bluuebunny/crossref_metadata_2025 huggingface dataset:
Large enhancement of the giant magnetoresistance in inhomogeneous semiconductors: Dependence on magnetic field direction
Authors: Solin S. A. , Thio Tineke , Hines D. R. , Kawano M. , Oda N. , Sano M.
Published: July 2002

In high-mobility Hg1−xCdxTe (x=0.10), the physical giant magnetoresistance (GMR), due to the orbital motion of the carriers in the applied magnetic field, is apparent at high magnetic field (H>1/μe). At low field (H<1/μe), this is enhanced by a geometrical GMR associated with conducting inhomogeneities in the semiconductor. In previous work, we have presented a model which accounts quantitatively for the GMR enhancement. Here we report on the dependence of the GMR on the direction of the magnetic field. The transverse GMR shows the low-field boost expected; in the longitudinal case both the physical and the geometrical MR are expected to vanish, and indeed the measured longitudinal GMR is very small.
Hysteresis loop of planar Hall effect in CoCr films
Authors: Wu J. C. , Wu C. S. , Wu Te-ho
Published: July 2002

Micrometer devices of Co85Cr15 film with Hall bar shape were made using standard electron beam lithography and lift-off process. The film was thermally deposited without bias field and the sample was kept at room temperature during evaporation. The Hall voltage, measured at room temperature, showed square hysteresis loops with an external magnetic field applied in the direction close to the film plane. Based on the variation of the Hall voltage measured under different directions of external magnetic field, the magnetic easy axis was verified to be along the long axis of the device. The formation of the easy axis along the narrow wire due to the physical confinement as an intrinsic mechanism is discussed.
Spin-dependent electron transport at the ferromagnet/semiconductor interface
Authors: Hirohata A. , Xu Y. B. , Guertler C. M. , Bland J. A. C.
Published: July 2002

A search for spin-dependent electron transport at the ferromagnet/semiconductor interface has been made by measuring the bias dependence of a photon excited current through the interface. A circularly polarized laser beam was used to excite electrons with a spin polarization perpendicular to the film plane. In samples of the form 3 nm Au/5 nm Ni80Fe20/GaAs (110), a significant transport current was detected with a magnitude dependent on the relative orientation of the spin polarization and the magnetization vector. At perpendicular saturation, the bias dependence of the photocurrent is observed to change in the range 0.7–0.8 eV when the helicity is reversed.
Magnetic properties of ion beam deposited CoCrPt hard bias films for magnetoresistive heads
Authors: Leng Q. , Mao M. , Miloslavsky L. , Simion B. , Hung C.-Y. , Qian C. , Miller M. , Basi R. , Tong H. C. , Wang J. , Hegde H.
Published: July 2002

Ion beam deposition techniques have been employed to prepare Cr/CoCrPt bilayers for hard bias applications in giant/anisotropic magnetoresistive recording heads. The bilayers were deposited at different deposition angles, which is defined as the angle between deposition beam direction and substrate plane. The magnetic properties of CoCrPt films, such as coercivity (Hc), remnant magnetization–thickness product (Mrt), and coercive squareness (S*) as a function of CoCrPt layer thickness and deposition angle have been studied and correlated to film microstructures. Typical values of Hc, Mrt, and S* for a Cr100 Å/CoCrPt490 Å bilayer deposited at 42° are 1580 Oe, 2.90 memu/cm2, and 0.90, respectively. The increase in Mrt with increasing CoCrPt layer thickness at a fixed deposition angle is accompanied by a linear decrease in Hc. We have observed, however, that for the same bilayer geometry both Hc and Mrt of CoCrPt layer increase with decreasing deposition angle, whereas S* remains almost unchanged. High angle x-ray diffraction measurements indicate that the hexagonal-close-packed (101̄0) crystallographic orientation of CoCrPt films is enhanced with decreasing deposition angles. This suggests that the improvement in both Mrt and Hc for CoCrPt films deposited with decreasing deposition angle results from an enhancement in in-plane c-axis texture, and crystallinity. This could be attributed to an enhancement in in-plane adatom mobility at low deposition angles.
Analysis of the effect of write head field rise time on signal, noise, and nonlinear transition shift
Authors: Xing X. , Bertram H. N.
Published: July 2002

The effect of finite flux rise time on the recording process has been examined by a comparison of theoretical analysis and experimental measurement. It was shown that a finite flux rise time increases the transition width, transition noise, and nonlinear transition shift. The effect of rise time on these recording phenomena was shown to scale with the product of disk velocity and rise time divided by the total magnetic spacing (ντ/y). Comparison of the experiment and theory yields reasonable flux rise time.
Write bubble and field rise time measurements
Authors: Luo Yansheng , Garfunkel Glen
Published: July 2002

An experimental technique has been developed for studying the write bubble created by the thin film recording heads. From the measurements of the write bubble size, we can estimate the write field magnitude and the write field gradient at the write bubble boundaries. Using the same experimental setup, we can also study the write field high frequency characteristics.
Magnetic properties of Sc-substituted erbium iron garnet single crystals in high magnetic field
Authors: Ostoréro J. , Guillot M.
Published: July 2002

Magnetic properties of garnet single crystals Er3Fe5−zSczO12 (z=0.10 and 0.30) are reported under high magnetic field up to 264 kOe in the 2–320 K temperature range. Below the compensation temperature Tcomp (71 and 52 K for z=0.10 and 0.30, respectively), in the spontaneous state, the magnetic anisotropy is very weak for z=0.10 and more important for z=0.30. The “easy” axis is [100] for both Sc concentrations as in pure ErIG. These results are compared to Faraday rotation anisotropy. From the analysis of the temperature dependences of the spontaneous magnetizations, the molecular field coefficient Ndc related to the Fe–Er superexchange interactions has to be, as are Ndd and Nad, a function of z. In the high magnetic field and at low temperature, the onset of the usual canted magnetic phase takes place for H applied parallel to the “easy” [100] axis for both concentrations, whereas an unusual behavior is observed for z=0.30 and H parallel to [111] direction: below 30 K, three temperature dependent field induced transitions are observed.
Magnetization and ferromagnetic resonance studies in implanted and crystal ion sliced bismuth-substituted yttrium iron garnet films
Authors: Rachford F. J. , Levy M. , Osgood R. M. , Kumar A. , Bakhru H.
Published: July 2002

Bismuth-substituted yttrium iron garnet (Bi-YIG) is the material of choice in the fabrication of optical isolators. Recently some of us have demonstrated an ion-implantation based technique for detaching single-crystal Bi-YIG films from their gadolinium gallium garnet growth substrates, and for their subsequent bonding onto semiconductor wafers. In this article we study the magnetic properties of bubble-type Bi-YIG films with large out-of-plane uniaxial anisotropy in various stages of the separation process. We find that the implantation reduces the perpendicular anisotropy field from approximately 1435 to 750 Oe as a result of increased residual strain. Annealing partially restores the anisotropy to 900 Oe depending on annealing conditions. Chemical etching of the implanted sample separates the Bi-YIG film from the substrate at the sacrificial layer. Upon detachment the perpendicular anisotropy of the sample is nearly fully restored to its original value. The 9 GHz ferromagnetic resonance (FMR) linewidth of these films is large (440–520 Oe) and is only weakly affected by the processing.
Phase diagram and magnons in quasi-one-dimensional dipolar antiferromagnets
Authors: Hummel M. , Schwabl F. , Pich C.
Published: July 2002

We investigate antiferromagnetic spin chains, which are coupled by a weak antiferromagnetic exchange interaction. The spins are located on a hexagonal lattice, i.e., frustration is present when three-dimensional order sets in. Typical realizations of such systems are the halides ABX3. In this work we particularly study the role of the long-range dipolar interaction within the framework of a Heisenberg model with nearest-neighbor exchange and additional dipolar interaction. We perform a classical ground-state analysis and show that the spin configuration is sensitively dependent on κ′, the ratio of the dipolar interaction to the interchain interaction, as a consequence of their competing character. Several commensurate and incommensurate phases arise in the different regions of the parameter space. The ground-state investigations are supplemented by a stability analysis by means of a linear spin-wave calculation. From the magnon spectra we can show that all commensurate phases are stable against fluctuations. In comparison with experiments (CsMnBr3, RbMnBr3) we obtain good agreement for the energy gaps. From this we conclude that the dipolar interaction is the most important source of anisotropy in these Mn compounds.
Influence of the phase transition of Tb on the magnetic properties of Fe/Tb multilayers
Authors: Freitag A. E. , Chowdhury A. R.
Published: July 2002

The impact of the phase transition between para- and ferromagnetic states of Tb on the magnetic properties of Fe/Tb multilayers has been studied by taking low temperature Mössbauer measurements. 26 Fe/Tb multilayer samples with a wide range of thickness were fabricated by planar magnetron sputtering on polyester and glass substrates. The magnetic and structural properties of these samples were examined using x-ray diffraction, Mössbauer spectroscopy, and torque magnetometry. At room temperature, Mössbauer spectra show perpendicular magnetic anisotropy (PMA) of the Fe magnetic moments for samples that consist of Fe layers and mixed interfaces only. The formation of an unmixed Tb layer starting at about 12 Å Tb layer thickness leads to parallel alignment of the Fe magnetic moments. For samples for which discrete Tb layers are formed, these Tb layers are probably paramagnetic at room temperature as can be deduced from low temperature Mössbauer measurements and room temperature torque measurement. The hyperfine field of Fe in these multilayers as determined from Mössbauer spectroscopy shows a relative minimum around the Curie temperature of Tb of 220 K indicating reorientation of the Fe moments due to spin polarization of Tb.
Temperature dependence of the magnetization reversal in Co(fcc)–BN–Co(poly hcp) structures
Authors: Pust Ladislav , Wenger Lowell E. , Lukaszew Rosa A. , Sheng Yongning , Litvinov Dmitri , Wang Yonghua , Uher Ctirad , Clarke Roy
Published: July 2002

The magnetic properties of multilayer structures with two magnetic layers of the same metal (Co) but with different crystallographic structures separated by an insulating BN layer have been studied. These structures were prepared on Si (001) substrates by a combination of molecular beam epitaxy (metallic layers) and electron cyclotron resonance-assisted sputtering (BN layer). An fcc Co single-crystal layer (60 Å) was first stabilized by growing it on a copper fcc buffer layer and subsequently a polycrystalline Co layer (70 Å) with hcp structure was grown on top of the insulating BN layer. A CoO antiferromagnetic layer, formed adjacent to this hcp Co layer, significantly influenced the magnetic behavior of the polycrystalline hcp Co layer. The magnetic hysteresis loops for these structures were measured at temperatures ranging from 5 to 350 K with the magnetic field applied along the easy (110) in-plane axis of the fcc Co. A very sharp flipping of the magnetization was found for the fcc Co layer with a nearly temperature-independent coercive field that increased from 14 mT below 100 K to 16 mT at 300 K. In contrast, the magnetization reversal in the hcp Co layer was smoother and its coercivity varied significantly with temperature depending on the strength of the exchange coupling with the adjacent CoO layer. At 5 K the coercivity was greater than 0.2 T and decreased with increasing temperature, becoming essentially zero above room temperature. When cooling in a magnetic field, an exchange offset was observed below 150 K that increased to about 0.1 T at 5 K.
Tunneling controlled electroluminescence switching in bilayer light emitting diodes
Authors: Nikitenko V. R. , Bässler H.
Published: July 2002

It has been reported that in bilayer light emitting diodes in which large internal energy barriers for holes and electrons exist, electroluminescence features a step function-like onset upon applying the voltage. A theory is advanced to explain that effect. It is based upon time-dependent accumulation of interfacial charge followed by tunneling across the interface. Good agreement between theory and experiment is obtained. The effect may be exploited for signal addressing in flat panel displays.
Spin gap in low-dimensional Mott insulators with orbital degeneracy
Authors: Guidoni L. , Santoro G. , Sorella S. , Parola A. , Tosatti E.
Published: July 2002

We consider the exchanged Hamiltonian HST=−J∑〈rr′〉(2Sr⋅Sr′−12)(2Tr⋅Tr′−12), describing two isotropic spin-1/2 Heisenberg antiferromagnets coupled by a quartic term on equivalent bonds. The model is relevant for systems with orbital degeneracy and strong electron-vibron coupling in the large Hubbard repulsion limit. To investigate the ground state properties we use a Green’s Function Monte Carlo, calculating energy gaps and correlation functions, the latter through the forward walking technique. In one dimension we find that the ground state is a “crystal” of valence bond dimers. In two dimensions, the spin gap appears to remain finite in the thermodynamic limit, and, consistently, the staggered magnetization—signal of Néel long range order—seems to vanish. From the analysis of dimer–dimer correlation functions, however, we find no sign of a valance bond crystal. A spin liquid appears as a plausible scenario compatible with our findings.
One-dimensional heavy fermion lattice model
Authors: Zvyagin A. A. , Schlottmann P.
Published: July 2002

We consider a one-dimensional supersymmetric t−J model and introduce a finite concentration of impurities of arbitrary spin S without destroying the integrability. Using the Bethe ansatz equations diagonalizing the correlated host with impurities, we discuss the ground state properties as a function of magnetic field H and the Kondo exchange coupling. While an isolated impurity of spin S>1/2 has a magnetic ground state of effective spin (S−12), a finite concentration introduces a heavy impurity band and yields a singlet ground state with antiferromagnetically correlated impurities. A field H first polarizes the narrow impurity band. The impurities have mixed valent properties induced in part by correlations in the host. Some aspects of the model are related to heavy fermion alloys.
Inelastic neutron scattering study of the spin dynamics of Yb1−xLuxAl3
Authors: Osborn Raymond , Goremychkin Eugene A. , Sashin Igor L. , Murani Amir P.
Published: July 2002

We present the results of a systematic inelastic neutron scattering study of the spin dynamics of the mixed valent compound YbAl3 doped with nonmagnetic lutetium. The aim of the investigation is to clarify the origin of the unusual gap-like magnetic response observed in YbAl3, which can be modeled by two inelastic peaks: a narrow peak at 34 meV with half width at half maximum, Γ=6.4±0.8 meV and a broad peak at 44 meV with Γ=30±1 meV. Lutetium substitution leads to a substantial increase in the linewidth (Γ=9±1 meV at x=0.1) and a decrease in the intensity (down by 60% at x=0.1) of the narrow component, with a negligible effect on the broad inelastic peak. This trend is confirmed with higher doping resulting in the complete suppression of the narrow peak at x⩾0.35. The results indicate that the narrow component arises from coherent excitation processes within the hybridized 4f band.
Synthesis and magnetic property of condensation product of bis(oxamide oximato)nickel and 1,1′-dicarbaldehydeferrocene
Authors: Ji Tianhao , Zhao Jiangao , Wen Gehui , Shen Baogen , Zhan Wenshan
Published: July 2002

A new multinuclear metals condensation product (polymer C) was prepared by a condensation reaction between bis(oxamide oximato)nickel and 1,1′-dicarbaldehydeferrocene. Its structure was investigated by using elemental analysis, Fourier-transform infared, x-ray diffraction, and thermogravimetric analysis techniques. All these results show that the polymer is a three-dimensional layer, large π-conjugated part order structure and its initial decomposition temperature is about 230 °C. The magnetic susceptibility was measured from 5 to 160 K by a superconducting quantum interference device magnetometer under the applied magnetic field of 10 kOe. The plot of 1/χM vs T is linear in the range of 5–30 K, but deviates from the line above 30 K. The Weiss constant θ calculated according to the equation χM=C/(T−θ) in the 5–30 K was −2.1 K, indicating presence of a weak antiferromagnetic coupling between the most near neighboring two metal ions. The polymer C can also easily complex with metal ion Cu2+, but is difficult with Mn2+.
Magnetite film growth at 30 °C on organic monomolecular layer, mimicking bacterial magnetosome synthesis
Authors: Abe M. , Ishihara T. , Kitamoto Y.
Published: July 2002

At 30 °C magnetite (Fe3O4) films were successfully grown, ∼5 μm in thickness, by flowing for 20 h a mixed gas of O2/N2=1/2000 on lipid monomolecular layers of arachic acid spread on an aqueous solution of FeCl2. The ordered array of the OH groups of the lipid layer stimulated the nucleation and growth of the Fe3O4 films, similar to that proposed for the bacterial magnetosome (Fe3O4 crystals) synthesis in which the OH groups on the cytoplasmic membrane stimulates the surface growth of Fe3O4. The Fe3O4 films obtained in this study contained a small amount of ferric hydroxyl impurity phases of α-FeOOH and γ-FeOOH, as shown by x-ray and Mössbauer analyses.
Prediction and identification of the magnetic field close to electric machines
Authors: Papadopoulos Peter J. , Ioannides Maria G.
Published: July 2002

The magnetic field around an electric machine operating at full load is predicted numerically and identified experimentally. A precise model of a three-phase induction machine is constructed considering the machine geometry, the windings and the stator and rotor materials. The field distribution close to the machine is determined using the discrete element technique, and actual measurements are carried out for identification purposes. An approximate human model is constructed and the magnetic field inside the human body is computed for a distance of 1 and 5 m from the machine. The magnetic field density is determined at specific points, and the induced current densities are computed.
Enhancement of magnetic properties in CoCrPt longitudinal recording media by Cr75Ti25/CoTi bilayer
Authors: Hong S. Y. , Lee T. D. , Shin K. H.
Published: July 2002

Effects of the Cr75Ti25/CoTi bilayer on the magnetic and crystallographic properties in CoCrPt longitudinal recording media were investigated. CoCrPt(30 nm)/Cr75Ti25(X nm)/CoTi(50 nm) films were deposited on glass substrates heated at 250 °C by a dc magnetron sputtering method. Coercivity increased from 2200 to 3200 Oe by incorporating a 50 nm thick Cr75Ti25 intermediate layer between the CoCrPt magnetic layer and the CoTi underlayer. From x-ray analyses, the rather strong Co(101̄0) diffraction was developed in addition to Co(112̄0) diffraction in CoCrPt/Cr75Ti25/CoTi films, while only Co(112̄0) diffraction was observed in CoCrPt/CoTi films. Transmission electron microscopy image confirmed that the small grain size of 18 nm in both types of films. This enhancement of the coercivity can be attributed to the increase of magnetic decoupling effect through the Cr segregation at the grain boundaries of the magnetic layer. In addition, the presence of the Co(101̄0) plane, and small grain size due to the B2 structure intermetallic compound underlayer should influence the magnetic properties of CoCrPt/Cr75Ti25/CoTi films.
Structural and magnetic studies of fcc Fe films with self-organized lateral modulation on striped Cu(110)–O(2×1) substrates
Authors: Li Dongqi , Diercks Verena , Pearson J. , Jiang J. S. , Bader S. D.
Published: July 2002

Face-centered cubic (fcc) Fe wedges of 0–12 ML were grown by means of molecular beam epitaxy onto a novel substrate: flat Cu(110) with an oxygen-induced, long-range ordered striped phase, and studied in situ with medium energy electron diffraction (MEED) and the surface magneto-optical Kerr effect (SMOKE). In contrast to Fe growth on either clean or oxygen-saturated Cu(110), the films on the striped substrates retain a layer-by-layer growth mode up to 6–7 ML and are fcc at least up to 12 ML. In addition, satellite peaks were observed on both sides of the MEED (0, 0) streak, indicating a long-range-ordered lateral modulation of the Fe surface. We postulate that the Fe films grow conformally onto the original striped substrate. SMOKE studies show that these fcc Fe wedges are ferromagnetic with an easy axis along the original stripes for Fe thickness >4 ML and a remanant magnetization that increases linearly with thickness beyond 4 ML.
Temperature dependence for the metastable phase formation in Au50Ni50 thin films under ion bombardment
Authors: Tamisier F. , Jaouen C. , Rivière J. P. , Durner R. , Heinz B. , Ziemann P.
Published: July 2002

The transformation of crystalline (111) oriented Au50Ni50 thin films produced by ion beam mixing of Au/Ni multilayers was studied under krypton ion irradiation in the temperature range from 4 to 100 K. The volume fraction of the amorphous phase was determined as a function of the ion fluence from in situ resistivity measurements, while ex situ x-ray diffraction was used to analyze the residual crystalline fraction. The experimental results indicate that the total Kr+ fluence necessary for complete amorphization of a metastable Au50Ni50 solid solution exponentially increases with increasing bombardment temperatures up to a critical value Tc of approximately 100 K, above which an ion induced amorphization is no longer possible. Furthermore, within the amorphization regime, two different mechanisms can be distinguished: A direct amorphization process via dense cascades at very low temperatures, while close to Tc overlapping of sequentially damaged regions is required to obtain the amorphous phase. The fact that amorphization is never observed under light ion irradiation even at 4 K demonstrates that the idea of a lattice collapse by defect accumulation cannot explain the amorphization behavior in this system. This points to the importance of energetic dense cascades for the amorphization process. Additional experiments on nanocrystalline Au50Ni50 films suggest that the lack of structural relaxation and/or the reduction of crystalline regrowth at amorphous/crystalline interfaces are necessary conditions for the stabilization of the amorphous clusters formed within the cores of dense cascades. Thus, the observed kinetic behavior is explained by considering the balance between dynamic annealing and amorphous phase formation. The importance of interface processes for crystalline growth is corroborated by studying the ion induced crystallization of an amorphous film.
Homoepitaxial growth and hydrogen incorporation on the chemical vapor deposited (111) diamond
Authors: Sakaguchi Isao , Nishitani-Gamo Mikka , Loh Kian Ping , Haneda Hajime , Ando Toshihiro
Published: July 2002

A systematic study of the effect of growth conditions (temperature, microwave power, and pressure) on the hydrogen incorporation and growth rate of (111) homoepitaxial diamond synthesized by microwave plasma assisted chemical vapor deposition is carried out by secondary ion mass spectrometry. Significant quantities of hydrogen incorporation in the (111) homoepitaxial diamond are detected for the growth conditions employed. Hydrogen concentration and growth rate are found to increase with an increase in the substrate temperatures. The increase of microwave power decreases the hydrogen incorporation and increases the growth rate. The effect of pressure rise is similar to that of microwave power. However, the effect of pressure rise over 50 Torr on hydrogen incorporation and growth rate is small. The decrease of hydrogen incorporation can be explained by the suppression of defects in the growing film by atomic hydrogen in the gas phase.
Effect of I− impurity on the efficiency of silver cluster formation on AgBr microcrystal surfaces
Authors: Hailstone R. K.
Published: July 2002

Computer simulation is used to study the effect of iodide impurity on silver cluster formation on AgBr microcrystals. The simulation is based on a nucleation-and-growth model of silver cluster formation in competition with recombination of electrons and holes. The efficiency of silver cluster formation is calculated as a function of microcrystal size and shown to increase with size for the impurity-free control, contrary to experimental data. This behavior is due to a partitioning of the hole between free and trapped states which favors the free side as size increases. As a result, recombination decreases and efficiency increases at large microcrystal size. Iodide impurity decreases efficiency relatively more at larger sizes because it introduces an internal recombination pathway not present in the control simulation. Because of their larger volume-to-surface-area ratios, the larger microcrystals are affected more by this additional recombination pathway than smaller microcrystals.
Carrier range and photocarrier generation efficiency on thin-film crystalline-conjugated polymers
Authors: Donovan K. J. , Scott K. , Spagnoli S.
Published: July 2002

The range of carriers in thin crystalline films of the conjugated polydiacetylene derivatives 3 and 4BCMU [with the side groups R1=R2=–(CH2)3–OCO–NH–CH2–COOC4H9 and –(CH2)4–OCO–NH–CH2–COOC4H9] are demonstrated to be greater than 110 and 90 μm, respectively, and the quantum efficiencies for carrier generation are determined. The films are created as a result of electron-beam-induced topochemical polymerization of a thin surface layer on a crystalline monomer substrate. The polymer films are subsequently separated from the underlying monomer and deposited onto a series of interdigitated electrodes with varying interelectrode separation. The range and generation efficiencies are deduced by measuring the photocharge induced on the electrode system as a result of a laser pulse. Comparisons are made with bulk single crystals.
One-dimensional effects in quantum wires made from a double heterojunction based on the AlAs/GaAs system
Authors: Sander T. H.
Published: July 2002

Narrow quantum wires of length 1–4 μm were fabricated by wet chemical etching from a heterostructure consisting of a 10 nm GaAs quantum well embedded into a δ-doped AlAs/GaAs short-period superlattice. Studying the magnetotransport at T=0.3 K the diffuse boundary scattering effect is observed. The position of the occurring peak depends strongly on the top gate voltage in contrast to previous reports. The depopulation of magnetoelectric subbands is identified in power spectra of the oscillatory magnetoresistance. The experimental results are used to estimate an electrical wire width and the Fermi wave vector. It is found that the wire conductance is mainly controlled through a narrowing of the potential from the sides. This is attributed to the stronger Fermi level pinning in the barrier layer of a double heterostructure compared to a single heterojunction. A wire of length 1 μm shows conductance steps at zero field. They vanish on applying a weak magnetic field.
Theory of transient spectroscopy of multiple quantum well structures
Authors: Ershov M. , Ruda H. , Shik A. , Perera A. G. U.
Published: July 2002

A theory of the transient spectroscopy of quantum well (QW) structures under a large applied bias is presented. An analytical model of the initial part of the transient current is proposed. The time constant of the transient current depends not only on the emission rate from the QWs, as is usually assumed, but also on the subsequent carrier transport across QWs. Numerical simulation was used to confirm the validity of the proposed model, and to study the transient current on a larger time scale. It is shown that the transient current is influenced by the nonuniform distribution of the electric field and related effects, which results in a step-like behavior of the current. A procedure of extraction of the QW emission time from the transient spectroscopy experiments is suggested.
Magnetization hystereses in trilayer systems with an in-plane component defect
Authors: Yorozu Takehiko , Hu Xiao
Published: July 2002

Hystereses loops during magnetization reversal processes are calculated numerically for systems of trilayer magnetic materials having an in-plane anisotropy defect between two perpendicular anisotropy layers. The shape of the hysteresis loop depends critically on the magnetic parameters and the thickness of the in-plane layer. The sum of anisotropy, exchange and Zeeman energy is also pursued in accordance with the reversal processes. When the energy required for the magnetization reversal is plotted as a function of the defect layer thickness, the energy accumulated in the defect indicates maximum value where the nucleation field bifurcates from the field necessary for the entire magnetization reversal.
Magneto-optical properties of chromium-alloyed manganese bismuth thin films
Authors: Bandaru Prabhakar R. , Sands Timothy D. , Weller Dieter , Marinero Ernesto E.
Published: July 2002

MnBi thin films have been considered for short-wavelength rewritable optical recording media due to the very large magneto-optic Kerr rotation and perpendicular anisotropy (Ku) of the hexagonal magnetic low-temperature MnBi phase. However, coincident structural and magnetic transformations near the Curie temperature (360 °C) result in poor thermal cycling behavior, preventing the application of MnBi as rewritable media. We have previously hypothesized that the substitution of Cr for Mn would reduce the ferromagnetic coupling along the c axis, thereby lowering the Curie temperature and possibly decoupling the magnetic and structural transitions. Preliminary experimental data reported earlier [P. R. Bandaru et al., Appl. Phys. Lett. 72, 2337 (1998)] supported this hypothesis. In this article, the effects of Cr substitution are further explored and the feasibility of Mn1−xCrxBi (0<x<0.15) films for magneto-optical recording applications analyzed. It is shown that 5% Cr is sufficient for decoupling the phase transitions with no significant loss in the magneto-optic figure of merit. Transmission electron microscopy studies indicate a small grain size (∼50 nm) for the Cr-alloyed films, which could be beneficial for reducing media noise.
A theoretical investigation of the effect of the oblique anisotropy axis on the ferromagnetic resonance linewidth
Authors: Layadi A.
Published: July 2002

The effect of the oblique anisotropy axis on the ferromagnetic resonance (FMR) linewidth in a thin film is studied theoretically. The angular dependence of the frequency linewidth is investigated in the plane containing the anisotropy axis and in the film plane. The effect of the tilt angle on the frequency linewidth is shown. Some unusual broadening of the lines in the FMR spectra and certain features of the variation of the frequency linewidth Δf with field orientation and field intensity could be interpreted as being due to the fact that the magnetocrystalline anisotropy axis, i.e., the easy or hard direction of the magnetization, is tilted from the normal with an angle δ. The tilt angle δ as well as the Gilbert damping coefficient α can be deduced from experimental data through this analysis.
Direct and inverse electromechanical effect in ferroelectric liquid crystalline elastomers
Authors: Lehmann Walter , Hartmann Lutz , Kremer Friedrich , Stein Peter , Finkelmann Heino , Kruth Holger , Diele Stefan
Published: July 2002

Ferroelectric liquid crystalline elastomers (FLCEs) form a novel class of materials, in which the properties of macroscopically oriented ferroelectric liquid crystals can be combined with those of elastomers. The (direct and the inverse) piezoelectric effect in these systems is measured and analyzed on a molecular level. Backed also by x-ray measurements, a model is suggested that interprets the observed piezoelectricity caused by electrically or mechanically induced motions of smectic layers which are inclined with respect to the sample surface. The strength of the observed electromechanical effects compares well with (or exceeds) that of classical piezoelectric materials like barium titanate, lead zirconate titanate or poled polymers like poly(vinylidene fluoride), making FLCEs an interesting candidate for applications in microsystems technology.
A new mechanism for deflagration-to-detonation in porous granular explosives
Authors: Gifford M. J. , Luebcke P. E. , Field J. E.
Published: July 2002

An investigation has been carried out into the differences between the deflagration-to-detonation (DDT) process as it occurs in low density [∼30% theoretical maximum density (TMD)] columns of conventional grain size (∼180 μm) pentaerythritol tetranitrate (PETN) and in ultrafine PETN with a grain size ∼1 μm. The principle technique for observing the process utilized charges confined within a steel housing fitted with a polycarbonate slit window. This allowed direct recording of the transition using high speed streak photography. The explosive was thermally ignited using a pyrotechnic mixture with low gaseous emission to minimize any prepressurization of the charge. In addition to the photographic records of the events, the outputs of photodiodes along the length of the column were monitored in order to determine the rate at which the reaction proceeds. The results obtained show that the DDT process in the larger grain PETN at low density was similar in structure to the DDT process at higher densities. In contrast a different mechanism leads to detonation in columns composed of the smaller grain size PETN when packed to densities less than 50% TMD. After ignition hot gases propagate along the column both compacting and igniting material as they pass. After the gases have reached the downstream end of the column, the column continues to burn and the pressure and temperature increase. Some time later initiation takes place at a point along the burning column, and detonation waves propagate in both directions from this point. The detonation waves propagate from the initiation point at speeds that would normally be associated with material compacted to around 60% TMD. The process appears to be in effect a deflagration-to-localized thermal explosion detonation transition.
Structural and vibrational properties of GaN
Authors: Deguchi T. , Ichiryu D. , Toshikawa K. , Sekiguchi K. , Sota T. , Matsuo R. , Azuhata T. , Yamaguchi M. , Yagi T. , Chichibu S. , Nakamura S.
Published: July 2002

Structural and vibrational properties of device quality pure GaN substrate grown using a lateral epitaxial overgrowth (LEO) technique were studied using x-ray diffraction, Brillouin, Raman, and infrared spectroscopy. Lattice constants were found to be a=3.1896±0.0002 Å and c=5.1855±0.0002 Å. Comparing the results with those on GaN epilayer directly grown on sapphire substrate, it is shown that the GaN substrate is indeed of high quality, i.e., the lattice is relaxed. However the GaN substrate has a small enough but finite residual strain arising from the pileup of the lateral growth front on SiO2 masks in the course of LEO. It was also found that the elastic stiffness constants C13 and C44, are more sensitive to the residual strain than the optical phonon frequencies. The high frequency and static dielectric constants were found to be 5.14 and 9.04. The Born and Callen effective charges were found to be 2.56 and 0.50.
Transverse strain responses in electrostrictive poly(vinylidene fluoride-trifluoroethylene) films and development of a dilatometer for the measurement
Authors: Cheng Z.-Y. , Bharti Vivek , Xu T.-B. , Wang Shexi , Zhang Q. M. , Ramotowski T. , Tito F. , Ting R.
Published: July 2002

A dilatometer based on the cantilever beam concept has been developed. The dilatometer is easy to use and capable of measuring transverse strain response of soft polymer films in a broad strain range (from 10−7 to 10−1) without mechanical constraining of the sample. It is capable of detecting strain over a relatively wide frequency range from mHz to above 100 Hz under different load and temperature. Using the setup, the electric field induced transverse strains of the electrostrictive poly(vinylidene fluoride-trifluoroethylene) copolymer films were characterized which shows that a large transverse strain can be achieved in this class of polymer. In addition, the effect of mechanical tensile load on the transverse strain was also evaluated and the results show that the strain response will be affected by the load. However, depending on the load level, the strain response of the polymer film under a given electric field may increase or decrease with load. Based on the phenomenological theory, it is shown that for a ferroelectric based material, the mechanical load will shift the Curie temperature. Hence, to a large extent, the change of the strain response with load observed here can be understood by linking it to the strain change with temperature.
Effect of Mo doping on accelerated growth of C-54 TiSi2: Evidence for template mechanism
Authors: Gribelyuk M. A. , Kittl J. A. , Samavedam S. B.
Published: July 2002

The phase sequence of the rapid thermal processing induced reaction at T=650 °C has been studied by a combination of high resolution transmission electron microscopy and image simulations. We found that pre-amorphization of poly-Si substrates does not change the reaction path, i.e., Ti5Si4 and C-49 TiSi2 phases were formed with the latter growing upon further annealing. In the Mo doped poly-Si/Ti system the C-54 TiSi2 phase forms along with Ti5Si4 and two Mo silicide phases, MoSi2 and Mo5Si3; no C-49 TiSi2 was observed. We provide direct evidence that the Ti–Si reaction in the Mo doped system follows the template mechanism with MoSi2 and Mo5Si3 based phases acting as template phases for accelerated growth of C-54 TiSi2. Direct formation of C-54 TiSi2 at lower temperatures bypassing C-49 TiSi2 is very promising for application of the Ti salicide process in the future generation of deep-submicron complementary metal–oxide–semiconductor devices.
Electrostatic measurement of plasma plume characteristics in pulsed laser evaporated carbon
Authors: Mayo R. M. , Newman J. W. , Sharma A. , Yamagata Y. , Narayan J.
Published: July 2002

A triple Langmuir probe measurement has been implemented to investigate plasma plume character in low fluence (∼3.0 J/cm2) pulsed laser evaporation (PLE) discharges and has been found to be an extremely valuable tool. Absolute plasma plume density estimates are found to reside in the range 1.0×1013–2.0×1014 cm−3 for vacuum pulses. A simple heavy particle streaming model for vacuum pulses allows estimates of the plume ionization fraction of ∼10%. This is consistent with typical deposition inventory suggesting that high kinetic energy ions may play an important role in diamond-like carbon (DLC) film deposition. Electron temperature inferred from the electrostatic probe is found to consistently reside in the range 0.5–3.0 eV, and appears to be uninfluenced by operating conditions and large variations in Ar and N2 fill gas pressure. Consistent with strong plume ion and neutral particle coupling to the background fill, constancy of Te suggests expulsion of background gas by the energetic plume. The leading edge ion plume speed is measured via temporal displacement of spatially separated probe signals on consecutive PLE pulses. Flow speeds as high as 5.0×104 m/s are observed, corresponding to ∼156 eV in C+. The ion flow speed is found to be a strongly decreasing function of fill pressure from an average high of ∼126 eV in vacuum to ∼0.24 eV at 600 mTorr N2. Raman scattering spectroscopy indicates DLC film quality also degrades with fill pressure suggesting the importance of high ion kinetic energy in producing good quality films, consistent with earlier work demonstrating the importance of energetic particles. Optical emission indicates an increase in C2 molecular light intensity with fill gas pressure implying a reduced, if any, role of these species in DLC production. Ion current signal anomalies are often seen during high pressure pulses. It is suggested that this may indicate the formation of high mass carbon clusters during plume evolution in the presence of background gas. Mass diffusivity estimates, based on density decay, suggest the presence of C2+ under these conditions. Demonstration and control of such cluster formation may provide method(s) for controlling novel advanced materials properties.
A configuration of the laser cavity for simultaneous dual wavelength Q-switch pulsed Nd:YAlO3 laser
Authors: Lin Wenxiong , Shen Hongyuan
Published: July 2002

On the basis of the investigation on the rate equations of the Q-switch simultaneous dual wavelength laser of Nd:YAlO3(Nd:YAP), we have developed a new configuration of the cavity by adjusting the ratio of the cavity lengths for different wavelength lasers according to their emission cross sections. Therefore, the competitive ability and the net gain for the two different wavelengths will be equal at anytime and anywhere in the laser rod, and the optimal spatial and temporal overlap of the two different wavelengths can be easy to realize for optimal sum frequency mixing. In addition, this configuration can overcome several disadvantages of the previously used configurations, and its photon density product integral of the two different wavelengths is about one order larger than the previous ones.
Prediction of the evolution of the erosion profile in a direct current magnetron discharge
Authors: Okazawa Katsunori , Shidoji Eiji , Makabe Toshiaki
Published: July 2002

We have performed a simulation of ion and fast neutral transport in the sheath region of a direct current magnetron discharge under different pressures by using the Boltzmann equation and the database from the two-dimensional (2D) results of a plasma structure, which was given by a hybrid model. Evolution of the erosion profile on the target surface has been predicted by using the 2D energy distributions of ions and fast neutrals incident on the target (cathode) surface. We confirmed that an accurate prediction of the erosion profile can be obtained by assuming that the constant sputtering yield corresponds to the cathode voltage under conditions of low pressures that make use of the film deposition processes.
Modeling of ionization of argon in an analytical capacitively coupled radio-frequency glow discharge
Authors: Bogaerts Annemie , Yan Min , Gijbels Renaat , Goedheer Wim
Published: July 2002

A hybrid Monte Carlo-fluid model has been developed for the description of electrons, argon ions, and fast argon atoms in a capacitively coupled radio-frequency (rf) glow discharge used in analytical spectroscopy. Typical operating conditions are about 6 Torr pressure and 10 W electrical power. The discharge cell is rather small and is characterized by a much smaller rf-powered electrode than grounded electrode, which yields a high dc bias voltage. The electron density at these conditions is in the order of 1013 cm−3. The computation time to simulate all these electrons with a Monte Carlo or a particle-in-cell method was found to be too long. Therefore, the electrons are subdivided in two groups. The fast electrons emitted from the rf electrode, as well as the ones formed by ionization with sufficiently high total (=kinetic+potential) energy for further ionization, give rise to so-called γ ionization; these are described with a Monte Carlo method. The slow electrons, which can, however, be heated again by the fluctuating electric field, give rise to so-called α ionization; they are described with a fluid approach, which also treats the argon ions. Moreover, the fast argon ions and atoms are treated with a Monte Carlo model in the rf sheath. Typical results of this model include the electrical characteristics (i.e., dc bias and rf amplitude voltages, electrical current, potential, and electric field distributions), the electron densities and mean energies, the ionization rates due to the electron impact α and γ ionization and fast argon ion and atom impact ionization, and the relative contributions of these ionization mechanisms to the overall ionization.
Macroscopic migration of implanted deuterium along a gas-bubble network in silicon
Authors: Katabuchi Tatsuya , Kawachi Naoki , Tagishi Yoshihiro
Published: July 2002

We observed experimentally the macroscopic migration of implanted deuterium in silicon with a new method in which the implanted deuterium concentrations were measured by counting the protons emitted from the nuclear reaction H2(d,p)H3 in the implanted region. The experimental results support a new model in which a macroscopic migration of gas bubbles containing the implanted deuterium in the form of D2 molecules exists in the implanted region and D2 molecules migrate macroscopically along the network. We describe the growth of the macroscopic network with percolation theory. From quantitative analysis of the experimental results, the density critical exponent β was obtained and found to be close to the theoretical value for two-dimensional systems predicted by the scaling theory of percolation.
Evaluation of tailored electrode (Ba,Sr)RuO3 for (Ba,Sr)TiO3
Authors: Choi Duck-Kyun , Kim Boum-Seock , Son Seung-Young , Oh Se-Hoon , Park Kyung-Woong
Published: July 2002

The conducting oxide electrode (Ba,Sr)RuO3 (BSR), which is expected to be advantageous for (Ba,Sr)TiO3 (BST) dielectric materials due to its similarity in structure and chemical composition, was evaluated. Ba/Sr and (Ba+Sr)/Ru ratios in BSR films were varied to investigate the effect of composition in BSR films on the BST properties. BSR films deposited at 1/9 ratio of O2/Ar resulted in the lowest resistivity. BST films on stoichiometric BSR electrodes demonstrated the best electrical properties. The dielectric constant was 450 and the leakage current density was 10−8 A/cm2,which is about two orders of magnitude lower than BST on RuO2 processed in the same laboratory conditions. The dielectric constant of BST films was affected more by the chemical composition, while the leakage current was influenced both by the chemical composition and the lattice constant of BSR. Minimization of reaction at the interface between BST and BSR through a suppression of diffusion due to the compositional similarity seems to result in such outstanding electrical properties.
Thickness-deconvolved structural properties of thermally grown silicon dioxide films
Authors: Ishikawa Kenji , Ogawa Hiroki , Fujimura Shuzo
Published: July 2002

We have studied structural and optical properties of thermally grown silicon dioxide films. These properties were examined by incrementally etching back the oxide films and performing infrared reflection absorption spectroscopy measurement at each thickness. We have divided the oxide into incremental layers and calculated the dielectric function of each of the layers. This study shows that the incremental layer, whose structure differs from that of bulk oxide, with thickness below 1 nm exists at the silicon and silicon dioxide interface. In addition, farther from the interface, the transverse optical mode frequency shifts slightly in wave number to a higher value.