bluuebunny/crossref_metadata_2025 huggingface dataset:
Giant magnetoresistance in evaporated NiFe/Cu and NiFeCo/Cu multilayers (abstract)
Authors: Zeltser A. M. , Smith Neil
Published: July 2002

The magnetic and transport properties of electron beam evaporated (Ni83Fe17/Cu)10 and (Ni66Fe16Co18/Cu)10 multilayers (ML) were studied as a function of the Cu spacer and magnetic layer thicknesses (tCu and tNiFe), annealing conditions and Ta buffer layer thickness. The ML were evaporated in a magnetic field at deposition rates ∼ 2 Å/s and background pressure <5×10−8 mbar on Si/SiO2 substrates at Ts=200 °C. These ML exhibited two unique features: (1) ΔR/R and the interlayer coupling did not show oscillatory behavior as a function of tCu; and (2) after magnetic post annealing, ΔR/R increased from <0.3% in the as-deposited state, to up to ∼6% and 7% in Ta/(NiFe/Cu) and (NiFeCo/Cu), respectively. The coupling between the NiFe layers changed from ferromagnetic in the as-deposited state Mr/Ms∼0.9k;20 to essentially antiferromagnetic Mr/Ms<0.2) after appropriate annealing, and the ML became virtually isotropic in-plane.

This is quite different from strong oscillatory behavior of giant magnetoresistance (GMR) previously reported in (NiFe/Cu) as-deposited ML made by ion-beam sputtering. After annealing at 300° and 325 °C for 2 h, the ΔR/R became ∼4.5% and ∼6.5% in (NiFe/Cu) and (NiFeCo/Cu) ML, respectively, and remained approximately constant for tCu=20 to 40 Å. The coupling field generally decreased with an increase in Cu and NiFe and after annealing at 300 °C dropped to as low as ∼25 and 45 Oe in (NiFe/Cu) and (NiFeCo/Cu) ML, respectively. The of ΔR/R Ta/(NiFe/Cu) ML increased with the thickness of Ta buffer layer from 30 to 70 Å. The high-angle θ–2θ x-ray scans of (NiFe/Cu) ML showed (111) texture, essentially independent of annealing temperature.

The low-angle x-ray diffraction did not reveal roughening of the Cu–NiFe interfaces as a result of annealing. In many respects the GMR behavior of these ML is similar to that reported in sputtered ‘‘discontinuous’’ NiFe/Ag. However, in contrast to the latter, the resistivity of NiFe/Cu monotonically increases with annealing temperature. This suggests that lattice interdiffusion is more prominent in the NiFe– system, consistent with a greater equilibrium compared solubility of Cu in the NiFe matrix compared to that of Ag. It is believed that Cu diffusion along the NiFe grain boundaries creates intra-layer magnetic discontinuity in NiFe and promotes inter-layer antiferromagnetic coupling between adjacent NiFe layers, which then gives rise to the observed GMR. Evaporated NiFe/Cu ML showed very small hysteresis and uniform GMR properties throughout the thickness, which makes them good candidates for GMR–DMR heads.
A new type of spin glass in spin-density-wave CrMn alloys (abstract)
Authors: Galkin V. Yu. , de Camargo P. C. , Ali Naushad , Schaf J. , Fawcett E.
Published: July 2002

The temperature dependence of the susceptibility χ(T)=M(T)/H of Cr1−xMnx(≪0.1<x<8% Mn) alloys in the range 5<T<400 K and in fields up to 55 kOe, measured with a SQUID magnetometer, and hysteresis of the magnetization M(H) show typical spin-glass (SG) behavior. After zero-field cooling (ZFC), χ(T) exhibits the low-T maximum typical of a SG, while cooling in the measuring field (FC) gives quite different behavior. In some cases, when measuring at low field, χ(5 K) is 10× larger in the FC state than in the ZFC state. The temperature of the irreversibility limit decreases with increasing field. All our CrMn alloys show nonlinear field dependence of M(H), with pronounced hysteresis and decay of the remanent M with time. On the other hand, these alloys exhibit properties that are essentially different from those of all other metallic spin glasses: (1) the linear scaling law based on the RKKY interaction between magnetic impurities in a typical SG is not obeyed, and indeed the temperature of the maximum in χ(T) is essentially independent of Mn concentration, which shows that formation of the SG state does not depend on the distance between the Mn atoms; (2) the maximum in χ(T) in the alloy containing only a trace of Mn (≪0.1%) is at about 40 K, a temperature at least an order of magnitude larger than that in CuMn and other metallic SG with about 0.1% impurity concentration; (3) χ(T) obeys a Curie–Weiss law above the Neel temperature, but not below, which shows that the Mn moment is frozen in the spin-density-wave (SDW) matrix

We propose a model to explain this unusual behavior, in which the SG state is formed, not as a result of frustration of the Mn impurity moments, but through the frustration of the moments of the itinerant electrons of the host Cr. At low temperatures the frozen Mn moments pin the SDW, which gives rise to frustration surfaces between adjacent domains having different phase of the SDW. This effect depends only on the T-dependent interaction between the Mn moment and its neighbors, and thus is independent of the Mn concentration.
Nonlinear susceptibility measurements at the spin-glass transition of the pyrochlore antiferromagnet Y2Mo2O7
Authors: Gingras M. J. P. , Stager C. V. , Gaulin B. D. , Raju N. P. , Greedan J. E.
Published: July 2002

We have measured the magnetic field and temperature dependence of the nonlinear dc susceptibility, χnl, of the frustrated pyrochlore antiferromagnet Y2Mo2O7 close to and above the temperature, Tg∼22 K, where this material exhibits irreversible (spin-glass like) magnetic behavior. Our results suggest that the observed irreversible magnetic properties in this material are due to a thermodynamic spin-glass transition signaled by a divergence of the nonlinear magnetic susceptibility coefficient χ3∼(T/Tg−1)−γ at Tg with γ=3.3±0.5. χnl shows two power-law behavior at Tg, χnl∼H2/δ, with δ∼2.8 for H<1000 Oe and δ∼4.1 for H≳2000 Oe. These values for γ and δ, as well as the crossover behavior of χnl at Tg, is consistent what is found in common disordered Heisenberg spin glass materials.
Second-order magneto-optic effects in anisotropic thin films (abstract)
Authors: Osgood R. M. , Clemens B. M. , White R. L.
Published: July 2002

The signal measured in a magneto-optic Kerr effect (MOKE) experiment is normally assumed to be proportional to the component of the magnetization (M) along a certain axis. We have observed significant effects from the M2 term in the magneto-optic response (i.e., in the rotation and/or ellipticity, which can be thought of as the real and imaginary parts of the same quantity) from films with in-plane magnetization and in-plane magnetic anisotropy. These films were: epitaxial Fe(110)/Mo(110) bilayers and multilayers, thin Fe films with ion-bombardment induced anisotropy, Co(110) thin films, and NiFe films with anisotropy induced with a field during deposition. In all of these films, the in-plane anisotropy created a coherently rotating magnetization (as evidenced by measurement of the component of the magnetization perpendicular to the applied field) that contributed a large M2 term to the magneto-optic response when an external field was applied close to parallel to the hard axis of the film. The M2 term in the magneto-optic response changed sign as H was rotated through the hard axis and reached a minimum when H was applied nearly parallel (±0.5°) to the hard axis, thus providing a sensitive indication of the location of the sample’s hard axis.
In situ and ex situ optical characterization of electro deposited magneto-optic materials
Authors: Hilfiker James N. , Glenn Darin W. , Heckens Scott , Woollam John A. , Wierman Kurt W.
Published: July 2002

Electrodeposition is being investigated as a novel and low-cost method to prepare magneto-optic thin film and nanostructured materials. This deposition method allows precise control over thin-film properties and permits deposition of novel magnetic geometries. Multilayers and alloys can be deposited and controlled by adjusting deposition potentials and ion concentrations in the bath. Nickel/cobalt alloys have been electrodeposited from sulfamate, sulfate, and chloride solutions onto Au substrates. The optical properties were monitored in situ with a real-time spectroscopic ellipsometer measuring simultaneously at 44 wavelengths in the 410–750 nm spectral range. In situ measurements have the advantage of determining the material microstructural properties (thickness, density, and roughness) before the films are oxidized in the air ambient. Ex situ variable angle spectroscopic ellipsometry measurements were taken over the spectral range from 205 to 1000 nm.
Modeling MFM images of periodic magnetization patterns (abstract)
Authors: Madabhushi R. , Gomez R. D. , Burke E. R. , Mayergoyz I. D.
Published: July 2002

Magnetic force microscopy (MFM) is a powerful technique that has gained wide acceptance in the imaging of magnetization patterns. However, MFM images are ‘‘raw’’ images, and they are not necessarily identical to the original images of the magnetization patterns. In an attempt to investigate the relationship between the raw images and the magnetization patterns, we present a theoretical analysis of the MFM images through numerical computations. In this way, we show the dependence of the images on both the orientation of the probe’s magnetic moment and the interaction between the probe moment and the sample magnetic fields. For a pure sinusoidal magnetization pattern, a vertical orientation of the MFM probe’s magnetic moment will yield images that follow the magnetic charge density distribution, while a horizontal orientation of the probe will yield MFM images that follow the magnetization patterns themselves. We investigate the deviation from this correlation between the images and the magnetization distribution when magnetization patterns are not purely sinusoidal. Computations are performed for periodic patterns with linear and arctangent transitions. To model the interaction of the tip with the sample’s magnetic fields, we model the tip as a Stoner–Wohlfarth particle. This allows us to investigate how the tip–sample interaction may distort the MFM image. Our modeling suggests that in general there is no direct correlation between the raw MFM image and the actual magnetization distribution. This makes a strong case for the implementation of image reconstruction techniques.
Magnetic and electronic properties of the magnetically ordered quasicrystalline alloys Al70–xPd15Mn15Bx
Authors: Yewondwossen M. , Koziol Z. , Bahadur D. , Yang Z. J. , Foldeaki M. , Dunlap R. A.
Published: July 2002

The magnetic susceptibility, magnetization and the temperature and field dependence of the Hall coefficient have been measured for the magnetically ordered icosahedral quasicrystals Al70–xPd15Mn15Bx (x=0,2,4,6,8,10). The temperature dependence of the real and imaginary part of the complex ac susceptibility indicates re-entrant magnetic behavior and dc magnetization measurements are interpreted in the context of this behavior. The present Hall resistivity measurement shows the existence of both normal and anomalous Hall effects. The normal Hall coefficient is independent of temperature in all the alloys and changes from negative to positive with increasing boron concentration. This can be explained by the effects of s–d hybridization. The anomalous Hall coefficient is also found to be temperature independent and has a compositional dependence correlated to the spin–orbit scattering rate.
High coercivity single-domain particles in glass matrix
Authors: Tsang C. , Gafney H. D. , Sunil D. , Rafailovich M. , Sokolov J. , Gambino R. J.
Published: July 2002

Impregnating transition metallic compounds into porous glass is a simple way to form ultrafine particles in a glass matrix. In our studies, ultrafine particles were prepared by impregnating Fe compounds into porous glasses (pore size 70±21 Å) and annealing. After annealing, the samples were studied at different temperatures with a vibrating sample magnetometer. In the annealed glasses two types of particles were formed: (a) superparamagnetic particles and (b) ferromagnetic particles with exceedingly high coercivity (Hc=13 kOe at 298 K). The ratio between superparamagnetic and high coercivity particles depends on the annealing temperature and time. In samples annealed at lower temperatures or at high temperature for short times, only superparamagnetic particles were found. In those annealed at high temperatures after certain times, only high coercivity particles were found. It was also found that the blocking temperature TB of the high coercivity particles depends on the annealing process. Moreover it was found that the Curie temperature of the high coercivity particles is below 520 K.
Field dependence of magnetic transitions for magnetic superconductors RNi2B2C (R=Dy, Ho)
Authors: Ku H. C. , Lin M. S. , Hsu Y. Y. , Shieh J. H. , You Y. B.
Published: July 2002

High field (up to 5 T) magnetization measurements were performed on both bulk and field-aligned powder samples of RNi2B2C (R=Dy, Ho) magnetic superconductors. For Tc=3.8 K, a DyNi2B2C superconductor with antiferromagnetic Néel temperature TN of 10–11 K, and anomalous field-induced weak-ferromagnetic transitions were observed above the superconducting upper critical field Hc2(2 K) of 1.75 kG. The aligned powders show distinct hysteresis characteristics for both fields applied parallel and perpendicular to the tetragonal basal plane. For Tc=7.8 K, HoNi2B2C has an incommensurate magnetic ordering temperature Tm of 6 K and TN of 5.2 K. Anomalous field-induced weak-ferromagnetic transitions were also observed below TN. No hysteresis can be detected for TN<T<Tm.
Rotational magnetic measurements of vortex pinning in polycrystalline superconductors (abstract)
Authors: Hasan M. K. , Park S. J. , Kouvel J. S.
Published: July 2002

Magnetization-vector measurements were made at various temperatures on polycrystalline disks of YBa2Cu3O7 and (Ba,K)BiO3 as each was rotated about its axis in a fixed field H along the disk plane. For hysteretic starting states, the vortex flux density vector B is found to bifurcate into a BR component that rotates rigidly with the sample and a BF component that stays at a fixed angle (θF) relative to H, thus turning frictionally relative to the sample, as seen earlier. With increasing H, BR decreases and BF increases in size, indicating a distribution in the strength of the vortex pinning torques. After BR has vanished, the frictional angle θF decreases rapidly. Thus, the quantity Hμ sin θF (μ being the quantized vortex moment), which equals the average pinning torque (τp) on each vortex, does not remain constant but diminishes with increasing H. This decrease of τp is consistent with the collective pinning phenomenon known as vortex bundling. At fixed H, τp diminishes rapidly with increasing temperature, reaching very low values well below Tc. These results are compared with those derived from critical-state model interpretations of conventional hysteresis loop data.
Domain structures at the cross sections of thin film inductive recording heads
Authors: Liu Francis H. , Tong Hua-Ching , Milosvlasky Lena
Published: July 2002

Using a scanning Kerr effect microscope with external ac-tickle field excitation, zigzagged and slanted intersections of domain walls were observed at the symmetry-axis cross sections of thin film inductive heads. These domain walls are believed to be the intersections of edge-closure domain walls, with the magnetization on the two sides of the walls (or lines) oriented in ‘‘head-to-head’’ or ‘‘tail-to-tail’’ configurations. They are thus slanted or zigzagged in order to spread the charges along the walls over a larger region, thereby reducing magnetostatic energy. This interpretation is further evident by the observation that upon applying a small dc bias field in the symmetry-axis direction, the slanted or zigzagged intersections of domain walls were observed to displace according to the field direction, and sometimes deformed to different configurations. Upon lapping the heads from their air-bearing surfaces, slanted domain walls at the sloped backgap cross sections were occasionally observed.
Anomalous time-induced curvature in Henkel plots based on the Preisach model
Authors: Mitchler P. D. , Dahlberg E. Dan , Wesseling E. , Roshko R. M.
Published: July 2002

We have used a finite-temperature version of the Preisach model, in which thermally activated switching events supplement those induced by an applied field ha, to calculate the magnetizing and demagnetizing remanences, ir(ha) and id(−ha), respectively, assuming a Preisach distribution that is Gaussian in both the coercive field hc and the shift field hs. Since T≠0, the time t that the magnetization is permitted to relax influences the shape of the hysteresis loop and the Henkel plots constructed from the remanences. If the effective time for relaxation is specific to a given branch of the cycle, due perhaps to the way the system reacts to a given experimental procedure, it is possible to generate Henkel plots with curvature suggestive of mean field interaction effects, even if no such effects are actually present, or with such extreme demagnetizing-like curvature that the plot actually crosses the nominal lower boundary id=−ir. Experimental instances of this behavior are discussed.
Engineering coercivities by dual ion beam sputtering for synthesizing soft permalloy-based spin-valve multilayers (abstract)
Authors: Gutierrez C. J. , Dail R. , Selestino R. , Tristan L. , Khater M. , Michel Steve
Published: July 2002

Thin permalloy (Ni80Fe20) single-layer films and multilayers were fabricated onto Ta-buffered oxidized silicon substrates using a prototype dual ion beam sputtering system (Commonwealth Scientific Corporation). The room temperature substrates were placed in a uniform in-plane magnetic field during growth (∼150 Oe) yielding a uniaxial magnetic anisotropy in the permalloy films (typically ∼6 Oe). An examination of single-layer permalloy films with thicknesses from 20–50 nm indicated that the presence of an ∼100 eV ion-beam assist beam focused directly onto the substrate during film growth consistently modified the coercive and remnant characteristics of the films. Typically, the nonion beam assist modified NiFe films exhibited a coercivity ∼1 Oe, while the ion beam assist modified films exhibit a slightly larger coercivity (∼1.5 Oe).

Based on this apparent influence of the assist ion beam treatment on the film coercivity, the fabrication of soft spin-valve multilayer structures exploiting differences in the ion beam treatment of chemically identical ferromagnetic layers seemed plausible. To test this idea ,two parallel series of multilayers composed of [2.5 nm permalloy]-[Cu (2.5 nm, 4 nm, or 5.5 nm)] or [0.3 nm Co-1.9 nm permalloy -0.3 nm Co]-[Cu (2.5 nm, 4 nm, or 5.5 nm)] were fabricated. In these 30 cycle multilayers, alternating ferromagnetic layers were subjected to a 150 eV assist ion beam treatment during growth (higher coercivity layer), or to NO assist ion beam treatment during growth (lower coercivity layer). VSM analysis revealed that the simplest uncoupled spin-valve multilayer configuration consisted of the [2.5 nm permalloy]-[4 nm Cu] multilayer with Hc1 ∼0.8 Oe and Hc2 ∼2.2 Oe, and the [0.3 nm Co - 1.9 nm permalloy - 0.3 nm Co] - [4 nm Cu] multilayer with Hc1 ∼ 3 Oe and Hc2 ∼ 15 Oe. Film analysis by low-angle x-ray diffraction and reflectivity, as well as the effect of the Co interface layer on GMR properties will also be reported.
Signals and nonlinearities in thin metal-particle tape
Authors: Wei Dan , Bertram H. Neal , Friedmann A.
Published: July 2002

A new type of metal-particle tape, whose thickness is thinner and whose particles are better oriented than traditional thick particulate media, is simulated and analyzed. Signals at high densities are much larger in the thin tape than in the traditional tape. Nonlinearities at high densities are dominated by transition width increases. Signals and nonlinearities in even thinner media are studied versus record gap length to investigate proper system parameters.
Spin configurations in VBr2 supported by uniaxal anistropy and quantum fluctuations (abstract)
Authors: Rastelli E. , Tassi A.
Published: July 2002

Small single ion anisotropy is believed to play an important role in selecting the plane where the spins lie in hexagonal anitferromagnets on the basis of the classical approximation. For instance, in VBr2 a small easy-axis single ion anisotropy is expected to force the spins in a plane containing the c axis and the classical approximation suggests that the spin configuration should be an ‘‘umbrella’’ configuration with axis along the external magnetic field perpendicular to the c axis. However, the classical expectation is questionable because the zero point motion energy supports spin configurations with the spins lying in a plane containing the magnetic field. We find that the ‘‘planar’’ configuration with the spins in the plane containing the c-axis and the external magnetic field is stable for interplane coupling and anisotropy sufficiently small. Since this is the case of VBr2, our results rise some doubts about the interpretation of experimental data in VBr2 when an external magnetic field is applied.
Nanocrystalline Fe-M-B-Cu (M=Zr, Nb) alloys with improved soft magnetic properties (abstract)
Authors: Makino A. , Bitoh T. , Inoue A. , Masumoto T.
Published: July 2002

We have recently succeeded in producing good soft magnetic properties and high saturation magnetic density (Bs) for nanocrystalline Fe-M-B (M=Zr, Hf, or Nb) ternary alloys prepared by melt spun and sputtered technic. The alloys have the mixed structure consisting of α-Fe particles with 10–20 nm in size embedded in a remaining amorphous phase. The nanocrystalline bcc Fe90Zr7B3, Fe89Hf7B4 and Fe84Nb7B9 alloy ribbons subjected to the optimum annealing exhibit high Bs of 1.5–1.7 T as well as high effective permeability (μe) of 22 000–32 000 at 1 kHz. The soft magnetic properties of the nanocrystalline Fe-M-B alloys were found to be improved by adding small amounts of Cu and by optimizing the chemical composition. The addition of Cu to the alloys decreases the bcc grain size. The excellent soft magnetic properties (high μe of 100 000 at 1 kHz combined with high Bs of 1.53 T) can be achieved in the region where small grain size as well as nearly zero magnetostriction are obtained, which is attained in the compositional range around Fe84Nb3.5Zr3.5B8Cu1. The soft magnetic properties can be further improved by preannealing before crystallization treatment, probably resulting from the decrease in the distribution of the grain size at the crystallized state. Consequently, the μe reaches the maximum value of 120 000 for the nanocrystalline Fe84Nb3.5Zr3.5B8Cu1 alloy.
Crystallization behavior of a mechanically alloyed amorphous Fe80Zr8B11Cu1 alloy
Authors: Zhang Jianqiang , Wu Bingyao , Wu Xiaohua , Wang Guiqin , Zhao Jiwan
Published: July 2002

An amorphous alloy of Fe80Zr8B11Cu1 has been prepared by mechanical alloying, milled for 72 h, and annealed in the temperature range from 673 to 1173 K. The crystallization behavior of this sample at different annealing temperatures has been investigated by x-ray diffractometry, electron microscopy, and Mössbauer spectroscopy. With the increase of annealing temperature, about a 15 nm bcc phase is formed in the amorphous base. Upon further increasing the annealing temperature to 1173 K, in addition to the metastable α -Fe (Zr,B) phase, a ZrB2 phase appears. Furthermore, the results from Mössbauer spectroscopy reveal that there are two different magnetic areas in these amorphous phases at different annealing temperatures.
Application of Co-based amorphous ribbon to a noise filter and a shielded cable
Authors: Ishii Osamu , Senda Masakatsu , Takei Koji , Koshimoto Yasuhiro , Mori Toshinori
Published: July 2002

This paper describes the application of a Co-based amorphous alloy ribbon to a common-mode noise filter in the 1 MHz–1 GHz frequency range. Its saturation magnetization, and relative permeability were 6000 G and 1450, respectively. The 10-μm-thick amorphous ribbon exhibits high loss characteristics. The resistance of a 2-mm-diam roll of amorphous ribbon (12.5 mm wide and 30 cm long) increases from 3 Ω at 1 MHz to 50 Ω at 350 MHz. It is confirmed that a filter 1/17th the volume of a conventional (ferrite core) filter exhibits the same noise attenuation. Moreover, the noise emission from a twisted pair cable is greatly reduced without any increase in the cable volume by employing the amorphous ribbon as a shielding material.
Aharonov–Bohm oscillations at finite temperature
Authors: Schlottmann P. , Zvyagin A. A.
Published: July 2002

The Aharonov–Bohm effect is the quantum interference of charged particles in mesoscopic rings enclosing a magnetic field. The wavefunction acquires a phase due to the field flux φ and gives rise to flux-dependent oscillations in persistent charge currents. The period and amplitude of the oscillations are associated with the properties of the Fermi surface of the elementary excitations. For systems with one Fermi surface the groundstate persistent current has the form of a saw-tooth. The temperature reduces the amplitudes of oscillation by smearing the Fermi surface. The amplitude of higher harmonics decreases faster with T than the fundamental one, changing the saw-tooth to a more sinusoidal form with much smaller amplitude. The controlling parameter is LT/zvF, where L is the length of the ring, vF is the Fermi velocity and z the dressed generalized charge. Our calculations are performed within the framework of Bethe’s ansatz.
Magnetic properties of Zn–Mg ferrite fine powders (abstract)
Authors: Hamdeh H. H. , Ho J. C. , Oliver S. A. , Willey R. J. , O’Shea M. J.
Published: July 2002

Several fine powder samples of ZnxMg1−xFe2O4, 0≤x≤1, were prepared by the supercritical sol-gel (aerogel) process, and were annealed at 500 °C for 2 h. The structure, crystallite size and inversion parameter were determined by x-ray diffraction. The crystallite diameter of the as-prepared powders ranged from 6.5 to 8.5 nm. The minimum size was achieved for the samples with 0.25≤x≤0.50. After annealing the crystallite size increased by an average of 3 nm. The values of the inversion parameters, before and after annealing, reflect roughly the chemical preference of Zn cations to the tetrahedral sites and Mg cations to the octahedral sites of the spinel structure. However, these values indicate that the as-prepared materials were not in the equilibrium state. The magnetic properties were studied by 57Fe Mössbauer spectroscopy over a temperature range of 25 K to room temperature. Taking advantage of their sensitivity to the superparamagnetic behavior induced by the size effects, the Mössbauer spectra were used to determine the blocking temperatures. The calculated values peaked at x=0.50 for both the as-prepared and annealed powders. SQUID magnetometry measurements are currently underway.
Deposition condition and thickness dependence on magnetic properties of sputtered NiFeCo thin films
Authors: Tsoukatos A. , Gupta S. , Kim Y. K.
Published: July 2002

The soft magnetic properties of polycrystalline NiFeCo films have been investigated in the thickness range of 20–2400 Å. The films were deposited by dc magnetron sputtering, at 5 Å/s, from a Ni66Fe16Co18 high purity target, and under an applied magnetic field of 85 Oe. The effects of deposition pressure on the film crystal structure and magnetic properties have been investigated via x-ray diffraction and magnetic measurements. Both easy and hard axis coercivity values decrease significantly as the deposition pressure was changed from 7 to 3 mTorr, with the corresponding effects observed in the magnetoresistance and anisotropy field values, respectively. The magnetic data at 3 mTorr showed a nonlinear behavior at thicknesses below 1000 Å, while at 7 mTorr the nonlinearity is present in both the low and high end thicknesses.
The magneto-optical and optical properties of ultrathin Fe films in the short-wavelength range
Authors: Zhou Shi-Ming , Chen Liang-Yao , Wang Ya-Dong , Wang Yu , Zhu Wei-Rong , Zheng Yu-Xiang , Jin Qing-Yuan , Qian You-Hua , Shen Xiao-Liang , Xia Hui
Published: July 2002

The magneto-optical and optical properties of ultrathin Fe films sandwiched between Au layers were studied. A pronounced Kerr rotation peak is observed respectively in the photon energy range of 3.6–3.8 eV when the Fe layer is thinner than 1.5 nm, and in the range of 3.4–3.5 eV for thicker Fe layers. In simple calculations, the Kerr rotation peak is demonstrated to originate from the variation of the off-diagonal element in dielectric tensor of the Fe layers. The relative peak height Δθk, the normalized Kerr rotation θkn, and the effective optical constants at the photon energy of 3.5 eV are found to oscillate as a function of dFe.
Cathodoluminescence study of ArF excimer laser-induced planarization of large grain diamond films
Authors: Cremades A. , Piqueras J. , Solís J.
Published: July 2002

Planarization of large grain diamond films has been induced by 193 nm excimer laser irradiation. Secondary emission images and cathodoluminescence (CL) in the scanning electron microscope have been used to characterize the irradiated area. Irradiation causes changes in the structure of defects involving nitrogen and vacancies. Evolution of the CL signal with the number of pulses indicates that the luminescence intensity tends to stabilize when a smooth film surface is obtained.
Impurity band conductance through oxygen vacancy donor states in bismuth iron molybdate
Authors: Sears W. M. , McIntyre S. M.
Published: July 2002

Under chemical reduction at 620 K in an atmosphere of methanol vapor in a nitrogen carrier gas, bismuth iron molybdate [Bi3FeO4(MoO4)2] will give up lattice oxygen and form highly mobile vacancies. These vacancies act as donors and sit about 0.4 eV below the conduction band edge: The band gap is 2.7 eV. Because of the ability to remove up to 1% of the lattice oxygen and still maintain crystallographic stability, it is relatively easy to produce high donor carrier densities (up to 1020 cm−3). Under these conditions, and taking into account that this is a wide gap semiconductor with well compensated deep donor levels, it is possible to measure an electrical conductivity dominated by carrier movement through the impurity band. Both phonon assisted hopping and the transition to metallic impurity conduction have been observed. The crossover temperature from conduction dominated by the impurity band to conduction dominated by the conduction band was seen to increase from 125 to 380 K with increasing oxygen depletion.
A magnetic, neutron diffraction, and Mȯssbauer spectral study of the Ce2Fe17−xAlx solid solutions (abstract)
Authors: Mishra Sanjay R. , Long Gary J. , Pringle O. A. , Hu Z. , Yelon W. B. , Middleton D. P. , Buschow K. H. J. , Grandjean F.
Published: July 2002

The magnetic properties of a series of Ce2Fe17−xAlx solid solutions with x equal to 0.0, 0.88, 2.06, 2.80, 3.98, 5.15, 6.08, 7.21, 8.20, 9.08, 9.84, and 10.62 have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. Neutron diffraction data indicate that the compounds all crystallize in the rhombohedral Th2Zn17-type structure. The aluminum atoms are excluded from the 9d site and show a distinct preference for the 6c site for x greater than 6. The substitution of aluminum leads to an expansion of the a and c axis by 0.5% and 0.4% per aluminum atom. The unit cell volume increases by approximately 1.4% per aluminum atom. The magnetic moment per formula unit, measured at 295 K, shows very little change for x less than or equal to 4, but decreases rapidly with increasing aluminum content for higher values of x, indicating that aluminum acts as a magnetic hole at the lower concentrations. The Curie temperature increases from 238 K in Ce2Fe17 to a maximum of 384 K in Ce2Fe14Al3. The Ce2Fe17−xAlx solid solutions behave as spin glasses for x greater than 7.

The Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments in terms of a maximum hyperfine field, Hmax, for an iron atom with zero aluminum near neighbors, and a decremental field, ΔH, per aluminum near neighbor. Mössbauer spectral results indicate both that the samples are ferromagnetically ordered in the basal plane for x values between 0.2 and 7 and that a magnetic phase transition occurs from helimagnetic in Ce2Fe17 to ferromagnetic in Ce2Fe16.8Al0.2. However, at 295 K ferromagnetic ordering is observed only for x values between 2 and 5. The average value of Hmax decreases by about 25 kOe per aluminum atom. The isomer shift increases with aluminum content, an increase which can be explained by the screening of the 4s conduction electrons by the 3d band electrons. ΔH decreases by about 1.0 kOe per aluminum atom, a decrease which can be explained by the indirect exchange interaction between iron atoms modified by the presence of non-magnetic aluminum atoms through the 4s conduction band. Note added in proof: A full account of this work has been published in J. Appl. Phys. 79, 3145 (1996).
Structural and magnetic studies of heteromagnetic multilayers by reflectivity of circularly polarized soft x rays (abstract)
Authors: Idzerda Y. U. , Chakarian V. , Gutierrez C. J. , Prinz G. A. , Kao C.-C. , Park J.-H. , Meigs G. , Chen C. T.
Published: July 2002

The reflectivity of circularly polarized (CP) soft x rays is a technique which combines the power of x-ray scattering and magnetic circular dichroism and allows the structural and magnetic characterization of heteromagnetic multilayers. In this technique, the energy and angular dependence of the differential reflectivity of left- and right-circularly polarized soft x rays at the relevant absorption edges of the constituent elements of the multilayers are measured. As examples the reflectivity curves for single films of Fe, Co, ordered and disordered Fe50Co50 alloys, and for a trilayer consisting of two single crystal Fe30Co70 films separated by an 8.7 Å Mn spacer layer media utilizing complex dielectric tensors with no free parameters show that the rich reflectivity curves can be used to determine the multilayer magnetic ordering, film thicknesses, interface quality, as well as differentiating between ordered and disordered alloys.
Surface filamentation in semi-insulating silicon
Authors: Gradinaru G. , Sudarshan T. S.
Published: July 2002

A new field limit of silicon devices stressed at pulsed high fields is proposed, namely, the threshold field for the onset of the phase-shifted current response of the device. While the classical limit by the breakdown of the semiconductor-dielectric system, generally by surface flashover, causes a total damage of the semiconductor, the new limit prevents any damage of the device, even at high applied fields. Depending on the overall quality of the device under test, catastrophic failure of the device can be avoided even at large fields of ∼70 kV/cm by not exceeding the threshold for the onset of the phase-shifted current response, the proposed new field limit. Beyond this limit the device response is unstable and a surface filament, distinctly different from the surface flashover tracks, may appear on the device surface, permanently degrading its quality. Experimental results are presented supporting the new predamage high-field limitation. Scanning electron microscopy and optical micrographs are presented for a clear description of the nature of the damage. The present results are discussed using a recent physical model of prebreakdown and breakdown phenomena in high-field semiconductor-dielectric systems [G. Gradinaru and T. S. Sudarshan, J. Appl. Phys. 73, 7643 (1993)].
Fractal analysis of atomic force microscopy pictures of slip lines on a GaAs/GaAlAs heterostructure plastically deformed to obtain quantum wires
Authors: Ressier L. , Voillot F. , Goiran M. , Peyrade J. P. , Vieu C.
Published: July 2002

A fractal analysis of slip lines on the surface of a plastically deformed GaAs/GaAlAs heterostructure has been performed using atomic force microscopy. The high resolution of atomic force microscopy allowed us to analyze a very large interval of magnifications ranging from 1100 to 1 110 000. The fractal behavior of the slip line pattern was only observed in a decade of magnification. At the highest magnifications (40 000–1 110 000), the less fractal tendency of the slip line pattern confirms that the slipping of dislocations is an efficient method to cut a quantum well into quantum wires with a ‘‘natural’’ reasonable degree of homogeneity.
Laser-induced crystallization phenomena in GeTe-based alloys. III. GeTeSe alloys for a CD compatible erasable disk
Authors: Jongenelis A. P. J. M. , Coombs J. H. , van Es-Spiekman W. , Jacobs B. A. J.
Published: July 2002

A novel multipulse laser technique has been used to study the crystallization behavior of the GeTeSe ternary system, and the dependence of the nucleation and growth rates on composition has been related to the optical and structural properties. We show that the Se content is dominant in determining both the nucleation rate and the amorphous optical constants, while the crystal-growth speed depends primarily on the single or multiphase nature of the resultant crystalline state. In this respect compositions on the GeTe-GeSe line are unique in that they are single-phase for a Se content up to 25%. This characterization of both the optical and crystallization behavior has allowed us to identify materials suitable for use in a CD-compatible erasable optical disk based on the phase change principle, in which local amorphous regions in a crystalline film represent data bits. Readout compatibility with a CD imposes extremely stringent conditions on the reflection from the amorphous and crystalline states as well as the crystallization rate, which previous materials had been unable to satisfy. We present optical recording data on a disk using a composition on the GeTe-GeSe line, Ge50Te25Se25, in a five-layer stack, and show that the criteria for a fully CD-compatible erasable disk are fulfilled.
Mode reduction, Q loss, gain saturation, and bandfilling modification of the light versus current characteristics of thin (∼0.9 μm) quantum well heterostructure lasers
Authors: Evans P. W. , Holonyak N. , Ries M. J. , Chen E. I. , Minervini A. D.
Published: July 2002

Data are presented demonstrating the effect that cavity length, and thus Q, has upon quantum well heterostructure edge-emitting laser diodes that are reduced to microcavity thickness. The lasers, with reduced mode density and enhanced spontaneous emission, are defined vertically by a Ag top-contact mirror and a closely spaced (∼0.9 μm) high-contrast AlAs native oxide-GaAs distributed Bragg reflecting bottom mirror. For shorter and shorter diode lengths (700→70 μm, and still lesser mode density) the light versus current (L–I) characteristic below threshold is at first steeper and steeper (amplified stimulated emission), until, at a diode length of ∼100 μm, the loss in Q and insufficient gain are manifest as a downward bend in the L–I curve and a shift to higher threshold current where bandfilling to a higher state (shorter wavelength) contributes more gain.
Variation of cobalt silicide resistivity with temperature
Authors: Naem A. A. , Chee L. Y.
Published: July 2002

The variation of cobalt silicide film resistivity in response to the operating temperature and current level was investigated. The silicide films were formed on various sizes (1×200 μm or 2×200 μm) of n-type polycrystalline silicon resistors. Two different silicide thicknesses were used: 800 Å and 1200 Å. The silicide film resistivity was characterized at various temperatures ranging from 15 °C to 195 °C. At each temperature step, samples were subjected to a sweep of increasing current density, in the range of 1–15 mA for 1 μm and 1–30 mA for 2 μm wide samples, to evaluate the joule-heating effect. The behavior of cobalt silicide was similar to that of titanium silicide where resistivity increases linearly with temperature. Empirical models that can accurately estimate the resistivity and temperature coefficient of resistivity at a certain ambient temperature and operating current level were developed. The models were experimentally verified over a wide range of temperatures and current levels.
High resonance frequency of push-pull distributed feedback lasers
Authors: Chen Jianyao , Maciejko Roman , Makino Toshihiko
Published: July 2002

Using spatially dependent multimode rate equations, we perform the small signal analysis of the push-pull distributed feedback laser. A closed form expression for the amplitude modulation response has been obtained. It shows explicitly that the resonance frequency of push-pull distributed feedback lasers is determined by the spacing between the lasing mode and its closest antisymmetric side mode which has design implications.
Effect of exchange anisotropy on susceptibility of one-dimensional spin 1/2 ferromagnets
Authors: Liu Ying , Drumheller John E.
Published: July 2002

We have studied several quasi-one-dimensional spin 1/2 ferromagnets ranging from Heisenberg to Ising anisotropy to test the Johnson and Bonner prediction that the low temperature thermodynamic behavior of these systems in external fields depends on the exchange anisotropy and that there are certain ranges of the anisotropy and fields in which bound magnons or spin waves dominate thermodynamic properties. The experimental results confirm that for the samples tested bound magnons dominate susceptibility across available range of the anisotropy (γ<1.05) and for fields up to 9 T near the Heisenberg limit.
Thermal conductivity of Bi2Sr2CanCun+1Ox bulk superconductors in high magnetic fields (abstract)
Authors: Nakamae S. C. , Schwartz J.
Published: July 2002

The study of the thermal conductivity of high temperature superconductors can provide a theoretical understanding of the nature of the charge carriers, phonons, and scattering processes between them. From the technological point of view, thermal conductivity influences the growth rate of a local hot spot in a superconductor and therefore it is an important parameter in thermal stability analysis. Recently, the magnetothermal conductivity has been considered to be a key factor in the understanding of the vortex mechanisms. Here we report thermal conductivity measurements of bulk Bi2Sr2CanCun+1Ox superconductors in an applied magnetic field up to 20 T. The measurements are taken with field directions both parallel and perpendicular to the a-b plane of the samples for a wide range of temperatures. The phonon scattering mechanism at vortex sites, thermal transport mechanism in both a-b and c directions, as well as across grain boundaries will be discussed.
Grating coupled intersubband transitions in microcavities
Authors: Duboz J. Y.
Published: July 2002

Microcavity effects can apply to intersubband transitions if the polarization selection rules are satisfied. We thus propose to insert a grating in a planar microcavity and to take advantage of both the field enhancement due to the cavity and the coupling with intersubband transitions due to the grating. The electromagnetic coupling is reinforced when both the grating and the cavity are resonant with the light wavelength. Accordingly, the spectral width is reduced where the coupling is efficient, which may provide additional advantages for some applications. Various configurations are envisioned and calculations are carried out for dielectric and metallic gratings. These architectures can be used for infrared detection, emission or modulation. Efficient and realistic devices are proposed and optimized.
Theory of operation of high temperature Josephson fluxon-antifluxon transistor
Authors: Terzioglu Esin , Beasley M. R. , Zhang Y. M. , Berkowitz S. J.
Published: July 2002

We provide two qualitative models for the operation of Josephson fluxon–antifluxon transistor based on the pendulum model and image currents induced in the junction. This device, which is a variant of a Josephson vortex flow transistor, utilizes high temperature superconductor long Josephson junctions with a control line on top of the junction. Our models are consistent with the experimental observation that the coupling efficiency increases with this geometry. We also provide a quantitative model using numerical simulations to confirm the static and dynamic characteristics predicted by the intuitive models.
Cholesteric liquid-crystal displays illuminated by diffuse and partially diffuse light
Authors: St. John W. D. , Lu Z.-J. , Doane J. W. , Taheri B.
Published: July 2002

We report on the photometric and colorimetric properties of surface and polymer network stabilized reflective cholesteric displays. Both diffuse and partially diffuse illumination are used, the latter being an experimental approach to emulating typical room light conditions. It is shown that addition of polymer increases the field of view while decreasing angular dependence of the color quantities: hue, chroma, and lightness. Total luminance and contrast ratio, however, are also decreased. Therefore, it is concluded that optimum polymer concentration is dependent on the viewing geometry. Luminance and contrast ratio of the surface stabilized cells exceeded that of polymer stabilized cell when viewed in a geometry void of specular reflection. Colorimetric quantities in surface stabilized cells are less sensitive to illumination geometry. This suggests that for displays in which specular reflection has been suppressed, surface treatment represents the best method of stabilization. If the viewing angle allows specular reflection, polymer stabilization yields the largest luminance and contrast ratio. This behavior is explained in terms of angular distribution of helical axes due to presence of the stabilizer.
Growth of iodine-doped ZnS0.07Se0.93 disordered alloys and electron mobility enhancement by ordered structures in (ZnS)3(ZnSe)42
Authors: Fujiwara Hiroyuki , Kiryu Hideaki , Ii Toshihiro , Shimizu Isamu
Published: July 2002

Carrier transport properties of (ZnS)3(ZnSe)42 ordered and ZnS0.07Se0.93 disordered alloys are studied. Iodine-doped ZnS0.07Se0.93 was grown by hydrogen radical-enhanced chemical vapor deposition at a low temperature of 200 °C. These iodine-doped ZnS0.07Se0.93 are characterized by sharp x-ray diffraction peaks and dominant blue band-edge emissions in photoluminescence spectra. It was found that (ZnS)3(ZnSe)42 ordered alloys show higher electron mobility over 100 cm2/V s than that of ZnS0.07Se0.93 disordered alloys at a low electron concentration of ∼1016 cm−3, even though these crystals have the same sulfur composition and crystallinity. This mobility enhancement in the ordered alloys is attributed to the elimination of ‘‘disorder scattering’’ originated from random atomic fluctuations in the disordered alloys. Carrier scattering mechanisms and donor activation processes in the ordered and disordered alloys were further determined by a temperature dependence of Hall measurement.
Photoconductivity of photorefractive Sr0.61Ba0.39Nb2O6:Ce crystals at high light intensities
Authors: Simon M. , Buse K. , Pankrath R. , Krätzig E. , Freschi A. A.
Published: July 2002

A method is presented to perform direct photoconductivity measurements of Sr0.61Ba0.39Nb2O6:Ce crystals at high intensities: A high voltage is applied to the sample and subsequent illumination yields a photocurrent through the crystal which charges a capacitor. Finally an electrometer measures the voltage of this capacitor. From voltage, capacity and exposure time the photoconductivity is calculated. Measurements with light pulses of a Q-switched frequency-doubled Nd:YAG laser (pulse duration 20 ns, light wavelength 532 nm) are performed. The photoconductivity σph increases sublinearly with light intensity I according to σph∝Ix with x<1. The experimental results point to Ce3+/4+ as the dominant photo- refractive center in Sr0.61Ba0.39Nb2O6:Ce at high intensities.