bluuebunny/crossref_metadata_2025 huggingface dataset:
Interfacial reactions of Ni-In and Ni/In/Ni ohmic contacts to n-GaAs
Authors: Fu H. G. , Huang T. S.
Published: July 2002

The metallurgical characteristics of codeposited Ni-In and sequentially deposited Ni/In/Ni ohmic contacts to n-GaAs with a total Ni:In atomic ratio of about 1 were investigated in this study. After annealing below 400 °C, only NiIn phase was detected and no interfacial reaction was observed in Ni-In contact, whereas NixGaAs phase was formed at the interface in Ni/In/Ni contact. An extensive interfacial reaction occurred and some epitaxial phases were formed after annealing at 400 °C and above. NiGa and InxGa1−xAs were the final main phases in both the contacts. Furthermore, both the contacts became ohmic after annealing at 400 °C and the above temperatures. The appearance of ohmic behavior was correlated with the formation of interfacial InxGa1−xAs.
The formation of ternary compound Fe3Mo3C by ball milling
Authors: Di L. M. , Calka A. , Li Z. L. , Williams J. S.
Published: July 2002

Ball milling of Mo in nitrogen and ammonia gases has been studied. After heating, the formation of an intriguing ternary compound, Fe3Mo3C1−x, was observed as a result of Fe (and C) contamination from the milling apparatus. For a comparison, mechanical alloying of Fe–Mo–C powders with a stoichiometric composition of Fe3Mo3C was also performed and again an Fe–Mo–C alloy was obtained after milling. Thermal analysis shows a single transition peak for the formation of both the Fe3Mo3C phase in the latter case and also for Fe3Mo3C1−x formed as a result of contamination from the mill. The lattice parameter for the Fe3Mo3C1−x phase obtained by Mo-gas reaction is smaller than for the Fe3Mo3C phase obtained by mechanical alloying of stoichiometric Fe–Mo–C powders. It is suggested that, in the Fe3Mo3C1−x phase produced by milling of Mo in gases, a high concentration of C vacancies exists.
Modeling of O-18 tracer distribution during ‘‘double oxidation’’ experiments for inward growing scales
Authors: Appannagaari N. , Basu S. N.
Published: July 2002

Quantitative modeling of oxygen tracer (18O) concentration profiles obtained during double oxidation experiments has been carried out. An existing model in the literature, involving combined grain-boundary and lattice diffusion of oxygen with exchange, has been extended to cases where the grain size of the oxide is allowed to vary over its thickness. The model predicts tracer profiles in the entire oxide for the case of scale growth by inward oxygen diffusion. A computer program has been developed along with a graphical user interface for easy generation and visualization of simulated tracer profiles. The sensitivity of the simulated profiles to variations in volume (Dv) and grain-boundary (Db) diffusivities of oxygen in the oxide scale as well as the oxide grain-size variation have been studied. The results indicate that the normalized tracer profiles are sensitive to variations in Db, although to a lesser extent as compared to similar variations in Dv. Also, incorporation of variable grain size leads to profiles very different from those obtained from a constant oxide grain-size assumption. The computer program has been used to fit an experimental profile reported in the literature to obtain values of Dv and Db.
Analytical calculation of the surface potential and the band profile within the i region of a semi-infinite p/i semiconductor junction with arbitrary thickness and surface charge coverage of the intrinsic top layer
Authors: Ristein J.
Published: July 2002

An analytical calculation scheme for the band profile in the i region of a semi-infinite p/i or n/i junction with arbitrary thickness of the intrinsic top layer is presented. Using this algorithm the surface potential as a function of i-layer thickness as well as the band profile within the top layer can be calculated without mathematical approximations as long as the surface potential is held a few kT away from its intrinsic position by the doped sublayer, i.e., up to a critical thickness. Above that thickness the band profile can be approximated with sufficient accuracy by that of the corresponding infinite junction for which the exact solution will be also given in analytical terms. In a second step an arbitrary surface charge layer is incorporated into the algorithm which can modify the band profile considerably. The method whose mathematical derivation is presented in detail in this article was motivated by experiments on the surface-near passivation of dopants in silicon by hydrogen but should be generally useful for the discussion of p/i or n/i junctions.
Experimental study on the Er/p-InP Schottky barrier
Authors: Chen W. X. , Yuan M. H. , Wu K. , Zhang Y. X. , Wang Z. M. , Qin G. G.
Published: July 2002

Rare-earth element Er was deposited onto (100) oriented Zn-doped p-type InP to form Schottky barriers. The Er/p-InP Schottky barrier have been studied by current-voltage (I-V), temperature dependence of current-voltage (I-V-T), and capacitance-voltage (C-V) methods and Schottky barrier heights (SBHs) measured by I-V and I-V-T methods are in the range 0.83–0.87 eV, while SBHs measured by the C-V method are in the range 0.98–1.06 eV. Ideality factor n and series resistances R are in the range 1.08–1.11 and 30–50 Ω, respectively. Combining the experimental results of SBHs reported in the literature for Schottky barriers with various metals on p-InP (100), we conclude the Fermi level pinning for InP with (100) orientation is much stronger than that for Si or GaAs.
Temperature dependence of the minority carrier lifetime in GaAs/AlGaAs double heterostructures
Authors: Bergman J. P. , Hallin C. , Janzén E.
Published: July 2002

We have studied the photoluminescence decay time in a series of high quality GaAs/AlGaAs double heterostructure samples, grown by liquid phase epitaxy, with different p-type doping. We have compared the experimentally observed decay time as a function of temperature from 100 to 700 K, with a complete calculation of the radiative recombination rate, including the temperature dependence of the reabsorption factor. We conclude that the observed decay time is well explained by a dominating radiative recombination up to temperatures of about 500 K. The internal quantum efficiency is hence close to unity. At higher temperatures we observe a deviation from the expected values for the radiative recombination, attributed to a nonradiative recombination channel.
The Néel temperature of nanocrystalline chromium
Authors: Fitzsimmons M. R. , Eastman J. A. , Robinson R. A. , Lynn J. W.
Published: July 2002

Wide-angle neutron diffraction measurements taken at temperatures from 6 to 250 K indicate that the portion of a chromium sample with a mean grain size of 73 nm is antiferromagnetically ordered at temperatures below 100 K. The Néel temperature (about 120±10 K) for this nanocrystalline sample is suppressed considerably below that of strain-free single-crystal chromium (311 K).
Beryllium ion implantation in GaAsSb epilayers on InP
Authors: Merkel K. G. , Bright V. M. , Cerny C. L. A. , Schuermeyer F. L. , Solomon J. S. , Kaspi R. A.
Published: July 2002

Beryllium ion implantation was used to form high acceptor concentrations in GaAs1−xSbx (0.47≤x≤0.49) epilayers on semi-insulating InP substrates. Two implant doses were tested: Q0=5×1014 cm−2 and Q0=1×1015 cm−2 at an implant energy of E=50 keV. Electrochemical profiling and secondary-ion-mass spectrometry (SIMS) results confirm acceptor concentrations of NA≥1×1019 cm−3 and NA≥2×1019 cm−3 within 1000 Å of the GaAs1−xSbx surface for the lower and higher implant dose, respectively. These results provide a p+ surface layer for low-resistance ohmic contact to GaAsSb-based devices. Optical microscopy and SIMS demonstrate rapid thermal anneal (RTA) temperature limits of T=650 °C for Q0=5×1014 cm−2 and T=600 °C for Q0=1 ×1015 cm−2. The temperature limitation is imposed by destabilization of the GaAs1−xSbx surface through Ga sputtering during implantation, and Ga and As outdiffusion during RTA.
Photoacoustic determination of the optical and thermal properties of isovalent S-substituted polymeric Ge–Se system
Authors: Ghosh A. K. , Chaudhuri B. K.
Published: July 2002

Isovalent sulfur (S) -substituted glassy Ge0.05SxSe0.95−x (0≤x≤0.15) alloys have been prepared by quick quenching from the melt. The concentration (x) dependences of optical absorption, band gap, and thermal diffusivity of these alloys have been investigated at room temperature (300 K) by the photoacoustic spectroscopic (PAS) method in the spectral range of 350–800 nm. The small variation of the optical gap E0 with composition is analyzed on the basis of Kastner’s model [in the small Ge limit of M. Lannoo and M. Bensoussan, Phys. Rev. B 16, 3546 (1977)], considering the electronegativity difference between the substituting atoms (Se and S) and bond energy calculation. The observed exponential edge (Urbach edge) can be considered as an internal Franz–Keldysh effect arising from the charged impurity-generated as well as ‘‘frozen-in’’ optical-phonon-generated fields. This can be described in the framework of the Tauc and Dow–Redfield model. The phonon-assisted indirect transition at the band-tail region is also obtained from the present studies. The concentration-dependent thermal diffusivity, estimated from the PAS studies, showed a minimum around x=0.05 similar to the behavior of the observed optical energy gap.
Electron paramagnetic resonance of Ni+ impurities in AgGaSe2
Authors: Halliburton L. E. , Giles N. C. , Schunemann P. G. , Pollak T. M.
Published: July 2002

Electron paramagnetic resonance (EPR) has been used to detect Ni+ impurities substituting for Ag+ ions in a single crystal of AgGaSe2. The observed EPR spectrum has axial symmetry about the c axis of the crystal and is described by g∥=2.635 and g⊥=2.241. A partially resolved hyperfine pattern in the c-axis spectrum is consistent with equivalent interactions with four I=3/2 nuclei (e.g., 69,71Ga). The concentration of Ni+ ions in the undoped crystal was approximately 7×1017 cm−3. These substitutional Ni+ impurity ions are suggested to be responsible for the polarized 2.2 μm absorption band that is presently limiting the use of AgGaSe2 as a 2 μm pumped optical parametric oscillator with mid-infrared output.
High-resolution transmission electron microscopy of structural changes at and near surfaces of ZnTe crystals
Authors: Ning X. G. , Avalos-Borja M. , Ye H. Q.
Published: July 2002

Structural changes on and near surfaces of ZnTe crystals were observed by the surface profile imaging method in a high-resolution transmission electron microscope (HRTEM) with 200 kV accelerating voltage. It was found that ZnO particles form under electron irradiation on surfaces originally covered with a thin amorphous layer. For amorphous coverages thicker than about 7 nm, ZnO particles were not observed. Under the ZnO particles, 2×1 and 2×2 superstructures (about 10 nm in size) were often found in the ZnTe matrix. These superstructures can be described as a metastable Zn2Te3 having a structure similar to the ordered Ga2Te3 structure. This change can be accomplished by considering the diffusion of zinc from the ZnTe matrix to the surface to form the oxide and evaporation of tellurium out of the surface, leaving behind a matrix with an altered stoichiometry. The HRTEM computer-simulated images for the proposed Zn2Te3 superstructure matched very well the experimental images. The 2×1 and 2×2 superstructures corresponded to the HRTEM images of the new Zn2Te3 structure viewed along its [211] and [100] directions, respectively.
Frequency-induced angular sweep of the electromagnetic wave diffracted by weakly curved periodic media in the dynamical regime
Authors: Melone S. , Francescangeli O. , Caciuffo R.
Published: July 2002

The effects connected with the dynamical scattering of electromagnetic waves by weakly curved periodic layered media are discussed. A simple physical model is used to take into account the effects of the curvature in the framework of the dynamical theory of diffraction. A peculiar effect consisting of the possibility of a continuous sweep of the direction of the diffracted beam over a relatively wide angular range, under the control of the frequency of the incident wave, is seen and analyzed. The design of a device realizing such a frequency-direction conversion in the frequency range of 82–107 GHz is presented and discussed as a prototype of a new possible class of instruments.
Alloy nonrandomness in diluted magnetic semiconductors
Authors: Harrison P. , Fatah J. M. , Stirner T. , Hagston W. E.
Published: July 2002

A simple model of alloy nonrandomness is introduced within a framework where the effective concentration of spin singlets as a function of the nominal concentration of magnetic ions in a nonrandom alloy can be obtained by transformations of the corresponding function in random alloys. The theory shows that a given system that is appreciably nonrandom can have a magnetic response almost identical with that of a random distribution. Possible ways of identifying alloy nonrandomness in diluted magnetic semiconductor structures are described.
Magnetic microstructure observations of Co-Cr-Ta films using neutron scattering
Authors: Maeda Yasushi , Rogers David J. , Song Ohsung , Suzuki Jun-ichi , Morii Yukio , Takei Koji
Published: July 2002

Using small-angle neutron scattering (SANS), we examined the magnetic microstructure of Co-12 at. % Cr-2 at. % Ta films sputter-deposited at 200 °C. Nuclear magnetic resonance confirmed the presence of highly Co-enriched regions produced by compositional separation. The SANS spectrum exhibited a main magnetic scattering peak corresponding to a size of several nm in the magnetic microstructure. This is indicative of a fine and uniform distribution of Co-enriched regions suitable for high-density magnetic recording.
Magneto-optical Faraday effect of discontinuous magnetic media with a one-dimensional array structure
Authors: Inoue Mitsuteru , Isamoto Keiji , Yamamoto Takeshi , Fujii Toshitaka
Published: July 2002

To elucidate unique Faraday spectra observed in porous magnetic films, we theoretically analyzed the magneto-optical Faraday effect of discontinuous media with an ideal one-dimensional array structure. The media are constituted by magnetic materials (width DM) and air gaps (width DG) which are aligned interchangingly. Propagation properties of optical waves in the media, such as the dispersion characteristics and the polarization states, were explored circumstantially, and apparent Faraday rotation and ellipticity spectra of the discontinuous media were obtained. The analysis reveals that the apparent Faraday spectra of the media vary to a large extent from those of a continuous medium depending strongly on the media structures. The key factors which govern the magneto-optical effect of the discontinuous media are structural elements of DM and 2(DM+DG): The Faraday spectra are subject to change only when DM<λ (λ: the wavelength of optical wave in a vacuum). Then, the magneto-optical effect of the magnetic materials is related to mode conversion from TE to TM when 2(DM+DG)<λ. If DM<λ<2(DM+DG), the mode conversions of both directions from TM to TE and TE to TM are possible. The mode conversion dominates the apparent Faraday spectra of the media, causing the enhancement in the Faraday rotation and systematic shift of the spectra toward shorter and/or longer wavelength of the optical waves. These results explain very well the unique Faraday spectra of the porous films.
Remanent magnetization of ceramic and single-crystal high-T
c superconductors in tilted magnetic fields
Authors: Bugoslavsky Yu. V. , Minakov A. A. , Vasyurin S. I.
Published: July 2002

Dependence of the remanent magnetization (Mr) anisotropy on the structure and shape of the superconductor and on the magnetizing procedure was studied for a number of high-Tc superconductor ceramics and single crystals. The experiments were done by means of a vibrating-sample magnetometer with a rotatable sample holder. It was found that the main contribution to the anisotropic behavior of Mr is due to the surface screening currents, and therefore the anisotropy is subject to variation when sample shape is changed. The question is resolved, why the effective demagnetization factors for decoupled ceramic samples are different from those calculated in the inscribed-ellipsoid approximation. Influence of inhomogeneous grain magnetization and global bulk currents on the angular dependencies of Mr in ceramic samples is investigated. The evolution of remanence in YBCO single crystals with an increase of the magnetizing field is described within an extended Bean model.
Comparative optical studies of Ce2S3 and Gd2S3 compounds
Authors: Witz C. , Huguenin D. , Lafait J. , Dupont S. , Theye M. L.
Published: July 2002

A detailed comparative study of the optical properties of two rare-earth (RE) sesquisulfide compounds γ−Ce2S3 and γ−Gd2S3, shows that if the two compounds have about the same main absorption edge corresponding to band-to-band S 3p-RE 5d transitions, Ce2S3 exhibits an additional absorption band at lower energies. This band, which is ascribed to the presence of Ce 4f states at the top of the valence band, explains the difference in color between the two compounds.
Monitoring of intermixing and interdiffusion by x-ray diffraction of ion-implanted quantum-well structures
Authors: Karla I. , Hogg J. H. C. , Hagston W. E. , Fatah J. , Shaw D.
Published: July 2002

The intermixing (and associated interdiffusion) resulting from ion implantation of argon ions into Cd1−xMnxTe quantum-well structures has been investigated. The experimental value of the mixing parameter of 1.5×103 Å/eV is large compared with the values reported for this parameter in metallic superlattices, and is consistent with an appreciable degree of inter diffusion accompanying the implantation process.
Field-effect transistors as tunable infrared detectors
Authors: Gusmão M. S. , Mahan G. D.
Published: July 2002

We present a theory of tunable infrared response in field-effect transistors. The linear α(ω) and second-order β(ω) polarizability are calculated numerically as functions of frequency of an applied electric field. Transitions between electron subbands in the inversion layer cause resonances in the frequency response, which are tunable with a gate voltage.
Numerical simulation of wide band-gap AlGaN/InGaN light-emitting diodes for output power characteristics and emission spectra
Authors: Shah Pankaj , Mitin Vladimir , Grupen Matt , Song G. Hugh , Hess Karl
Published: July 2002

We present results from numerical simulations of AlGaN/InGaN double-heterostructure light-emitting diodes. A highly convergent, fast, and memory efficient algorithm necessary for wide band-gap device simulation was developed and is described here. Charge carrier tunneling currents and a band to impurity recombination mechanism are included. The results compare favorably to experimental results. The results demonstrate that the saturation of power at high currents, the high rate of increase in currents at high voltages, and the reduced broadening of the optical emission spectrum at high biases, with only band-to-acceptor recombination occurring in the active region, are due to carriers leaving the active region by thermionic emission rather than recombining.
Determination of valence band offsets from Si/Si1−xGex/Si using temperature-dependent current–voltage characteristics
Authors: Chretien O. , Souifi A. , Apetz R. , Vescan L. , Lüth H. , Popescu C.
Published: July 2002

We have determined the valence band discontinuity ΔEv of strained Si1−xGex on unstrained (100) Si using temperature-dependent current–voltage characteristics (I–V-T) of Si/Si0.83Ge0.17/Si heterostructures. In a first step, the measurements were performed on a Schottky diode, and in a second step, on a sample with ohmic contacts. The values of ΔEv obtained by these two different procedures are comparable. Moreover, they are in good agreement with the theoretical value of ΔEv=0.84x=143 meV predicted by Van de Walle measurements.
DX center in AlGaAsSb:Te and a mechanism of its ionization
Authors: Terziev N. S. , Hubík P. , Krištofik J. , Mareš J. J. , Šmíd V. , Yakovlev Y. P.
Published: July 2002

The Te-related DX center in Al0.3Ga0.7As0.02Sb0.98 layers of p-n junctions and avalanche photodiodes has been studied by capacitance–voltage, deep level transient spectroscopy, and direct capacitance transient measurements. A process of DX center ionization under high reverse bias conditions has been observed at temperatures where the thermal emission is negligible. The kinetics and reverse bias dependence of this process has been studied and analyzed. A mechanism of photon generation in the high-electric-field region of the devices and a subsequent photoionization of the DX center has been proposed.
Shallow Si/Pd-based ohmic contacts to n-Al0.5In0.5P
Authors: Hao P. H. , Wang L. C. , Chang J. C. P. , Kuo H. C. , Kuo J. M.
Published: July 2002

Si/Pd-based contact schemes based on the solid-phase regrowth (SPR) process have been developed to form low-resistance ohmic contacts to n-Al0.5In0.5P (Eg=2.3 eV) with a minimum contact resistivity of about 6×10−6 Ω cm2. The SPR process responsible for the ohmic contact formation was verified using cross-sectional transmission electron microscopy. The contact resistivity of the Si/Pd-based contacts remained in the range of 2–3×10−5 Ω cm2 after aging at 400 °C for 25 h. Furthermore, a lateral modulation disordering phenomenon as a result of the SPR process in the regrown AlInP layer has been observed. These ohmic contacts may be useful in some novel Al0.5In0.5P-related device fabrication schemes.
Transmission electron microscopy study of microstructure and misfit dislocations in epitaxial LiTaO3 thin films grown on sapphire by a metalorganic chemical vapor deposition process
Authors: Xie H. , Lu Y.-C. , Raj R.
Published: July 2002

Epitaxial lithium tantalate thin films were grown on the prismatic plane of sapphire substrates by metalorganic chemical vapor deposition. The film contained two crystallographic variants, both epitaxially oriented with respect to the substrate. The variants were related to each other by 180° rotation about the c axis (which lies in the interface plane). A rectangular misfit dislocation network at the interface was imaged by cross-sectional high-resolution transmission electron microscopy, along two orthogonal directions, and by plan-view weak beam dark field transmission electron microscopy. The Burgers vectors of these two orthogonal sets of misfit dislocations were determined to be 1/3[11̄00] and 1/3[0001]. The spacing of the dislocations was nearly exactly equal to the ideal values suggesting a very high degree of epitaxy, which was also confirmed by nearly equal values of the full width at half-maximum of x-ray peaks in the ω scans obtained from the overgrowth and the sapphire substrate. The perfection of the epitaxy is attributed to the lack of multiplicity of slip systems in the interface plane for accommodating the misfit.
Bragg scattering and effective force-balance equations in superlattice miniband transport
Authors: Lei X. L.
Published: July 2002

The two existent balance-equation methods for electron transport, which incorporate Bragg scattering and give rise to bulk negative differential mobility, are further investigated using the Boltzmann transport equation for Bloch electrons. We point out that the ansatz employed in the earlier Büttiker–Thomas (BT) scheme to relate the average drift velocity and the average momentum, should be modified for a general energy band. Numerical calculation of superlattice miniband conduction shows that the modified BT force-balance method yields a velocity–field behavior almost equivalent to that predicted by the acceleration-balance method of Lei, if the appropriate momentum function is used in the calculation of the BT frictional force, such that Bragg scattering is properly taken into account in the process of electron scattering.
Impedance of a coil near an imperfectly layered metal structure: The layer approximation
Authors: Satveli Radhika , Moulder John C. , Wang Bing , Rose James H.
Published: July 2002

Changes in the impedance of a coil next to a one-dimensional layered conductor due to three-dimensional changes in the conductivity are studied. Eddy current probes are often used to inspect layered one-dimensional, nonmagnetic metal structures whose electrical conductivity varies primarily with depth beneath the surface. We present a perturbation method, the ‘‘layer approximation,’’ which yields simple and readily evaluated formulas for changes in the impedance of a small coil due to localized three-dimensional variations in the conductivity. The layer approximation is constructed to be accurate when the conductivity change due to the defect is small or the defect is nearly one-dimensional. The impedance is calculated and reported for a variety of defects in layered metal structures: voids, inclusions, interfacial roughness, and fasteners. We test the ‘‘robustness’’ of the layer approximation using an extreme case, a flat-bottom hole in an aluminum plate, as a ‘‘benchmark.’’ Both experimental measurements and more exact theoretical calculations are reported. Impedance measurements were made with a Hewlett–Packard 4194A impedance analyzer for a right-cylindrical flat-bottom hole in a 1-mm-thick 2024 aluminum alloy plate; the hole was on the side opposite to the coil. Frequencies were varied from 2.5 to 50 kHz. We also calculated the change in the impedance for this benchmark problem using the numerically exact volume integral method. For this benchmark problem, the layer approximation is in good agreement with experiment and more exact theory.
Highly crystallized (La,Sr)MnO3 films deposited by facing targets sputtering apparatus
Authors: Matsushita N. , Noma K. , Nakagawa S. , Naoe M.
Published: July 2002

La1−xSrxMnO3 films about 4000 Å thick were deposited on thermally oxidized Si wafers, and then their crystalline orientation and magnetic characteristics were investigated. Orientation of (111) in which large ions such as O2−, La3+, and Sr2+ are most closely packed became preferential with increase of total gas pressure Ptotal and partial oxygen pressure PO2. Although spontaneous magnetization was not detected, even the film deposited at substrate temperature Ts as low as 330 °C revealed obvious orientation of (110) in which metallic ions are most closely packed. The film deposited at Ts of 500 °C, Ptotal of 2.0, and PO2 of 0.1 mTorr, and the film postannealed at 900 °C for 3 h in oxygen atmosphere possessed the saturation magnetization 4πMs of 1.2 and 3.5 kG at 77 K and their Curie temperatures were 217 and 313 K, respectively. B–H curves at 77 K revealed that the easy magnetization direction of these films was in-plane.
The three-dimensional RFIM: Existence of a tricritical point? (abstract)
Authors: Hernandez Laura , Diep H. T.
Published: July 2002

The critical behavior of the RFIM has been a subject of controversy for a long time. After the qualitative analysis of Imry and Ma and the rigorous proof given by Imbrie, it is nowadays accepted that the lower critical dimension of the system is d1=2. Though some general results exist in the mean field approach, the nature of the transition for the real system for d≳2 remains less clear. In this work we study, using the Monte Carlo histogram technique and finite size scaling, the three-dimensional RFIM with a symmetric bimodal field distribution. Our results strongly suggest the existence of a tricritical point where the transition crosses over from a second-order transition at low but finite fields to a first-order transition for high fields, in contradiction with previous suggestions issued from standard Monte Carlo simulations.
Domain structure in NiO biasing layers (abstract)
Authors: Erwin R. W. , Borchers J. A. , Lind D. M. , Lochner E. , Shaw K. A. , Singer B. , Stoyonov P. , DiBari R. C. , Berry S. D.
Published: July 2002

Fe3O4/NiO multilayers exhibit long-range antiferromagnetic order with the magnetite ferrimagnetic correlations confined to a single layer due to stacking faults of the spinel structure at the interfaces. We are studying the field dependence of the interlayer coupling and magnetic structure in a series of Fe3O4/NiO multilayers using neutron diffraction. Both NiO and Fe3O4 thin films were included in the measurements for comparison. In a single thick Fe3O4 film, intensity changes with magnetic field are consistent with the alignment of the net ferrimagnetic moment parallel to the applied field. In a single thick NiO film, we observe no intensity changes with magnetic field. For multilayers where the ratio of NiO to Fe3O4 is far from unity, the field dependence approximates that of the thick film of the majority constituent. However, for a Fe3O4 (68 Å)/NiO (34 Å) multilayer the NiO antiferromagnetic intensity decreases with increasing field, applied parallel to a [110] axis in the film plane. This indicates a domain repopulation in the NiO, concomitant with the magnetization of the magnetite, where the antiferromagnetic modulation direction prefers the [111] axes that are closest to the applied field direction. A broadening of the NiO peak shows that the repopulated domains are smaller. We are pursuing a complete measurement of the domain populations by measuring a series of NiO antiferromagnetic diffraction peaks.
Effects of high-temperature annealing on the optical absorption of Ca:YIG films
Authors: Bornfreund R. E. , Wigen P. E.
Published: July 2002

The effect of high-temperature annealing on the optical properties of a calcium doped yttrium iron garnet (Ca:YIG) film were studied. The optical absorption was examined for a film of Ca0.18:YIG reduced in an atmosphere of N2 at temperatures above 900 °C. During the annealing, the optical absorption initially decreased below the as-grown value as expected, but subsequently increased to a value greater than that for the as-grown film as the annealing temperatures were increased. The changes observed in the optical absorption are believed to be due to the introduction of oxygen vacancies in the film as a result of the annealing. Initially, the added oxygen vacancies simply compensate the Ca2+, while successive annealings introduce excess vacancies which lead to a new absorption mechanism. The results of this study and the expected mechanism for the effect are reported.
First-order magnetic phase transition in (Er, Tb)M2(M=Co, Ni) (abstract)
Authors: Takeuchi A. Y. , Garcia F. , da Cunha S. F.
Published: July 2002

The rare-earth intermetallic compounds RCo2, cubic Laves phases, are very suitable to study the magnetism of the 3d. Depending on the R magnetism we have an induced moment for the Co. For R=Dy, Ho, or Er the magnetic transition is of first order, whereas for Gd and Tb it is of second order. This behavior has been observed in this work by electrical resistivity and dc magnetization measurements in function of concentration in the system Er1−xTbx)Co2 where the internal field of the nearest R neighbors can induce the Co moment. Magnetization measurements of field cooling (FC) and zero field cooling (ZFC) samples are different for T<Tc. This difference collapses for higher applied magnetic fields. The minimum for T just above Tc in ρ vs T curves disappears for x=0.2 with the onset of the second-order transition up to x=1.0. This minimum can result from the conduction electrons scattering on the spin fluctuations, enhanced by the 4f moments and the scattering by phonons. The idea of the metamagnetism of Co moments in the origin of this minimum is discarded. From our results we can conclude that a small amount of Tb in ErCo2 changes the first-order to second-order transition. Since the localization of the Fermi energy in the density of state of the 3d band is very important, a comparison study has been done with the system Er1−xTbx)Ni2. The concentration dependence of the lattice parameter differs a little from Vegard’s law. Here we can infer from our results that all the magnetic transitions are second-order type.
Magnetic properties of monodomain Nd-Fe-B-C nanoparticles
Authors: Brunsman E. M. , Scott J. H. , Majetich S. A. , McHenry M. E. , Huang M.-Q.
Published: July 2002

Nanoparticles of Nd-Fe-B-C with their crystal structure similar to the Nd2Fe14B phase were generated in a carbon arc. With an average diameter of less than 40 nm, they are monodomain. They have a smaller room-temperature coercivity than would be predicted from the bulk magnetocrystalline anisotropy. However, their coercivity is greater than was previously observed in particles <5 μm prepared by spark erosion. While the carbon arc process is useful for making small carbon-coated particles which resist oxidation, here dispersion of excess Nd in the carbon matrix results in a significant paramagnetic signal. The dc demagnetization curves enable the paramagnetic and ferromagnetic contributions to be distinguished.
Dependence of media noise on grain size and intergranular coupling in thin-film media
Authors: Kikuchi Akira , Kawakita Shinya , Nakai Junichi , Shimatsu Takehito , Takahashi Migaku
Published: July 2002

The media noise performance is discussed in connection with grain size and intergranular coupling for the Co85.5Cr10.5Ta4, Co78Cr17Ta5, and Co62.5Ni30Cr7.5 thin-film media fabricated under the ultraclean sputtering process (UC process). It is clarified that the value of S/Nm is quantitatively represented as functions of the value of Hc/Hkgrain and the grain size in every kind of media. The value of signal-to-media noise ratio (S/Nm) increases with the increment in Hc/Hkgrain and also with the decrement in grain size. In the media with grains adequately separated by segregated grain boundaries fabricated by the UC process, grain-size reduction, taking account of the decrease in intergranular magnetostatic coupling, is required to obtain even higher S/Nm values.
Using synchrotron radiation x-ray multiple diffraction to examine the lattice coherency of semiconductor surfaces and epitaxial layers
Authors: Sasaki J. M. , Cardoso L. P. , Campos C. , Roberts K. J. , Clark G. F. , Pantos E. , Sacilotti M. A.
Published: July 2002

An experimental facility for carrying out x-ray multiple diffraction (XRMD) studies in parallel-beam geometry using the Daresbury synchrotron radiation source and its application in the study of coherency of an epilayered sample are described. Experimental high-resolution Renninger scans (RS) about GaAs(006) are presented and the pseudoforbidden ‘‘Aufhellung’’ eight-beam (000,006,020,042,044,026,02̄4,02̄2) case has been fully resolved for the first time using the setup which involves a double-crystal six-circle scattering geometry and data acquisition providing optimal conditions for these scans to be obtained. A sample of InGaAs/AlGaInAs/InP(001) epilayered material was also examined and high-resolution RS from the bulk, substrate, and epitaxial overlayers were obtained. The diffraction wavelength used was determined directly from the bulk RS as λ=(1.4695±0.0005) Å. The data clearly reveal a number of interesting XRMD features which allow for the sample characterization. The layer parallel lattice parameter can be determined either from the epilayer tetragonal distortion in the layer RS or from the MORSI (modulation of the RS intensity due to the presence of the epilayers) [Greenberg & Ladell, Appl. Phys. Lett. 50, 436 (1987)] dips in the substrate RS. These dips can also provide simultaneous information on relative tilt and rotation between both layer/substrate lattices. The fitting of the position and profile of the three-beam surface peaks allows the determination of the layer parallel lattice parameter and the layer mosaic spread on the sample surface plane.
Sub-Doppler spectroscopy of 89Y
Authors: Lynds L. , Woody B. A.
Published: July 2002

Sub-Doppler resolution of atomic transitions is obtained by spatially resolved laser induced fluorescence in a diverging atomic beam created by laser ablation. Resonances are spatially separated by relativistic effects experienced between the probing radiation and the moving particles thus proving that it is a high-quality atomic beam. Fast neutral atomic beams of 89Y having centroid energies up to 12 eV are interrogated with tuned, single-frequency laser light. Narrow, spatially separated points of light move along the probe axis and are imaged by detecting only the zero-velocity components of the fluorescence. Accordingly, the Y 2D3/2→Y 2D03/2 transition centered near 16 146 cm−1 is resolved into its hyperfine components with line widths on the order of 55 MHz and with an equivalent resolution of ±5 MHz (±0.0002 cm−1). The upper level hyperfine interaction constant derived from level splitting is +89±5 MHz. An equivalent collimation ratio of 9 mrad is estimated for the expanding pulsed atomic beam. We propose that laser ablation of selected targets provides a unique source of neutral or ionic ground-state atoms and molecules for spectroscopic studies. Details of these experiments are discussed and analyzed.
Performance modeling of a 4 K active magnetic regenerative refrigerator
Authors: Johnson J. W. , Zimm C. B.
Published: July 2002

A model was used to compute the performance of an ErNi-based active magnetic regenerative refrigerator as a function of the hot and cold temperatures, applied magnetic field strength and mass flow rate. A device spanning 4 to 10 K with an applied field of 3 T and a mass flow rate of 1.3 g/s is predicted to have a cooling power of 2.8 W. Approximately 1 W of cooling is predicted with a magnetic field of 1 T. The device performance as a function of the mass flow rate is studied. The performance is not strongly dependent on the hot temperature but is very sensitive to changes in the cold temperature. This is attributed to the material properties of ErNi. Comparisons of the predicted performance of ErNi and three other magnetic refrigerants are also made.
Advantage of short over long annealing to activate As implanted in metastable pseudomorphic Ge0.08Si0.92 layers on Si(100)
Authors: Im S. , Lie D. Y. C. , Nicolet M.-A.
Published: July 2002

Metastable pseudomorphic Ge0.08Si0.92 layers grown by chemical-vapor deposition on Si(100) substrate were implanted at room temperature with 90 keV As ions to a dose of 1×1013 cm−2. The samples were subsequently annealed for short 40 s durations in a lamp furnace with a nitrogen ambient, or for long 30 min periods in a vacuum tube furnace. For samples annealed for a 30-min-long duration at 700 °C, the dopant activation can only reach 50% without introducing significant strain relaxation, whereas samples annealed for short 40 s periods (at 850 °C) can achieve more than 90% activation without a loss of strain. We conclude that it is advantageous to anneal a low-dose As-implanted pseudomorphic and metastable GeSi epilayers briefly at an elevated temperature, rather than to anneal it for a 30-min-long period at a lesser temperature, when high activation without a strain loss is desired.
Quasioptics of second-order Bragg interaction in a thin film optical waveguide
Authors: So Daniel W. C. , Seshadri S. R.
Published: July 2002

For the transverse magnetic mode, the second-order Bragg interaction of the guided and the radiated waves is considered for a thin film dielectric waveguide with a corrugated film–cover interface. A set of four canonical equations for the deexcitation and the excitation of the guided waves are derived by employing a quasioptical technique. The governing equations are applied for the treatment of a surface-emitting distributed feedback laser. Some design criteria are obtained for optimizing the performance of the laser. When these design criteria are used, the laser characteristics are insensitive to small changes in the physical parameters about their designed values. Numerical results are presented to illustrate the performance of the laser. The shape of the surface relief grating is found to play a vital role in determining the gain threshold and the gain margin of the laser.
Characterization of TaSi2–Si composites for use as wide-bandpass optical elements for synchrotron radiation
Authors: Stock S. R. , Rek Z. U. , Goorsky M. S.
Published: July 2002

The wide rocking curves of matrix reflections of the in situ eutectic composite TaSi2–Si make wafers of this material attractive for use as wide-bandpass monochromators for synchrotron radiation, and characterization of wafers of TaSi2–Si for use with energies normally accessible at storage rings (i.e., 5–40 keV) is the focus of the present report. A wafer with [111]Si orientation and a wafer with [110]Si orientation are studied. The high degree of preferred orientation of the TaSi2 rods relative to the Si matrix is examined using synchrotron Laue patterns, and the 100TaSi2, 003TaSi2, 101TaSi2, and 102TaSi2 reflections are used to establish the orientation relationship and to determine that the spread of rod orientations is at least 5° and probably no greater than 6°. Double-axis diffractometry with Cu Kα radiation reveals matrix reflections with rocking curve widths that are about 20 times broader than those from perfect Si and with peak reflectivities approaching 20%. The rocking curves widths are found to be relatively insensitive to irradiated area, thus indicating that most of the observed width is not due to long-range bending. Triple-axis diffractometry with Cu Kα radiation reveals that considerable compressive strain exists in the matrix and that much of the width of the diffraction peak is due to mosaicity. The performance of the [111]Si TaSi2–Si wafer and a perfect [111] Si wafer as monochromators for microradiography are compared, and a gain of an order of magnitude in x-ray intensity delivered to the sample is demonstrated with the composite crystal.
Contact magnetoresistive head for perpendicular magnetic recording
Authors: Takano Ken-ichi , Muraoka Hiroaki , Nakamura Yoshihisa
Published: July 2002

Contact-type magnetoresistive (MR) inductive head designs and recording characteristics have been discussed in perpendicular magnetic recording. The shielded-type MR head offered a high D50 of 100–135 kFRPI, and the unshielded MR head had the largest output in a low density region. However when the recorded bit length became shorter than the shield-to-shield spacing, the shield had little effect and the output of the shielded head was no longer almost equal to that of the unshielded MR head. In result, the shielded-type and even unshielded-type MR heads have good high density recording characteristics in the head-to-medium contact state.
Magnetic properties of CoFeB sputtered films for high Bsat applications (abstract)
Authors: Ravipati Durga , Tong Hua-Ching , Miloslavsky Lena
Published: July 2002

High density recording requires pole material with a large saturation magnetization and high permeability. To realize the high data rate and high frequency (in the range of 40–100 mHz) applications in thin film heads, high resistivity materials are desired. Amorphous materials such as CoZrNb, CoHfNb, etc., and crystalline materials such as FeAlN, FeN, etc., were studied for high Bsat applications. CoFeB films were rf sputter deposited on Al2O3–TiC substrates. The as-deposited Hce (easy axis coercivity), Hch (hard axis coercivity) anisotropy field Hk and magnetiostriction λs were measured to be 0.05–0.10, 0.19–0.31, 16–21 and −6.5×10−7, respectively, with a saturation magnetization of 15.4 kG. The as-deposited magnetic anistropy Hk was reduced to 2.5 Oe by vacuum annealing the wafers in a rotating external magnetic field of ∼6000 Oe at 200 °C. Reduction of Hk by RFA can be attributed to magnetic field thermally induced pair ordering. A 1 μm thick CoFeB film exhibited resistivity of 66 μΩ cm compared to 22 μΩ cm for NiFe plated films. The high resitivity helps in reducing eddy current losses which do impede the uniform reversal of magnetization in the films. The amorphous structure was observed to be stable to 250–275 °C and changes to crystalline structure above 300 °C. Details on corrosion properties and x-ray diffraction data will be presented.
65 Gbits/in.2 quantum magnetic disk (abstract)
Authors: Chou Stephen Y. , Krauss Peter R.
Published: July 2002

Quantum magnetic disk (QMD), proposed recently, offers a new paradigm for ultrahigh density magnetic recording. In a QMD, each bit is represented by a prepatterned nanoscale single-domain magnetic pillar or bar that was uniformly embedded in a nonmagnetic material on a disk. The size and shape of each bit is well controlled during the fabrication to ensure single magnetic domain formation, therefore the magnetic moment of each bit has only two quantized states: equal in magnitude but opposite in direction. Compared to conventional magnetic disks, the QMD offers many unique advantages in writing, reading, and tracking. In this article, we report the fabrication of both longitudinal and perpendicular magnetization QMDs and their investigation with scanning electron microscopy, atomic force microscopy, and magnetic force microscopy. Both QMDs were fabricated using electron beam nanolithography, reactive ion etching, and chemical mechanical polishing. We will also discuss QMD fabrication techniques which do not involve conventional lithography. The perpendicular QMD structure consists of 50-nm-diam nickel pillars uniformly embedded in 200 nm thick SiO2 with a surface roughness of 0.5 nm rms. The pillar array has a period of 100 nm which corresponds to a magnetic storage density of 65 Gbits/in2—over two orders of magnitude greater than the current state-of-the-art magnetic storage density.
Evidence for crossover effects in the spin dynamics of the two-dimensional antiferromagnet Sr2CuO2Cl2 from 35Cl nuclear magnetic resonance
Authors: Suh B. J. , Borsa F. , Miller L. L. , Johnston D. C. , Torgeson D. R. , Corti M.
Published: July 2002

35Cl nuclear magnetic resonance (NMR) and relaxation measurements have been performed in stacked Sr2CuO2Cl2 single crystals with Néel temperature TN=257 K for two crystal orientations with respect to the applied field: H∥c and H⊥c. All the parameters of 35Cl NMR are found to exhibit a strong orientation dependence varying in temperature. In particular, the unconventional anisotropy and strong temperature dependence of the 35Cl nuclear spin–spin relaxation rate, T−12, are presented and discussed in terms of a crossover of the Cu2+ spin dynamics from Heisenberg to XY-like correlation for T→T+N. The onset of anisotropic broadening and shift of the 35Cl NMR line was observed well above TN, which is in agreement with the previous results.
Coercivity mechanism in CoPt alloy films with perpendicular magnetic anisotropy (abstract)
Authors: Kleinefeld Th. , Heimel J. , Weller D.
Published: July 2002

The coercivity is one of the key parameters for defining a suitable magneto-optic recording material. In order to get insight into coercivity mechanisms we have studied the magnetization reversal processes by means of magneto-optic methods in CoPt alloy films. These films were prepared in UHV by coevaporation with a typical composition ratio of Co28Pt72 and show excellent perpendicular magnetic anisotropy at elevated substrate temperatures during deposition. STM and x-ray measurements reveal characteristic grain size of about 15–20 nm and dispersion of crystallite axis of the order of 0.5°, respectively, which varies only slightly with total film thicknesses of 5–50 nm. The magnetization reversal processes are investigated by using polar Kerr microscopy and were found to be dominated by domain formation and domain wall propagation. A micromagnetic model was developed which describes the film as an ensemble of single domain particles. The size of these single domain particles was chosen to be of the order of the average grain size. In the numerical simulation we include dipolar interaction and domain wall energy contribution. The comparison of the results of the simulation with the experimental results clearly points out that the microstructure is responsible for the magnetization reversal behavior. A thermal as well as thermally activated magnetization processes contribute to the reversal mechanism. The coercivity is predominantly determined by pinning on grain boundaries.