bluuebunny/crossref_metadata_2025 huggingface dataset:
Image method for field computation in prismatic yoked magnets
Authors: Bertora Franco , Trequattrini Alessandro , Abele Manlio G.
Published: July 2002

Exact solutions of the field equations cannot be obtained in permanent magnets. Complex geometries and numerical methods have to be used to compute the field generated by the magnetic material. Permanent magnets for magnetic resonance imaging clinical applications can be designed as prismatic structures, composed of high-energy-product magnetic materials having a quasilinear demagnetization characteristic, which are enclosed by high-permeability yokes. The field generated by these structures can be computed by means of an image technique, whereby the field is computed as the sum of the fields generated by the structure itself and by its images outside of the interfaces between the yoke and magnetic material. The formulation of the algorithm, which takes into account the real demagnetization characteristics of the magnetic materials, is presented. As an example, the method is employed to compute the field generated by a yoked prismatic magnet and the result is compared with the numerical results obtained from a conventional three-dimensional finite-element method.
The physics of particulate magnetic recording media: Areas for exploration (invited)
Authors: Mularie W. M. , Sharrock M. P.
Published: July 2002

Information densities recorded on particulate media have continued to increase, with no obvious end in sight. However, many key physical phenomena critical to the advancement of magnetic recording in particulate media are not understood at a fundamental level. This is especially true in the area of surface modification and other interfacial phenomena. These can modify magnetization intensities and anisotropy energies, the crucial parameters in particulate recording materials. As particles are made smaller, in the quest for higher information densities, surface effects become increasingly important. Further studies are needed, aided by measurements on thin-film surfaces and on particles even smaller than those of practical recording use. Also, there exist independent bodies of material science, such as fracture mechanics and corrosion science, where chemisorption in submonolayer quantities substantially modifies the measured response of covalent and ionic materials. Understanding of the phenomena from these collateral sciences may lead to improved modeling of the magnetic behavior of magnetic particles.
Magnets without moments: Spin nematics and beyond (invited)
Authors: Chandra P. , Coleman P. , Ritchey I.
Published: July 2002

Spin nematics are magnets with long-range tensor order in the absence of a sublattice magnetization; the formation of this novel spin state has been studied within the new quantum fluids perspective of magnetism. Here this recent work on spin nematics is reviewed, and experimental signatures and a candidate system are discussed.
A force transducer based on leakage flux variations
Authors: Garshelis I. J.
Published: July 2002

A novel construction for a magnetoelastic transducer for sensing tensile or compressive loads is described. The device requires neither coils nor exciting currents, relying instead on the field from a permanent magnet to polarize a magnetostrictive core element. A field intensity sensor, such as a Hall-effect device, detects the leakage flux from the magnet/core combination. Variations in this flux by the stress-dependent bulk moment of the core are used to signal the applied force. The core moment affects the flux both by sourcing an additional field and by altering the operating point of the magnet. The moment in the core is altered by the redistribution of domain orientations by the stress anisotropy. Data from experimental sensors using tubular cores with internal magnets show good linearity and a small hysteresis that can be of either sign. The design allows for the construction of small, low-cost force transducers.
Neutron and magnetization study of La1.8Sr0.2NiO4
Authors: Lander G. H. , Brown P. J. , Stassis C. , Gopalan P. , Spalek J. , Honig G.
Published: July 2002

Magnetization and neutron-diffraction experiments with both polarized and unpolarized neutrons have been performed on single crystals of La1.8Sr0.2NiO4. The susceptibility is highly anisotropic at low temperature, χab (field applied perpendicular to long axis) showing a peak at ∼18 K. However, no evidence has been found for long-range magnetic order. It is speculated that the low-temperature state is a spin glass, with 18 K being the freezing temperature. Polarized neutron diffraction has been used to measure the induced magnetization density in a field of 4.6 T and at 20 K. The results can be understood by using an expanded Ni2+ form factor and invoking a small positive susceptibility at the La site. There is also evidence for covalency between the Ni and O(2) atoms along the c axis. The material distorts from tetragonal (I4/mmm) to orthorhombic (Bmab) at 120±5 K. The structural parameters are similar to those of La2NiO4.
Observation of an unusual ESR signal in antiferromagnetic Eu2CuO4
Authors: Vier D. C. , Schultz S. , Rettori C. , Rao D. , Oseroff S. B. , Tovar M. , Fisk Z. , Cheong S-W.
Published: July 2002

We report the observation of an unusual electron spin resonance (ESR) signal in single crystals of Eu2CuO4. The signal appears to be associated with a resonance mode of the CuO2 planes, similar to the midfield and low-field absorptions we have reported previously [Phys. Rev. B 41, 1934 (1990)]. However, it is only observed when the projection of the applied dc magnetic field in the CuO2 plane is within a few degrees of the 〈110〉 crystallographic direction. Additionally the sample must be field cooled in the CuO2 plane, but with a component of the cooling field perpendicular to the 〈110〉 ESR observation direction. Both the field for resonance and the linewidth exhibit a 1/cos θ dependence, where θ is the angle of the applied dc field between the c axis and the 〈110〉 observation direction. Additional constraints for observation of the resonance are that the microwave rf magnetic field must have a component in the CuO2 plane, but perpendicular to the dc field. The signal disappears above ∼215 K, which we assume is associated with the antiferromagnetic ordering temperature.
Spin waves and spontaneous magnetization in La2CuO4 and YBa2Cu3O6
Authors: Rastelli E. , Sedazzari S. , Tassi A.
Published: July 2002

La2CuO4 and YBa2Cu3O6 are two famous compounds that become high-Tc superconductors by suitable doping. Here we compare neutron-scattering data with spin-wave theory to assess whether the low-temperature behavior of these compounds can be traced back to long-wavelength elementary excitations. In particular, we consider the thermal behavior of the magnetization. Although the overall agreement is assured, we conclude that conventional spin-wave theory seems to overestimate the low-temperature demagnetization in contrast with the customary finding.
Orientation, temperature, and frequency dependence of nonresonant microwave absorption in HTSC powders
Authors: Gould A. , Huang M. , Bhagat S. M. , Tyagi S.
Published: July 2002

Hysteresis in the microwave-power absorption of HTSC powders was studied as a function of temperature (T), field-sweep amplitude (Hmax), and orientation between the dc field (Hdc) and the wave vector of the microwaves (k). It was found that (i) the sizable low-temperature hysteresis effects occur only if Hdc∥k, (ii) the temperature and frequency dependence of the hysteresis is strongly affected by Hmax, (iii) the high- and low-temperature virgin curves are quite different, and (iv) the minimum of the absorption signal increases with Hmax and T. The low-temperature hysteresis loops were found to be similar to loops obtained from nonlinear equations describing cusp catastrophes.
Resistivity of multiphase high-T
c superconductors
Authors: Dionne Gerald F.
Published: July 2002

Resistivity versus temperature (ρ-T) behavior of high-Tc superconductors in the normal state was recently modeled semiempirically according to conventional electron hopping theory for mixed-valence metal oxides. One important feature of this work was the interpretation of a resistivity minimum (i.e., the transition between insulator and metal) in the temperature regime near Tc, based on the decrease of activation energies Ehop with increases in magnetic dilution reported in the NiO and CuO systems. Quantitative accuracy has now been established by fitting ρ-T data of the La2−xSrxCuO4 and YBa2Cu3O7 high-Tc compounds, which serve as the basis for interpreting the varying slopes of multiphase superconductors. Calculated curves fitted to reported experimental data show that the metallic slope (∂ρ/∂T≳0 above Tc) and the ρ-axis intercept of the linear extrapolation may be used to estimate the volume ratio of effective superconducting phase to normal phase within a particular specimen. From the application of this theory, it is determined that: (i) ρ-axis intercepts of single-phase superconductors are proportional to Ehop(1 − x)/x; (ii) normal phases cause increases in ρ, but decreases in slope of ρ(T)/ρ(300); and (iii) metallic slopes may be achieved with less than 10% effective volume of superconducting phase.
Iron substitution in Y2BaCuO5 and YBa2Cu3O7−y
Authors: DeMarco M. , Trbovich G. , Wang X. W. , Hao J. , Naughton M. , White M.
Published: July 2002

A few atomic percent of 57Fe has been substituted for Cu in the YBa2Cu3O7−y system. Air-annealed samples are the superconducting 1-2-3 compound YBa2Cu3−x57FexO7−y while nitrogen-annealed samples are mainly the nonsuperconducting 2-1-1 compound Y2BaCu1−x57FexO5. X–ray, Meissner effect, and thermogravimetric measurements have been made on both compounds. The site dependence has been studied and compared in these two compounds by the Mössbauer effect with the result that Fe prefers the Cu(l) site in the 1-2-3 compound.
Measurement of anisotropy constant in US with polarized neutrons (invited)
Authors: Lander G. H. , Brooks M. S. S. , Lebech B. , Brown P. J. , Vogt O. , Mattenberger K.
Published: July 2002

Uranium compounds can have an anisotropy that is considerably greater than that found in rare-earth compounds. Early estimates of K1 in ferromagnetic US (Tc = 178 K), for example, were that K1 ≳ 108 erg/cm3. We have re-examined this cubic material and determined K1 in the range of reduced moment (μ/μ0) from 0.1 < (μ/μ0) < 0.7 and find that it varies logarithmically over almost three orders of magnitude. The highest measured K1 is 2 × 108 erg/cm3 at (μ/μ0) = 0.7, but an extrapolation, which we anticipate on arguments of symmetry, to (μ/μ0)=1, (T=0 K) gives K1 ∼ 1010 erg/cm3, some 20 times more than found in TbFe2 at 0 K. The method we have used is with polarized neutrons. Because the neutron interaction with the magnetic moment is vectorial in nature we can determine individually the magnitude and direction of the moment in an applied field. In many cases this method has advantages over conventional methods, especially when the anisotropy is large.
Identification of parameters in a moving model
Authors: Oti John , Vajda Ferenc , Della Torre Edward
Published: July 2002

Although the problem of the identification of parameters in the classical scalar Preisach model, and in some field-dependent models, has been solved, it is still an open question for the magnetization-dependent models such as the moving model. In this paper we illustrate a solution for a scalar bivariate moving model for the case where the Preisach function is Gaussian with equal standard deviations in the two directions. The identification described is limited to the case where the nonzero values of a bivariate Preisach function, including its motion, are limited to one quadrant.
Magnetic, recording, and crystalline properties of multilayered longitudinal thin-film media
Authors: Ranjan R. , Miller M. S. , George P. K. , Lu M.
Published: July 2002

Multilayered longitudinal CoCrTa and CoCrPt media were evaluated for magnetic and recording properties. Depending on the alloy, multilayering produced 150–500 Oe increases in coercivity over equivalent single-layer media. These coercivity increases were also found to depend on the chromium underlayer and spacer layer thickness. The coercive squareness of the CoCrPt media was found to decrease upon multilayering, while S* of the CoCrTa double layer showed a slight increase with thicker Cr spacer layers. Double layering of the CoCrPt alloy produced up to a 4-dB increase in signal-to-noise ratio, while double-layer CoCrTa media demonstrated a 2.7-dB increase. Differences between the magnetic and recording properties of CoCrPt multilayers deposited on Cr underlayers of varying thicknesses are analyzed in terms of changes in crystalline anisotropy introduced by the multilayering process.
Role of atomic mass of underlayer material in the transition noise of longitudinal media (abstract)
Authors: Yogi Tadashi , Nguyen Thao , Lambert Steven E. , Gorman Grace L. , Kakalec Michael A. , Castillo Gil
Published: July 2002

Magnetic and recording properties of CoPtCr on Mo and W underlayers were investigated and compared to Cr underlayers over the sputtering pressure range of 3–24 mTorr. Normalized media transition noise decreases with increasing pressure in all cases, and the reduction can be attributed to a reduced intergrain exchange coupling due to isolation of the magnetic grains. In the low-pressure regime, the transition noise increases with increasing atomic mass of the underlayer, Cr<Mo<W. This trend can be attributed to the growth features of the underlayers which change from a partially coupled structure for Cr to closely coupled grains for W. We attribute this behavior in the low-pressure regime to increasing mobility with increasing atomic mass since the energy flux of the sputtered atoms and the back-reflected argon atoms increases with the atomic mass of the target. In the high-pressure regime, all underlayers produce an isolated grain morphology in the magnetic layer, which indicates sufficient thermalization of the atoms during the transport resulting in similarly low values of transition noise.
Rapid thermal annealing of CeDyGaIG films for magneto-optical disk storage
Authors: Bechevet B. , Challeton D. , Rolland B. , Armand M. F. , Valon B. , Mouchot J.
Published: July 2002

CeDyGaIG is a potential candidate for magneto-optical storage media because of its very good properties on a wide scale of wavelength. The high crystallization temperature of oxide media poses difficulties with the classical materials used for magneto-optical disks. In order to solve this problem we have used a rapid-thermal-annealing (RTA) process to crystallize amorphous CeDyGaIG thin films deposited by facing-target sputtering method on glass substrates. We have succeeded in crystallizing CeDy2Ga0.4Fe4.6O12 amorphous sputtered thin films on these substrates by RTA. The films made by this method have the same optical properties as those crystallized in situ. With the RTA method, we can adjust, by an appropriate process, the coercive field between 250 and 4000 Oe. The influence of the sputtering parameters on the grain size and boundaries was analyzed by transmission electron microscopy. On RTA crystallized films, bit information was written. In conclusion, we discuss the evolution of the magnetic properties versus the temperature and conditions of RTA annealing.
Magnetic structure of the Kondo lattice compound CeCu2
Authors: Trump R. , Thierfeldt S. , Loewenhaupt M. , Chattopadhyay T.
Published: July 2002

The magnetic structure of the Kondo lattice compound CeCu2 has been investigated by unpolarized and polarized neutron diffraction. CeCu2 orders at TN = 3.5 K to an antiferromagnetic structure with the wave vector k=(0,0,0). Two Ce atoms of the primitive unit cell have oppositely oriented spin directions which are parallel and antiparallel to the c axis. The susceptibility measurements show strong anisotropic magnetic behavior which may be attributed to the crystal field. Only the susceptibility along the c axis shows a broad maximum at the Néel temperature. The magnetization curve with the magnetic field parallel to the a axis shows a metamagnetic behavior at 1.1 T at 2 K.
Magnetic properties of CeCuX compounds
Authors: Yang Fuming , Kuang J. P. , Li Jingyuan , Brück E. , Nakotte H. , de Boer F. R. , Wu Xiucheng , Li Zhenxiao , Wang Yupeng
Published: July 2002

The temperature dependence of the magnetic susceptibility, the magnetization at 4.2 K in fields up to 35 T, and the specific heat between 1.3 and 40 K in fields up to 5 T of the ternary Ce intermetallic compounds CeCuX (X = Si, Ge, Sn) have been studied. All three compounds form in ordered ternary structures related to the hexagonal AlB2 type. Ferromagnetic order is found below 14.9 K for CeCuSi and below 10.2 K for CeCuGe. The magnetic order of CeCuSn below 8.6 K is of antiferromagnetic type but probably complex, as indicated by a double peak in the specific heat and the occurrence of a very low zero-field moment.
Electrical and magnetic properties of light rare-earth Heusler single crystals RInCu2, R=La, Pr, Nd, and Sm
Authors: Sato K. , Mizushima T. , Kondo K. , Isikawa Y. , Mori K. , Umehara I.
Published: July 2002

Electrical resistivity ρ and magnetic properties of light rare-earth Heusler single crystals RInCu2, R=La, Pr, Nd, and Sm, have been measured in the temperature range 1.3–300 K. No anomalies due to magnetic transitions were found in the temperature dependence of ρ and magnetic susceptibility χ above 4.2 K. Pr and Nd Heusler compounds have large negative paramagnetic Curie temperatures −19 and −29 K, respectively. The temperature dependence of χ is discussed on the basis of the crystalline field effect. For Sm compound, χ(T) is discussed by means of Van Vleck paramagnetism.
Calculation of magneto-optic Kerr effect in CeSb (abstract)
Authors: Lim S. P. , Cooper B. R.
Published: July 2002

It is well known that CeSb displays the largest ferromagnetic Kerr rotation known with a peak value of over 14°, which is about 30 times that of the strongly magnetic material iron. Conventional magneto-optical theory for magnetically ordered materials ascribes the magneto-optic effects to spin-orbit coupling. We have suggested1 that in such highly correlated partially-delocalized  f-electron systems where the magnetization is orbitally driven, the electromagnetic wave couples directly to the electronic magnetization. We believe that such a coupling avoids the weak linkage provided by spin-orbit coupling and is responsible for the giant magneto-optic rotation in CeSb. We first did a conventional itinerant electron (band magnetism) calculation to see if it gives the correct Kerr rotation. The band structure used is obtained from a self-consistent spin-polarized full-potential LMTO calculation with a true interstitial. As a test, we have also calculated the magneto-optic rotation in both bcc and fcc iron where the conventional theory provides good agreement with experiment.
Ferromagnetic resonance studies of (Ag/Fe)n multilayers (abstract)
Authors: Hurdequint H.
Published: July 2002

We report a detailed ferromagnetic resonance (FMR) study, at X band, of magnetic films constituted of (Ag‖Fe multilayer‖Ag) sandwiches deposited by sputtering on heated mica substrates, where the base and the top Ag layers are 550 Å thick. For this (Ag/Fe) system the corresponding series of single Fe layers is reported in Ref. 1 (structural characteristics, FMR study). The resonance spectrum is studied as a function of the orientation [angle θH = (N̂,Ĥ) with respect to the film normal N̂] of the applied dc field H in a plane perpendicular to the film. We will present results relative to a series of Fe multilayers (Ag‖Fe)n=9, with spacer tAg=70 Å, corresponding to a set of individual Fe layer thicknesses (tFe=24, 36, and 48 Å). In summary, at room temperature, all the features (line shape, signal phase, position, linewidth) which characterize the resonance spectrum of a Fe multilayer are found different from the ones relative to the single resonance line observed for the corresponding single Fe layer (same tFe). More precisely, the angular variation of the resonance spectrum of the (tFe=24 Å) multilayer shows the presence of a unique line in the range (90°<θH ≲ 30) and then the splitting in two lines for θH ∈ (0°,30°), with Hres(θH) of the higher-field line following closely the single Fe layer Hres whereas the second line remains in low field (∼3.2 kOe). These observations provide evidence for the presence of a dynamical coupling between the precessing magnetizations of adjacent Fe layers. We suggest that the mechanism responsible for the Fe layers coupling is to be associated with the diffusion (or transport) of the microwave-induced magnetization of the conduction electrons of the whole metallic structure.
Impurity states and magnetic order in layered copper oxides (invited)
Authors: Rabe K. M. , Bhatt R. N.
Published: July 2002

The influence of impurity potentials on vacancies in the t-J model is studied through the exact diagonalization of systems with 8 and 16 sites. For a wide range of model parameters, the ground state transforms according to the two-dimensional vector representation of C4v and has S =1/2. The inclusion of symmetry breaking effects present for an isolated impurity in an infinite system yields an anisotropic ground state which may couple to local lattice distortions. Implications for magnetic order, spectroscopy, and the metal-insulator transition in the CuO2 planes of the cuprate superconductors are discussed. The relationship to the behavior of free vacancies and the role of finite size effects are considered. A mean-field Hubbard model analysis is proposed as a supplementary approach.
Magnetic and transport properties of sputtered Gd-Y multilayers
Authors: Freitas P. P. , From M. , Melo L. V. , Plaskett T. S.
Published: July 2002

Gd-Y-Gd multilayers were prepared that show a magnetoresistance enhancement for an Y layer separation of 30 Å. This magnetoresistance enhancement is an interface effect and occurs in samples where some degree of antiferromagnetic coupling is present.
Micromagnetic model of an exchange-coupled NiFe-TbCo bilayer (abstract)
Authors: Smith Neil , Cain William C.
Published: July 2002

A micromagnetic model for NiFe-TbCo exchange-coupled bilayers which can quantitatively predict and explain the major macroscopic features observed in measured M-H characteristics is presented. Comparison of theoretical and experimental results shows conclusively that the strong interfacial exchange coupling in NiFe-TbCo is essentially indistinguishable from that of a perfect, homogeneous interface. Commonly invoked assumptions concerning the existence or origin of a substantially weakened exchange coupling at a highly imperfect interface are neither necessary, nor even consistant with experimental measurements. The mechanism of the unidirectional exchange anisotropy is the formation of Bloch-type domain walls in an ≂0.08-μm-thick TbCo sublayer of uniaxial, in-plane anisotropy adjacent to the NiFe interface. The manner in which the observable magnetic behavior of NiFe-TbCo bilayers depends on film thicknesses, TbCo anisotropy, interfacial exchange coupling strength, as well as the previously unconsidered large hysteretic effects due to the small net magnetization in ferrimagnetic TbCo, are discussed in detail. It is demonstrated, for a strongly coupled system such as NiFe-TbCo, that the often used single parameter ‘‘exchange field’’ description of a shifted NiFe M-H loop is inadequate. A quantitatively accurate description requires that one take into account the spatial variations in the micromagnetic magnetization distributions of both layers.
Field-effect conductance of YBa2Cu3O6
Authors: Levy A. , Falck J. P. , Kastner M. A. , Gallagher W. J. , Gupta A. , Kleinsasser A. W.
Published: July 2002

Metal-insulator-semiconductor field effect transistors have been fabricated using laser ablation to deposit YBaCuO thin films onto SrTiO3, MgO, LaAlO3, and LaGaO3 substrates. The substrates were used as gate insulators. The conductivity of two films on SrTiO3 could be modulated, while for other samples the conductivity was independent of the gate voltage. The field-effect mobility was extracted and found to be of magnitude comparable to the mobility of metallic YBa2Cu3O7.
On the proximity effect between normal metals and cuprate superconductors
Authors: Deutscher G. , Simon R. W.
Published: July 2002

We consider the proximity effect between high-temperature superconducting cuprates and ordinary metals in the context of superconductor-normal-superconductor (S-N-S) microbridges. The theory of the proximity effect for conventional superconductors predicts that superconductivity can only be weakly induced by the coupling of the cuprates with metals such as gold or silver. Experimental investigations of this system have yielded results consistent with this prediction. We discuss the possibility of fabricating S-N-S microbridges with desirable properties by using more appropriate normal layers in the devices.
Electronic transport analysis by electron-beam-induced current at variable energy of thin-film amorphous semiconductors
Authors: Najar S. , Equer B. , Lakhoua N.
Published: July 2002

The electron-beam-induced current technique, at variable energies, is applied to the determination of charge-carrier collection properties in amorphous silicon devices (solar cells, photodetectors, and x-ray or particle detector). Different calculation techniques have been developed and tested both on simulated and real measurements to demonstrate the efficiency of the technique. Investigations of the accuracy of the method, given different approximations of the electron-hole generation function, have been carried out. An estimated value for the mean electron-hole pair creation energy, extrapolated at high polarization, is also obtained: 4.8±0.5 eV. Applications of the method to p+-i-n+ and n+-i-p+ devices are also presented.
Fundamental and practical aspects of reactive N+2-ion sputtering in Auger in-depth analysis
Authors: Kawabata T. , Okuyama F. , Tanemura M.
Published: July 2002

Polycrystalline Al, Mo, Ti, and Ta films deposited on Si wafers were systematically depth profiled by Auger electron spectroscopy using Ar+ and N+2 ions as projectile. In agreement with the traditional concept of reactive sputtering, N+2 sputtering of Al and Mo films textured the surface less, thereby improving depth resolution significantly. N+2-sputtered Ti and Ta films, on the other hand, were characterized by microprojections thickly covering the bombarded area, forming a striking contrast to Ar+-sputtered Ti and Ta displaying less-enhanced surface texturing. As revealed by electron spectroscopy for chemical analysis, the N+2-bombarded surface was nitrided for every material, and the nitride zone was confined to the surface or near-surface for Al and Mo and extended far below the surface for Ti and Ta. It is thus considered that three-dimensional nitridation lay under the enhanced surface texturing observed for N+2-sputtered Ti and Ta.
Effects of rotation and magnetic field on a horizontal high-pressure Hg arc
Authors: Chang Peggy Y. , Dakin James T.
Published: July 2002

A numerical model is used to study the combined impact of gravity, rotation, and magnetic field on convection in a horizontal high-pressure arc. The model solves the Navier–Stokes equations for laminar compressible flow combined with the electric current continuity equation in a three-dimensional generalized curvilinear coordinate system. Model calculations are presented for a 1-atm, 400-W Hg arc in a 3.33-cm i.d. arctube with 15.5 cm electrode separation. The model results are compared with experimental data from the literature.
p-d hybridization of the valence bands of copper indium disulfide
Authors: Hsu T. M.
Published: July 2002

The p-d hybridization of the valence bands of copper indium disulfide has been studied by photoreflectance. A simple model has been used to measure the separation E between the p and d levels, the interaction strength M between these levels, the d-electron contributions x0 to the E0 energy level, and x1 to the E0 + Δ0 energy level. Our results confirm that the d-electron contributions decrease when the temperature is increased. Our measurements of x0 and x1 agree with the calculation of Yoodee et al. [Phys. Rev. B 30, 5904 (1984)].
Analysis of asymmetric fringe patterns of third-harmonic generation in a molecular crystal
Authors: Kobayashi Hideki , Kubodera Ken’ichi
Published: July 2002

The asymmetric Maker fringe pattern of third-harmonic generation in an anisotropic molecular crystal is analyzed to evaluate the third-order optical susceptibility χ(3). Analysis of a diethyl-amino-nitro-stilbene crystal shows that the molecular axes are not oriented parallel to the crystal surface, but are about 45° relative to the surface. The χ(3) value parallel to the orientation of the molecular axis is estimated to be 1.0×10−11 esu, corresponding to about ten times the χ(3) value perpendicular to that axis.
Gettering of bulk impurities by surface damage in Hg0.8Cd0.2Te wafers
Authors: Wang Jue , Liu Jiming , Yu Zhenzhong , Tang Dingyuan
Published: July 2002

The residual impurities in Hg0.8Cd0.2Te crystal have been investigated by using secondary-ion mass spectrometry. It was observed that the impurities are richer in near-surface layer than bulk material. This phenomenon was proved to be related to defect gettering by surface damage. In addition, the measurements of Hall effect and infrared spectra in gettered and ungettered Hg0.8Cd0.2Te wafers have been performed. It was found that the electrical properties of gettered Hg0.8Cd0.2Te wafers were superior to ungettered wafers. The absorption edges moved towards shorter wavelengths and the average transmittance increased in gettered wafers. These can probably be explained by assuming the residual impurity concentration in gettered wafers is relatively low. The results indicate that the defect gettering technique can be used to effectively reduce residual impurities in Hg0.8Cd0.2Te wafers.
Diffusion effects in p-type PtSi Schottky diodes under reverse bias
Authors: Chin Vincent W. L. , Storey John W. V. , Green Martin A.
Published: July 2002

At low temperature, the dominant current transport process of a p-type PtSi Schottky diode on a moderately doped Si substrate is thermionic emission over the barrier. In this paper, current has been measured as a function of temperature at a fixed reverse bias for several Schottky diodes. It is found that diffusion effects due to the limitation upon the diffusion rate through the space-charge region can become significant under some conditions. At moderate field, these conditions are calculated to be at a temperature of 150 K or greater, depending on the doping density of the devices. I-V-T measurements were carried out on several Schottky devices fabricated in slightly different ways, and the diffusion effect was observable in some of these devices. A numerical routine is utilized to fit the experimental data to a combined thermionic emission-diffusion theory in these regions. It is found that the experimental results fit this theory well. In addition, the acceptor density at which diffusion becomes the dominant current at 80 K under moderate field is calculated to be around 5×1011 cm−3.
A study of structural, electrical, and optical properties of low-pressure chemical vapor deposition tungsten silicide films processed at elevated temperatures
Authors: Shenai Krishna
Published: July 2002

Tungsten silicide films deposited by the low-pressure chemical vapor deposition technique on polysilicon are studied at elevated processing temperatures. It is shown that annealing of as-deposited silicon-rich tungsten silicide films in nitrogen at process temperatures exceeding 1000 °C results in severe silicide surface morphology. Silicide capping using a layer of dielectric is shown to be attractive to circumvent this problem and to avoid silicide peeloff. Structural, electrical, and optical properties of tungsten silicide films are studied as a function of annealing parameters. It is shown that silicide annealing ambient and activation temperature result in a considerable modification of silicide thin-film properties. Correlation among structural, electrical, and optical silicide film properties is made based on extensive experimental data.
Optical properties of finely structured metal-insulator superlattice particulates
Authors: Chui S. T. , Zhou M. Y. , Sheng P. , Chen Z.
Published: July 2002

We investigate the optical properties of new ‘‘superlattice particles’’ that can be made of precisely controlled metal-insulator layers fabricated by sputtering or evaporation techniques. We found that there are two limits to the reflectivity behavior corresponding to wide and narrow particles. In the narrow-particle limit, a series of sharply defined absorption peaks at frequencies controlled by the width is observed. In the wide-particle limit, most of the light incident on the particles will be absorbed. Materials of this type possess desirable infrared absorption characteristics and can be used for new polarization sensitive infrared absorbers and detectors.
Measurement of electrical avalanches and optical radiation near solid insulators in high pressure (up to 0.3 MPa) nitrogen gas
Authors: Mahajan S. M. , Sudarshan T. S.
Published: July 2002

Electron and ion avalanches have been recorded near a variety of insulators (plexiglas, teflon, high-density polyethylene, low-density polyethylene, polypropylene, delrin, polyvinyl chloride, and nylon) in nitrogen gas at pressures of 0.1, 0.2, and 0.3 MPa. With the exception of nylon, suppression of avalanches has been observed in the presence of insulators. In addition to electron and ion avalanches, simultaneous measurement of optical radiation associated with an electron avalanche was successfully carried out. Qualitative explanations have been provided for the suppression of avalanches near most insulators and an anomalous growth of avalanches near nylon insulators. Photoemission from nylon surfaces appears to be responsible for the enhanced growth of avalanches near nylon insulators. More precise measurements of optical radiation are needed to better understand the electron-photon interactions near a solid insulator in a gaseous dielectric medium.
Analytic potential in a linear radio-frequency quadrupole trap with cylindrical electrodes
Authors: Melbourne R. K. , Prestage J. D. , Maleki L.
Published: July 2002

We have analyzed a linear radio-frequency ion trap of finite length employing a four-sectored hollow cylinder enclosed between two end caps. Solutions to Laplace’s equation determine the effective potential inside the trap due to the rf trapping field, the static confining potential from dc-biased end caps, and the potential arising when a dc bias is present on the cylinder sectors. The finite length of a linear trap results in field distortion at the trap ends due to fringing effects. We have demonstrated that near the ends, the effective potential arising from the rf fields acts to propel ions out of the trap. We have further shown that the addition of a dc bias on two neighboring cylinder sectors generates an inhomogeneous field in the trap which produces a force on the ions along the trap’s long axis in a direction dependent on the sign of the bias. An exact analytic determination of the potential can be used to determine the profile of the trapped ion cloud.
A calibration curve for room-temperature resistivity versus carrier concentration in n-type CdSe
Authors: Levy Miguel , Sarachik M. P.
Published: July 2002

A calibration curve is given of the room-temperature resistivity as a function of carrier concentration for n-type CdSe, where net carrier concentrations between 1017 and 3×1018 cm−3 were determined from room-temperature Hall coefficient measurements.
Comment on ‘‘Surface acoustic wave properties of aluminum gallium arsenide’’ [J. Appl. Phys. 6
6, 90 (1989)]
Authors: Fliegel F. M. , Steele Victor E. , Hunt William D.
Published: July 2002

The conclusions reached in the previous work [Steel et al., J. Appl. Phys. 66, 90 (1989)] are essentially correct, but are in error in that exponents of four and two should not have been used in Eqs. (31) and (32), respectively. The effect on power reduction requirements to achieve a given surface acoustic wave electrical potential are thus proportional to the coupling coefficient (k2 ) ratio for the two materials being compared, rather than the square of the coupling coefficient ratio. Additionally, factors reflecting other differences in properties for different substrate materials or materials systems are included in the modified result. Finally, it is observed that the expressions derived do not necessarily allow the performance of acoustic charge transport (ACT) devices to be determined, but do allow such estimates to be made for heterostructure ACT (HACT) devices.
Dependence of pseudomorphic semiconductor band gap on substrate orientation
Authors: Hinckley J. M. , Singh J.
Published: July 2002

For a given misfit we examine the band-gap variation of a pseudomorphic overlayer on a thick substrate as a function of substrate orientation. The strain tensor is found to be a strong function of the substrate orientation. For both direct and indirect band-gap overlayers, this results in a significant variation in the band gap as the substrate orientation is changed. However, for indirect band-gap layers, such as SiGe alloys grown on Si substrates, the change in band gap is accompanied by a lifting of conduction-band-edge degeneracies. The magnitude of this splitting may be as large or larger than the change in the band gap.  
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u observation of the orthorhombic-tetragonal phase transformation in YBa2Cu3O7−δ
Authors: Monteiro Othon R. , Evans James W. , Johnson Sylvia M.
Published: July 2002

We have carried out in situ heat treatments of YBa2Cu3O7−δ at temperatures between 25 and 500 °C and oxygen pressure of 10 Torr. It was found that the original orthorhombic (OI phase) YBa2Cu3O7−δ transforms into the OII phase (still orthorhombic, but with a doubling of the a-axis, and with a δ=0.5) at about 250 °C. As the temperature is increased further, the tetragonal phase of YBa2Cu3O7−δ forms. As the tetragonal phase is slowly cooled to room temperature, the orthorhombic phase OI reappears. Cycling the temperature repeatedly has indicated the regenerability of the OI phase.
Reduced thermal budget processing of Y-Ba-Cu-O films by rapid isothermal processing assisted metalorganic chemical vapor deposition
Authors: Singh R. , Sinha S. , Hsu N. J. , Ng J. T. C. , Chou P. , Thakur R. P. S. , Narayan J.
Published: July 2002

Metalorganic chemical vapor deposition (MOCVD) has the potential of emerging as a viable technique to fabricate ribbons, tapes, coated wires, and the deposition of films of high-temperature superconductors, and related materials. As a reduced thermal budget processing technique, rapid isothermal processing (RIP) based on incoherent radiation as the source of energy can be usefully coupled to conventional MOCVD. In this paper we report on the deposition and characterization of high quality superconducting thin films of Y-Ba-Cu-O (YBCO) on yttrium stabilized zirconia substrates by RIP assisted MOCVD. Using O2 gas as the source of oxygen, YBCO films deposited initially at 600 °C for 1 min and at 745 °C for 25 min followed by deposition at 780 °C for 45 s are primarily c-axis oriented and zero resistance is observed at 89–90 K. The zero magnetic field current density at 53 and 77 K are 1.2×106 and 3×105 A/cm2, respectively. By using a mixture of N2O and O2 as the oxygen source substrate temperature was further reduced in the deposition of YBCO films. The films deposited initially at 600 °C for 1 min and than at 720 °C for 30 min are c-axis oriented and with zero resistance being observed at 91 K. The zero magnetic field current densities at 53 and 77 K are 3.4×106 and 1.2×106 A/cm2, respectively. To the best of our knowledge this is the highest value of critical current density, Jc for films deposited by MOCVD at a substrate temperature as low as 720 °C. It is envisioned that high energy photons from the incoherent light source and the use of a mixture of N2O and O2 as the oxygen source, assist chemical reactions and lower overall thermal budget for processing of these films.
Determination of the lithium Frenkel energy in lithium tantalate
Authors: Birnie III Dunbar P.
Published: July 2002

The ferroelectric to paraelectric phase transition is modeled as an order-disorder process with lithium atoms jumping between their normal sites and adjacent interstitial sites. This lithium jump corresponds to lithium Frenkel defect formation. At temperatures significantly away from the Curie temperature it is found that the lithium Frenkel energy is about 0.165 eV. As the temperature approaches Tc, disorder abounds and eventually the lithiums become equally distributed between the two available sites. The previously reported spontaneous polarization, Ps, data for lithium tantalate are analyzed in terms of this order-disorder model. The slope of P2s with temperature is directly in line with the model’s prediction.
Thin-oxide dual-electron-injector annealing studies using conductivity and electron energy-loss spectroscopy
Authors: Dori L. , Bruley J. , DiMaria D. J. , Batson P. E. , Tornello J. , Arienzo M.
Published: July 2002

A process to deposit in situ a dual electron injector structure (DEIS) with 5-nm SiO2 between two Si-rich SiO2 (SRO) layers of ∼20 nm each has been developed. The excess silicon, as evaluated by Auger spectroscopy and Rutherford backscattering, was of the order of 15%–17%, in agreement with previously reported values under similar deposition conditions. Thin cross-sectioned samples of DEIS structures, both as-deposited and annealed at 1000 °C with Ar in an oxygen and water-moisture-free atmosphere, were examined by spatially resolved energy-loss spectroscopy (EELS) in a scanning transmission electron microscope. The analysis has shown that the excess silicon is present either as nanometer-sized silicon islands or as submicroscopic silicon oxides of varying stoichiometry resulting from intermediate oxidation states (i.e., Si+3, Si+2, and Si+1). Additionally, the thermal anneal at 1000 °C did not appear to have any effect on silicon island size of the SRO layer in contact with the silicon substrate. This suggests that the driving force of silicon clustering might not be diffusion limited, but could be related to the conditions under which the anneal was performed (in our case, in oxygen and water-moisture-free atmosphere). From the EELS analysis, the annealing procedure caused the loss of ≊30% of Si and O atoms from the top SRO layer, which could have contributed to the observed degradation of the electrical properties of the annealed DEIS structure. Ramped current tests performed by injecting electrons either from the gate or from the substrate, demonstrate extremely high breakdown voltages (VBD). The thermal anneal at 1000 °C, performed under a very low oxygen and water partial pressure, lowered the SRO film conductivity. Also, for either the annealed or as-deposited structures, a reduced injection asymmetry has been observed.
Effects of temperature on structural properties of hydrogenated amorphous silicon-germanium and carbon-silicon-germanium alloys
Authors: Battezzati L. , Demichelis F. , Pirri C. F. , Tresso E.
Published: July 2002

The temperature dependence of the structural properties of a-SiGe:H and a-CSiGe:H films has been studied. The samples were characterized by differential scanning calorimetry, conductivity measurements, electron-spin resonance, infrared, optical, and photoacoustic spectroscopy. The disagreement of previous results of differential scanning calorimetry on hydrogenated amorphous semiconductors samples (a-Si:H and a-Ge:H) is examined and discussed. The equilibration temperature determined by the temperature dependence of dc conductivity agrees well with that of the first exothermic process detected with differential calorimetry. The second exothermic effect occurs when hydrogen evolution starts, as confirmed by electron-spin resonance and infrared spectroscopies.
Imaging of polydiacetylene on graphite by scanning tunneling microscopy
Authors: Leung M. S. , Ives N. A. , Stupian G. W.
Published: July 2002

Scanning tunneling microscopy has been applied to obtain atomically resolved images of polydiacetylene on graphite and to observe hydrogen bonding between polymers directly. Polydiacetylenes have a pseudo-one-dimensional conjugated pi-electron system which gives rise to nonlinear optical behavior of both fundamental and practical significance. Under low surface coverage, the images show a single polymer of CH3(CH2)11—C≡C—C≡C—(CH2)8—COOH. At high coverage, the images show a well-ordered monolayer of polymers with the nonpolar side groups pointing toward the hydrophobic graphite surface. The spatial electron distribution indicates substantial hydrogen bonding between polymers in the plane containing the polar side groups.
High-temperature growth of epitaxial NiAl thin films on AlAs by molecular-beam epitaxy
Authors: Kamigaki Kousei , Yuda Shinji , Kato Hiromu , Ishida Masaya , Terauchi Hikaru , Sano Naokatsu , Hiyamizu Satoshi
Published: July 2002

Good NiAl single-crystal films were successfully grown on AlAs at high temperature (600 °C) by means of molecular-beam epitaxy. It was found that there existed three characteristic temperature regions in the crystal growth. Below 300 °C, the in situ formation of NiAl failed. Above 300 °C, NiAl was formed in situ, while other Ni-Al intermetallic compounds (Ni3Al2, NiAl3, Ni3Al) also appeared. Above 400 °C, remarkable improvements of the crystallinity, epitaxy, and monocrystallinity were found. The best NiAl film was obtained at 600 °C, where no interfacial disorder attributed to the interdiffusion and no film discontinuity due to balling up were observed. It is suggested that high-temperature growth is powerful for synthesis of high-quality NiAl-(Al,Ga)As heterostructures.
Design analysis of a short wavelength laser in an unstable resonator cavity
Authors: Basu Santanu , Hagelstein Peter L.
Published: July 2002

The use of diffractive output coupling and potentially easy fabrication make unstable resonators suitable for extreme ultraviolet and soft x-ray lasers. We illustrate the design of an unstable resonator cavity for a proposed electron collisionally excited Ni-like Mo laser at 191 Å. Saturated output in a near-diffraction limited beam is predicted in a six-pass synchronously pumped cavity with a gain-length product of 6.
Free-electron laser diagnosis by polarization analysis of Thomson backscattered light
Authors: Ruschin S.
Published: July 2002

The polarization properties of backscattered radiation from a relativistic electron beam are analyzed. It is shown that the polarization state of the backscattered radiation is changed with respect to that of the incoming radiation whenever a transverse component is present in the velocity distribution of the electron beam. Changes of polarization properties are evaluated in order to serve as a diagnostic tool for electron-beam parameters relevant to the performance of free-electron lasers, namely the beam emittance and the wiggler parameter. A measurable dimensionless ‘‘depolarization parameter’’ is introduced which directly measures these two parameters.