bluuebunny/crossref_metadata_2025 huggingface dataset:
An experimental study on the influence of planar anisotropy on the critical behavior of a Fe17Dy2 single crystal
Authors: Coelho A. A. , Gama S. , Mohan Ch. V. , Kronmüller H.
Published: July 2002

In order to check the influence of planar anisotropy on the magnetic phase transition which occurs at the Curie temperature, we have made a detailed experimental study on the critical behavior of a high quality Fe17Dy2 single crystal which has a strong planar anisotropy. This study includes deducing various critical exponents from extensive magnetization measurements. The asymptotic and effective critical exponents and their corresponding amplitudes have been derived from the magnetization data measured with the applied field along the easy plane using the conventional methods to analyze phase transitions with and without the inclusion of the “correction-to-scaling” terms originating from the nonlinear irrelevant scaling fields. Inclusion of the correction-toscaling terms could only influence the critical amplitude values. The values of the critical exponents obtained in this way are found to be in excellent agreement with those obtained from the magnetization data taken with the applied field parallel to the hard axis, indicating that the planar anisotropy has not influenced the critical behavior. These results are in agreement with the observations made on Fe17Ho2 and Fe17Er2 single crystals.
Isotope effect on the reactivation of neutralized Si dopants in hydrogenated or deuterated GaAs: The role of hot electrons
Authors: Constant E. , Bernard-Loridant D. , Mezière S. , Constant M. , Chevallier J.
Published: July 2002

Room-temperature bias stress and annealing experiments have been performed on hydrogenated and deuterated Si doped n-type GaAs Schottky diodes. From a careful analysis of capacitance–voltage characteristics, we have studied the variation of the active doping concentration and the reactivation of neutralized dopants. In thermal annealing experiments at 250 °C, the Si–H complex dissociation rate does not vary significantly by incorporating deuterium rather than hydrogen. On the contrary, by applying high reverse bias voltages to the Schottky diodes at room temperature, a strong isotope effect is observed on the dissociation rate. In this case, the dopant reactivation should be due to hot electron excitations.
Magnetoresistance effects at the metamagnetic transition in R6Fe14−xAlx (R=Nd, La)
Authors: Jonen S. , Rechenberg H. R.
Published: July 2002

The La6Co11Ga3-type intermetallic compounds Nd6Fe10Al4, La6Fe10Al4, and La6Fe9Al5 have been studied by magnetization and electrical resistance measurements in applied fields up to 9 T. All compounds were found to be antiferromagnetic, i.e., no residual magnetization was found, in contrast to most compounds of the R6Fe14−xMx type. A metamagnetic transition was observed for all samples at a critical field smaller than 3 T, and a ∼3%–5% resistivity drop was found to occur concomitantly. Both the magnetization (M) and the magnetoresistance (MR) exhibited a large hysteresis for the Nd compound, but the MR vs M relationship was history independent in the upper part of the magnetization curve. The metamagnetic transition field exhibited an anomalous temperature dependence for Nd6Fe10Al4 which was ascribed to crystal-field effects.
Effects of copper deficiency on the magnetic response of La1.85Sr0.15Cu1−εO4+δ
Authors: Lisboa Filho P. N. , Ortiz W. A. , Zanetti S. M. , Leite E. R.
Published: July 2002

In this article we study the magnetic behavior of a series of copper-deficient spin-glass compounds La1.85Sr0.15Cu1−εO4−δ. High-quality ceramic samples were prepared by a sol–gel technique. Stoichiometry and structural characterization of the specimens were verified by x-ray diffraction, inductively coupled plasma spectrometry, atomic absorption spectrophotometry, and thermogravimetric analysis. Low-field ac and dc magnetic measurements, carried out in a superconducting quantum interference device magnetometer for samples with ε=0.05, 0.15, and 0.25, revealed all major characteristics of a spin-glass system: thermal irreversibility, magnetic remanence, time-relaxation and frequency-dependent ac susceptibility.
On the use of x-ray reflectivity and the second-harmonic magneto-optic Kerr effect for characterizing buried magnetic interfaces
Authors: Talghader V. , Kief M. T. , Al-Jumaily G.
Published: July 2002

This article examines the characterization of buried magnetic interfaces using a relatively new technique, the second-harmonic magneto-optic Kerr effect (SHMOKE), and a more established technique, x-ray reflectivity (XRR). Results are presented on the interdiffusion and interfacial roughness of Ta/NiFe and TaN/NiFe interfaces. The samples were characterized as deposited and after a 270 °C, 2 h anneal. SHMOKE analysis indicates that the Ta/NiFe interfaces are more susceptible to interdiffusion than the TaN/NiFe interfaces, during deposition as well as anneal. These results are consistent with XRR analysis of the samples and suggest that further development of SHMOKE may be of enormous benefit to interfacial magnetic metrology.
Magnetization reversal mechanism of Co–O films with oblique anisotropy
Authors: Sugita R. , Watahiki T. , Muranoi T. , Ishida T.
Published: July 2002

The angular dependence of the coercivity Hc for vacuum evaporated Co–O films with oblique anisotropy is studied both in an in-plane and in a normal plane in order to clarify the magnetization reversal mechanism. The angular dependence of Hc in the in-plane is remarkably different from that in the normal plane. The Co–O films have columnar structure, and the columnar grains look almost the same in both planes. The anisotropy field Hk in the normal plane is much higher than in the in-plane. Simulated curves of the angular dependence of Hc using the curling incoherent magnetization reversal model on the basis of measured values for Hc of the easy axis direction and Hk are similar to the experimental results in both planes. These results indicate that the magnetization reversal mechanism of the Co–O films is the incoherent rotation and that the interaction among the grains is different in both planes.
Magnetostatic interactions in films for perpendicular recording media
Authors: Bottoni G. , Candolfo D. , Cecchetti A.
Published: July 2002

The magnetostatic interactions in CoCr films for perpendicular recording are studied, based on the measurement of ordinary magnetization curves. The method used does not need the evaluation of the remanent magnetization curves, which is difficult to obtain in these media because of the presence of demagnetizing field in the anisotropy direction. The obtained interaction effects are related with the film thickness, Cr content, the presence of a Fe81Ni19 underlayer, and the magnetic properties of the recording media. The interactions are strong in films with large thickness and they are weak when the thickness is small and even weaker in double layer FeNi/CoCr films. This result is correlated with the magnetization reversal mode, which evolves towards incoherent mechanisms when the film thickness decreases and even more in double layers, as a consequence of the greater dispersion in the vertical columnar structure of the CoCr films.
Magnetic effects of atomic short-range order on PdMn alloys
Authors: Parra R. E. , González A. C.
Published: July 2002

The magnetic properties of a material are affected by the type of atomic order present in the sample. In particular, the atoms of one kind change their magnetic behavior depending on the composition of its first-neighbor shell. The analysis of magnetic samples by neutron diffuse scattering allows us to separate the atomic short-range order component from the magnetic component, but simultaneously we can study the correlations that affect the magnetic properties of materials. Using neutron-scattering data, we have analyzed the atomic short-range order in several samples where the host is Pd and the impurity is Mn. This analysis allows us to see the effects that this order has on the magnetic properties.
Three level, six state multilevel magnetoresistive RAM(MRAM)
Authors: Jeong Won-Cheol , Lee Byung-Il , Joo Seung-Ki
Published: July 2002

Using four magnetic layers, NiFe, NiFeCo, NiFeCo*, and Co with different coercivities, NiFe/Cu/NiFeCo/Cu/NiFeCo*/Cu/Co multilayered structure has been prepared on 4° tilt-cut Si(111) substrate by radio frequency magnetron sputtering method. Co, NiFeCo*, and NiFeCo are used as storage layers and NiFe used as a sensing layer. With proper external magnetic fields applied three memory levels can be separated. After separating the memory levels, six memory states can be distinguished with the magnetization reversal of the sensing layer, NiFe. As the spacer Cu thickness decreases, the interaction with the other magnetic layers increases gradually to ferromagnetic interaction. With 2 nm Cu spacer, we could see that the switching field of the hardest magnetic layer decreased by about 30%, compared to other cases. In order to demonstrate the industrial applicability, samples have been patterned by conventional photolithography technology into small size and its switching characteristics in small size have also been confirmed.
Effects of film thickness and lattice mismatch on strain states and magnetic properties of La0.8Ca0.2MnO3 thin films
Authors: Rao R. A. , Lavric D. , Nath T. K. , Eom C. B. , Wu L. , Tsui F.
Published: July 2002

The effects of strain relaxation on the crystallographic domain structure and on the magnetic and transport properties of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 (LCMO) thin films have been studied. LCMO films in the thickness range of 100–4000 Å were grown on (001) SrTiO3 and (001) LaAlO3 substrates, which impose an in-plane tensile and an in-plane compressive biaxial stress in the films, respectively. On (001) SrTiO3 substrates, the films can be grown coherently up to a thickness ∼250 Å, then strain relaxation occurs at a thickness of ∼500 Å. In contrast, even the 100 Å film grown on (001) LaAlO3 is partially relaxed, and the critical thickness for complete strain relaxation is ∼750 Å. The very thin films (<250 Å) show a pure (001)T normal orientation for growth on SrTiO3 and a pure (110)T texture for growth on LaAlO3. As thickness increases, the lattice strain relaxes, resulting in mixed (001)T and (110)T textures for growth on both substrates. Both the Curie and peak resistivity temperatures increase with increasing film thickness, but they do not exhibit a correlation to strain states of the film.
Easy-axis transition in epitaxial face-centered-cubic Ni80Fe20/Ni/Cu(100)
Authors: Zhang Wei , Vaz Carlos A. F. , Hirohata Atsufumi , Bland J. A. C.
Published: July 2002

We have studied the effects of ferromagnetic overlayers on the magnetic properties of step-wedged fcc (0–100 Å) Ni80Fe20/50 Å Ni/2000 ÅCu(001) structures. The perpendicular magnetic anisotropy (PMA) was found to be sensitively dependent on the NiFe thickness. An easy-axis transition from out-of-plane to in-plane was found to occur at a NiFe thickness of 36 Å with a corresponding domain structure observed by magnetic force microscopy. The absence of strain in the NiFe film as evidenced by reflection high energy electron diffraction suggests that the volume anisotropy of the NiFe films comes entirely from the shape anisotropy. The easy-axis transition in the fcc NiFe/Ni/Cu(100) system can be understood to result from an additional volume anisotropy which forces the Ni magnetization to lie in-plane. Our findings open a new path to controlling the critical thickness for PMA in this system via ferromagnetic overlayers.
Layer-resolved magnetometry of a magnetic bilayer using the magneto-optical Kerr effect with varying angle of incidence
Authors: Pufall M. R. , Platt C. , Berger A.
Published: July 2002

We determined the hysteresis loop of each layer in a magnetic bilayer of NiFe–SiO2–CoFe using the magneto-optical Kerr effect. Our method utilizes the fact that varying the angle of incidence of the optical beam changes the magnitude and sign of the polarization rotation signal contributed by each layer to the total signal. The magneto-optical signal of each layer varies differently with the angle of incidence, enabling us to determine the hysteresis loop of each. We measured the hysteresis loop of the bilayer for a range of angles of incidence, and from these loops determined the magnetization of the individual layers. We found that the magnetization of the soft NiFe layer coupled strongly to the CoFe layer for SiO2 interlayer thicknesses of <2 nm. The NiFe loop shears with decreasing interlayer thickness, increases in coercivity, and develops a two-stage switching behavior at an interlayer thickness of 1 nm.
Magnetic properties of ultrathin Fe films grown on stepped W(001) and Pd(001) substrates
Authors: Choi Hyuk J. , Kawakami R. K. , Escorcia-Aparicio Ernesto J. , Qiu Z. Q. , Pearson J. , Jiang J. S. , Li Dongqi , Osgood R. M. , Bader S. D.
Published: July 2002

In both Fe/W(001) and Fe/Pd(001) systems, the atomic steps induce an in-plane uniaxial magnetic anisotropy with the easy magnetization axis perpendicular to the step edges. The strength of the step-induced anisotropy was found to have a power law dependence on the step density: a quadratic dependence in the Fe/W system but a linear dependence in the Fe/Pd system. In addition, the Curie temperature is found to be higher on the stepped surface in the Fe/Pd system as compared to the flat surface. The enhancement of the Curie temperature is attributed to the step-induced Pd moments which is supported by the increased surface magneto-optic Kerr effect signal on the stepped surface. No such enhancement of either Curie temperature or magnetic moment was observed in the Fe/W system.
Low temperature dependence of the elastic properties of Ti3SiC2
Authors: Finkel P. , Barsoum M. W. , El-Raghy T.
Published: July 2002

In this work, the Young’s, E, and shear, μ, moduli of Ti3SiC2 are measured ultrasonically in the 20–300 K temperature range. At room temperature, Young’s, ERT, and shear, μRT, moduli are 322±2 and 133.6±0.8 GPa, respectively. Poisson’s ratio is 0.2. Both moduli increase slowly with decreasing temperature and plateau out at temperatures below ≈130 K. A least square fit of the data yields, μ/μRT=1−1.42×10−4(T−298) and E/ERT=1−0.95×10−4(T−298) for temperatures greater than 140 K. The elastic Debye temperature is estimated to be 427 K, which is significantly lower than the value of 620 K calculated from heat capacity measurements.
High mass positive ions and molecules in capacitively-coupled radio-frequency CF4 plasmas
Authors: Schwarzenbach W. , Cunge G. , Booth J. P.
Published: July 2002

The positive ions and neutral radicals arriving at the earthed walls of a capacitively-coupled radio-frequency pure CF4 plasma were analyzed using a quadrupole mass spectrometer adapted for high masses. Experiments were performed at 50 and 200 mTorr, in an empty reactor and with Si and SiO2-coated Si substrates on the powered electrode. High mass ions and neutrals were detected, up to 500 and 300 amu, respectively. The abundance of high-mass species was greatest in the presence of silicon wafers and at higher pressure. The observed ion masses can be separated into distinct series, originating from different initial bases to which successive CF2 units have been added. We, therefore, propose that these high-mass species are the result of a gas phase polymerization process consisting of CF2 addition reactions, in agreement with a model proposed recently by our group. The influence of a silicon substrate derives primarily from the strong decrease that it induces in the concentration of F atoms, which otherwise limit the concentration of CF2 and of chain initiating species.
Effects of composition on the formation temperatures and electrical resistivities of C54 titanium germanosilicide in Ti–Si1−xGex systems
Authors: Lai J. B. , Chen L. J.
Published: July 2002

The effects of alloy composition on the formation temperature and electrical resistivities of C54 titanium germanosilicide formed during the Ti/Si1−xGex (x=0, 0.3, 0.4, 0.7, 1) solid state reaction have been investigated. Ti5(Si1−yGey)3, C49– and C54–Ti(Si1−zGez)2 were observed to form in the Ti/Si1−xGex (x⩾0.4) systems. On the other hand, Ti6(Si1−yGey)5 and C54–Ti(Si1−zGez)2 were found in the Ti/Si1−xGex (x≧0.7) systems. For both cases, the relationship of x>y>z was found. The appearance and agglomeration temperature of low-resistivity C54–Ti(Si1−zGez)2 were both found to decrease with the Ge concentration. The resistivities of C54–Ti(Si1−zGez)2 were measured to be 15–20 μΩ/cm. The segregation of Si1−wGew (w>x) was found in all samples annealed above 800 °C. The effects of thermodynamic driving force, kinetic factor, and composition of the micro-area are discussed.
Ion behavior and gas mixing in electron cyclotron resonance plasmas as sources of highly charged ions
Authors: Melin G. , Drentje A. G. , Girard A. , Hitz D.
Published: July 2002

This article deals with ion behavior in small open-ended magnetic devices, the electron cyclotron resonance ion sources (ECRIS) that were developed for multicharged ion production. The ECRIS are basically ECR-heated plasma confinement machines with hot electrons and cold ions. The main parameters of the ion population in ECRIS plasmas are successively analyzed: temperature, collisions, losses, confinement times, followed by the gas mixing effect, a specific technique to improve the performance as an ion source. A series of experiments is described for the systematic analysis of this effect. It is experimentally shown that high charge state optimization by gas mixing results from an ion confinement time improvement due to ion cooling, and relies on a compromise between three criteria, ion losses, mass effect, and ionization rates.
Magneto-optical spectra of Fe/Au artificial superlattices modulated by integer and noninteger atomic layers
Authors: Sato Katsuaki , Takeda Eishi , Akita Masanori , Yamaguchi Masatake , Takanashi Koki , Mitani Seiji , Fujimori Hiroyasu , Suzuki Yoshishige
Published: July 2002

Spectra of the magneto-optical Kerr effect, in a series of [Fe (xML)/Au (xML)]N superlattices with integer and noninteger values of x (1⩽x⩽15), were measured. For x=1 the superlattic forms an L10-type ordered alloy structure. The magneto-optical spectrum of the superlattice exhibits a prominent structure around 4 eV, which shows a systematic shift toward higher energies as x increases (1⩽x⩽5). No such structure can be reproduced by an optical calculation assuming a simple stack of thin Fe and Au layers. Ab initio band calculations have shown similar magneto-optical spectra and a similar peak shift with an increase in x (1⩽x⩽6). This suggests that electronic structures which differ from that of a simple stack of thin Fe and Au are realized in the superlattices. For noninteger values of x, oscillatory behavior with a period of one monolayer was observed in the low energy region of the magneto-optical spectra.
Steady-state and time-resolved photoconductivity measurements of minority carrier lifetime in ZnTe
Authors: Bose D. N. , Ahrenkiel R. K. , Bhunia S.
Published: July 2002

Minority carrier lifetime in ZnTe has been determined from steady-state and time-resolved photoconductivity (PC) measurements. Three types of single crystal p-ZnTe (EG=2.26 eV) grown by the Bridgman technique were studied: (i) as-grown, before and after hydrogen passivation, (ii) Zn annealed, and (iii) In-doped semi-insulating. Steady-state photoconductivity was studied between 80 and 300 K and showed that for as-grown samples, the lifetime went through a sharp maximum of 4.5×10−7 s at 220 K, decreasing to 2.5×10−8 s at 300 K. For hydrogen passivated samples, the low temperature behavior was similar but the lifetime remained high at 4.5×10−7 s at 300 K, due to passivation of deep acceptors OTe. Time-resolved photoconductivity measurements gave a value of 4.6×10−8 s for as-grown ZnTe in reasonably good agreement and 3.2×10−7 s for Zn annealed and 3.1×10−7 s for SI-ZnTe. The radiative recombination constant B was thus found to be 1.4×10−9 and 4×10−10 cm3 s−1, respectively, at 300 K for as-grown and Zn-annealed samples. PC spectral response studies showed a maximum at 2.41 eV at 300 K corresponding to the main valence–conduction band transition as well as a feature near 3.2 eV corresponding to the splitoff band. Slight shift in peak energy to 2.43 eV occurred on H surface passivation due to reduction of surface recombination velocity.
Migration of constituent atoms and interface morphology in a heterojunction between CdS and CuInSe2 single crystals
Authors: Soo Y. L. , Huang S. , Kao Y. H. , Deb S. K. , Ramanathan K. , Takizawa T.
Published: July 2002

Angular dependence of x-ray fluorescence (ADXRF), x-ray absorption fine structure (XAFS), and grazing incidence x-ray scattering measurements were carried out using synchrotron radiation for a study of the interface morphology and migration of constituent atoms in a heterojunction formed between CdS and CuInSe2 single crystals. The advantage of using a single crystal for this study is to avoid the usually complicated problems arising from multiple phases of the Cu–In–Se compounds. By a comparison of the results obtained with a bare CuInSe2 single crystal, the changes of interface microstructures in the CdS/CuInSe2 heterojunction system with well-defined stoichiometry can therefore be investigated. Prominent features in the ADXRF data clearly demonstrate that both Cu and Se atoms have migrated into the CdS layer in the heterojunction while In atoms remain intact in the CuInSe2 single crystal. The local structures around Cu in the system also show a significant change after the deposition of CdS, as manifested by the appearance of new Cd near neighbors in the XAFS spectra.
Structural and luminescence characteristics of cycled submonolayer InAs/GaAs quantum dots with room-temperature emission at 1.3 μm
Authors: Krishna S. , Zhu D. , Xu J. , Linder K. K. , Qasaimeh O. , Bhattacharya P. , Huffaker D. L.
Published: July 2002

Quantum dots were grown by molecular beam epitaxy on GaAs substrates using a cycled submonolayer InAs/GaAs deposition technique. Their structural and luminescence characteristics have been compared with conventional self-organized dots. The room-temperature luminescence spectra are characterized by a ground state transition at 1.3 μm and additional transitions corresponding to excited states. Cross-sectional transmission electron microscopy indicates that no dislocations are formed if the total InAs thickness is less than 5–6 monolayers. Temperature dependence of the photoluminescence indicates that both types of quantum dots may have nonradiative defects, caused by segregation and related phenomena.
Coulomb effects on the optical gain in semiconductor superlattices and multiquantum wells
Authors: Je K.-C. , Seo K.-C. , Kim Yup
Published: July 2002

Based on the two-band semiconductor Bloch equations with screened Coulomb interactions, optical gains in a semiconductor superlattice and a multiquantum well are calculated to study the many-body Coulomb effects on the gains. Gain peaks, threshold density of gain and peak-gain frequencies with Coulomb effects are compared to those without Coulomb effects. The Coulomb effects are also studied by varying the carrier density and temperature. The specific Coulomb effects such as Coulomb enhancement, band filling effects, band gap renormalizations and the screening of Coulomb potential on the gain are also considered. It is found that at low temperature the Coulomb enhancement affects the optical gain sensitively, but it has little effect at high temperature.
Monte Carlo simulation of electron transport in 2H-SiC using a three valley analytical conduction band model
Authors: Nilsson H-E. , Hjelm M.
Published: July 2002

A Monte Carlo study of the electron transport in 2H-SiC is presented. The study is based on a three valley analytical band model that has been derived from an ab initio band structure calculation. The scattering models have been extrapolated from recent Monte Carlo simulations of 4H-SiC and 6H-SiC. The low field mobility in the c-axis direction is higher than in 4H-SiC and 6H-SiC, while the mobility perpendicular to the c axis is similar. The saturation velocity at 300 K obtained from the Monte Carlo simulation is 2.3×107 cm/s for field applied in the c-axis direction and 1.9×107 cm/s for field applied perpendicular to the c-axis direction. The difference in mean energy as a function of electric field between 2H-SiC and 4H-SiC or 6H-SiC is large. The energy spectrum along the c axis in 2H-SiC is not discontinuous as in the case of 4H-SiC and 6H-SiC, which gives 2H-SiC a higher mean energy for electric fields applied in the c-axis direction. This indicates that the electron impact ionization coefficients should be higher for 2H-SiC.
Contactless characterization of melt-textured superconducting junctions using micro-Hall sensor arrays
Authors: Karapetrov Goran , Cambel Vladimir , Kwok W. K. , Nikolova R. , Crabtree G. W. , Zheng H. , Veal B. W.
Published: July 2002

We use spatially resolved Hall-probe magnetometry and magneto-optical imaging to study current-carrying properties of melt-textured high-temperature superconducting welds at high magnetic fields. Magneto-optical images show no deterioration of the current-carrying properties of the sample in the presence of the superconducting weld. The study of the vortex dynamics near the junction using high-resolution Hall-probe magnetometry revealed enhanced relaxation of the magnetization at the junction at high temperatures. We attribute this behavior to an increased local concentration of defects near the weld that, depending on the temperature range, either facilitates enhanced relaxation or strengthens the local pinning properties.
Effects of Co addition on magnetic properties and nanocrystallization in amorphous Fe84Zr3.5Nb3.5B8Cu1 alloy
Authors: He Shuli , He Kaiyuan , Shen Baogen , Zhang Hongwei , Zhang Shaoying , Guo Huiqun
Published: July 2002

The structure, Curie temperature, and magnetization of (Fe1−xCox)84Zr3.5Nb3.5B8Cu1 (x=0.0–0.8) alloys were investigated by x-ray diffractometer, differential scanning calorimeter, magnetic balance, and extracting sample magnetometer, respectively. The bcc grains are embedded in the amorphous matrix after controlled annealing. The crystallization temperature Tx of the amorphous alloy decreases with the increasing Co content, whereas the Curie temperature Tc of the amorphous alloy is enhanced by an increase in Co content. The magnetization measured under the field of 60 kOe at a temperature of 1.5 K is enhanced first by the increase of Co addition, and exhibits a maximum at the composition where x=0.2, then decreases while Co content increases.
Analytical model of transient photoresponse of quantum well infrared photodetectors
Authors: Ershov M. , Satou S. , Ikebe Y.
Published: July 2002

This article presents a theory of the transient photoresponse of quantum well infrared photodetectors (QWIPs). As has been shown recently [M. Ershov, Appl. Phys. Lett. 69, 3480 (1996)], the photocurrent in a QWIP in response to a step-like infrared illumination is composed of fast and slow transients. Analytical models for both fast and slow transients in the linear regime are proposed. The amplitudes and time constants of the transients are expressed in terms of the basic QWIP characteristics — capture time to the QWs, photocurrent gain, and differential conductivity of the emitter contact. Closed-form analytical expressions for the transient photocurrent in the time and frequency domains are obtained. The validity of the proposed analytical model is confirmed by comparison with numerical simulations.
Modifying the temperature dependence of magnetic garnet film coercivity by etching
Authors: Vértesy G. , Keszei B.
Published: July 2002

The temperature dependence of the domain-wall coercive field of epitaxial magnetic garnet films was modified in a defined temperature range by removing the surface layer of the films. Outside the given temperature range the coercivity versus temperature curve did not change. The result supports a model of coercivity according to which different sets of material imperfections are responsible for pinning the domain walls in different temperature regions. Appropriate processing of the samples enables some of the pinning sets to be modified independently of each other.
Enhancement of amplification characteristics in a plasma-filled Cherenkov laser at millimeter wavelengths
Authors: Thumvongskul T. , Hirata A. , Zaginaylov G. , Shiozawa T.
Published: July 2002

On the basis of the fluid model of the electron beam, amplification characteristics of a two-dimensional plasma-filled Cherenkov laser at 100 GHz bands are analyzed. By injecting a dense plasma into the background of a Cherenkov laser, the coupling between a relativistic electron beam and an electromagnetic wave at millimeter wavelengths has been improved, and the weak beam–wave interaction problem has become less serious. In particular, we show that due to electron–ion collisions in the background plasma, proper choices of parameters are essential for good operation in this regime. The numerical results indicate that a Cherenkov laser with background plasma can generate higher output power with a greatly enhanced growth rate at a more reasonable interaction length. The plasma injection, in addition, enables the device to gain more output power with a higher beam current.
Lifetime of excess electron–hole pairs measured with white noise performance of the photocurrent
Authors: Kuan C. H. , Tu C. H.
Published: July 2002

A modified method to measure the lifetime of the excess electron–hole pairs generated by laser beams is proposed. Under the applied bias, excess carriers pass through the sample within the transit time and form the photocurrent Ip. The related white-noise component of the photocurrent can be written as 4egIp, where g is the ratio of the lifetime over the transit time. The lifetime can then be obtained by determining g from the white noise performance. Since the transit time can be adjusted with the applied biases and the sample length, the measured lifetime may be as small as 100 ns with the measurement system even having the analytic frequency range of 10 kHz only. A GaAs sample grown by molecular beam epitaxy is taken as an example to measure the excess carrier’s lifetime. It is concluded from the experiments that the lifetime of excess carriers in our sample is primarily caused by the indirect recombination via the deep trap centers within the band gap. The nonradiative lifetime is not really a constant and its variation versus the laser intensity indeed follows the prediction of the Shockley–Read–Hall recombination theory. This confirms the accuracy of our measurement.
Thermal stability of a Si/Si1−xGex quantum well studied by admittance spectroscopy
Authors: Lin Feng , Gong Da-wei , Sheng Chi , Lu Fang , Wang Xun
Published: July 2002

The thermal stability of a SiGe/Si quantum well grown by molecular-beam epitaxy is studied by using the admittance spectroscopy technique. The values of activation energies of hole emission from the subbands in the SiGe/Si quantum well are derived from the admittance spectra. After annealing the sample at different temperatures, the activation energy varies in different behaviors. There is no significant change of the activation energy after annealing at 700 °C for 40 min. At the annealing temperature of 900 °C, the decrease of the activation energy with annealing time could be attributed to the interdiffusion of Ge, Si atoms at the heterointerfaces and the strain relaxation effect. An unexpected phenomenon is observed at the annealing temperature of 800 °C, i.e., the activation energy increases with the annealing time. This extraordinary phenomenon is supposed to be caused by the change of the well potential shape due to the B out-diffusion from the well to the Si barrier.
Recoil implantation method for ultrashallow p+/n junction formation
Authors: Liu Henley L. , Gearhart Steven S. , Booske John H. , Cooper Reid F.
Published: July 2002

A recoil implantation technique is investigated for ultrashallow p+/n junction formation. In this method, a 3–35 nm thick B layer is deposited on the wafer by magnetron sputtering. Then a medium energy (10–40 keV) Ge implant drives the boron atoms into Si by means of ion beam mixing. The remainder of the boron film is chemically etched away prior to the annealing step. Sub-60 nm deep p+/n junctions with sheet resistance less than 1000 Ω/sq and test diodes with leakage current density below 2 nA/cm2 have been formed using this method.
Low temperature oxidation of SiGe in ozone: Ultrathin oxides
Authors: Madsen Jon M. , Cui Zhenjiang , Takoudis Christos G.
Published: July 2002

X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and spectral ellipsometry have been used to study ozone/atomic oxygen assisted direct oxidation of single crystalline SiGe. In situ Fourier transform infrared spectroscopy is used to monitor changes in the gas phase composition of O3/O2 gas mixtures. At all temperatures studied, 125–530 °C, XPS results indicate that Ge is incorporated into the growing oxide as GeO2. Increasing the oxidation temperature causes a decrease in the percentage of Ge incorporated in the growing oxide and an increase in the silicon content. Further, at oxidation temperatures of 400 °C and above, substantial Ge segregation is observed using SIMS. The change in composition is described using a temperature dependent flux model. Also, as the oxidation temperature increases, the oxide/SiGe interface becomes more abrupt with 1.9 monolayer (ML) of suboxide (Si+1, Si+2, Si+3) detected at 125 °C and 0.8 ML of suboxide at 530 °C. Based on thickness measurements from XPS and spectral ellipsometry, initial growth rates of 5 and ∼1 Å/min were achieved for ozone oxidation of Si0.85Ge0.15 at 530 and 125 °C, respectively. For the ultrathin regime (oxide thickness <25 Å), the effective activation energy for SiGe oxidation in 950 ppm of O3 is determined to be approximately 0.12 eV.
Elastic properties of a unidirectional SiCf/Ti composite: Acoustic-resonance measurements and micromechanics predictions
Authors: Ogi Hirotsugu , Dunn Martin L. , Takashima Kazuki , Ledbetter Hassel
Published: July 2002

Using the mode-selective contactless resonance ultrasound spectroscopy, we measured the complete elastic-stiffness tensor Cij and the corresponding internal-friction tensor Qij−1 of a silicon-carbide fiber unidirectionally reinforced Ti–6Al–4V-matrix composite. This material shows orthorhombic symmetry and possesses nine independent Cij and Qij−1. We independently excited and detected an unique vibration-mode group among the eight vibration groups of a orthorhombic rectangular parallelepiped. This provides a great advantage in identifying the vibration modes of the observed resonance peaks, whose frequencies yield the Cij by solving an inverse problem. To verify the resultant Cij, we also determined Cij by point-contact resonance ultrasound spectroscopy and pulse-echo methods. Then, using the composite Cij, we inversely calculated the transverse-isotropic fiber elastic stiffnesses with micromechanics theory. With the fiber Cij and the isotropic matrix Cij, we reconstructed the composite Cij as a function of fiber-volume fraction.
Micromagnetic study of dynamic switching constant in longitudinal thin film media
Authors: Torabi Adam F. , Zhou Hong , Bertram H. Neal
Published: July 2002

Dynamic switching time in longitudinal thin film media is studied by numerical simulation. An exponential head field with various rise times is assumed. A linear relation (with slope Sw) to correlate the inverse of switching time and the maximum reverse field is found to apply only for rise times larger than 0.3 ns. The switching constant Sw increases with increasing rise time, decreasing damping constant, and increasing intergranular exchange. A wide range of Sw, from about 250 to about 3500 Oe ns, is found for rise times between 0.05 and 1 ns.
Novel magnetic and heat capacity properties of the Cr(IV) diperoxides
Authors: Cage Brant , Leniek Rick , Dalal Naresh
Published: July 2002

This study presents, to our knowledge, the first observation of long range magnetic order in a simple Cr(IV) organo-metallic complex. Magnetic susceptibility and heat capacity results are given for an Cr(IV) ethylenediamine diperoxo complex, [Cr(En)(O2)2(H2O)]⋅H2O. For the investigated compound, the spin state of S=1 was determined from the analysis of susceptibility data above 20 K. Below this temperature the susceptibility was observed to deviate from Curie–Weiss behavior. We find evidence for long range ordering similar to ferromagnetism around 4.5 K, and the appearance of short range spin interactions at higher temperatures. The mechanism for the ferromagnetic like peak has tentatively been attributed to canted anti-ferromagnetism. Analysis of the heat capacity determined that the majority of the spin entropy is engaged at temperatures above 10 K.
Growth and magnetic properties of Fe films on vicinal to (001) substrates
Authors: Frank A. R. , Jorzick J. , Rickart M. , Bauer M. , Fassbender J. , Demokritov S. O. , Hillebrands B. , Scheib M. , Keen A. , Petukhov A. , Kirilyuk A. , Rasing Th.
Published: July 2002

We report on investigations of the crystallographic structure and the magnetic anisotropies of epitaxial iron films deposited onto periodically stepped Ag(001) surfaces using low energy electron diffraction, x-ray diffraction, second harmonic generation (SHG), as well as the Brillouin light scattering (BLS) technique. The focus of the present study lies on the interrelation between the surface morphology of the buffer layers and the magnetic properties of the Fe films, epitaxially grown onto them. Especially the symmetry breaking at the atomic steps is found to create an uniaxial magnetic anisotropy measured by BLS and a very strong anisotropic signal in magnetic SHG.
Annealed Co thin films: Pit formation and magnetic anisotropy
Authors: Shi H. , Lederman D.
Published: July 2002

We present a study of the morphological and magnetic transformation of hcp(0001) Co thin films, grown via sputtering on Al2O3(112̄0), which occurs when they are annealed in vacuum in the temperature range of 500 °C<TA<600 °C. In this temperature range, the films undergo a reentrant smoothening transition. For very thin films (thickness t<20 nm), this coincides with a fcc(111) to fcc(001) recrystallization, and with the appearance of rectangular pits. For thicker films, the recrystallization does not occur, even though the surface smoothens out leaving hexagonal pits behind. The number of pits per unit area decreases exponentially as the thickness of the film is increased, demonstrating that interactions at the film surface and film–substrate interface cause the smoothening and pit formation during the annealing process. Whereas the hcp(0001) and fcc(111) phases are magnetically isotropic in the film’s plane, the fcc(001) films exhibit a biaxial in-plane anisotropy.
Nanophase dispersed SmCo films with high remanence and corrosion resistance
Authors: Chen Li , Li Biao , Cadieu C. F. , Theodoropoulos T. , Cadieu F. J.
Published: July 2002

Nanophase dispersions of SmCo5 in either an Al or Co matrix have been synthesized by using pulsed laser deposition to deposit the SmCo aggregated grains while simultaneously sputtering either Al or Co. Smooth single phase type hysteresis loops were measured for both types of dispersed systems. SmCo5 plus Al dispersions were made that exhibited room temperature coercivities of 15 kOe with ≈5 at. % Al and decreased to 5.7 kOe with 21 at. % Al. The SmCo5 plus Al dispersions were only marginally affected by heating in air for 30 min to 300 °C. It was possible to make SmCo5 plus Co films with loop squareness of ≈0.83 and with room temperature coercivity ≈10 kOe by using a Co matrix.
Exchange coupling in Nd2Fe14B/Fe/Nd2Fe14B sandwich films and their magnetic properties
Authors: Yang C. J. , Kim S. W.
Published: July 2002

Sandwich magnetic films of Nd2Fe14B/Fe/Nd2Fe14B have been grown by a KrF excimer laser (λ=248 nm) ablation technique. Magnetic properties were characterized by varying the thickness and volume fraction of hard (Nd2Fe14B) and soft (Fe) magnetic layers. In the (y)nm[NdFeB]/(x)nm[Fe]/(y)nm[NdFeB]/(100)Si structure, the thickness (y) varied from 3.6 to 54 nm, and (x) from 15 to 112 nm. At (x)=15–23 nm where the volume fraction of Fe corresponded to 51–75%, the sandwich structure exhibited an enhanced Mr/Ms and Hci from the result of the exchange coupling between the magnetic layers. Experimentally calculated exchange constant (As) of As=2.5×10−10 J/m was estimated using the measured thickness (tcs) and reference crystalline anisotropy constant (Kh) of Nd2Fe14B crystal.
Microstructure and magnetic properties of the (FePt)100−xCrx thin films
Authors: Kuo P. C. , Yao Y. D. , Kuo C. M. , Wu H. C.
Published: July 2002

The (FePt)100−xCrx alloy thin films with x=0–16 at. % were fabricated on natural-oxidized Si(111) substrate by dc magnetron sputtering. The as-deposited films were annealed between 300 and 750 °C in order to transform the soft magnetic fcc γ-FePt phase to the hard magnetic fct γ1-FePt phase. The addition of Cr in the FePt thin films will reduce its saturation magnetization and coercivity, however, it could inhibit the grain growth during annealing of the samples. The optimum condition for high-density magnetic recording purpose of the (FePt)100−xCrx alloy films was found with x=5 at. %, annealing at 650 °C for 15 min, and ice water quench cooling. According to the transmission electron microscopy study, the average grain size in the annealed (FePt)100−xCrx alloy thin films decrease from 60 to 5 nm with increasing x from 0 to 16.
Simulation of ultrathin lubricant films spreading over various carbon surfaces
Authors: Vinay Stephen J. , Phillips David M. , Lee Young Sil , Schroeder Charles M. , Ma Xiaoding , Kim Min C. , Jhon Myung S.
Published: July 2002

The mathematical modeling of the dynamics of ultrathin perfluoropolyalkylether (PFPE) films, taking into consideration both the disk carbon surface composition and lubricant endgroup functionality, is described. Theoretical development based on the Monte Carlo method was employed to emulate experimental spreading data. In this model, we construct a system Hamiltonian based on a lattice-gas model by explicitly incorporating four classes of interactions: molecule/molecule, molecule/surface, endgroup/endgroup, and endgroup/surface, where a molecule is denoted as a backbone in the absence of endgroups. Spreading properties are investigated by tuning the lubricant interactions to model PFPE Z (without polar endgroups) and PFPE Zdol (with polar endgroups) on several surfaces. The simulations qualitatively describe the spreading profiles for molecules with and without polar endgroups. Acquired from N-frame animations, L-t plots are constructed and provide a qualitative comparison with the experimental data obtained from scanning microellipsometry.
Ultrathin diamond-like carbon coatings used for reduction of pole tip recession in magnetic tape heads
Authors: Scott William W. , Bhushan Bharat , Lakshmikumaran Anand V.
Published: July 2002

Diamond-like carbon (DLC) coatings were deposited using a commercial direct ion beam deposition technique on thin-film Al2O3–TiC inductive write heads. The coating thicknesses used were 5, 10, and 20 nm. Accelerated wear tests were conducted with metal particle tapes in a linear tape drive. Atomic force microscopy was used to image the thin-film regions to measure pole tip recession (PTR), relative wear of the pole tip with respect to the air bearing surface. It is found that the coating wears off of the head substrate to a significant extent in the first 1000 km of sliding distance. The coating is worn off the substrate long before it wears off of the thin-film region. The existence of the coating on the thin-film region provides close enough wear characteristics between the substrate and thin film that the two wear at similar rates. This results in little growth in pole tip recession. Early in the wear test, the coated substrate wears at a slightly higher rate than the DLC coated thin-film region due to the difference in tape contact pressure between the two materials; decreasing PTR is the result. As the coating on the substrate wears significantly, PTR begins to increase with sliding distance. Failure does not actually occur until the coating has worn off of the thin-film region. Near failure, the coating delaminates locally. Results indicate that coatings of 20 nm thickness may provide protection against PTR in future tape drives.
Mössbauer study on intergranular phases in the bcc-Fe/NdFeB nanocomposite alloys
Authors: Kobayashi T. , Yamasaki M. , Hamano M.
Published: July 2002

Magnetic properties of the intergranular phase in the bcc-Fe/NdFeB nanocomposite alloys containing Nb and Co were investigated with Mössbauer spectroscopy. The alloys were prepared with melt-spinning and post-annealing method. It is revealed that the intergranular phase is not amorphous as considered so far and consists of several thin layers outside the nanoscale grains of bcc-Fe and Nd2Fe14B. The hyperfine field in the layer on the surface of the grain is rather strong and the structure of this layer is similar to the grain. The field in the outer layers is reduced because the structure becomes indistinct gradually as the layer is apart from the grain. The high remanence and coercive field of the nanocomposite alloys may originate from the magnetic coupling between grains via the intergranular ferromagnetic phase.
Grain boundary structures in La2/3Ca1/3MnO3 thin films
Authors: Miller D. J. , Lin Y. K. , Vlasko-Vlasov V. , Welp U.
Published: July 2002

As with many other oxide-based compounds that exhibit electronic behavior, structural defects have a strong influence on the electronic properties of the CMR manganites. In this work, we have studied the effect of grain boundaries on the transport properties and on the local orientation of magnetization. Thin films of the perovskite-related La2/3Ca1/3MnO3 compound were deposited onto bicrystal substrates using pulsed laser deposition. Transport measurements showed an enhancement of magnetoresistance across the grain boundary. The structure of the boundary was evaluated by electron microscopy. In contrast with the highly meandering boundaries typically observed in bicrystals of high-temperature superconductors, the boundaries in these films are relatively straight and well defined. However, magneto-optical imaging showed that the local magnetization was oriented out of the plane at the grain boundary while it was oriented within the plane in the grains on either side. This coordinated reorientation of local magnetization near the grain boundary leads to enhanced magnetoresistance across the boundary in low fields.
Thickness and oxygen pressure dependent structural characteristics of BaTiO3 thin films grown by laser molecular beam epitaxy
Authors: Zhao Tong , Chen Fan , Lu Huibin , Yang Guozhen , Chen Zhenghao
Published: July 2002

A series of BaTiO3 thin films with various thicknesses from 10 to 400 nm were epitaxially grown under various oxygen pressures from 2×10−4 to 12 Pa on SrTiO3 (001) substrates using laser molecular beam epitaxy. Being confirmed by reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, and high resolution transmission electron microscopy the epitaxial single crystal BaTiO3 thin films are highly c-axis or a-axis oriented with a root-mean-square surface roughness of 0.14 nm. The observed thickness and oxygen pressure dependent structural characteristics of the BaTiO3 thin films are discussed by taking into account both the misfits in thermal expansion and lattice constants between BaTiO3 films and SrTiO3 substrates, and the effect of the energy of the sputtered particles, which is consistent with the established strain relaxation mechanism. An abnormal expansion of lattice volume of a BaTiO3 unit cell is found and attributed to the effect of oxygen vacancies in the BaTiO3 films.