bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystal structure characterization of ion-beam-synthesized CoxY1−xSi1.7 silicide
Authors: Wu M. F. , Vantomme A. , Hogg S. , Pattyn H. , Langouche G.
Published: July 2002

Heteroepitaxial ternary CoxY1−xSi1.7 silicide (x>0.27) has been formed by Co implantation into YSi1.7/Si(111). The formation of this compound is confirmed by an x-ray symmetric θ–2θ scan. However, the θ–2θ scan alone cannot discriminate between the possible phases (tetragonal, orthorhombic, or hexagonal) of this compound. On the other hand, Rutherford backscattering (RBS)/channeling confirms that this silicide is hexagonal and that its azimuthal orientation is CoYSi1.7[0001]//Si[111] and CoYSi1.7{112̄0}//Si{110}. In addition, the lattice constants of the ternary silicide aepi=0.3989 nm (which means that the lattice mismatch is >3.9% relative to the Si substrate) and cepi=0.3982 nm have been determined by RBS/channeling and x-ray diffraction.
Influence of imperfections on the dynamical response in model quantum cellular automata
Authors: Yakimenko I. I. , Zozoulenko I. V. , Wang C.-K. , Berggren K.-F.
Published: July 2002

The time-dependent behavior of quantum cellular automata (QCA) with imperfections is examined. We have studied a chain of quantum cells with the imperfections introduced by (i) variations of intercellular distances, (ii) variations of intercell tunneling strengths, and (iii) stray charges. We find that imperfections like these may easily impair the switching properties of semiconductor based QCAs.
Transverse magnetoresistance of metal interfaces with Hg1−xCdxTe
Authors: Hu Xiaoning , Fang Jiaxiong , Gong Haimei , Zhao Jun , Lu Huiqing , Li Xiangyang , Hu Xinwen
Published: July 2002

A magnetic field perpendicular to a two-terminal semiconductor will increase the sheet resistance under the contact as well as that of the noncontact region. A method using magnetoresistance is presented in this article to study the carrier’s behavior at the metal–Hg1−xCdxTe interface, in which we consider the disruption of the interface by the deposition of metal. Experimental results show that an inversion layer exists between p-Hg0.783Co0.217Te and the deposited Sn/Au.
The role of Ag in the pulsed laser growth of YBCO thin films
Authors: Kalyanaraman R. , Oktyabrsky S. , Narayan J.
Published: July 2002

We have studied systematically the role of silver in improving microstructure and properties of Y1Ba2Cu3O7−δ (YBCO) thin films. We have more than doubled the grain size to nearly 1.8 μm and reduced processing temperatures by incorporating Ag in the YBCO films, which is accomplished by using a composite target containing 15% by weight of Ag. These films show approximately four times higher Jc than the best films obtained on MgO(001) substrates deposited from stoichiometric Y1Ba2Cu3O7−δ targets. Study of the silver content in the film as a function of the deposition temperature shows clearly a decreasing concentration with increasing temperature and a segregation of the Ag to the surface. The increased oxygen content in the films is also observed at lower processing temperatures, providing strong support for the efficient oxygenation of YBCO via the presence of silver. A qualitative model suggests that the formation of silver oxide, rapid surface diffusion of Ag on MgO surfaces, and the nonreactivity of Ag with YBCO are the key aspects to the improvement in microstructure. The possibility of extending these ideas to the growth of oxides is also discussed, along with the fabrication of in-situ superconducting-metal junctions with 3D geometries.
In-plane anomalies of the exchange bias field in Ni80Fe20/Fe50Mn50 bilayers on Cu(110)
Authors: Riedling S. , Bauer M. , Mathieu C. , Hillebrands B. , Jungblut R. , Kohlhepp J. , Reinders A.
Published: July 2002

We report on the exchange bias effect as a function of the in-plane direction of the applied field in twofold symmetric, epitaxial Ni80Fe20/Fe50Mn50 bilayers grown on Cu(110) single-crystal substrates. An enhancement of the exchange bias field, Heb, up to a factor of 2 is observed if the external field is nearly, but not fully aligned perpendicular to the symmetry direction of the exchange bias field. From the measurement of the exchange bias field as a function of the in-plane angle of the applied field, the unidirectional, uniaxial and fourfold anisotropy contributions are determined with high precision. The symmetry direction of the unidirectional anisotropy switches with increasing NiFe thickness from [11̄0] to [001].
X-ray photoelectron spectra of low temperature plasma anodized Si0.84Ge0.16 alloy on Si(100): Implications for SiGe oxidation kinetics and oxide electrical properties
Authors: Riley L. S. , Hall S.
Published: July 2002

The material properties of low-temperature plasma-grown oxide on gas-source molecular beam epitaxial Si0.84Ge0.16 have been investigated. X-ray photoelectron spectra studies show that plasma anodization leads to no segregation of Ge species during thin oxide growth with the onset of partial segregation occurring for thicker oxides. Depth profiling shows that the plasma oxide is stoichiometric in form with the exception of a small percentage of Ge atoms left in their unoxidized state. The density of these Ge atoms agrees with that measured in previous electron trapping studies. In addition, oxide growth rate enhancement of SiGe is observed. These phenomena are explained using a qualitative model for the mechanism of oxide growth of SiGe which is consistent with published results for SiGe: oxides grown with other systems.
Determination of the barrier heights for electron injection in organic light emitting devices
Authors: Jonda Ch. , Mayer A. B. R. , Grothe W.
Published: July 2002

The performance of organic light emitting devices is strongly dominated by the cathode materials, because the injection barrier for electrons is largely affected by the electronic properties of the used metal. We report internal photoemission measurements of the barrier height existing between aluminum tris(8-hydroxyquinoline) and different cathode materials. It is shown, that a linear relationship exists between the measured barrier height and the work function of the cathode material. However, the gradient in this phenomenological equation indicates, that the barriers cannot be calculated as usual merely as the difference of the lowest unoccupied molecular orbital of the organic material, and the work function of the cathode, but that surface states play an important role. The barrier height remains unchanged after storage, even though the contact exhibits clearly visible degradation effects and numerous “dark spots.”
Effects of partial surface anisotropy on a fine magnetic particle
Authors: Zhang Kezhao , Fredkin Donald R.
Published: July 2002

Surface effects on fine magnetic particles become very pronounced because of the large surface-to-volume ratio. Experiments have shown that surface modification of fine γ-Fe2O3 particles can enhance the coercivity [see, for example, A. E. Berkowitz et al., IEEE Trans. Magn. 24, 2871 (1988) and Z. Lin, J. Magn. Magn. Mater. 116, 147 (1992)]. Using micromagnetic simulations, we have demonstrated that surface anisotropy with the proper sign contributes to the increase of coercivity [K. Zhang and D. R. Fredkin, J. Appl. Phys. 79, 5762 (1996)]. The situation where only part of the particle surface has anisotropy has not yet been investigated. To investigate this effect, we studied a single domain γ-Fe2O3 fine particle (with a length of 3000 Å and an aspect ratio of 4.6:1) using micromagnetics and the finite element method. Two regions of the particle surface have been modified by the surface anisotropy: (1) the region centered around the equator, and (2) the region around one end of the particle. In both cases the area of the modified region is about 20% of the total surface area. The surface anisotropy energy, axially symmetric about the surface normal, is added to the total magnetic energy of the particle. Hysteresis loops were obtained by minimizing the total energy at a given applied field. Numerical results show that the addition of positive surface anisotropy in case (1) enhances the coercivity while in case (2) the coercivity decreases. In both cases the surface magnetization tends to be tangential to the surface and the magnetization reversal is incoherent and starts from the surface. Further, in case (2) the incoherence of the magnetization is more prominent than in case (1), which explains the different effects of surface modification in regions (1) and (2) on coercivity.
Micromagnetic simulation of the domain structure of a flat rectangular permalloy prism
Authors: Hertel Riccardo , Kronmüller Helmut
Published: July 2002

Magnetic domain states of a permalloy prism 1 μm×500 nm×250 nm are calculated by means of three-dimensional finite element modeling. We obtain the four-domain Landau structure and the seven-domain “diamond” structure by using different starting conditions. Both domain patterns are sheared on the surfaces. This shearing is attributed to bulk effects of the magnetic structure. As an example, the role of head-to-head domain walls in the diamond structure is discussed. Adaptive mesh refinement methods are used to minimize the discretization errors.
Numerical investigation of stripe domain dynamics in grooved bubble garnet films
Authors: Morrissey R. P. , Redjdal M. , Ruane M. F. , Humphrey F. B.
Published: July 2002

The dynamic behavior of an isolated stripe domain attracted to a single rectangular groove on the top surface of a 1.6-μm-thick bubble garnet film is analyzed using a numerical model based on the Landau–Lifshitz–Gilbert equation. The groove creates an effective field which interacts with the stripe domain, such that the azimuthal angle of magnetization in the wall no longer rotates uniformly through the film thickness. The wall shape becomes highly distorted near the groove, forming head-on walls at the groove corners, and cleaving when pushed beneath the groove.
Structure and element-specific magnetic moments of epitaxial face-centered-cubic FexNi1−x alloy ultrathin films
Authors: Schellenberg R. , Meinert H. , Takahashi N. , Hillebrecht F. U. , Kisker E.
Published: July 2002

By core level x-ray photoelectron spectroscopy using an x-ray tube, ultrathin films of FexNi1−x alloys deposited epitaxially on a Cu(001) substrate have been investigated. Structural information is obtained by means of core level photoelectron diffraction. The angular dependent intensity scans reveal peaks corresponding to a face-centered-cubic lattice in the covered concentration range 0.08<x<0.9. Information on the local magnetic moments is obtained by observing the intensity asymmetry occurring when switching between directions of opposite magnetization (MLDAD). Both the Fe 2p3/2 and Ni 2p3/2 asymmetries show a maximum at about 50% Fe concentration.
Magnetic properties of a ferromagnetic double spin chain compound LaCrOS2
Authors: Takano Yoshiki , Takase Kouichi , Kitamura Goh , Itoi Chigak , Sekizawa Kazuko
Published: July 2002

Magnetic properties of LaCrOS2, in which Cr ions form zigzag chains, well separated with each other, have been studied. Because the temperature dependence of the inverse magnetic susceptibility above 200 K obeys Curie–Weiss law, we obtain a Weiss temperature Θp of 75 K. The large positive Weiss temperature and the steep increase of the magnetization below 80 K show that ferromagnetic interaction between Cr ions is dominant in this compound. The magnetization curves, however, show no spontaneous magnetization down to 4.2 K. This indicates that Cr ions do not order three dimensionally at least above 4.2 K. The saturation magnetization at 4.2 K is about 2.5μB per Cr ion. In the low temperature range, the differential magnetic susceptibility χd is expressed approximately as χd=(C/T)exp Δ/(kBT), where Δ is equal to 131 K. The temperature dependence of χd at low temperature suggests that a zigzag chain of Cr spins in this compound behave like a one-dimensional ferromagnetic Ising chain which has an excitation gap Δ. A numerical calculation for the ferromagnetic Ising chain with s=3/2 using the transfer matrix method reproduces the temperature dependence of the magnetic susceptibility for the whole temperature range using a g value of 1.91 and a coupling constant J of 28 K well.
Determination of temperature dependence of GaSb absorption edge and its application for transmission thermometry
Authors: Yang M. J. , Moore W. J. , Yang C. H. , Wilson R. A. , Bennett B. R. , Shanabrook B. V.
Published: July 2002

Transmission spectra of GaSb have been obtained over a temperature range from 10 to 470 °C. Using this information, transmission thermometry is applied to obtain accurate measurements of sample temperature during molecular beam epitaxy growth on GaSb substrates. A GaSb surface reconstruction transition is determined as a function of Sb flux and substrate temperature, establishing a laboratory-independent temperature standard.
Dynamics of lateral grain growth during the laser interference crystallization of a-Si
Authors: Aichmayr G. , Toet D. , Mulato M. , Santos P.V. , Spangenberg A. , Christiansen S. , Albrecht M. , Strunk H. P.
Published: July 2002

Laser interference crystallization of amorphous silicon (a-Si) thin films, a technique that combines pulsed laser crystallization with holography, enables the fabrication of periodic arrays of polycrystalline silicon (poly-Si) lines with lateral dimensions between 0.5 and 20 μm. The lines consist of grains with well-defined grain boundary locations and lateral dimensions that are appreciably larger than the thickness of the initial a-Si:H film (up to 2 μm for a 300 nm thick film). We investigated the dynamics of the crystallization process by two-dimensional finite element computer simulations of the heat transport and phase transitions during laser crystallization. The theoretical results were compared to: (i) measurements of the crystallization kinetics, determined by recording the transient changes of the reflectance during laser exposure, and to (ii) the structural properties of the crystallized films, determined by scanning force and transmission electron microscopy. The simulations indicate that the crystallization front responsible for the large grains propagates laterally from the edges of the molten silicon lines to their centers with a velocity of ∼14 m/s. A substantial lateral growth only occurs for laser intensities large enough to melt the a-Si film around the center of the lines down to the substrate. Vertical crystallization, which is substantially slower (0.5 m/s), also participates in the solidification process. Using a transfer matrix approach, we converted the time-dependent phase and temperature distributions generated by the simulation program into values for the reflection and transmission of the film as a function of time during and after the laser exposure. A good agreement between the simulated and measured transient reflection was obtained both in the case of homogeneous crystallization as well as that of laser interference crystallization.
Photocurrent spectroscopy and study of subband parameters for heavy holes in nanoscale In0.53Ga0.47As/In0.52Al0.48As multiquantum well structures
Authors: Tanaka K. , Kotera N. , Nakamura H.
Published: July 2002

Nanoscale In0.53Ga0.47As/In0.52Al0.48As multiquantum wells structures were studied by photocurrent spectroscopy. Photocurrent spectra showed clear steplike structures accentuated by exciton peaks. Many interband transitions were assigned from the spectral structure. As peaks of forbidden transitions, which appeared in large reverse bias voltages, were extrapolated to zero bias voltage on the photocurrent spectra, transition energies were estimated in a square potential well. New estimation methods of valence band parameters, heavy hole effective mass and valence band offset, were derived from a saturation of the heavy hole subband in the valence potential well, using the envelope function model in the effective mass approximation. The heavy hole effective mass in a direction normal to the quantum well plane and the valence band offset were 0.38m0 and 0.22 eV.
Anisotropy of the sublattice magnetization and magnetoresistance in Co/Re superlattices on Al2O3(112̄0)
Authors: Charlton T. , Lederman D. , Yusuf S. M. , Felcher G. P.
Published: July 2002

[ Co (20 Å)/ Re (6 Å)] 20 superlattices were grown on a (112̄0) surface of a Al2O3 single crystal, with the [0001] direction of their hcp structure in the plane of the film. The Co layers were found to be antiferromagnetically coupled (AF), with a saturating field of 6 kOe. Polarized neutron reflectivity was used to determine the direction of the sublattice magnetization. In zero applied field, the AF moments are aligned along the Co[0001] axis. In a magnetic field H perpendicular to the Co[0001] axis, the sublattices moments evolve to a canted arrangement, with the AF component always perpendicular to the field. With H along the Co[0001] axis, the AF moments flop in a direction perpendicular to Co[0001] axis. The spin flop transition is not abrupt, but can be described as a gradual rotation that is completed at 2 kOe. The anisotropy of the sublattice magnetization is related to the anisotropy of the magnetoresistance. This has the conventional dumbbell behavior with the field applied perpendicular to the Co[0001] axis, but exhibits an extended plateau near H=0, and a slight increase up to H∼2 kOe, when H is parallel to Co[0001] axis.
Giant magnetoimpedance of glass-covered amorphous microwires of Co–Mn–Si–B and Co–Si–B
Authors: Nie H. B. , Zhang X. X. , Pakhomov A. B. , Xie Z. , Yan X. , Zhukov A. , Vázquez M.
Published: July 2002

A study of magnetic hysteresis and giant magnetoimpedance (GMI) in amorphous glass-covered Co–Si–B and Co–Mn–Si–B wires is presented. The wires, about 10 μm in diameter, were obtained by a glass-coated melt spinning technique. Samples with positive magnetostriction (MS) have a rectangular bistable hysteresis loop. A smooth hysteresis loop is observed for wires with nearly zero MS. When the MS is negative, almost no hysteresis is observed. The GMI was measured in the frequency range between 20 Hz and 30 MHz. The shapes of the impedance versus field curves are qualitatively similar to each other for both positive and zero MS samples. The impedance is maximum at zero field, and decreases sharply in the range of 10–20 Oe. For the negative MS wires, when the driving current is small, the impedance is maximum at a finite external field. The position of the maximum approaches zero with increasing current. The contributions of the moment rotation and domain wall motion in the three cases are discussed.
Magnetoresistance of ion-beam deposited Co/Cu/Co and NiFe/Co/Cu/Co/NiFe spin valves
Authors: Slaughter J. M. , Chen E. Y. , Tehrani S.
Published: July 2002

Giant magnetoresistance (GMR) films produced by ion-beam deposition (IBD) have been studied much less than material produced by other techniques such as magnetron or diode sputtering. Although the process control aspects that are unique to IBD seem ideal for such work, the reported MR values for IBD material are often lower and the material resistivities higher than for conventional sputter-deposited films. We have used a carefully optimized ion-beam process to produce high-quality spin valves of Co/Cu/Co and NiFe/Co/Cu/Co/NiFe. The MR characteristics were studied as a function of the layer thickness for structures that were top pinned with FeMn and bottom-pinned structures grown on NiO. Using a 25 Å thick Cu layer, the MR of top-pinned Co/Cu/Co reached 8.4% over a fairly wide range of top and bottom Co thickness. In contrast, the MR of such films grown on NiO exhibited a strong dependence on the thickness of both Co layers and peaked at 9.9%. The thickness dependence is strong evidence for specular scattering effects at the external interfaces, although it is at least partly an indirect result of the weak exchange bias provided by the NiO in some samples. The symmetric spin valve structure, NiO/Co/Cu/Co/Cu/Co/FeMn, produced our highest MR of 12.1%. The NiFe/Co/Cu/Co/NiFe structures we studied were not optimized for maximum MR, but instead had much different top and bottom NiFe thicknesses as is appropriate in pseudospin-valve material for memory elements. Spin valves of this type had a maximum MR of 7.2%.
High magnetoresistance ratio and high coercivity observed from epitaxial Co/Cu/Co/NiO spin valve sandwiches on MgO grown by molecular beam epitaxy
Authors: Chern G. , Lee D. S. , Hou T. Y. , Chang H. C. , Wu Te-ho
Published: July 2002

Bottom and symmetrical Co/Cu/Co/NiO sandwiches are fabricated on single crystal substrates including MgO(100), MgO(110), and MgO(111) by molecular beam epitaxy without introducing a bias field. Although most of samples only show very weak exchange anisotropy, high magnetoresistance (MR) ratios (5%–8%) accompanied by high coercivities (200–2500 Oe) are observed. Strong angular dependence on Hc and MR has been also observed, which represents a uniaxial symmetry. The minor loops on the MR measurements have shown clear local MR maxima (minima) indicating that some intermediate antiparallel (parallel) spin states may exist.
Drastic change of giant magnetoresistance of Co/Cu multilayer by decreasing residual impurities in sputtering atmosphere
Authors: Miura Satoshi , Tsunoda Masakiyo , Nagatsuka Toshiya , Sugano Satoshi , Takahashi Migaku
Published: July 2002

In order to clarify the influence of residual impurities in the sputtering atmosphere on the microstructure and the giant magnetoresistance (GMR), Co/Cu multilayers were fabricated by changing the chamber pressures, Pb, just before introducing processing gas. Pb was controlled by changing the pumping time after venting the chamber with air. A drastic change of magneto resistance (MR) ratio from 48% to 14% was observed, when Pb was changed slightly from 7×10−8 to 3×10−8 Torr. In that Pb region, the root mean square (rms) roughness increased discontinuously from 3.7 to 4.6 Å as Pb was lowered. The abrupt drop of the MR ratio was accounted for by the decrease of the antiferromagnetic coupling accompanied by the increase of interfacial roughness.
Bifurcation to chaos in auto-oscillations in circular yttrium-ion-garnet films
Authors: Piskun N. Y. , Wigen P. E.
Published: July 2002

One of the most intriguing features of any nonlinear system is its dynamic evolution. Starting from the steady state the system evolves to a linear periodic motion and then experiences a series of “phase transitions” known as bifurcation which lead to a final chaotic state. In order to better understand the nonlinear behavior in magnetic systems, an experimental and numerical study of bifurcation to chaos in auto-oscillations at ferromagnetic resonance is presented. Experiments have been performed with thin circular iron garnet films using perpendicular high-power ferromagnetic resonance. Taking the magnetic field as a variable parameter the bifurcation route to chaos has been investigated. The system has been modeled by the usual three magnetostatic mode interaction model derived from the macroscopic Hamiltonian. Along with the modeled signal, the time dependence of the complex Lyapunov exponent has been investigated for the three magnetostatic modes. The possibilities for controlling chaos with periodic perturbation have been studied and the reason for desynchronization bursts is discussed.
Polarized neutron scattering study of the magnetic response across TN in a single crystal of UO2
Authors: Amoretti G. , Caciuffo R. , Santini P. , Lander G. H. , Kulda J. , de V. Du Plessis P.
Published: July 2002

We present the results of an inelastic neutron scattering experiment with polarization analysis performed on a single crystal of UO2. A magnetic inelastic response is observed above TN in the energy region between 3 and 10 meV, at least up to 6.5TN. This result provides evidence of the fact that in the time scale of our experiment the uranium triplet ground state is split into three singlets because of a dynamical Jahn–Teller distortion of the oxygen cage, reducing the point symmetry at the uranium site. The position of the peaks and their dispersion are compatible with monoclinic 1-k distortions along the three directions of the 〈100〉 star. This suggests a picture in which as TN is approached, a correlation builds up between the phases of the corresponding vibrations until, eventually, a static 3-k distortion is obtained below TN.
Many-spin model and the spin Hamiltonian of Mn12 clusters
Authors: Katsnelson M. I. , Dobrovitski V. V. , Harmon B. N.
Published: July 2002

In this work, we study the many-spin interactions taking place in Mn12 large-spin clusters using the eight-spin model Hamiltonian, for which we determine the possible parameters based on experimental data. Account of the many-spin excitations satisfactorily explains positions of the neutron scattering peaks, results of electron paramagnetic resonance measurements and the temperature dependence of magnetic susceptibility. Among others, strong Dzyaloshinsky–Morya interactions are found to be important for description of neutron scattering data. These interactions can be important for the relaxation of the magnetization in Mn12 clusters.
Electrical resistivity of UNi2Si2 in magnetic fields
Authors: Syshchenko A. , Sechovský V. , Andreev A. V. , Dremov R. V. , Šebek J. , Menovsky A. A.
Published: July 2002

We report on electrical resistivity measurements for an UNi2Si2 single crystal in the temperature range 4.2–140 K and magnetic fields up to 8.4 T parallel to the c axis. The anomalies in the temperature and field dependencies of the resistivity are associated with magnetic phase transitions and allow one to complete the B–T magnetic phase diagram. The relationship between the electrical resistivity and the periodicity of particular magnetic phases is discussed in light of magnetoresistance behavior in other antiferromagnetic U compounds.
FeTaN films with ultrafine grain structure
Authors: Chen Yingjian , Hossain Syed , Qian Chester , Miller Mark , Tong Hua-Ching
Published: July 2002

FeTaN films with moderate Bs of 14 kG and high resistivity of 100–150 μΩ cm have been investigated. Even with a low Ta incorporation of only 3 at. %, films with the above characteristics were obtained by using rf diode and dc magnetron sputtering. When the soft magnetic properties were optimized, only a body-centered-cubic α-Fe phase was found, and it had a significantly expanded crystal lattice. The films exhibited a nearly amorphous grain structure. It was found that low atomic mobility of the sputtered atoms was needed in order to promote formation of this phase.
Growth of magnetic overlayers on a polarized substrate: The case of Fe/Cr
Authors: Wille L. T. , Dreyssé H.
Published: July 2002

A computational framework is outlined for the determination of typical configurations in a solid-on-solid model of the growth of Cr on Fe(001). The relevant interactions are obtained from surface segregation parameters in FeCr. The concentration profile is determined and related to the system’s magnetic properties.
Effect of magnetic anisotropy on magnetic shaking
Authors: Paperno E. , Sasada I.
Published: July 2002

The effect of magnetic shaking on both the transverse and axial shielding factors (TSF and ASF) is investigated using open-ended cylindrical shields made of Metglas 2705M amorphous ribbons. Shaking enhancement is found to be strongly dependent on the orientation of the magnetic anisotropy of the shielding material, that is, the anisotropy axis should be aligned along the corresponding shielding direction to achieve a greater enhancement of the TSF or ASF. Magnetic shaking provides an ∼40-fold increase in the TSF and only an ∼twofold increase in the ASF for a shield consisting of a helical structure of the ribbons. The situation is almost completely different if the ribbons’ structure is axial: ∼twofold increase in the TSF and ∼20-fold increase in the ASF. The shaking field intensity (∼320 mOe at 1 kHz) for axial shielding is found to be about 10 times larger than that for transverse shielding. Experiments with a three-shell axial structure shield show an ∼350-fold increase in the ASF (∼40 000, which is one order larger than that of similar conventional shields). The TSF of this shield is, however, about one tenth of its ASF. Reorientation of the ribbons in the innermost shell, from an axial structure to a helical one, increases the total TSF (∼50 000) while still maintaining a large ASF (∼20 000). Hence, combining shells of helical and axial structures and having a proper distribution of the shielding material between them may allow the construction of an open shield with a large (>20 000) total ASF and TSF.
Permanent magnetic properties of nano-composite Sm2Fe15Si2C/α-Fe with Cu additive
Authors: Zhang Hong-wei , Zhang Shao-ying , Shen Bao-gen , Lin Chin
Published: July 2002

The ribbons of nano-composite Sm2Fe15Si2C/α-Fe with Cu additive were prepared by melt spinning at substrate velocity v=20 m/s. The analysis of x-ray diffraction patterns shows that the relative content of α-Fe phase increases with increasing Fe percent in samples, and the average grain size of 2:17 carbides and α-Fe in the ribbons determined by the Scherrer method is about 50 and 30 nm, respectively. The ribbons exhibit a single hard magnetic phase behavior and remanence enhancement, due to inter-grain exchange coupling between hard and soft magnetic phases. The remanence increases with the increase of iron content in samples, while the coercivity decreases rapidly. The highest maximum energy product around 10.7 MGOe was obtained for Sm8Fe76Cu4Si8C4 ribbons. The reversible and irreversible magnetization portions were analyzed from the recoil loops, and an irreversible nucleation field around 4.8 kOe for hard magnetic phase was observed in Sm8Fe76Cu4Si8C4 ribbons.
T-induced changes of magnetization profiles and reversal in exchange-coupled trilayers
Authors: Sbiaa R. , Le Gall H. , Pogossian S. , Bechevet B.
Published: July 2002

Thermomagnetic phenomena are investigated in appropriate exchange-coupled trilayers at the basic of a magnetic amplification of the magnetically-induced super-resolution signal used for ultrahigh magneto-optical storage. The changes of the magnetization profiles and reversal behavior between room temperature and the Curie temperature are determined from the mean field theory and a micromagnetic model based on a discontinuous distribution of the magnetization and compared in bi- and trilayers F1/F3 and F1/F2/F3. These multilayers correspond to a TbFeCo layer F1 with perpendicular anisotropy exchange-coupled with either one GdFeCo (F3) or with a double GdFe/GdFeCo (F2/F3) in-plane magnetic layer. Depending on the magnetic parameters of the in-plane layers associated with appropriate choice of their composition, it is possible to stimulate sequential magnetic reorientation in the layers F2 and F3 at different switching temperatures Ts2 and Ts3 which can induce a dynamic expansion of the magnetic domains in the upper readout layer.
Optical contrast, topographic contrast and artifacts in illumination-mode scanning near-field optical microscopy
Authors: Valle P. J. , Greffet J.-J. , Carminati R.
Published: July 2002

We use a two-dimensional exact numerical simulation and a three-dimensional perturbative analysis to study the coupling between dielectric contrast and topography in the images obtained by illumination-mode scanning near-field optical microscopy. We use a model for the emitting tip, which describes the polarization and confinement effects of a real tip. We analyze the image formation, especially the coupling between topographic and dielectric contrast. In the case of weakly scattering samples, we introduce rigorously the concepts of impulse response and equivalent surface profile. This tool may be useful to describe and understand quantitatively experimental images. Finally, we study the presence of artifacts in the images, due to the coupling between optical scattering and the z motion of the tip in constant-distance operating mode. We put forward the difficulty of predicting the relative weight of the artifact and the purely optical contributions.
Time dependence of the oxygen exchange O2↔SiO2 at the SiO2–Si interface during dry thermal oxidation of silicon
Authors: Åkermark T. , Gosset L. G. , Ganem J.-J. , Trimaille I. , Vickridge I. , Rigo S.
Published: July 2002

During dry thermal oxidation of silicon oxygen exchange reactions may occur between oxygen molecules (O2↔O2, catalyzed by the SiO2) or between oxygen from the gas phase and the oxygen in SiO2 (O2↔SiO2), both at the surface and at the Si–SiO2 interface. We found that the oxygen exchange rate at the Si–SiO2 interface is at least 25% of the oxygen uptake rate, requiring the movement of oxygen both from the surface to the interface, and from the interface to the surface. The oxygen exchange at the interface is further evidence in favor of the presence of a reactive interfacial layer between the growing oxide and the silicon substrate.
Time-resolved photoluminescence measurements of quantum dots in InGaN multiple quantum wells and light-emitting diodes
Authors: Pophristic M. , Long F. H. , Tran C. , Ferguson I. T. , Karlicek R. F.
Published: July 2002

We have used time-resolved photoluminescence to examine InGaN/GaN multiple quantum wells (MQWs) and light-emitting diodes (LEDs) before the final stages of processing at room temperature. The photoluminescence kinetics are well described by a stretched exponential exp[−(t/τ)β], indicating significant disorder in the material. We attribute the disorder to nanoscale quantum dots of high local indium concentration. For the three MQWs examined, the stretching parameter β and the stretched exponential lifetime τ were found to vary with emission energy. The stretching parameter β for the emission peak of the three MQWs was observed to increase from 0.75 to 0.85 with apparently increasing indium phase segregation. A higher degree of indium phase segregation is consistent with more isolated quantum dots inside the two-dimensional quantum well. The time-resolved photoluminescence from a LED wafer, before the final stages of processing, suggests the importance of quantum dots of high indium concentration on the LED operation.
Large and small angle x-ray scattering studies of CdTe/MgTe superlattices
Authors: Hartmann J. M. , Eymery J. , Carbonell L. , Wang Y.
Published: July 2002

We have performed large and small angle x-ray scattering measurements on CdTe/MgTe superlattices. The individual thicknesses of the CdTe and MgTe layers, together with the period dispersion and the crystallographic quality of the stacking, were extracted from large-angle x-ray diffraction. The Fresnel optical method and the distorted wave Born approximation were used to analyze the small angle x-ray scattering data. Specular reflectivity shows that the interface roughness is quite large for the two CdTe/MgTe superlattices grown either by conventional molecular beam epitaxy or by atomic layer epitaxy with however in the latter case a strong asymmetry between the direct and inverted interfaces. The effective MgTe concentration is determined from the refractive index. A model of correlated interface profiles is successfully used to simulate the diffuse scattering, and to gain access to the lateral correlation length of the roughness (Λ∥=1500±750 Å for both samples); moreover, we demonstrate that the layers are almost completely correlated over the sample thickness in the growth direction.
High resolution electron microscope analysis of lattice distortions and In segregation in highly strained In0.35Ga0.65As coherent islands grown on GaAs (001)
Authors: Kret S. , Benabbas T. , Delamarre C. , Androussi Y. , Dubon A. , Laval J. Y. , Lefebvre A.
Published: July 2002

Lattice distortions in three-dimensional coherent In0.35Ga0.65As islands grown by molecular beam epitaxy at 510 °C on GaAs have been imaged by high resolution electron microscopy. The strain fields are determined from the corresponding digital images, either by direct measurement of the lattice distortions or by combining real space and Fourier space information, with an uncertainty Δε=2×10−3. The strain fields are also simulated through finite-element calculations, taking into account the strain relaxation due to the low thickness of the electron-transparent specimens. The significant differences found between experimental and calculated strain fields are attributed to In segregation within the islands. Bidimensional compositional maps are then established showing that the In concentration in the central part of the islands (up to ∼50%) is significantly higher than the nominal concentration (35%), whereas it is lower (down to ∼20%) at the edges of the islands.
Magnetic properties of nanostructured γ-Ni–46Fe alloy synthesized by a mechanochemical process
Authors: Qin X. Y. , Lee J. S. , Kim J. G.
Published: July 2002

Magnetic properties and their relations to microstructures of nanostructured γ-Ni–46Fe alloy (nano γ-Ni–46Fe), synthesized by a mechanochemical process, were investigated at temperatures from ∼300 down to 8.5 K. The results indicated that the nano γ-Ni–46Fe with average grain sizes of 20–220 nm displayed ferromagnetism, with no superparamagnetic phenomenon being observed in the temperature range investigated. Coercivity increased with decreasing grain size obeying the law Hc∝1/d well in the grain size regime d>∼100 nm. Contrary to prediction from the random anisotropy model, however, the coercivity at both 300 and 8.5 K was not found to decrease with further decreasing grain size in the size range d=20–100 nm, although the grain sizes were then obviously smaller than the evaluated value (102 nm) of ferromagnetic exchange length, suggesting random anisotropy did not dominate its magnetic behavior. Instead, it was demonstrated that the further increase of coercivity with decreasing grain size as d<100 nm can be attributed to the effect of shape anisotropy in the nano γ-Ni–46Fe. Saturation magnetization Ms of the nano γ-Ni–46Fe was found to decrease with decreasing grain size, with a magnitude of ∼11% smaller than that of the coarse-grained standard being reached at d≈20 nm. Experiments revealed that this decrease of Ms was closely related to the increase of oxygen content in the specimens, implying oxidation would be responsible for the decrease of Ms. By assuming that the oxygen existed in the form of surface oxidation layers wrapping the nano-grains, a thickness of ∼0.8 nm for the oxidation layers was derived from the magnitudes of Ms corresponding to different grain sizes.
Modeling of gettering of precipitated impurities from Si for carrier lifetime improvement in solar cell applications
Authors: Plekhanov P. S. , Gafiteanu R. , Gösele U. M. , Tan T. Y.
Published: July 2002

Physical and numerical modeling of impurity gettering from multicrystalline Si for solar cell production has been carried out using Fe as a model impurity. Calculated change of nonradiative recombination coefficient of minority carriers in the course of gettering is used as a tool for evaluating the gettering efficiency. A derivation of the capture cross section of impurity precipitates, as compared to single atom recombination centers, is presented. Low efficiency of the conventional application of the gettering process is explained by the modeling results. The variable temperature gettering process is modeled and predicted to provide high gettering efficiency and short needed gettering times.
Structure and magnetic properties of Zr-doped nanocrystalline Sm2Fe15Ga2Cx ribbons
Authors: Zhang Jun-xian , Kleinschroth I. , Cheng Zhao-Hua , Goll D. , Kronmüller H.
Published: July 2002

The phase formation and magnetic properties of melt-spun Sm2Fe15Ga2Cx ribbons with a Zr addition of 1.5 at. % have been investigated. A metastable phase with the disordered TbCu7-type structure is formed in the samples annealed at temperatures between 620 and 740 °C. Upon heating above 750 °C, a structural transformation from the hexagonal TbCu7- to the rhombohedral Th2Zn17-type structure occurs. It was found that the Zr addition improves the thermal stability of the TbCu7 structure. Transmission electron microscopy observations show that the grain sizes in 1:7-type ribbons are much smaller than that in 2:17-type samples. The highest value of the maximum energy product of 88.7 kJ/m3 is obtained in a 1:7-type ribbon annealed at 740 °C for 15 min. The corresponding remanence and coercivity amounts to 0.8 and 0.43 T, respectively. In contrast, coercivities higher than 0.8 T obtained in 2:17-type ribbons are accompanied by remanences lower than 0.66 T. The 1:7-type ribbons with grain sizes in the range of 15–45 nm show significant remanence enhancement which can be attributed to the exchange coupling between adjacent grains.
Analysis of Schottky gate electron tunneling in polarization induced AlGaN/GaN high electron mobility transistors
Authors: Sierakowski Andrej J. , Eastman Lester F.
Published: July 2002

( gate=nickel)/(barrier=GaN/Al (y) Ga (1−y) N)/(buffer=GaN)/(substrate=SiC ) polarizationinduced high electron mobility transistors (PI-HEMTs) show promise for ultrahigh power microwave amplification. The polarization fields in these Ga-face, wurtzite structures are used to engineer the conduction band profile in the absence of any doping. One important attribute of these PI-HEMTs is their ability to withstand electric fields up to 3 MV/cm without avalanche breakdown. The combination of high electric field under the gate necessary to pinch off the channel, and the high electric field fringing to the gate from the large drain–source bias voltage, makes it possible for electron tunneling from the gate into and through the GaN/Al(y)Ga(1−y)N barrier into the GaN channel. A one-dimensional theoretical analysis of the tunneling current at pinchoff as a function of the aluminum fraction (0.05<y<0.45) in the barrier, the operating temperature of the gate metal at 300 and 573 K, and a uniform additional electric field of −1 and −2 MV/cm from drain bias reveals the potential for these GaN based devices to impose negligible current loading by the gate on a microwave driver. Unlike GaAs, avalanche breakdown induced by gate leakage is unimportant in these GaN based power devices.
Raman scattering investigation of the phase transition in nanocrystalline (Pb, Ca, La)TiO3
Authors: Tang X. G. , Zhou Q. F. , Zhang J. X.
Published: July 2002

Nanocrystalline (Pb0.95−xCa0.05Lax)Ti1−x/4O3 (PCLT) powders were prepared by the sol–gel technique. Doping-induced or grain size-induced structure phase transition was investigated by using Raman scattering and x-ray diffraction for the PCLT system. Experimental results show that the soft phonon mode E(TO1) displays a decrease in frequency but an increase in linewidth with increasing concentration of La. A discontinuous behavior of the phonon energy is associated with a phase transition from a ferroelectric tetragonal to a paraelectric cubic phase, and occurs at a morphotropic phase boundary of x≈0.30 for grain sizes being 20 nm. The dependence of Raman phonon modes on the size of particles has been analyzed for nanocrystalline (Pb0.88Ca0.04La0.08)Ti0.98O3 with grain sizes of 10–60 nm and a grain size-induced phase transition is observed for sizes below 14 nm.
An optical study on dislocation clusters in a slowly pulled silicon crystal
Authors: Nango Nobuhito , Iida Satoshi , Ogawa Tomoya
Published: July 2002

Since the defect density of slowly pulled Czochralski (CZ) Si crystals is fairly low, determination of the surveillance volume is very important to get correct information about defects in the crystals. Here, the volume is defined as that to be searched by a single surveillance trial from a viewpoint of economical quality control of the crystals and should be equal to or a little larger than the reciprocal density of the defects. The following two-step investigation resulted in successful and novel use of an ordinary IR light scattering tomograph (LST) instrument for this purpose. (1) The surveillance thickness of photoluminescence (PL) due to carriers optically injected by a laser beam for LST is estimated as 300 μm in the crystal, because the carriers diffuse out emitting PL about 150 μm from the position illuminated by the laser beam. Thus PL mapping is very effective to search for low defect density silicon crystals since defects such as dislocations act as PL killer centers. (2) The dark positions in the PL mapping were carefully surveyed by layer-by-layer tomography using the light scattered by defects, where a beam of 1.06 μm radiation from a Nd-YAG laser was focused into about 5 μm in diameter to scan inside the crystal. By complimentary combination of the above two methods, large entangled dislocation loops were clearly confirmed in a slowly pulled p-type CZ Si crystal.
Effect of Yb3+ doping on upconversion emission intensity and mechanism in Er3+/Yb3+-codoped fluorozirconate glasses under 800 nm excitation
Authors: Tsuda M. , Soga K. , Inoue H. , Inoue S. , Makishima A.
Published: July 2002

Er 3+ singly- and Er3+/Yb3+-codoped fluorozirconate glasses were prepared. The upconversion emission intensities of the Er3+ ion around both 550 and 660 nm were measured under 800 nm excitation. The absorption band of the Yb3+ ion is located around 980 nm. Though the absorption band of Yb3+ does not interact directly with the incident light at 800 nm, both upconversion intensities around 550 and 660 nm emission increased with the increasing YbF3 concentration. We calculated the upconversion intensities around 550 and 660 nm by using rate equations and evaluated the principal upconversion mechanism in Er3+ singly- and Er3+/Yb3+-codoped samples quantitatively. We also investigated the reason for the increase of the upconversion emission intensities with the codoping of Yb3+ under 800 nm excitation.
Characterization of GaAs/AlGaAs heterojunction bipolar transistor devices using photoreflectance and photoluminescence
Authors: Han A. C. , Wojtowicz M. , Block T. R. , Zhang X. , Chin T. P. , Cavus A. , Oki A. , Streit D. C.
Published: July 2002

Photoreflectance (PR) and photoluminescence (PL) techniques were used to characterize the AlGaAs/GaAs heterojunction bipolar transistor (HBT) wafers grown by molecular beam epitaxy (MBE). The line shape of the PR GaAs signal is closely related to the cleanliness of the MBE system. The Franz–Keldysh oscillations of the GaAs signal become sharper, well defined, and the oscillation amplitude increases slightly as the MBE system is cleaned up. The dc current gain of the HBT devices was observed to increase accordingly. The origin for this correlation is discussed. The PL spectra of the HBT device wafers indicate that the intensity of the free-to-bound transition corresponding to the donor to valence band becomes strong in high gain device wafers.
Electroluminescence and photoelectric properties of type II broken-gap n-In(Ga)As(Sb)/N-GaSb heterostructures
Authors: Moiseev K. D. , Mikhailova M. P. , Stoyanov N. D. , Yakovlev Yu. P. , Hulicius E. , Simecek T. , Oswald J. , Pangrác J.
Published: July 2002

Layers of n-InAs and n-InGaAsSb were grown by metalorganic vapor phase epitaxy and liquid phase epitaxy on N-GaSb substrates. The electroluminescence, current-voltage characteristics and photocurrent spectra of these heterostructures were studied at low temperatures. It was shown that GaSb/In(Ga)As(Sb) with InAs-rich narrow-gap solid solutions are broken-gap heterojunctions of type II at 77 and 300 K. Intense electroluminescence of the N-GaSb/n-In(Ga)As(Sb) heterostructures was found in the spectral range of 3–4 μm at 77 K. The origin of radiative recombination at the N-n type II broken-gap heterointerface is proposed and is in agreement with the experimental results for both systems.