bluuebunny/crossref_metadata_2025 huggingface dataset:
Conduction-band tailing in parabolic band semiconductors
Authors: Chakraborty P. K. , Biswas J. C.
Published: July 2002

Kane’s [Phys. Rev. 131, 79 (1963)] model for the density of states for energies in the band gap is in qualitative agreement with experiment. Halperin and Lax [Phys. Rev. 148, 722 (1966)] have also suggested a model for band tailing, which is applicable to the deep tailing states. In the present study, theoretical models have been proposed for the band tailing in the case of heavily doped semiconductors that are more general than those of Kane and others. In addition, the present study also offers an E-k̄ dispersion relation for band tails in the parabolic band based on Kane’s semiclassical theory. This study has helped us to obtain a different model for the density of states in the band-tailing conditions and the model is expected to agree better with experiment.
Accurate calculation of Auger rates in infrared materials
Authors: Krishnamurthy Srinivasan , Sher A. , Chen A.-B.
Published: July 2002

The Auger recombination rates in small-gap semiconductor alloys are calculated using full band structures with electron–electron interactions in Coulomb and phonon fields. We find that the results are sensitive to band structure details and the calculated minority carrier lifetimes can differ by two orders of magnitude depending on the approximations used to describe the energy bands and wave functions. The full band structure results agree well with experiments in Hg0.78Cd0.22Te. Similar calculations were carried out for lifetimes in In0.67Tl0.33P, In0.85Tl0.15As, and In0.92Tl0.08Sb as a function of temperature. The minority carrier lifetimes in In0.67Tl0.33P and In0.92Tl0.08Sb are shorter than that in Hg0.78Cd0.22Te at all temperatures. However, the low-temperature minority carrier lifetime in In0.85Tl0.15As is an order of magnitude longer than that in Hg0.78Cd0.22Te. Our calculations further suggest a possibility of increasing the lifetimes of minority carriers by decreasing the density of states inside a critical energy and momentum region and also by selecting donors with energies in the gap.
Specific heat, magnetization and C-isotope effect of Y2C2(Br,I)2 superconductors
Authors: Schnelle W. , Henn R. W. , Gulden Th. , Kremer R. K. , Simon A.
Published: July 2002

The specific heat cp(T) and the magnetization M(B,T) of the layered superconductors Y2C2I2, Y213C2I2 (Tc≈10 K), Y2C2Br2 (Tc≈5 K), and of Y2C2Br0.5I1.5 (Tc≈11.6 K) were measured from 2–80 K in magnetic fields up to 10 T. All samples exhibit sharp phase transitions in zero field. The jump Δcp at Tc is drastically reduced in amplitude and broadens significantly with increasing field. The temperature dependence of Δcp(T) and of Bc(T) is analyzed and is consistent with strong electron–phonon coupling. The C13/C12 isotope effect of Y2C2X2 is found to be nearly zero. In contrast to this finding the 3-dimensional parent compound YC2 shows weak-coupling BCS behavior. The anisotropic thermodynamic parameters of the materials are evaluated and discussed.
On the nature of the magnetic phase transition of the HoCo2 intermetallic
Authors: von Ranke P. J. , de Oliveira N. A.
Published: July 2002

In this work we calculate the itinerant and localized magnetization curves of the Laves phase intermetallic compound HoCo2 and discuss the nature of its magnetic phase transition. The numerical results were calculated from the self-consistent solution of magnetic coupled equations obtained through the combination of functional integral methods with molecular field approximation.
Growth and magnetism of one Mn monolayer on Ag(100)
Authors: Elmouhssine O. , Moraı̈tis G. , Parlebas J. C. , Demangeat C. , Schieffer P. , Hanf M. C. , Krembel C. , Gewinner G.
Published: July 2002

Structural properties of Mn films, with a thickness of one monolayer (ML), deposited on a Ag(100) substrate have been investigated both experimentally (photoemission and ion scattering spectroscopies) and theoretically (tight-binding linear muffin-tin orbital method). The magnetic structure of the films and the effect of magnetism on their relative stability have been investigated ab initio. We find that after Mn evaporation [at room temperature (RT)], a superficial MnAg alloy is formed. Mild annealing gives rise to the formation of an (almost) inverted Mn monolayer covered by a Ag plane, i.e., the second atomic plane of the sample is mainly constituted by Mn atoms, whereas the first atomic layer is almost a pure Ag plane. A complete inversion of the Mn ML can be obtained by direct sequential deposition of Mn and Ag at 80 K. Our calculations on the energetic stability of 1 ML of Mn on top of Ag(100) versus 1 ML of Mn covered by one Ag atomic plane, show that the second situation is energetically preferred. This is also true when this situation is compared to the formation of a 2-ML-thick MnAg alloy on Ag(100). We find that the inverted Mn monolayer tends to be ferromagnetic and that magnetism acts against interdiffusion.
Influence of the configurational degeneracy on the hysteretic behavior of a system of magnetostatically coupled magnetic moments
Authors: González J. M. , Chubykalo O. A. , Hernando A. , Vázquez M.
Published: July 2002

The dynamics of dipolarly coupled magnetic moment systems representing sets of microwires has been simulated in the cases of (i) moments coupled through first-neighbors dipolar interactions and in that of (ii) moments coupled through exact many-body dipolar ones. Our results show that, in the case of the small size systems and independently from their parity, the dynamics is based either on localized excitations (of the antiferromagnetic domain wall-like type) or in collective modes depending on the occurrence of either short- or long-range dipolar interactions, respectively. When large size systems are considered the dynamics is based on localized interactions independently of the parity and the type of the present interactions.
Thermal expansion of compounds Rn+1Co3n+5B2n (R=Y and Gd; n=1, 2, 3, and ∞)
Authors: Ido H. , Suzuki Y. , Suzuki T.
Published: July 2002

Thermal expansions of the lattice parameters a and c of the system in the title have been measured by x-ray diffraction technique to investigate the nature of magnetic interactions as well as chemical bond in these systems. Our data reveals that a spontaneous magnetostriction caused by the Co–Co intrasublattice interaction in YCo4B (n=1) occurs in the c plane unlike YCo5 whose magnetostriction occurs only along the c axis. It is also found that the positive spontaneous magnetostriction of GdCo4B in the c plane can be divided into three contributions due to the Co–Co, Co–Gd, and Gd–Gd interactions in the crystal. Thermal expansion coefficients in the c plane in the paramagnetic temperature region strongly decrease with increasing n in the Gd system. This implies a significant covalent bonding in the R–B layer in the present compounds. These results are discussed on the basis of the results of energy band calculation together with the previous magnetic data.
Immobilization diffusion in R2Fe17 nitrides
Authors: Skomski R. , Wirth S.
Published: July 2002

The diffusion of nitrogen in R2Fe17 intermetallics is investigated by an approach which reconciles the solid-solution and immobilization theories of nitrogen diffusion. It turns out that two-sublattice diffusions may yield sharp concentration profiles but leave the phase structure of the nitride unchanged. Beside the reaction time and a local relaxation time there exists a global relaxation time which governs the smoothing of concentration gradients. Due to the large number of unknown energy parameters involved it is difficult to make quantitative predictions, but in general the diffusion behavior depends on factors such as the use of N2 or NH3 as a nitrogen source.
Magnetism and colossal magnetoresistance in the compound Sr14MnSb11
Authors: Webb D. J. , Cohen R. , Klavins P. , Shelton R. N. , Chan J. Y. , Kauzlarich S. M.
Published: July 2002

The magnetization and resistivity of single crystals of the ferromagnetic compound Sr14MnSb11 are measured as functions of magnetic field, temperature, and crystal orientation. The uniform magnetic susceptibility is used to identify TC=39 K. The ferromagnetic state is found to have a hard axis (c axis) anisotropy. The resistivity data place the compound in the class of colossal magnetoresistance (CMR) materials such as Ca-doped LaMnO3. The data are also compared to the related CMR material Eu14MnSb11.
Magnetic susceptibility of Fe/Cu multilayers: Ferromagnetic, antiferromagnetic, and spin-glass phases
Authors: de Oliveira A. J. A. , Ortiz W. A. , Mosca D. H. , Mattoso N. , Schreiner W. H. , Teixeira S. R.
Published: July 2002

In this work we present low-field magnetic measurements on multilayers of Fe 1.5 nm and Cu 1.5 nm layers, repeated 10 times, grown on Si(111) and Fe 5 nm/CaF2 40 nm/Si(111) substrates. Magnetic characteristics of a spin-glass phase are exhibited by samples directly grown on Si(111), for which the glass temperatures (TG) depend strongly on the magnetic field. Above TG those samples exhibit a ferromagnetic behavior. In contrast, samples grown on Fe/CaF2 buffer structure also show the spin-glass characteristics, but are antiferromagnetic above TG. This work deals with the fundamental role played by the interface between Fe and Cu for these effects, which is clearly emphasized by the experimental results.
Magnetic memory effect in YBa2Cu3O7−x/(BiDy)3(FeGa)5O12 heterostructures
Authors: Mou D. , Grishin A. M. , Rao K. V.
Published: July 2002

YBa 2 Cu 3 O 7−x /(YBCO)(BiDy) 3 (FeGa) 5 O 12 heterostructures have been prepared on ceramic glass substrates by Nd-yttrium–aluminum–garnet pulsed laser ablation. Both YBCO and BiDy-IG garnet layers are found to be polycrystalline without any preferential crystallite orientation. Magnetic hysteresis loop measurements made before and after YBCO deposition reveal perpendicular magnetic anisotropy in the garnet film. The resistance versus temperature measurements indicate that the YBCO layer contains both superconducting and semiconducting phases and exhibits a broad superconducting transition with TC|R=0=22 K. The resistivity of the fabricated heterostructures was found to be very sensitive to external magnetic field and the remnant magnetization of the garnet layer. At 18 K, in a magnetic field of 0.5 T applied perpendicular to the surface of the sample, the magnetoresistance figure of merit ΔR/ΔH reaches the value of 0.26 Ω/Oe and the absolute resistance value of the YBCO channel changes from the mΩ range up to kΩ. In the wide fluctuation temperature range (from 22 to 80 K), the R(T) curve of the YBCO layer is strongly modulated by the remnant magnetization of the garnet layer, clearly exhibiting a magnetic memory effect. A reduction of as much as 9 K in TC|R=0 is observed when the garnet layer has remnant magnetization after having been magnetized. At a measuring current of 1 A/cm2, the output voltage obtainable is in the mV range, which is adequate for use in nonvolatile memory applications.
Magnetic extended x-ray absorption fine structure at the L3,2 edges of Fe and Co on Cu(001)
Authors: Srivastava P. , Lemke L. , Wende H. , Chauvistré R. , Haack N. , Baberschke K. , Hunter-Dunn J. , Arvanitis D. , Mårtensson N. , Ankudinov A. , Rehr J. J.
Published: July 2002

We report magnetic extended x-ray absorption fine structure (EXAFS) measurements on 30 monolayer Fe and Co films on Cu(001) substrates at L3,2 edges. For 3d transition metals, the magnetic EXAFS at the L edges is of great interest since it probes final d states which mainly contribute to magnetism. Magnetic EXAFS oscillations were detected up to 500 eV above the edge corresponding to 11.5 Å−1 in k space. Over such a large range, the long wavelength wiggles from the Fe and Co nearest neighbor backscattering and previously observed fast oscillations are clearly seen. It is shown that without using any deconvolution procedure a meaningful analysis can be performed despite an interference of L3 and L2 edges. In the case of Fe, experimental data are compared with multiple-scattering calculations.
Fully relativistic theory for the magnetic extended x-ray absorption fine structure
Authors: Ahlers D. , Schütz G. , Popescu V. , Ebert H.
Published: July 2002

A fully relativistic theoretical description of the magnetic extended x-ray absorption fine structure (MEXAFS) is presented that has been set up within the framework of relativistic multiple scattering theory on the basis of the Dirac equation for spin-polarized, magnetic solids. The multiple scattering processes in the final state are treated using the scattering path expansion technique for finite atomic clusters. Results of the approach for the Fe K and the Gd L edges are presented and are compared with experiment. An analysis of the theoretical spectra clearly shows that the MEXAFS spectra can be interpreted on the basis of the so-called sum rules in a way completely analogous to the case of near-edge x-ray magnetic circular dichroism. This means that the K- and L1-edge spectra directly reflect the orbital polarization of the final states with p character. The L2,3-edge spectra, on the other hand, give, after suitable superposition, access to the spin and orbital polarization of the d states.
Specific-heat studies of RCu4Al8 compounds (R=Er,Y)
Authors: Hagmusa I. H. , Kayzel F. E. , Brück E. , Buschow K. H. J.
Published: July 2002

Specific-heat measurements have been performed on the tetragonal ThMn12-type compound ErCu4Al8 and YCu4Al8 in the temperature range 1.5–200 K. A sharp peak in the specific heat, signals the antiferromagnetic transitions at TN=5.18 K. In order to separate out the effect of the Er sublattice we have also performed measurements on the YCu4Al8 compound in the same temperature range. From the temperature dependence of the magnetic contribution to the specific heat of ErCu4Al8 we obtained the temperature dependence on the magnetic entropy and discuss our results in terms of the crystal field level scheme proposed earlier from inelastic neutron scattering.
Maximum operating temperature of the 1.3 μm strained layer multiple quantum well InGaAsP lasers
Authors: Smetona S. , Elenkrig B. B. , Simmons J. G. , Makino T. , Evans J. D.
Published: July 2002

We report on an experimental verification of a phenomenological model describing high-power and high-temperature operation of a compressively strained, InGaAsP multiple quantum well ridge-waveguide laser. The model, based on an assumption that mainly crystal heating defines optical output power saturation, has been proved to adequately describe the experimental results on maximum operating temperature of the lasers.
Early stage electromigration in gold thin films
Authors: Bai S. , Roenker K. P.
Published: July 2002

The early stage of electromigration in thin gold films on polyimide has been investigated at room temperature using the resistometric technique. While the resistance increase is initially linear, a saturation tendency is observed for longer stressing times at all stress current densities. A simple model is described which relates the saturation behavior in the resistance change to the buildup of mechanical stress gradients, which produce a counterflux of metal ions proportional to the stress gradient. The stress gradients arise due to nonuniformity in the grain size in the polycrystalline thin metal films which produces cluster regions of small grain size alternating with regions of large or near-bamboo grain size, which have larger and smaller metal ion diffusivities, respectively. The dependence of the maximum level of the resistance’s change and the rate of resistance increase on the stress current density are experimentally characterized and compared with the model’s predictions with good agreement.
Evaluation of the energy-transfer rate between an Er 4f shell and a Si host in Er-doped Si
Authors: Taguchi A. , Takahei K. , Matsuoka M. , Tohno S.
Published: July 2002

We estimated the energy-transfer rate between an erbium (Er) 4f shell and a Si host by using two independent measurements. The first method involved measuring the temperature dependence of the decay time of Er 4f-shell luminescence and obtaining the energy-transfer rate by assuming that the energy transfer is assisted by nonradiative multiphonon processes and that thermal quenching is due to an energy back-transfer mechanism. The estimated value was 2×108 s−1. The second method involved measuring the time response for luminescence intensity after pulsed host photoexcitation. Although some tens of μs luminescence delay after host excitation has been reported, we found that the slow response time of the detection system may cause spurious delay. We measured the luminescence decay curve using a system with a fast response time and analyzed the data, taking into account the system response time. The energy transfer rate was estimated to be at least 107 s−1. This estimation is consistent with the result obtained by the first method, mentioned above, confirming a rather large energy-transfer rate between the Er 4f shell and Si host.
Two-dimensional numerical investigation of the impact of material-parameter uncertainty on the steady-state performance of passivated 4H–SiC thyristors
Authors: Shah Pankaj B. , Jones Kenneth A.
Published: July 2002

We calculated the impact of inaccurate values of the free-carrier lifetime, SiC/SiO2 interface recombination velocity and interface charge, dopant ionization energy, electric permittivity, impact ionization rates, saturation velocity and field-dependent mobility on the holding current, switching current, and breakover voltage at different applied gate currents for an interdigitated 4H–SiC thyristor with two-dimensional steady-state numerical simulations. We observed that free-carrier lifetimes, interface charge and recombination velocity, and impurity ionization energy have the greatest impact on thyristor performance. As the carrier lifetime increases, the holding current decreases and the results logarithmically approach a single I-V curve in the negative differential resistance region. Comparing interface charge and recombination velocity, we demonstrated that recombination velocity has a larger effect on the holding current, but interface charge has a larger effect on the breakover voltage. Also, deeper acceptors increase the holding current, while deeper donors decrease the holding current. Our results demonstrate that mapping the negative differential resistance region by plotting the breakover voltage for different applied gate currents can give valuable information on the device and material properties. The experimentally observed nonuniform changes in the breakover voltage and switching current for uniform gate current steps are explained. The results indicate that interface properties must be included when simulating turn-on and turn-off by gate control. The results also indicate that the free-carrier lifetime variations that occur across a wafer can lead to large differences in the performance of two devices that are expected to have undergone identical processing.
Effects of p-doping and nonradiative processes on the current–voltage characteristics of long wavelength quantum-well laser diodes
Authors: Lau Pey-Kee , Makino Toshihiko
Published: July 2002

This article presents a model that combines a pair of simplified Fermi functions and an electrical derivative technique to study the effects of the following on the ideality factor of lattice-matched long wavelength InGaAsP/InP quantum-well laser diodes: the p-doping concentration level, the nonradiative recombination process (n0.52) within the active region, the Auger recombination process, and the drift carriers leakage process. The model showed that the ideality factor increases with the p-doping level. It also showed that the traditional three terms recombination model was inadequate for predicting the change in ideality factor with respect to variation in doping level.
Electron attachment to boron trichloride
Authors: Tav Cumali , Datskos Panos G. , Pinnaduwage Lal A.
Published: July 2002

Low-energy electron attachment to BCl3 was measured using an electron swarm technique. The parent negative ion, BCl3−, was observed within a narrow electron range close to thermal energy. Previous negative ion measurements in BCl3 discharges, which yielded seemingly inconsistent results, can be shown to be self-consistent based on the present observations.
Al
x
Ga
1−x
As
native-oxide-based distributed Bragg reflectors for vertical cavity surface emitting lasers
Authors: Evans P. W. , Wierer J. J. , Holonyak N.
Published: July 2002

The problems associated with constructing AlxGa1−xAs native-oxide-based distributed Bragg reflectors (DBRs) for vertical cavity surface emitting lasers are investigated. Reflection and stability measurements are performed on structures with central λ/2 cavities (λ∼980 nm) of GaAs surrounded by two periods of native-oxide-based DBRs on the top and 2.5 periods on the bottom. Prior to crystal oxidation (H2O vapor+N2, 430 °C) a period of the DBRs consists of a ∼λ/4 optically thick layer of GaAs and a thicker (oxidation) layer of AlxGa1−xAs (x=0.95, 0.96, 0.97, 0.98, 1.00) surrounded by thinner (∼100 Å) buffer layers that are AlyGa1−yAs (y=0, 0.25, 0.50, 0.65, 0.070, 0.75, 0.80, 0.85). The DBRs are formed after oxidation of the high Al composition AlxGa1−xAs layers, and to some extent the AlyGa1−yAs buffer layers, forming a ∼λ/4 optically thick layer of the native oxide. For comparison, more complicated DBRs are created by oxidizing superlattice layers. It is found that the AlxGa1−xAs composition, x, of the oxidation layer, choice of oxidizing or nonoxidizing AlyGa1−yAs buffer layers (y), oxidation parameters, and post-processing parameters determine the DBR quality and stability, as well as the possibility of reoxidation.
Structural and dielectric characterization of cadmium tartrate
Authors: Torres M. E. , López T. , Peraza J. , Stockel J. , Yanes A. C. , González-Silgo C. , Ruiz-Pérez C. , Lorenzo-Luis P. A.
Published: July 2002

Polycrystalline samples of dimeric cadmium tartrate, [(Cd,C4H4O6)2H2O)]3H2O [labeled CdT(I)], were studied using impedance measurements and x-ray powder diffraction. The dependence of the real part of the dielectric constant on temperature showed a sharp peak at about 65 °C, revealing a structural phase transition, while the other broad peak in the temperature range (70<T<85 °C) was due to the loss of water molecules. The x-ray powder diffraction patterns at three temperatures (25, 60, and 70 °C) are consistent with three nonequivalent space groups. According to these results, it seems that this compound undergoes two successive phase transitions: P212121→P21→Pnmn, suggesting an intermediate ferroelectric behavior, labeled CdT(II) between a paraelectric CdT(I) and an anhydrous phase, labeled CdT(III).
Ferroelectric films prepared by laser ablation directly on SiC substrates
Authors: Mou D. , Linnros J. , Petersson C. S. , Rao K. V.
Published: July 2002

Because of the common physical properties of silicon carbide (SiC) and lead base ferroelectric films [such as lead zirconate titanate (PZT) and lead titanate (PTO)], it is interesting to investigate ferroelectrics and SiC based integrated devices for potential high radiation environments, high speed, high temperature, and high density memory applications. Here we first report our primary results of depositing lead base ferroelectric films directly onto bare SiC substrates. Ferroelectric films including undoped and doped PZT and PTO were directly deposited on bare 3C-SiC and 4H-SiC substrates. X-ray diffraction analyses indicate that all the PTO films deposited at substrate temperatures from 600 to 650 °C are polycrystalline of the pure perovskite phase. PZT films deposited at a substrate temperature of 570 °C are dominated by the pyrochlore phase and even at a higher substrate temperature of 650 °C both the perovskite and the pyrochlore phases coexist. The pyrochlore phase of the PZT films is able to be converted into the perovskite phase by rapid thermal annealing at 800 °C for 30–60 s. The formation of an interfacial layer of a nonferroelectric phase (Pb2Ti2O6), which is caused by interdiffusion, is one of the most serious problems during the fabrication of lead based ferroelectric films directly onto the Si wafer. However, there is no such interdiffusion observed between the ferroelectric films and the SiC substrates. Capacitance versus voltage (C–V) curves of metal/ferroelectric/semiconductor diode structures demonstrate typical C–V hysteresis loops and the capacitance versus temperature measurements further confirm the ferroelectric properties of the films.
Dynamic properties of relaxor ferroelectrics
Authors: Glinchuk M. D. , Stephanovich V. A.
Published: July 2002

We calculate the dynamic order parameter and dielectric susceptibility for the relaxor ferroelectrics like PbMg1/3Nb2/3O3, PbSc1/2Ta1/2O3 and Pb1−xLaxZryTi1−yO3. Calculations of temperature and frequency dependence of dielectric susceptibility show the existence of low- and high-temperature group of maxima obeyed to Vogel–Fulcher (VF) and Arrhenius laws, respectively. The VF law and non-Debye relaxation behavior as well as the stretched exponential law of susceptibility time decay were shown to be the consequence of random fields distribution which transforms single dipole Arrhenius and Debye laws into VF and non-Debye, stretched exponential laws, respectively. Comparison of observed and calculated characteristics of the relaxors is carried out.
Magnetic order and spin reorientation in Nd0.45Ca0.55MnO3
Authors: Li W.-H. , Wu S. Y. , Lee K. C. , Lynn J. W. , Liu R. S. , Wu J. B. , Huang C. Y.
Published: July 2002

Neutron diffraction and ac magnetic susceptibility measurements have been performed to study the magnetic ordering of the Mn spins in polycrystalline Nd0.45Ca0.55MnO3. Three peaks around 260, 90, and 40 K were observed in the temperature dependence of the ac susceptibility. Structural analysis using high-resolution neutron diffraction patterns shows that the peak at 260 K is associated with the charge ordering of the Mn ions. Neutron magnetic diffraction shows that the peak at 90 K originates from the ordering of the Mn spins. Both ferromagnetic and antiferromagnetic couplings between the Mn spins were found. The spin structure in the basal plane consists of tilted ferromagnetic chains (along the [110] direction) aligned antiferromagnetically, whereas the interlayer coupling is ferromagnetic. A spin reorientation occurs below 15 K, which results in a change in the title angle. The Mn spins order at TN≈120 K, and the moment saturated at 30 K at 〈μZ〉=2.29(4)μB.
Longitudinal and transverse Hall resistivity of epitaxial SrRuO3 films
Authors: Yang H. C. , Liu S. H. , Wang L. M. , Horng H. E.
Published: July 2002

We report the longitudinal ρxx and transverse Hall ρxy resistivities of epitaxial SrRuO3 (SRO) under varied applied fields. The Hall resistivity, ρxy, is proportional to 1/(T−Tc) in the paramagnetic state of SRO films, where Tc is the asymptotic Curie temperature. The Lorentz resistivity Δρ(H)Lorentz in a fixed magnetic field in the ferromagnetic regime of SRO films is proportional to A(1−T/Tc)p with p∼0.5. RH is positive for T>125 K and changes sign at T∼125 K for SRO films. The RH remains negative down to low temperatures.
Microstructure investigation of Fe/Al2O3/Fe tunneling junction
Authors: Chen H. , Xu Q. Y. , Ni G. , Lu J. , Sang H. , Zhang S. Y. , Du Y. W.
Published: July 2002

A series of Fe/Al2O3/Fe tunneling junction samples were prepared by ion beam sputtering technique. After deposited, the Al layers in a series of samples were exposed to the air for 24–100 h to obtain Al2O3 layers (100 Å) with different oxidation quality. A tunneling magnetoresistance (TMR) value of 5.89% was observed in the sample with the Al layer oxidized for 72 h. The I–V curve of this sample shows a nonlinear relationship, indicating the existence of well metal–insulator contact in the sample. The microstructures of these samples were analyzed by using x-ray photoelectron spectroscopy. The results show that good Al2O3 layers are formed in all samples, but there exists an interdiffusion interface between the Fe and Al2O3 (with Al metal inclusions) layer which greatly influences the TMR effect.
Shield-related signal instability in magnetoresistive heads
Authors: Nakamoto K. , Narumi S. , Kawabe T. , Kobayashi T. , Fukui H.
Published: July 2002

Magnetoresistive (MR) heads with various upper shield materials were fabricated and their read-write performance was tested to clarify the shield-related effect on the signal instability in MR heads. Comparison of a head with an upper shield layer of higher magnetostriction and one with lower magnetostriction showed that the latter had better stability in the output signal of a repeated read-write test. The output amplitude of a head with an upper shield layer of Co52Ni27Fe21 film, which had a high magnetostriction of about +3×10−6, was varied by applying a low external longitudinal field, which affected just the shield layers. This change in the output corresponded well to the output variation in the repeated read-write test. The spin scanning electron micrograph image of this head revealed a distinct domain wall in the air bearing surface near the MR sensor. These results indicated that instability of the domain structure in a shield layer was one of the causes of the signal instability in MR heads; an unusual bias field from a domain wall of the shield layer, which could be moved easily by a repeated writing operation, caused a variation in the biased state of the MR layer which resulted in the signal variation, and that low magnetostriction was required for a shield material to achieve a stable head.
Impact of write current wave form on high frequency magnetic recording
Authors: Shi Rick C. , Zhu Jian-Gang , Schuelke Robert
Published: July 2002

In this article, we present an experimental study of the impact of the write current wave form on high frequency magnetic recording. It has been found that, at high recording frequencies, current overshoot reduces transition broadening, overwrite degradation, and nonlinear transition distortion. Results show that, to achieve fast flux reversal time, increasing write current overshoot is more effective and efficient than increasing write current amplitude. While overdriving the write head may bring the head into deep saturation, and introduces additional nonlinear transition shift, current overshoot significantly eliminates this nonlinear transition distortion by achieving fast head field rise time yet preventing the write head from deep saturation.
X-ray photoemission characterization of La0.67(CaxSr1−x)0.33MnO3 films
Authors: Broussard P. R. , Browning V. M. , Cestone V. C.
Published: July 2002

The Curie temperature and x-ray photoemission spectra of thin films of La0.67(CaxSr1−x)0.33MnO3 have been studied as a function of the Ca/Sr ratio. The films were grown by off-axis cosputtering from individual targets of La0.67Ca0.33MnO3 (LCMO) and La0.67Sr0.33MnO3 (LSMO) onto (100) oriented NdGaO3 substrates. The films grow with a (100) orientation, with no other orientations observed by x-ray diffraction. For the alloy mixtures, the Curie temperature, TC, varies slowly as the Ca/Sr is decreased, remaining ≈300 K, while for the LCMO and LSMO films TC is 260 and 330 K, respectively. The Mn-O valence structure is composed of two dominant peaks, whose positions undergo a change as the Ca fraction is decreased. The core lines behave as linear combinations of lines from pure LCMO and LSMO.
Ferromagnetism in a Cd1−xMnxTe double quantum well
Authors: Boselli M. A. , da Cunha Lima I. C. , Ghazali A.
Published: July 2002

The indirect exchange is treated for magnetic moments located in a thin semiconductor layer which is imbedded in the corresponding nonmagnetic quantum well host. The interaction is mediated by carriers provided by remote doping. The specific case of Cd1−xMnxTe is considered. A significant Curie–Weiss temperature is obtained for several possible arrangements of the magnetic layer and the quantum well widths, and the carrier (hole) density. We found a substantial contribution of virtual transitions involving continuum states for high enough carrier density.
Study of magnetic thin films deposited by closed-field unbalanced magnetron sputtering
Authors: Ormston M. W. , Petford-Long A. K. , Teer D. G.
Published: July 2002

Closed-field unbalanced magnetron sputtering, developed by TEER Coatings Ltd., uses a novel plasma confinement system, which allows controllable high-rate deposition from a wide range of target materials. We report the first use of this technique using ferromagnetic target materials to grow films of nanometer thickness. A study was carried out on a series of Py/Cu/Py and Py/Au/Py magnetic multilayer films, with and without underlayers of Ti or Ta. High-resolution electron microscopy showed that 5 nm of Ti or 15 nm of Ta did not change the structure of the trilayers. The use of Au as a spacer layer induced a texture in the upper Py layer, which decreased its saturation field by half. In situ experiments to observe the effects of an applied field on the domain structure of the films were carried out using Lorentz transmission electron microscopy. Variations in the switching field of the Py layers and of the coupling strength between the Py layers were observed when the thicknesses of the three layers were varied. Double domain wall structures with different wall intensities were observed in some cases. The roughness of the interfaces were increased by ion bombardment; this increased the saturation field of the Py layers.
Changes in parameters of auditory evoked magnetic fields with click train patterns
Authors: Nakagawa Seiji , Ueno Shoogo , Imada Toshiaki
Published: July 2002

N1m is an auditory evoked brain magnetic field with a magnitude of 100 fT order observed over the auditory cortex, 100 ms after the onset of auditory stimuli. The N1m is often used as a landmark of functional localization in the cortex. However, the mechanism of the N1m has not yet been clarified. The N1m peak amplitude and latency are dependent on the specifics of the stimulus; duration, intensity, and sequence of stimuli. In this study, we examined the dependency of the N1m peak amplitude and latency on the stimulus duration and frequency. Trains of 0.2 ms clicks were used for auditory stimuli by changing the number of clicks and the click interval. Auditory brain magnetic responses evoked by the click trains were recorded from seven human adult subjects by a dc superconducting quantum interference device magnetometer. In the results of this study, the N1m amplitudes significantly increased as the stimulus duration increased and the amplitudes leveled when the stimulus duration reached 32 ms. The amplitudes produced by the trains with the same number of clicks showed greater values for 4-ms-interval trains. The N1m latencies significantly decreased as the stimulus duration increased and leveled at 32 ms. It is concluded that all clicks received within 32 ms were integrated and that this integration mechanism is dependent upon the click interval. Increased synchrony of neuronal cells at the cortical level can explain this integration mechanism.
Control of the orientation of human erythrocytes by magnetic and electric fields
Authors: Suda T. , Ueno S.
Published: July 2002

The orientation of human red blood cells (RBCs) was controlled by the application of magnetic and electric fields. Because of their anisotropic diamagnetism, RBCs orient parallel to strong magnetic fields. The electric orientation of erythrocytes is also caused by electric dipoles induced by an electric field. The RBCs orientation is parallel to both the electric and magnetic fields. A 4–5 kV/m alternating current (ac) electric field (10–200 kHz, sine wave) was applied to RBCs suspended in a phosphate buffer solution using a pair of platinum black electrodes spaced 200–250 μm apart. An 8 T magnetic field was applied to the RBCs perpendicular to the direction of the electric field. It was observed that all RBCs were oriented in the same direction and parallel to the electric and magnetic fields. By the application of a horizontal 8 T magnetic field and a 4 kV/m ac electric field positioned perpendicular to one another, the RBCs oriented horizontally and their sedimentation rate was decreased by 18%. The flowing rate of the 10% RBCs suspension was decreased by 7.6% with the application of an 8 T magnetic field and a 4 kV/m ac electric field perpendicular to the direction of the suspension flow. It was observed that flowing RBCs were oriented perpendicular to the direction of the flow by the application of the fields, when the velocity of the suspension of RBCs was less than 300 μm/s.
Magnetism and structure of Fe/Cu multilayers studied by low-temperature conversion electron Mössbauer spectroscopy
Authors: Kuprin A. P. , Cheng L. , Lee D. W. , Altounian Z. , Ryan D. H.
Published: July 2002

Magnetic properties, structure, and morphology of a (Cu 39 Å/Fe 7 Å)×36 multilayer prepared by dc magnetron sputtering were studied mostly by low-temperature conversion electron Mössbauer spectroscopy (LT-CEMS) down to 90 K. LT-CEMS measurements in external magnetic field of 0.1 T rule out superparamagnetism of Fe islands. The multilayer exhibits strong perpendicular anisotropy below its Curie temperature (TC=185 K) and the linear temperature dependence of the magnetization suggests two-dimensional ferromagnetism.
Doubly exchange-biased NiCoO/NiFe/Cu/NiFe/NiCoO spin valves
Authors: Ambrose T. , Liu Kai , Chien C. L.
Published: July 2002

A new type of doubly exchange-biased Ni50Co50O/NiFe/Cu/NiFe/Ni30Co70O spin-valve structure, with two ferromagnetic layers exchange biased in opposite directions by two different antiferromagnetic layers is demonstrated. By field cooling in magnetic fields of opposite directions, the resultant hysteresis loop consists of two loops shifted in opposite directions from the zero magnetic field. The NiFe layers are in the antiparallel state in zero magnetic field, and the switching to the parallel state can be tuned by varying the exchange bias. The modified spin valves also show potential for suppressed Barkhausen noise.
Scanning electron microscopy study of carbon nanotubes heated at high temperatures in air
Authors: Lu Xuekun , Ausman Kevin D. , Piner Richard D. , Ruoff Rodney S.
Published: July 2002

Multiwalled carbon nanotubes (MWNTs) were dispersed in 2-butanol and dropped onto a V-ridge, lithographically patterned Si substrate that was coated with a thin layer of gold. These MWNTs were shown by scanning electron microscopy (SEM) to conform to the V-ridge surface topology at room temperature, which is thus useful for introducing kinks (at the apex of the V-ridge and the bottom of the trenches between V ridges). The substrate-supported MWNTs were then heated in air at temperatures from 673 to 1173 K for varying exposure times and were monitored with SEM. A 122 kJ mol−1 activation energy for complete oxidation was obtained, and preferential oxidation at kink sites was observed on some MWNTs at high temperatures. The dominant mode of oxidation was either thinning of the walls of the MWNTs or sequential oxidation of the component tubes in bundles. Some MWNTs, which at room temperature conformed to the V-ridge surface topology, detached (“sprang” away) from the substrate surface, demonstrating that the MWNTs are under tensile stress, but are held to the surface by van der Waals attractive forces, which can be overcome by exposure to higher temperatures.
Electronic structure and magnetization of Fe–Co alloys and multilayers
Authors: Paduani C. , Krause J. C.
Published: July 2002

The magnetic properties and electronic structure of bcc Fe–Co alloys and multilayers are investigated with the first-principles molecular cluster discrete variational method. The density of states and the contact interactions are obtained for the central atom of each cluster. Besides the local magnetic moment and the isomer shift the occupancies of 3d, 4s, and 4p shells are investigated when Co atoms are introduced in the immediate vicinity of iron sites. The calculations indicate a varying magnetic moment for Fe atoms and a constant value for Co atoms which is in agreement with experiments. For the superstructures, our results indicate a strong dependence of the local moment, contact field, and isomer shift for Fe atoms with the thick of iron layers. The internal field increases for thicker Fe layers while the local moment decreases which is also in accordance with experimental predictions.
Reactively sputtered Ru–Si–O films
Authors: Gasser S. M. , Kolawa E. , Nicolet M.-A.
Published: July 2002

Films of Ru–Si–O were synthesized by reactively sputtering a Ru1Si1 target in an Ar/O2 gas mixture. They were characterized in terms of their composition by 2.0 MeV He++4 backscattering spectrometry, their atomic density by thickness measurements combined with backscattering data, their microstructure by x-ray diffraction and transmission electron microscopy, and their electrical resistivity by four-point-probe measurements. The compositions indicate preferential sputtering with ruthenium enrichment of the films, and a saturation level of oxygen is determined at 67 at. % corresponding to the formation of SiO2 and RuO2. X-ray diffraction spectra reveal an amorphous structure for oxygen-saturated and nanocrystals for unsaturated as-deposited films. The crystallization temperature clearly increases with the oxygen concentration of the films, from 500 °C for oxygen-free films to 1000 °C for oxygen-saturated films, when annealed in vacuum for 30 min. Transmission electron micrographs of as-deposited oxygen-saturated films show few nanocrystals of 1–2 nm in diameter in an otherwise amorphous matrix. The atomic density is roughly 8×1022 atom/cm3 for all compositions. The resistivity of the ternary alloys scales with the terminal phases in the ternary phase diagram and reaches a maximum value when the Ru–SiO2 tie line is crossed near 50 at. % oxygen. The films are stable in vacuum up to thermal stressing at 800 °C for 5 h, and their decomposition starts near 1000 °C.
Optical second-harmonic generation in lead formate
Authors: Betzler K. , Hesse H. , Jaquet R. , Lammers D.
Published: July 2002

The optical properties of lead formate crystals are investigated, especially those affecting the usage for optical second-harmonic generation. The refractive indexes and the optical absorption are measured in the near-ultraviolet, visible, and near-infrared spectral regions. The data show that the material is suitable for the efficient generation of blue light when pumped with near-infrared laser wavelengths. Phase matching conditions for this application of lead formate are determined both experimentally and by a numerical evaluation of the index data.
Dwell-time related effects in focused ion beam synthesis of cobalt disilicide
Authors: Hausmann Stephan , Bischoff Lothar , Teichert Jochen , Voelskow Matthias , Möller Wolfhard
Published: July 2002

The influence of the high current density of a focused ion beam on the ion beam synthesis of CoSi2 layers has been investigated. After 35 keV Co+ or 70 keV Co2+ implantation into a heated Si(111) substrate and subsequent annealing, the layers have been investigated by scanning electron microscopy and Rutherford backscattering spectroscopy (RBS). It is shown that the mode of beam scanning influences the CoSi2 layer formation significantly. At a given substrate temperature, a sufficient low dwell time is required to obtain a continuous layer rather than a laterally disrupted structure. With increasing target temperature, the dwell-time window becomes less restricted. The results are discussed in terms of damaging and dynamic annealing of the silicon crystal. RBS channeling investigations demonstrate that continuous or disrupted CoSi2 layers are formed when the substrate remains crystalline or becomes amorphous, respectively.
Interdependence of elastic strain and segregation in metallic multilayers: An x-ray diffraction study of (111) Au/Ni multilayers
Authors: Labat S. , Gergaud P. , Thomas O. , Gilles B. , Marty A.
Published: July 2002

This study concerns the analysis of elastic strains in Au/Ni multilayers whose periods lie in the range 1–5 nm. Lattice parameter measurements have been performed by x-ray diffraction in various directions. The results are interpreted in terms of elastic strain and interfacial mixing. It is shown that considerable strains (several percent) are encountered within these very thin layers. In the Au sublayers, coherence is never observed with the underlying Ni film and the residual strain relaxes progressively as the Au film thickens. In the Ni sublayers, interfacial mixing controls the lattice parameter and the elastic strain is a function of this mixing. Out of equilibrium mixing of Au into Ni is ascribed to dynamic segregation during the growth of Ni on Au. All these results bring new insights on the still open question of stress relaxation mechanisms in ultrathin films.
Photoluminescence study of exciton–optical phonon scattering in bulk GaAs and GaAs quantum wells
Authors: Gopal A. Venu , Kumar Rajesh , Vengurlekar A. S. , Bosacchi A. , Franchi S. , Pfeiffer L. N.
Published: July 2002

We obtain the temperature dependence of the homogeneous linewidth of excitons in GaAs quantum wells (QWs) and bulk GaAs using photoluminescence measurements. The results indicate that exciton scattering rates with optical phonons are larger in bulk GaAs than in QWs.
Phase formation in hypostoichiometric Sm2Fe17 alloys modified with Ti and C
Authors: Shield J. E. , Meacham B. E.
Published: July 2002

The phase formation, crystallization products, microstructure, and magnetic behavior of Fe-rich Sm–Fe alloys modified with Ti and C have been determined. Alloys of nominal composition (Sm0.11−xFe0.89+x)94Ti3C3 with x=0, 0.02, and 0.04 were produced by melt spinning. Crystalline phases observed included SmFe7 and α-Fe and an increasing amorphous fraction was observed with increasing x. Upon crystallization the amorphous fraction converted to Sm2Fe17 and α-Fe, with a grain size of 30 nm for the sample with x=0.02. The magnetic measurements of nitrided alloys revealed a remanent ratio above 0.5 for x=0 and 0.02, although the coercivity was rather low.
Diffusion of implanted nitrogen in silicon
Authors: Shaik Adam Lahir , Law Mark E. , Jones Kevin S. , Dokumaci Omer , Murthy C. S. , Hegde Suri
Published: July 2002

Growth of thinner gate oxides and their thickness control is one of many challenges in scaling technologies today. Nitrogen implantation can be used to control gate oxide thicknesses. This article reports a study on the fundamental behavior of nitrogen diffusion in silicon. Nitrogen was implanted as N2+ at a dose of 5×1013 ions/cm2 at 40 and 200 keV through a 50 Å screen oxide into Czochralski silicon wafers. Furnace anneals at a range of temperatures from 650 to 1050 °C have revealed anomalous diffusion behavior. For the 40 keV implants, nitrogen diffuses very rapidly and segregates at the silicon/silicon-oxide interface. Qualitative modeling of this behavior is also discussed in terms of the time taken to create a mobile nitrogen interstitial through the kickout, Frenkel pair, and the dissociative mechanisms.
Activation volume and magnetic interaction in longitudinal CoCrPtTa magnetic recording media
Authors: Lauhoff G. , Suzuki Takao
Published: July 2002

The thermal activation volume (VA) is studied for CoCrPtTa longitudinal magnetic recording media. VA is found to strongly increase for smaller reversing fields. This trend is also continued in small positive applied fields. The activation area (SA) at the same reversing fields are found to be in reasonable agreement to each other for different media thicknesses. The thinner magnetic films show a larger SA near the coercivity, while δM remanence curves indicate that the intergranular exchange coupling becomes stronger for thinner media. Surprisingly comparable read/write performances are found for different media thicknesses.