bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic and mechanical properties of consolidated gas atomized and hydrogenation, disproportionation, desorption, and recombination Nd2Fe14B powders (abstract)
Authors: Horton J. A. , Heatherly L. , Sellers C. H. , Branagan D. J. , Ragg Oliver , Harris I. R.
Published: July 2002

Magnetic and mechanical properties of consolidated (hot pressed) gas atomized powders of Nd2Fe14B with TiC additions will be reported and the results correlated with microstructure. The fracture toughnesses obtained will be compared with similar tests on hot pressed hydrogenation, disproportionation, desorption, and recombination (HDDR) material, made by the HDDR process and with previously reported results on a variety of commercially available materials. TiC additions, by controlling grain growth, aid the gas atomization process and should result in better processability of consolidated magnets. Improvements in the fracture toughness and processability may lead to the use of the Nd2Fe14B magnets in special situations where it is necessary for the magnet to be highly stressed. For example, in electric vehicle drive motors, high rpm and small clearances preclude the use of supporting elements and result in high tensile forces on the magnet.
Interface anisotropy and magnetization reversal in epitaxial Fe(001)/Pd double layers
Authors: Demokritov S. O. , Mathieu C. , Bauer M. , Riedling S. , Büttner O. , Hillebrands B. , de Gronckel H.
Published: July 2002

Static and dynamic magnetic properties of epitaxial Fe(001) films with Pd overlayers have been measured by means of Brillouin light scattering and magneto-optic Kerr effect magnetometry. From low energy electron diffraction studies, the initial growth of Pd [dPd<2 monolayer (ML)] is identified as a self-formation process of quasione-dimensional Pd structures. Auger electron spectroscopy examinations proved that for dPd⩾2 ML the Pd overlayers cover the Fe films almost completely. The measured frequencies of the dipolar spin wave mode and the derived anisotropy constants show a linear change with increasing dPd up to 5 ML with following saturation. The M(H) curves show pronounced anomalies in the coercive field for dPd<2 ML accompanied by large tails in the hysteresis loops. The results are interpreted as a strong pinning of domain walls caused by large local changes in the surface anisotropy due to the inhomogeneous Pd cover layer in this regime.
59Co nuclear magnetic resonance studies of interfacial defects in molecular beam epitaxy grown Co/Cu(111) multilayer
Authors: Thomson T. , Riedi P. C. , Wellock K. P. , Hickey B. J.
Published: July 2002

59Co NMR and magnetotransport measurements have been undertaken on a series of molecular beam epitaxy grown Co 22 Å/Cu 32 Å(111) multilayers as a function of the number of repeat units in the multilayer stack. Examining the positions of the bulk [Co surrounded by 12 Co nearest neighbors (NN)] and ideal planar interface (Co surrounded by 9×Co and 3×Cu NN) lines showed that increasing the number of repeat units leads to greater strain in the Co layers indicating better epitaxial registry. Interfacial mixing was studied by examining the ratio of the relative signal strength of the bulk and ideal planar interface lines compared to the signal outside these regions. The relative intensity of the NMR signal outside the bulk and ideal interface lines clearly reduces as the number of repeat units is increased. This demonstrates that in these samples the interfacial quality improves as the number of repeat units is increased. Magnetotransport measurements show that the improvement in structural quality could be correlated with an increase in magnetoresistance ΔR/R.
Theory of far-infrared attenuated total reflection from surface magnetic polaritons in rare-earth superlattices
Authors: Albuquerque E. L.
Published: July 2002

We present a theory for the attenuated total reflection (ATR) spectra to probe surface magnetic polaritons that propagate in semi-infinite superlattices made up of a rare-earth material intercalated by a nonmagnetic layer. We consider both the spiral and the cone states of the rare-earth constituent, apart from its simple ferromagnetic (antiferromagnetic) alignment due to the nearest (next-nearest) exchange coupling. We use the framework of the effective medium theory to determine the magnetic polariton’s dispersion relation, since it is very suitable to explain experimental observations of excitations in the far-infrared. Numerical results are presented to illustrate the ATR spectra, with emphasis in the surface polariton’s nonreciprocal aspect shown in the cone state of the rare-earth material.
Lithium vanadium bronze thin films for electrochromic applications
Authors: Green Mino , Pita K.
Published: July 2002

Thin films of fine grained polycrystalline stoichiometric vanadium pentoxide (85% of bulk density) have been prepared by vacuum evaporation. These films have been made into lithium vanadium bronze, LixV2O5, by inserting lithium, either electrochemically or chemically. In addition, lithium vanadium bronze thin films have been prepared by co-evaporation of Li and V2O5. The optical properties, absorption and refractive index, have been measured from 2000 to about 200 nm. Strong absorption arises from indirect transitions across the main gap and is ≈2.2 eV for x=0. The variation of the energy gap up to an x value of 2 has been obtained from the absorption data and the importance of irreversible phase changes noted. High x value bronze is useful as a counter electrode material in glazings, having an energy gap of about 3 eV. There is a considerable, technologically significant, band tail in the absorption spectrum thought to arise from polaronic-type states, perhaps modified by the presence of guest species ions. The optical behavior upon lithium electrochemical insertion has also been examined. It is found that nonstoichiometry gives rise to an overall reduction in optical change per guest atom inserted. These effects are fast compared with electrochemical insertion times.
Magnetothermal conductivity of Bi2Sr2CaCu2Ox bulk superconductor in high magnetic fields
Authors: Nakamae S. C. , Schwartz J.
Published: July 2002

The magnetothermal conductivity of high-temperature superconductors (HTS) reflects the scattering processes between the charge carriers, phonons, and vortices. Therefore, the magnetothermal conductivity can provide information about the vortex dynamics. Here, we report thermal conductivity measurements of bulk Bi2Sr2CaCu2Ox superconductors in an applied magnetic field up to 20 Tesla. The measurements were taken over a wide range of temperatures around Tc. The superconducting parameters were successfully recovered from the obtained data using the theoretical model developed by Ausloos and Houssa [J. Phys. Condens. Matter 7, 193 (1995)], in which the change in thermal conductivity in the applied magnetic field is predominantly due to the scattering between the vortex lines in the superconductor and the normal electrons in the Cu–O planes.
Magnetism, charge ordering, and metal–insulator transition in the lanthanum manganites (abstract)
Authors: Mishra S. K. , Pandit R. , Satpathy S.
Published: July 2002

We examine the magnetic and structural properties of the lanthanum manganite-based double-exchange magnets exhibiting colossal magnetoresistance. A model Hamiltonian containing the double-exchange, superexchange, and the Hubbard terms, with parameters obtained from density–functional calculations (Ref. 1), is studied within a mean-field approximation both at temperature T=0 and T>0 and with the effects of the magnetic field included. The phase diagrams we obtain with magnetic and charge-ordered phases enable us to examine the competition between the double- and superexchange terms as functions of doping and temperature. Our theoretical study provides a qualitative understanding of the phase diagram observed in the experiments.
Magnetic switching in ultrashort field pulses (abstract)
Authors: Back C. H. , Weller D. , Heidmann J. , Mauri D. , Garwin E. L. , Siegmann H. C.
Published: July 2002

The Ginzburg–Landau–Lifshitz (GLL) equation, which describes the time dependence of spin precesssion in an external magnetic field1 relates the minimal field required to reverse the magnetization at fixed pulse length to the anisotropy field of the sample.23 We have systematically varied this parameter between 1.3 and about 5.0 T in a series of perpendicularly magnetized Co/Pt multilayer films and studied the magnetization reversal in picosecond in plane field pulses. Such pulses of several Tesla field strength and ultrashort duration were obtained in the final focus test beam section of the Stanford Linear Accelerator Center. The resulting magnetization pattern, which is reminiscent of the field during exposure, is subsequently analyzed with Kerr microscopy2 (see Fig. 1). As a prominent feature, we observe a beam field related switching radius from up to down magnetization which we compare to the theoretically expected field within the GLL formalism.
Effect of hydrogen termination on the work of adhesion between rough polycrystalline silicon surfaces
Authors: Houston Michael R. , Howe Roger T. , Maboudian Roya
Published: July 2002

A novel micromachined test structure has been used to measure the work of adhesion between polycrystalline silicon surfaces. The effects of several surface treatments, including a hydrogen- and an ammonium-fluoride-induced hydrogen termination and a hydrogen peroxide chemical oxidation, have been investigated with these test structures. A reduction in the average apparent work of adhesion by a factor of 2000 has been observed on the NH4F-treated surface compared to the oxide-coated surface. By using x-ray photoelectron spectroscopy and atomic force microscopy, the observed reduction is traced to the combined effect of the surface chemistry and topography. This work demonstrates that a hydrophobic, rough surface provides a significant reduction of the apparent work of adhesion in polysilicon micromachined devices.
Ising model on the Cayley tree: Finite size versus finite field (abstract)
Authors: Stosic Borko D. , Stosic Tatijana
Published: July 2002

Roughly every two decades it turns out that our understanding of the ferromagnetic Ising model on the Cayley tree is still not complete.1 Only recently new progress was made2 by recognizing glassy behavior of finite (but large) lattices, shedding new light on this already classical problem. In this work we complement the results of Ref. 2 by presenting evidence of the fine interplay between finite size and “finite field” effect on this model, where both the thermodynamic limit and the zero field limit should be taken with extreme caution. We first derive analytical closed expressions for zero field magnetization and susceptibility for a tree of arbitrary generation. We obtain the same expression for magnetization as in Ref. 2, and a new expression for susceptibility that has not yet been reported in literature. It is shown that although the spontaneous magnetization remains zero in the thermodynamic limit for all finite temperatures, systems with the number of spins comparable to the estimated size of the universe retain spontaneous magnetization up to rather large temperatures. Similarly, the susceptibility of very large systems is “practically divergent” at quite large temperatures. We then turn to the numerical analysis of the “finite field” effect. It is shown that even in the thermodynamic limit the system in inconceivably small fields displays nontrivial behavior. The exact real space renormalization equations are extremely sensitive to field, so that special precautions regarding the precision of numerical calculations need to be taken into account. The above nontrivial behavior of large but finite systems does not seem to stem solely from the infinite dimensionality of the Cayley tree, since similar behavior was recently found3 for the Ising model on the Sierpinski gasket with the fractal dimension df=ln 3/ln 2.
Effect of pressure on the magnetic phase diagram of the antiferromagnetic spin density wave alloy Cr–1.6% Si
Authors: Fernandez-Baca J. A. , Fawcett E. , Alberts H. L. , Galkin V. Yu. , Endoh Y.
Published: July 2002

A neutron diffraction experiment has been performed to study the effects of high pressure on the magnetic phase diagram of a single crystal specimen of Cr–1.6% Si. At applied pressures up to 0.49 GPa there are two clear transitions. At TNC there is a transition from a paramagnetic (P) to a commensurate spin density wave (CSDW), while at lower temperatures there is a transition to a mixed state where the CSDW seems to coexist with an incommensurate SDW (ISDW) state. TNC decreases with the application of pressure while the transition temperature TCI to the mixed C-I state seems to be insensitive to pressure. At 0.69 GPa the ISDW state disappears while the P-C transition changes drastically.
Magnetization process in perpendicular epitaxial Cu/Ni/Cu/Si (001) (abstract)
Authors: Hug H. , Stiefel B. , Güntherodt H.-J. , Bochi G. , Paul D. I. , O’Handley R. C.
Published: July 2002

Epitaxial Cu/Ni/Cu (001) films exhibit perpendicular anisotropy over an exceptionally wide thickness range (30 Å⩽Ni⩽145 Å).12 Magnetic force microscopy (MFM) has been used to reveal new details of the magnetization process;34 here we focus on two features. (1) the forces of attraction and repulsion between different segments of domain walls and (2) a bimodal distribution of magnetization response times to perpendicular applied fields. Domain images were taken using high-resolution scanning MFM in perpendicular fields up to 500 Oe. The following observations are explained. As high energy domains are reduced in area by a perpendicular applied field, the domain patterns (whose wall orientations in zero field have no correlation with the easy in-plane 〈110〉 directions) evolve to a serpentine pattern. Some lengths of the serpentine domains collapse completely while others shrink in width with opposite walls failing to annihilate each other even in fields up to 500 Oe. These “hard domains” show a preference for alignment with the easy 〈110〉 directions. We believe the annihilation and hard domain behavior to be due to the combination of opposing short-range exchange and long-range dipole interactions between lengths of domain wall having the same or opposite chirality; chirality along a domain wall changes at a Bloch line. The alignment of hard threadlike domains with the 〈110〉 directions is due to the magnetoelastic interaction between the misfit dislocation strain field and the in-plane magnetization of the domain wall. Films of Ni thickness up to 85 Å can be saturated and show a remanence ratio of unity by MFM, vibrating sample magnetometry and magneto-optic Kerr effect in perpendicular fields. On the other hand, for 100 Å of Ni, the remanence ratio is smaller and the MFM images at 500 Oe show a small fractional area of unreversed domains; thicker films show larger unsaturated fractions. The magnetization process in films of 75–100 Å Ni is revealed in the MFM images to have an instantaneous field response and a slower time response (in constant field) over a period of several minutes. The instantaneous response is due to the motion of glissile domain wall segments; the after effect appears to be due to the thermally activated motion of Bloch lines along the hard wall segments.
Anisotropy and domain structure of die-drawn Co–Si–B amorphous wire with negative magnetostriction (abstract)
Authors: Soeda M. , Takajo M. , Yamasaki J.
Published: July 2002

Domain observation was performed on die-drawn Co–Si–B amorphous wires with negative magnetostriction. We reported previously that die-drawn Fe–Si–B wire with positive magnetostriction shows the characteristic shell and core domain similar to that of as-quenched negatively magnetostrictive Co based wire.1 So it is expected that the domain observation of die-drawn Co based wire will help us understand the process of formation of the core and shell domain structure in die-drawn wires. It was found that the easy axis of magnetization of as-drawn Co based wire is normal to the wire surface and exhibits a maze domain configuration near the wire surface. Its maze domain width is finer than that observed on the as-quenched Fe based wire. The hysteresis loop shows a remanence ratio of around 0.5, from which it is inferred that the die-drawn Co wire has a core domain with longitudinal magnetization underneath the maze domain similar to Fe based wire. It was also found that the relaxation of the anisotropy due to annealing makes the magnetization of the shell domain point in the circumferential direction. This may be related to the two easy axes of anisotropy in the shell domain that are created by the longitudinal tension through negative magnetostriction. Thus, it is inferred that die drawing reverses the direction of the residual stress of the as-quenched wires,2 resulting in the characteristic shell and core domain structure.
Single-site properties of U impurities doped in La2Zn17 (abstract)
Authors: Suzuki H. , Anzai K. , Takagi S.
Published: July 2002

Thermodynamic and transport properties of heavy Fermion (HF) U compounds show similar behavior to HF Ce compounds. Although most of the magnetic properties of HF Ce compounds can be qualitatively understood on the basis of the impurity Kondo model, no such consensus for HF U compounds has been reached. In addition to this, the single-site properties of U impurities are not understood so well, in contrast to the case of Ce impurities. Recent works for dilute U systems reported new features as are not seen in dilute Ce systems. We have investigated a dilute-U2Zn17 system of (La1−zUz)2Zn17 in order to reveal the single U ion site properties of this system by preparing single crystals. The impurity contributions to various physical quantities such as ρimp(T), χimp(T), and Cimp(T) can be scaled by the U concentration between z=0.025 and 0.05, and the system is considered as in the dilute limit still for z=0.05. The electrical resistivity shows the typical impurity-Kondo upturn at low temperatures. The electronic specific-heat coefficient is strongly enhanced (γimp≈1.5 J/K2 mole U) and about 4 times as large as that for dense U2Zn17. Suppressions of the Kondo effect by the magnetic field are seen in γimp(H) and magnetoresistance. The relatively large anisotropy in χimp(T) indicates an existence of the crystal field. These features of this system will be explained in terms of the Kondo effect in the presence of the crystal field.
Thermodynamics of alternate Ising chains of spins 1 and 3/2 with dipolar, biquadratic, and single ion interactions
Authors: Fireman Elton C. , dos Santos Roberto J. V.
Published: July 2002

Within a recently developed extended decoration-transformation formalism we study the thermodynamic properties of a linear chain of alternate Ising σ=1 and S=3/2 spins. We allow for different anisotropy fields on each subchain of different spins. For some range of the parameter space we show the existence of a crossover from a ferromagnetic to an antiferromagnetic-like behavior of the model, as explicitly captured in the susceptibility results.
Correlation length of the isotropic quantum Heisenberg antiferromagnet
Authors: Cuccoli Alessandro , Tognetti Valerio , Vaia Ruggero , Verrucchi Paola
Published: July 2002

The quantum Heisenberg antiferromagnet on the square lattice is known to model the magnetic interactions in the copper ion planes of many high-Tc superconductors and their parent compounds. The thermodynamics of the model is approached by the pure-quantum self-consistent harmonic approximation, that reduces the quantum problem to the study of an effective classical antiferromagnetic system. The effective exchange, weakened by quantum fluctuations, enters as a temperature scale the classical-like expressions for the thermal averages, and the quantum spin correlation length is then obtained from its classical counterpart in a simple way. The theory compares very well, for any value of the spin and without need for adjustable parameters, with high temperature expansions, quantum Monte Carlo simulations, and recent neutron and nuclear quadrupole relaxation (NQR) experiments.
Superconductivity and magnetic order in (Pr0.55Nd0.45)Ba2Cu3O7
Authors: Li W.-H. , Lee K. C. , Ku H. C.
Published: July 2002

Crystal structure, superconductivity, and magnetic order of the Pr spins in polycrystalline (Pr0.55Nd0.45)Ba2Cu3O7 were studied by x ray and ac susceptibility measurements. The crystal structure was determined by applying the Rietveld refinement method to the high resolution x-ray diffraction pattern. The Pr and Nd atoms are randomly mixed to form (Pr0.55Nd0.45) layers rather than two separated PrBa2Cu3O7 and NdBa2Cu3O7 phases. Above 50 K the temperature dependence of the in-phase component of the ac susceptibility χ′(T) exhibits Curie–Weiss behavior. Below 50 K diamagnetic responses become evident, indicating the onset of superconductivity. A dc field of strength 2 kOe smears out 80% of the diamagnetic signals, showing the superconducting pairings are being destroyed by the applied field. Further reducing the temperature, χ′(T) shows an upturn at T≈15 K below which χ′(T) increases rapidly with decreasing temperature, presumably due to the Pr spin ordering. Both the onset of superconductivity and the spin ordering of the Pr ions are also evident in the temperature dependence of the out-of-phase component of the ac susceptibility χ′′(T).
Structure and magnetism of multiphase Sm0.080Co0.645Fe0.276 powders
Authors: Harris V. G. , Liou M. , Das B. N. , Browning V. M. , Snyder J. E. , Rubinstein M. , Lawrence S. H. , Littleton R. , Pappas D. P.
Published: July 2002

A Sm-poor mixture of Fe-substituted Sm2Co17, having the nominal stoichiometry of Sm0.080Co0.645Fe0.276, was ball-milled to explore the possibility of enhancing its remanence through direct microstructural refinement. With milling, the Sm2(Co0.7Fe0.3)17 compound disassociates to a body-centered-cubic supersaturated SmCoFe solid solution and a residual SmCoFe amorphous phase. Correspondingly, the coercive field values first increase, peaking at 0.83 kOe after 180 min of milling, then decrease with continued milling to <0.1 kOe after 1200 min. The remanence, as Mr/Ms, is measured to track closely the coercive field behavior, experiencing modest increases to 0.26 after just 120 min of milling. Magnetization values are found to increase first with the dissociation of the 2:17 phase, and again with the ejection of Sm from the bcc component.
Magnetic properties of Y(Fe0.8M0.2)11.3Nb0.7 compounds with M=Mn, Fe, Co, Ni, Al, and Ga
Authors: Wang Jianli , Ning Tang , Brück E. , Ruwen Zhao , Fuming Yang , de Boer F. R.
Published: July 2002

Structural and magnetic properties of Y(Fe0.8M0.2)11.3Nb0.7 compounds with M=Mn, Fe, Co, Ni, Al, and Ga have been investigated. The x-ray diffraction patterns of aligned samples show that, for M=Mn, Fe, Co, Ni, the compounds exhibit easy axis anisotropy while for M=Al and Ga the compounds have planar anisotropy. The anisotropy fields of Y(Fe0.8Mn0.2)11.3Nb0.7 increase monotonously with decreasing temperature. The influence on the Curie temperature and the magnetization due to the various substitutional atoms are discussed.
Electronic structure calculation for Pt substituted MnSb compound
Authors: Zhang Long , Zou Xian-Wu , Qiao Hao-Xue , Jin Zhun-Zhi
Published: July 2002

The electronic structure and local magnetic properties of the MnSb compound with or without substituted Pt atoms are calculated using the spin-polarized discrete variational method within local-density-functional theory and multiple embedded clusters models. Calculated charge transfer, spin moments and density of states of MnSb without doping Pt atoms agree well with experiments and augmented sphere wave calculations for MnSb crystal. Finally, the electronic structure and spin moments of MnSb doped with different concentrations of Pt atoms are calculated.
Magnetic phase diagram of CsNiF3
Authors: Pich C. , Schwabl F.
Published: July 2002

We study the phase diagram of CsNiF3, a quasi one-dimensional ferromagnet in the three-dimensional antiferromagnetically ordered phase (T<TN). Due to an isotropic antiferromagnetic exchange interaction and the dipole–dipole interaction the system has a collinear ordered ground state. For homogeneous magnetic fields perpendicular to the spin chain axis we investigated the stability of the Néel and the paramagnetic phase. For fields along the spin direction the Néel phase is metastable up to a critical field, which depends solely on the dipolar energy. In addition we calculated the transition of the paramagnetic phase to an intermediate phase. Depending on the direction of the field the system undergoes a transition to a commensurate or an incommensurate phase.
Field-effect trap-level-distribution model of dynamic random access memory data retention characteristics
Authors: Hiraiwa A. , Ogasawara M. , Natsuaki N. , Itoh Y. , Iwai H.
Published: July 2002

Dynamic random access memory data retention characteristics were investigated as a function of operating voltage. Based on a statistical process, called the trap-level-distribution model, which was proposed in our previous report, we further assumed that the junction leakage current by Shockley–Read–Hall process is enhanced by an electric field through the trap-assisted-tunneling process. We incorporated into the model two processes that cause variation in the electric field at the traps; the variation of electric field itself and the spatial trap distribution. By comparing the Monte Carlo and analytical calculations with the experimental results, we found that the retention time distribution is not only caused by the energy level and spatial distributions of the traps, but by the space-charge-region field distribution among the cells. A possible origin of the field distribution is the variation of dopant profile among the junctions.
Structures and defects in arsenic-ion-implanted GaAs films annealed at high temperatures
Authors: Chen Wen-Chung , Chang C.-S.
Published: July 2002

The structures and defects are studied in arsenic-ion-implanted GaAs(As+–GaAs) films annealed at temperatures higher than 600 °C by using transmission electron microscopy, deep level transient spectroscopy, temperature-dependent conductance, and photoluminescence. The estimated concentration of arsenic precipitates in films decreases from ∼4×1016 cm−3 to ∼6×1015 cm−3 and the corresponding size increases from ∼3 to ∼10 nm as the annealing temperature increases from 600 to 800 °C. A defect with an energy level at about 0.3 eV from the band edge is found and its concentration increases with the increasing annealing temperatures. The electrical transport of free carriers is replaced by hopping conduction, through the defect band at about 0.26 eV below conduction band, when the film is annealed at temperature 800 °C. It indicates that during high-temperature annealing the defect of the arsenic and gallium vacancies due to the diffusion of As and Ga atoms is the dominant factor to change its electrical and structural properties.
Study on the ions’ behavior in an electron cyclotron resonance plasma
Authors: Liu Minghai , Hu Xiwei , Wu Hanming
Published: July 2002

The energy, velocity, angle distribution of ions in magnetoactive electron cyclotron resonance plasma have been studied with a two-dimension hybrid mode. The dependence of these distribution functions versus position and pressure are discussed. Our simulation results are in good agreement with many experimental measurements.
Rare-earth effect on the site occupation of Al and C in R2Fe15Al2Cy alloys
Authors: Chang W. C. , Tsai S. H. , Yelon W. B. , Hu Z. , Luo H. , Chen M.
Published: July 2002

Neutron diffraction has been used to investigate the rare-earth effect on the site occupations of Al and C atoms in the combined substitutional–interstitial compounds, R2Fe15Al2Cy (R=Pr, Tb, Ho). All three samples have the rhombohedral Th2Zn17-type structure. As the composition changes from light rare-earth to heavy rare-earth, the site occupations of the substitutional Al atoms on the 6c and 18f increase while that on the 18h site decreases. However, the 18h site still has the highest Al occupation for all samples. The site occupation of the C atoms on the octahedral interstitial site (9e) is much higher in the heavy rare-earth (Ho) sample than in the light rare-earth (Pr) sample. However, the tetrahedral interstitial 18g site shows a reverse behavior. This 18g site occupation of C atoms has not been observed before except in gas phase interstitial compounds. The Al site occupation is determined by the crystal environment, while the C site occupation appears to be determined predominantly by site size.
Perpendicular giant magnetoresistance in Co/Cu and permalloy/Cu multilayered nanowires (abstract)
Authors: Dubois S. , Duvail J. L. , Fert A. , George J. M. , Maurice J. L. , Piraux L.
Published: July 2002

We have prepared Co/Cu and NiFe/Cu (Ni/Fe=permalloy) multilayered nanowires by electrodeposition into pores of membranes or holes made by e-beam lithography in PMMA layers. In both cases, pores and lithography, the diameter of the wires is around 100 nm and the layer thickness ranges between a few nm and several hundred nm. Transmission electron microscopy reveals that the nanowires are composed of long single-crystal grains with c axis (hcp Co) or (111) axis (fcc NiFe or Cu) perpendicular to the axis of the wire. A first series of samples is composed of conventional periodic multilayers. Their CPP-MR ratio can be as large as 80% at 4.2 K (NiFe 5 nm/Cu 5 nm) and giant magnetoresistance effects can be observed up to very large thicknesses (example: tco=1 μm). At small thicknesses, we find the conventional behavior of the so-called “long spin diffusion length limit.” More interesting are the results obtained out of this limit and used to determine the spin diffsion length SDL in Cu (140 nm at low T) and Co (44 nm at low T). For NiFe/Cu, the magnetic arrangement of successive layers is more difficult to control and we could not determine the SDL from data of the first series. A second series of samples is made with NiFe/Cu/NiFe trilayers (tCu=10 nm, 7 nm⩽tNiFe⩽30 nm), separated from each other by Cu layers of 100 nm. As shown by superconducting quantum interference device measurements, the magnetization of the two NiFe layers in a trilayer are approximately antiparallel at zero field. We use the CPP-MR data on these samples to derive the SDL in permalloy.
Integral relaxation time of single-domain ferromagnetic particles (abstract)
Authors: Garanin D. A.
Published: July 2002

The thermally activated escape rate of single-domain ferromagnetic particles over a potential barrier ΔU due to the uniaxial anisotropy K is described usually by the lowest eigenvalue of the Fokker–Planck equation Λ1, for which analytical results are available in the high-barrier, T≪ΔU, and high-temperature, T≫ΔU, cases.1 More convenient and physically appealing is, however, using the integral relaxation time τ determined as the area under the magnetization relaxation curve after an instantaneous infinitesimal change of the longitudinal magnetic field H, which can be calculated analytically from the Fokker–Planck equation in the whole range of temperatures.2 In Ref. 3 it was shown that in the unbiased case, H=0, the quantity τ−1 is everywhere very close to Λ1. On the contrary, the numerical results of Ref. 4 for H≠0 show a drastic behavior τ−1≫Λ1 at low temperatures, starting from some critical field Hc. Here, it is demonstrated that such a behavior is the consequence of the depletion of the upper potential well. In these conditions, the integral relaxation time consists of two competing contributions corresponding to the overbarrier and intrawell relaxation, which can be analytically separated in the general formula of Ref. 2. The critical field Hc is given by hc≡Hc/(2KMs)=3−22≈0.17.
Generalized method for measuring read-back nonlinearity using a spin stand
Authors: Wilson Bruce A. , Wang Shan X. , Taratorin Alex M.
Published: July 2002

A new method for measuring the read-back transfer function of a magnetic recording system is proposed. For this method, square wave patterns are recorded over a range of linear densities. The resulting read signals should exhibit certain superposition properties if the read process were linear. The transfer function of a magnetoresistive (MR) head distorts read-back signals and makes the read process nonlinear. The inverse transfer function is modeled using an arbitrary function such as a polynomial or cubic spline. This model is then used to linearize the distorted read-back signals. Superposition properties are used to evaluate the accuracy of this correction and optimize the parameters of the model. The transfer function is then obtained directly from the optimum inverse transfer function. The effectiveness of the proposed method is confirmed by comparing the read-back transfer function obtained experimentally using this method and a MR read head with a direct measurement of the transfer function of the same read head.
Effect of chlorine addition on diamond growth using methane/hydrogen reactants
Authors: Wu Jih-Jen , Chau-Nan Hong Franklin
Published: July 2002

The effect of Cl2 addition on diamond growth using CH4/H2 was studied in a hot-filament chemical vapor deposition reactor. Cl2 was mostly converted into HCl before arriving at the hot filament. Cl2 addition enhanced the diamond growth rate. The growth rate enhancement was more pronounced at a lower growth temperature and lower reactor pressure, implying that the CH3 and CH4 interconversion reactions were not in partial equilibrium below 5 Torr. Cl2 addition also reduced the activation energy by the same value for both pressures at 1 and 5 Torr, suggesting that the H and Cl exchange reaction was in partial equilibrium. However, Cl2 addition degraded the film quality, and the deterioration became more pronounced at a higher Cl2 concentration or lower substrate temperature. The effects of chlorine are suggested to include: (i) the fast reaction of Cl with surface bonded C–H; (ii) the fast reaction of Cl with CH4; (iii) the weak ability of Cl to adsorb on C=C; and (iv) the weak ability of Cl to break C–C to remove the improperly adsorbed CH3.
Influence of yttrium on the magnetic properties of amorphous Fe91−xYxZr9 alloys
Authors: Driouch L. , Kim K. S. , Rao K. V.
Published: July 2002

In this article, we investigate and discuss the influence of the substituting Fe with Y on the magnetic properties of amorphous Fe91Zr9 alloys prepared by single-roll melt spinning technique. The temperature dependence of magnetization for amorphous Fe91−xYxZr9 alloys, at low fields, shows the existence of antiferromagnetic (AFM) couplings between Fe atoms. On increasing the concentration of Y, the strength of the AFM interactions is suppressed and the Curie temperature TC increases, which can be explained on the basis of the atomic size effect of Y. From an analysis of the approach to saturation magnetization MS, the magnetic moment μFe per Fe atom, anisotropy field, and local random anisotropy constant K1 have been extracted. It is found that μFe increases from 1.57 to 2.04 μB and K1 decreases with increasing Y concentration. This result suggests that the substitution Fe with Y seems to favor the increase of magnetic order by reducing the random anisotropy and thus change the magnetic structure from being an asperomagnetic to a ferromagnetic one.
Structural and magnetic properties of epitaxially grown Fe–Sm and Fe–Sm–N films
Authors: Jang P. W. , Wang D. , Doyle W. D.
Published: July 2002

Thin Fe–Sm–N films with thickness ⩾50 nm and coercivities ⩾4 kOe were grown on an α-Ta (110) substrate by dc magnetron sputtering. Transmission electron microscopy diffraction data clearly showed that the films were polycrystalline with the ThMn12 tetragonal structure and with a preferential orientation of (011) Fe–Sm//(110) Ta. This resulted in a uniform distribution of the easy axis on a cone of 29° to the film normal. The structural data were also supported by the hysteresis loops measured as a function of angle in plane and out of plane. The high coercivity phase was obtained from targets with 15–20 at. % Sm. For the 10 at. % Sm target, only a soft α-Fe phase was formed. For films of <50 nm sputtered from 15 to 20 at. % Sm, a soft phase was clearly observed in the vibrating sample magnetometer and was the only soft phase formed in 10 nm thick films. However, films of >50 nm thickness showed high coercivities without a kink in the demagnetization loops and seemed to be composed only of magnetically hard phase, which was probably due to strong magnetic interactions between magnetically soft and hard phases at the Fe–Sm/Ta interface.
Four discrete magnetic states of keepered media
Authors: Reed W. B. , Cohen U. , Hollars D. R. , Zubeck R. B.
Published: July 2002

Keepered recording media includes a high permeability (μ) keeper layer overlying a conventional longitudinal thin-film magnetic layer. Vibrating sample magnetometer (VSM) hysteresis loops show four discrete magnetic states in the superposition of two exchange decoupled loops: one is the high coercivity storage layer and the other is the low coercivity keeper. The keeper layer provides an adjustable reluctance shunt across the head gap and a closure between recorded transitions. Keepered media output power is attenuated more than 20 dB until a bias current is applied to the reproduce head windings. Head bias current and recorded wavelength, λ, have a direct linear effect on both head output level and mirror-symmetric peak pair offset within a defined variable reluctance zone. Head output level within the variable reluctance zone is 1 dB per mA turn of bias and normalized peak pair offset is ±1% or ±3° per ±mA turn of bias, with phase reversal at zero current. Variable reluctance amplitude and phase functions are linear until keeper layer saturation at ±17 mA turns bias. A method of encoding binary data into the four magnetic states is described. Eight user bits are coded into five flux changes and recorded at 90° increments. Reproduce circuitry uses a 2F phase locked loop keeper head bias which decodes the four magnetic states into four voltage levels at 90° spacing. The described (1,5) four level code requires one less flux change per 8 user bits than (1,7) code, and is stable only within the variable reluctance zone of keepered media.
Three-dimensional simulation of the disturbance of magnetic domain walls by magnetic force microscope tips
Authors: Huo S. , Bishop J. E. L. , Tucker J. W. , Rainforth W. M. , Davies H. A.
Published: July 2002

The micromagnetics of the interaction of a strong 3D magnetic force microscope tip with a 180° asymmetric Bloch domain wall in an 80 nm {100} iron film has been simulated numerically using cubic elements on a 3D lattice. The distortion of the wall is found to be quite well localized on the scale of the tip dimensions, 60×60 nm2. The force of attraction between tip and film and the changes in self-energy of the wall and in the energy of the tip-sample system have been calculated for various positions of the tip. The strong attraction between tip and sample is slightly reduced (∼5%) when the tip is directly over the central vortex in the wall.
A new design approach to the shape and topology optimization of magnetic shields
Authors: Wakao S. , Onuki T. , Ogawa F.
Published: July 2002

A novel approach to the three-dimensional optimal design of magnetic shields is presented, in which both shape optimization and topology optimization are achieved with the aid of numerical magnetic-field analysis. These numerical procedures are implemented by means of the hybrid finite element and boundary element method combined with the genetic algorithm. This results in an overall increase in the optimization speed with acceptable computational accuracy. Finally, some numerical results that demonstrate the validity of the proposed approach are presented.
Magnetic domain structures of ferromagnetic thin films deposited on superconducting substrates (abstract)
Authors: Stankiewicz A. , Tarasenko V. , Robinson S. J. , Gehring G. A.
Published: July 2002

The influence of superconducting substrates and coatings on the domain structure parameters of ferromagnetic thin films was analyzed, assuming validity of the London equation in the superconducting volumes (characterized by finite penetration depths) with standard boundary conditions. Analytic expressions for the demagnetization energy of arbitrary magnetization distribution were found by direct integration of the field Fourier components. This result was then applied both for strong in-plane anisotropy (N=E9el-type domain walls) and strong perpendicular anisotropy (Bloch-type walls). For the first case it was shown that superconducting substrate changes the domain structure properties only if the layer is also coated with superconducting material. Otherwise the parameters of domain structure change due to superconducting substrate, but the qualitative behavior remains the same. However, in the case of perpendicular anisotropy, essential changes in domains parameters appear even for normal capping. In thicker films the domain structure period is smaller than for the same film deposited on normal substrate. For sufficiently thin films the period becomes larger than for the normal substrate1 and increases to infinity at critical thickness L. Domains are fully suppressed, when the film thickness becomes smaller than L. The L value dependencies on different parameters of magnetic film and superconducting substrate were found by consideration of an isolated Bloch wall with finite width. This approach was also extended on finding the critical behavior of domain period. The results of numerical calculations, as well as analytic approximations, based on the derived formulas, are presented. It was shown that for films thinner than domain wall width the finite value can significantly influence domain structure parameters. Estimations made for typical values of superconducting and magnetic materials show that the critical thickness can exceed 100 Å.
Difference of interface trap passivation in Schottky contacts formed on (NH4)2Sx-treated GaAs and In0.5Ga0.5P
Authors: Moon C. R. , Choe Byung-Doo , Kwon S. D. , Lim H.
Published: July 2002

The effects of (NH4)2Sx treatments on the interface traps in Au/n-GaAs and Au/n-In0.5Ga0.5P Schottky contacts are investigated by deep level transient spectroscopy measurements. The interface trap concentration in Au/GaAs increases when the residual S overlayer is sublimated after (NH4)2Sx treatments. But the trap concentration decreases when the S overlayer is rinsed by de-ionized water. In Au/InGaP, the phosphorus vacancy-related interface traps are passivated effectively by the (NH4)2Sx treatment. These results are attributed to the fact that S bonds with As but not with P.
Description in a local model of the magnetic field distributions of Fe1−xNix disordered alloys
Authors: Restrepo J. , Pérez Alcázar G. A. , Bohórquez A.
Published: July 2002

A local model based on the idea of atoms distributed within a crystalline lattice and following a binomial probability method for describing the hyperfine field distributions obtained from the room temperature Mössbauer spectra of the disordered Fe–Ni system is presented. This model allows one to calculate the mean hyperfine field for each concentration from 5 to 95 at. % Ni, to determine what are the most probable sites and to interpret why the mean hyperfine field initially increases and then decreases with the Ni concentration in the BCC phase. The results are in good agreement with the experimental results except for Invar composition and for 85 at. % Ni which demand a different treatment.
Nonlinearities in the current–voltage characteristics of neutron transmutation doped germanium at millikelvin temperatures
Authors: Perrin N.
Published: July 2002

We report on the electrical properties of neutron-transmutation-doped Ge at very low temperatures. The current–voltage characteristics calculated for different heat sink temperatures Ts in our model, based on the variable-range-hopping conduction model, are seen to be very nonlinear; the nonlinearities are shown to be due to the lack of thermal equilibrium between the electrons and the phonons. Comparisons are done with the experimental results obtained by Wang et al. [Phys. Rev. B 41, 3761 (1990)].
Modelling of the kinetics and parametric behaviour of a copper vapour laser: Output power limitation issues
Authors: Carman R. J.
Published: July 2002

A self-consistent computer model was used to simulate the plasma kinetics (radially resolved) and parametric behaviour of an 18 mm bore (6 W) copper vapour laser for a wide range of optimum and non-optimum operating conditions. Good quantitative agreement was obtained between modelled results and experimental data including the temporal evolution of the 4p2P3/2, 4s2 D5/22 and 4s2D3/22 Cu laser level populations derived from hook method measurements. The modelled results show that the two most important parameters that affect laser behaviour are the ground state copper density and the peak electron temperature Te. For a given pulse repetition frequency (prf), maximum laser power is achieved by matching the copper atom density to the input pulse energy thereby maintaining the peak Te at around 3 eV. However, there is a threshold wall temperature (and copper density) above which the plasma tube becomes thermally unstable. At low prf (<8 kHz), this thermal instability limits the attainable copper density (and consequently the laser output power) to values below the optimum for matching to the input pulse energy. For higher prf values (>8 kHz), the copper density can be matched to the input pulse energy to give maximum laser power because the corresponding wall temperature then falls below the threshold temperature for thermal instability. For prf >14 kHz, the laser output becomes highly annular across the tube diameter due to a severe depletion of the copper atom density on axis caused by radial ion pumping.
A limit of validity of the straight line hypothesis in shaped charge jet formation modeling
Authors: Curtis J. P. , Kelly R. J.
Published: July 2002

A particular problem in the field of shaped charge jet formation modeling concerns the collision of two fluid streams of different widths and speeds. It is commonly assumed that the flow is incompressible, and that the velocity of the fluid in any of the streams is constant across and normal to its cross section. Then the well-known classically indeterminate mathematical problem arises. In the shaped charge context the indeterminacy of the problem has been addressed by making three assumptions about the flow. Several models have assumed that conservation of kinetic energy holds, and have applied Bernoulli’s Law to equate the speeds of the jet and slug in a frame moving with the collision point. One natural choice for the third and final assumption is that the jet and slug lie in a straight line when viewed in this frame, the so-called straight line hypothesis. In this article the inclination of this line relative to the bisector of the two colliding streams is expressed as a function of the parameters of the incoming streams. It is shown that the angle between the jet and the incoming stream supplying momentum at the greater rate increases with the size of the angle between the incoming streams until it reaches a maximum value. It then decreases to zero. It is known that the straight line hypothesis is a good approximation for low values of the angle between the incoming streams, but becomes increasingly inaccurate as this angle increases. The above maximum appears to correspond to the limit of validity of the straight line hypothesis. Recommendations for the utilization of the existing formation models to achieve best accuracy are made, based on this limit.