structure etched into the top surface of a 1.6-μm-thick bubble garnet film on an isolated stripe domain underneath one of the grooves is analyzed using a numerical model based on the Landau–Lifshitz–Gilbert equation. Warping of the edge of the domain wall underneath the cross groove has been observed. The cross groove generates an in-plane field parallel to the stripe domain wall, and reverses from one edge of the groove to the other. The field is not uniform across the width of the wall, but reverses with the magnetization across the wall. The net containment field generated has been found to be as high as 150 Oe, varying with the width of the stripe domain and the dimensions of the deeper groove.
Pressure–volume relationship of Ta
Authors: Hanfland M. , Syassen K. , Köhler J.
Published: July 2002

Pressure-volume (P–V) data were measured for Ta up to 69 GPa at T=298 K using angle-dispersive synchrotron x-ray diffraction in combination with diamond anvil cell techniques. Nitrogen and sodium were used as quasihydrostatic pressure media. The obtained P–V relation is compared to experimental and recent theoretical results. Some advantages and limitations of using Ta as a pressure marker in high-resolution x-ray diffraction experiments with diamond anvil cells are discussed.
Generalized notion of spin-wave for large magnetization motion
Authors: Bertotti G. , Mayergoyz I. D. , Serpico C.
Published: July 2002

An attempt is presented to extend the notion of spin-waves to the case of large, far-from-equilibrium motions generated by rf fields of arbitrary amplitude. The spin-waves are treated as small perturbations around the exact analytical solution, previously derived for large magnetization motions in uniaxial systems subjected to a circularly polarized field. A generalized spin-wave dispersion relation is obtained which governs parametric instability effects under arbitrarily large motions and is reduced to the classical spin-wave dispersion law in the limit of small motions.
Switching field distribution of 40 Gbit/in.2 longitudinal media obtained by subtracting thermal agitation of magnetization
Authors: Uwazumi H. , Shimatsu T. , Kuboki Y.
Published: July 2002

Switching field distributions (SFDs), are presented for longitudinal media of ∼40 Gbit/in.2 and the influence of thermal agitation of magnetization is discussed. Two remanence curves were measured at different sweep rates of applied field of ∼10 and ∼108 Oe/s, respectively, and SFD curves were obtained by differentiating the remanence curves. The SFD becomes significantly narrower at the higher field sweep rate. A SFD curve without the effect of thermal agitation was calculated using Sharrock’s equation. The full width at half maximum of the SFD curve at 10 Oe/s, ΔHr/Hr, is 0.45 for an isotropic medium, while that estimated without the thermal agitation effect ΔH0/H0 is 0.24, indicating that the SFD at the recording frequency is nearly half that measured at vibrating sample magnetometer (VSM) time scales. The ΔH0/H0 of the ∼40 Gbit/in.2 medium is much smaller than that of a 15 Gbit/in.2 medium used for comparison, although the ΔHr/Hr is ∼1.4 times larger than that of the 15 Gbit/in.2 medium due to thermal agitation. The value of ΔH0/H0 is not influenced by the uniaxial anisotropy induced by substrate texture KuTX. This result indicates that the KuTX is not effective for reducing SFD at recording frequencies, although the KuTX strongly affects the SFD at VSM times scales. The improvement of recording performance with increasing KuTX may be related to suppression of thermal relaxation of the remanent magnetization at bit transitions, immediately after the writing process.
New experimental method for Preisach distribution identification
Authors: Cerchez M. , Bissell P. R. , Stancu Al.
Published: July 2002

A mixed (parametric/nonparametric) method to identify the Preisach distribution of particulate recording media is presented. The algorithm is tested on computer-generated data and is applied to Ba–Fe random oriented recording media. The results are in good agreement with the experimental data. We show that the Preisach distribution may not always be accurately described by analytical functions and that a discrete Preisach distribution could be used for better agreement between the simulations and the experimental data for complex magnetization processes.
Monte Carlo simulation of thermal activation volume in longitudinal hard-disk drives media
Authors: Igarashi Masukazu , Akagi Fumiko , Sugita Yutaka
Published: July 2002

Thermal fluctuation in longitudinal hard-disk recording media was investigated through micromagnetic simulation. The activation volume for a recording medium obtained by using the Monte Carlo simulation with field dependent attempt frequency was investigated. It was found that the activation volume should be obtained by using the remanence magnetization consisting of irreversible magnetization and the method using the total magnetization including the reversible magnetization leads to a wrong dependence of the activation volume on the field. It was also found that when the intergrain surface exchange coupling J is increased, the activation volume remains almost the same, close to the volume of a grain until J reaches 0.7×10−3 J/m2, and increases linearly further. Observation of the distribution of switched grains revealed that the activation volume obtained using our method agrees well with the size of the thermal-switching units.
Green phosphorescence of CaAl2O4:Tb3+,Ce3+ through persistence energy transfer
Authors: Jia D. , Meltzer R. S. , Yen W. M. , Jia W. , Wang X.
Published: July 2002

Single-crystal CaAl2O4 fibers doped with Ce3+ or Tb3+ or double doped with Tb3+ and Ce3+ were prepared by the laser-heated pedestal-growth method. Ce3+-doped CaAl2O4 was found to have persistent phosphorescence in the blue violet lasting more than 10 h, while Tb3+-doped CaAl2O4 showed only a weak afterglow. However, long persistent green phosphorescence from Tb3+ was observed in the doubly doped fiber, CaAl2O4:Tb3+,Ce3+. It was found that the enhanced green phosphorescence from Tb3+ is due to persistence energy transfer from the thermally released electrons to Ce3+ followed by energy transfer to Tb3+. The finding suggests a promising approach for controlling the color of persistent phosphors.
Evaluation of thermal parameters of bolometer devices
Authors: Neuzil P. , Mei T.
Published: July 2002

Thermal conductance G, thermal capacitance H, and the time constant τ are major parameters to be determined in the design of bolometer devices and, therefore, it is an important task to evaluate these thermal parameters experimentally for design verification. In this work, a technique has been developed to determine these parameters with high accuracy by convenient electrical measurement where a high-voltage short-duration pulse was used. The method also incorporates operation conditions in the measurement and thus reveals the bias dependency of the thermal parameters. The approach was experimentally confirmed by using metal–film microbolometer devices with different values of both G and H. Test results were in a good agreement with the theoretical analysis of the measurement technique.
Radiative singlet–triplet transition properties from coupled-cluster response theory: The importance of the S→T1 transition for the photodissociation of water at 193 nm
Authors: Christiansen Ove , Gauss Jürgen
Published: July 2002

Expressions for first-order induced electronic transition matrix elements are derived within the coupled-cluster response theory framework. When combined with electric–dipole and spin–orbit operators, these matrix elements allow the calculation of radiative transition probabilities between singlet ground and triplet excited states. An implementation employing an atomic mean-field representation of the spin–orbit operator is presented at the coupled-cluster singles and doubles level. The suitability of this operator for the calculation of radiative transition probabilities is checked in test calculations for BH and AlH which are compared to full configuration interaction results obtained with the full Breit–Pauli spin–orbit operator. In a first application, we investigate the importance of the S0→T1 transition relative to the S0→S1 transition in the first absorption band of the electronic spectrum of H2O. The potential importance of the S0→T1 transition for understanding the photodissociation in the low energy regime of this band is confirmed and accurate estimates are given for the energy difference between the S1 and T1 state as well as the transition dipole moments for excitations to these states. In addition, the geometry dependence of these properties is analyzed.
Electric quadrupole transitions of Rb2 observed in a pulsed molecular beam: The 1 1Δg–X 1Σg+ bands near 540 nm
Authors: Yoon Youngjee , Lee Yonghoon , Lee Sungyul , Kim Bongsoo
Published: July 2002

Electric quadrupole transitions of Rb2 are identified near 540 nm by resonance enhanced two photon ionization (RE2PI) study. The rotational structure shows O, P, Q, R, and S branches, which are quite similar to those of the electric quadrupole transition observed in K2 [Kim et al., J. Chem. Phys. 113, 2945 (2000)]. From the high resolution rotational analysis and comparison with the spectra of other alkali diatomic molecules we assign the observed bands to the 1 1Δg–X 1Σg+ transitions of Rb2, which are allowed by electric quadrupole radiation. We obtain molecular constants for the 1 1Δg state of Rb285 as ωe=49.6652(9) cm−1, Te=18 263.960(1) cm−1, Be′=0.021 357 9(5) cm−1, Re=4.311 71(5) Å.
Molecular hydrogen 3s,d 3Λg+ complex revisited
Authors: Pazyuk E. A. , Pupyshev V. I. , Stolyarov A. V. , Kiyoshima T.
Published: July 2002

The rovibronic term values, Landé factors, and radiative properties of all bound levels in the 3s,d 3Λg+ complex of H2, D2, and HD are evaluated in the framework of fully ab initio channel-coupling approach. The modified multichannel quantum-defect theory is employed to transform highly accurate ab initio Born–Oppenheimer electronic matrix elements borrowed from a literature to their diabatic counterparts avoiding explicit consideration of radial coupling effects. The radiative lifetimes of the most critical levels in the H2 complex are remeasured by a delayed coincidence method. The accuracy of the predicted term values, Landé factors, transition probabilities and experimental lifetimes is sufficient to indicate the sources of disagreement in existing theory and experimental data.
A study of water–water interactions in hydrophobic association by a molecular dynamics simulation with an optimized umbrella sampling method
Authors: Tsunekawa Naoki , Miyagawa Hiroh , Kitamura Kunihiro , Hiwatari Yasuaki
Published: July 2002

The association of a pair of methane molecules in water has been investigated by molecular dynamics simulations with an optimized umbrella sampling method. The potential of mean force, entropy, and internal energy of the association have been calculated, in which for the first time the energetic contribution of water–water interaction to hydrophobic association was estimated. It turns out that the entropic contribution to the association is attractive at short distances and the energetic contribution of water–water interaction is very little. These results suggest that the number of hydrogen bondings between water molecules little change in the process of hydrophobic association.
Prediction of helical peptide folding in an implicit water by a new molecular dynamics scheme with generalized effective potential
Authors: Pak Youngshang , Jang Soonmin , Shin Seokmin
Published: July 2002

Previously we demonstrated the effectiveness of the recently developed q-jumping molecular dynamics simulation method (q-jumping MD) in vacuo for fast conformational searches and optimization purposes. In this work we attempt to further investigate the conformational searching capability of this new scheme by applying it to folding problems of helical peptides containing fully charged side chains in an implicit water. With a slightly modified q-jumping MD method using the all-atom empirical force field and its generalized Born solvation model, the current simulations at T=300 K all lead to fast helix folding with broad potential energy fluctuations, starting from their extended (linear) conformations. The present study demonstrates that this new MD scheme greatly enhances the rate of conformational changes, making it possible to explore low energy conformations of peptides in aqueous environments in a reasonably short time. Therefore, the all-atom based theoretical prediction of native solution structures of more challenging systems, such as helix bundles, β-sheets, and even small proteins may be a realistic possibility.
Effect of foil target thickness on fast proton generation driven by ultrashort-pulse laser
Authors: Badziak J. , Woryna E. , Parys P. , Wołowski J. , Platonov K. Yu. , Vankov A. B.
Published: July 2002

The influence of the target foil thickness on the characteristics of a proton beam produced by the interaction of a 1-ps laser pulse with a plastic foil target at intensities near 1017 W/cm2 has been investigated. It is shown that, for maximizing the energies and/or the current of forward-emitted protons, the target thickness must be properly selected. It should be smaller than the hot electron range in the target but greater than the characteristic path length of the electron heat wave generated by the prepulse and the leading edge of the laser pulse. If the target thickness is smaller than the heat wave path length, both the maximum and the mean proton energies can be a decreasing function of laser energy.
Nanostructured icosahedral phase formation in Al70Cu20Fe10 by mechanical alloying: Comprehensive study
Authors: Barua P. , Murty B. S. , Mathur B. K. , Srinivas V.
Published: July 2002

A systematic study on the formation and the stability of Al70Cu20Fe10 quasicrystalline alloy has been carried out through mechanical alloying. The influence of the milling parameters, sequence of additive elements, and heat treatment on the formation of icosahedral phase has been investigated through x-ray diffraction analysis and transmission electron microscopy. A combination of milling and heat treatment of Al70Cu20Fe10 composition resulted in the formation of a single icosahedral phase (i-phase) quasicrystal with face-centered structure. We present evidence that suggests the i phase is of a nanoquasicrystalline nature. From the present investigations, we propose an i-phase formation mechanism in Al–Cu–Fe system.