bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma nitridation process for superconducting Nb wiring to improve their annealing stability
Authors: Shiota Tetsuyoshi , Imamura Takeshi , Hasuo Shinya
Published: July 2002

We developed plasma nitridation for superconducting Nb wiring to improve their annealing stability. Plasma nitridation was observed to be very effective in increasing the critical currents and improve annealing stability. We analyzed the nitride layer formed on the surface of Nb film by plasma nitridation, and found that it prevented oxygen diffusion in Nb. Plasma nitridation is a currently feasible process to improve the reliability of Nb wiring in Josephson circuit.
Strain tolerance of doctor-blade-processed Bi2Sr2Ca1Cu2O8−δ tapes
Authors: Togano K. , Kumakura H. , Kase J. , Li Qiang , Ostenson J. E. , Finnemore D. K.
Published: July 2002

The strain tolerance of doctor blade tapes of the high-temperature superconductor Bi2Sr2Ca1Cu2O8−δ have been studied to determine the suitability of these materials for magnet conductors. Critical currents that are on the order of 150 000 A/cm2 in zero magnetic field fall by about 30% with 0.1% bending strain and by a factor of 6 with 0.2% bending strain. At 20 T, critical currents fall with strain by approximately the same factor as at 4.2 K, but the critical currents fall with field somewhat faster. These bending strain results have a character quite different from Nb3Sn bronze process material in that there is essentially no prestress from the Cu jacket and there seems to be only a very small reversible range where the sample is within the elastic limit.
Force prediction in permanent magnet flat linear motors (abstract)
Authors: Eastham J. F. , Akmese R.
Published: July 2002

The advent of neodymium iron boron rare-earth permanent magnet material has afforded the opportunity to construct linear machines of high force to weight ratio. The paper describes the design and construction of an axial flux machine and rotating drum test rig. The machine occupies an arc of 45° on a drum 1.22 m in diameter. The excitation is provided by blocks of NdFeB material which are skewed in order to minimize the force variations due to slotting. The stator carries a three-phase short-chorded double-layer winding of four poles. The machine is supplied by a PWM inverter the fundamental component of which is phase locked to the rotor position so that a ‘‘dc brushless’’ drive system is produced. Electromagnetic forces including ripple forces are measured at supply frequencies up to 100 Hz. They are compared with finite-element analysis which calculates the force variation over the time period. The paper then considers some of the causes of ripple torque. In particular, the force production due solely to the permanent magnet excitation is considered. This has two important components each acting along the line of motion of the machine, one is due to slotting and the other is due to the finite length of the primary. In the practical machine the excitation poles are skewed to minimize the slotting force and the effectiveness of this is confirmed by both results from the experiments and the finite-element analysis. The end effect force is shown to have a space period of twice that of the excitation. The amplitude of this force and its period are again confirmed by practical results.
Analysis of high-energy plasma formation in hydrogen
Authors: Walsh Peter J. , Degnan J. H.
Published: July 2002

Analysis is presented on the position, velocity, acceleration, mass, size, and shape of the hydrogen plasma formed by a 88-kJ pulse within a coaxial injection and accelerated into a coaxial expansion chamber. Within the expansion chamber of the 1.25-m-long apparatus, the velocity was obtained near the positions of three sets of current probes and exceeds 44 cm/μs. The acceleration of the plasma in the acceleration chamber increases with time to values approaching 30 cm/μs2 and the plasma length averages nearly 30 cm. Numerical inversion of the circuit current measured by a current probe near the base of the injection chamber yields the time variation of the circuit inductance produced by the motion of the plasma. From the inductance the position of the plasma during the pulse is inferred throughout the whole of the apparatus up to the end wall of the expansion chamber. The inversion analysis agrees with the probe analysis if it is assumed that the plasma extrudes from the injection chamber at approximately the radius of that chamber. Mass is lost by the accelerated plasma and specific mass values are derived, versus time, for cases when plasma momentum is conserved and not conserved. Both values extrapolate, at zero time, to the measured mass of injected hydrogen. There is good indication that the plasma survives reflection from the end wall.
High-performance sintered R2Fe14BNx (R=Nd/Pr/Dy) magnets
Authors: Chin T. S. , Bai H. J. , Lin K. D. , King F. D. , Heh S. J.
Published: July 2002

High-performance R2Fe14BNx-type magnets (R=Nd and/or Pr/Dy) were successfully prepared by sintering. The optimum magnetic properties are coercivity iHc: 680–1440 kA/m; Hc: 500–1000 kA/m; Br: 1.0–1.3 T; and (BH)m: 180–250 kJ/m3 depending on the composition and processing parameters, especially the proper sintering conditions, including nitrogen intake at 1040 to 1050 °C. The magnets thus obtained have Tc values of 335 to 370 °C depending on the nitrogen content, which should be <6 at. % for thermal stability. The anisotropy field increases from 6.2 to 8 T with addition of nitrogen, and magnetization measured at 2 T increases by 18%. The weight loss after immersion in a 5% HCl solution for 30 min is 25% less for nitrogenated magnets than for those without nitrogen, due to the replacement of the Nd-rich phase at grain boundaries by more corrosion- resistant nitrides.
Successive growth of Ba-ferrite magnetic layers on c-axis oriented YBaCuO superconductive layers
Authors: Naoe Masahiko , Matsushita Nobuhiro , Nakagawa Shigeki
Published: July 2002

Successive growth of Ba-ferrite (BaM) layer on YBa2Cu3Ox (YBCO) layer has succeeded by using facing targets sputtering (FTS) method, which is suitable for preparing oxide films. BaM layer deposited on YBCO showed clear c-axis orientation of their crystallites normal to the film plane and they could be grown successively without interdiffusion. These bilayered films showed apparent perpendicular magnetic anisotropy originated from BaM layer. Preferred c-axis orientation in BaM layer could be clearly observed when its thickness was as small as 300 Å.
Suppression of large Barkhausen jumps in annealed amorphous wires with an imposed twist as a function of current, external axial stress, and axial field
Authors: Wun-Fogle M. , Restorff J. B. , Clark A. E. , Savage H. T.
Published: July 2002

The magnetization and small signal alternating-current permeabilities of transverse-field-annealed amorphous magnetostrictive wires (Fe77.5B15Si7.5) with a fixed twist of 400 deg/m were examined as a function of applied axial tensile stress and axial direct-current (dc) currents. We observe multiple-featured magnetization curves which we attribute to the inverse Wiedemann effect, as well as a near total suppression of the large Barkhausen jumps when a dc current is flowing through the wire. Several properties of the wire depend on stress and would be useful for sensor applications; figures of merit of 1700–2600 for torque sensors were obtained.
Elastic anomalies in iron-platinum invar alloys
Authors: Kawald U. , Schulenberg P. , Bach H. , Pelzl J. , Eckold G. , Saunders G. A.
Published: July 2002

Elastic anomalies in an Fe75Pt25 single crystal have been investigated by ultrasonic pulse echo and inelastic neutron scattering measurements in the temperature range 200 to 800 K. The single crystal which was grown by the Bridgman–Stockbarger process undergoes a magnetic transition at Tc = 370 K and a martensitic transformation at Ms = 200 K corresponding to a degree of order of S=0.7. Both ultrasonics and neutron scattering reveal a softening of the transverse modes below Tc. The excess elastic constants obey an M2 law as expected from a local magnetoelastic interaction model. The longitudinal ultrasonic velocities already start to soften at 700 K whereas the neutron data for the same acoustic branch do not show any anomaly down to 200 K. The observed q dependences of the acoustic branches point towards a softening confined to the very center of the Brillouin zone.
Neutron-diffraction study of YFe10−xCoxSi2
Authors: Lin Chin , Li Guo-Zhong , Liu Zun-Xiao , Jiang Hong-Wei , Wan Zhong , Yang Ji-Lian , Zhang Bai-Shen , Ding Yong-Fan
Published: July 2002

YFe10−xCoxSi2 (0≤x≤10) compounds were synthesized and studied by x-ray and neutron diffraction. The ThMn12-type structure can be formed in the entire range of composition. Magnetic properties were studied at fields up to 70 kOe in the temperature range of 1.5 K to room temperature. X-ray-diffraction analyses of aligned powders indicate that the easy magnetization direction is parallel to the c axis when x≤6, and deviates from the c axis when x≥ (R18)8. The site preference of Si and Co atoms in the structure was determined by neutron-diffraction experiments.
Uniformly high-performance Nd-Fe-B magnet powder (abstract)
Authors: Kim Andrew S.
Published: July 2002

Isotropic Nd-Fe-B bonded magnet powder is currently produced by the rapid quenching of the molten alloy by the melt spinning. An alternate route was investigated to produce spherical particles of Nd-Fe-B alloy having high magnetic properties, particularly coercivity, because spherical powders have excellent flowability. It was found that the magnetic properties of gas atomized spherical Nd-Fe-B powders were substantially improved by hydrogen absorption-desorption (HAD) treatment at 700–800 °C. However, the previous HAD treatment1 causes nonuniform magnetic properties as well as nonuniform microstructure across the sample location of the load in the furnace. Change in process variables does not substantially improve the uniformity in magnetic properties across the sample load. A unique HAD treatment resolves the nonuniformity in microstructure and magnetic properties, and further uniformly improves the magnetic properties. The uniform and high magnetic properties may be due to uniformly fine grain size produced by repeated phase transformation.
Coercivity enhancement in Pr-Fe-B magnets with small additions of Cu and Zn
Authors: Withanawasam L. , Zhang Y. J. , Hadjipanayis G. C.
Published: July 2002

Addition of small amounts of Cu and Zn was found to enhance the coercivity Hc of Pr-Fe-B melt-spun magnets. 1.5% Cu addition increases the coercivity by 6 kOe over the Cu free value of 18 kOe while 3% Zn increases Hc by 8 kOe. Both of the additions cause a reduction in grain size, and lead to disturbed lattice planes at grain boundaries and to thin PrCu(Zn) layers which are partially surrounding the 2:14:1 grains. The enhancement in coercivity is believed to be the result of these microstructural changes at the grain boundaries.
Replacement of Nd by an intermetallic phase in the intergranular region of Fe-Nd-B sintered magnets
Authors: Grieb B. , Pithan C. , Henig E.-Th. , Petzow G.
Published: July 2002

The microstructure of Fe-Nd-B based magnets consists of mainly Fe14Nd2B (Φ) grains embedded in a eutectic matrix of mainly Nd plus some Fe4Nd1.1B4 (η) and Fe-Nd phases stabilized by impurities. The addition of Al or Ga to Fe-Nd-B material leads to the formation of quite different ternary intergranular phases based on Fe, Nd and Al or Ga. The main phase stabilized by these additions is a phase (δ) with the composition Fe70−xNd30Tx (for T=Al: 8≤x≤25; for T=Ga: 3≤x≤8). In standard magnet compositions the pure Nd is still dominant in the intergranular region and causes corrosion problems due to high sensitivity to oxygen and humidity. By choosing an appropriate gross composition for the master alloy based on the knowledge of the phase relations, a new microstructure can be created consisting of Φ and an intergranular Fe-Nd (Al or Ga) phase almost without the residual Nd phase. The magnetic parameters of this material are comparable to or even better than those of conventional Fe-Nd-B materials. The δ phase crystallizes peritectically at the surface of Φ and leads to a better separation of the grains. The new two-phase material of two intermetallic phases shows better corrosion resistance combined with high coercivities of more than 15 kOe (1200 kA/m) after sintering without annealing, due to the formation mechanism of δ and the avoidance of Nd. Ga additions show better results than Al because of the very limited solubility of Ga in Φ.
Element specific magnetic moments of gadolinium iron garnet probed by soft-x-ray magnetic circular dichroism (abstract)
Authors: Rudolf P. , Sette F. , Tjeng L. H. , Meigs G. , Chen C. T.
Published: July 2002

The element specific magnetic moments of Gd and Fe in a gadolinium iron garnet (Gd3Fe5O12) were probed at various temperatures by measuring the soft-x-ray magnetic circular dichroism (SXMCD) at the Gd M4,5 and the Fe L2,3 adsorption edges in an external field of 5 kG. The soft-x-ray absorption spectra were recorded with the photocurrent method using highly circularly polarized soft x rays recently obtained at the AT&T Bell Laboratories Dragon synchrotron radiation beamline.1 The temperature dependence of the SXMCD signal at each edge allows one to independently determine the magnetic ordering of each specific ion. Our individually probed magnetic moment of the Gd3+ ion and of the two Fe3+ ions in different sites confirm the earlier calculations which predict a reversal of the local magnetic orientation at ∼280 K. This work demonstrates that SXMCD is a valuable tool for probing the element and site specific magnetic properties of multicomponent systems. The applications of this novel technique to the investigation of transition metals and rare earths in ferromagnetic alloys, compounds, disordered, and impurity systems will be addressed.
Anomalous dependence of Faraday rotation on doping concentrations in Gd-doped germanate infrared glasses
Authors: Yuan S. H. , Chee S. M.
Published: July 2002

This paper reports measurements of the Faraday rotation in Gd-doped germanate IR glasses in magnetic fields up to 10 kG at various light wavelengths and sample temperatures. The Faraday rotation shows an expected increase with decreasing wavelength, and an unexpected oscillation with increasing doping concentration. In addition, the magnetic susceptibilities of the glasses were measured at various magnetic fields and temperatures. The results show a paramagnetism and a low-temperature ferromagnetic interaction in the material. The experimental results cannot fit the single ion theoretical model. It is suggested that the formation of isolated ferrimagnetic clusters exhibiting superparamagnetic behavior causes the oscillation of the Faraday rotation. Consideration must be given to the ingredients and the technological process in order to avoid the formation of this cluster structure.
Effect of sodium doping of rare-earth iron garnet films on magnetic and magneto-optic properties
Authors: Ramesh M. , Gualtieri D. M. , Silliman S. D. , Peruyero J. , Stancil D. D.
Published: July 2002

Bismuth-doped lutetium iron garnet (BLIG) is an important material for implementation of the interaction between magnetostatic waves and optical guided modes. Sodium oxide is suggested to reduce the viscosity of the lead-free melt used for liquid-phase epitaxy (LPE) growth of the films, improving flux removal. The effect of the incorporation of sodium into the garnet lattice on the important magnetic and magneto-optic properties is studied, using yttrium iron garnet as a model system. Negligible effect on the Faraday rotation and magnetostatic wave passband is noted. The uniaxial anisotropy field is affected in the negative direction by a growth induced anisotropy due to sodium inclusion, but this effect may be removed by annealing.
Magneto-optical properties of (TbBi)3Fe5O12 and its application to a 1.5 μm wide-band optical isolator (abstract)
Authors: Tamaki T. , Kaneda H. , Kawamura N.
Published: July 2002

Wide-band optical isolators are indispensable for wavelength multiplexing optical fiber communication systems to eliminate noise induced by the reflected light from optical devices. To realize the wide-band optical isolator for wavelength of 1.5 μm, we have found a new composition of Bi-substituted terbium iron garnet (Tb3−xBixFe5O12) and developed a crystal growth method of the garnet.1 The composition of the crystals was determined by SEM and by x-ray fluorescence. Wavelength dependence of the Faraday rotation (FR) of Tb3−xBixFe5O12 single crystals was measured with a rotation-analyzer polarimeter and a spectrometer at room temperature. The wavelength coefficient of the FR of Tb3−xBixFe5−yGayO12 (x=0.25, y<0.3) is as small as 0.1%/0.2 μm at a wavelength of 1.5 μm. This is less than 1/10 of that (3%/0.2 μm) obtained for Gd2Bi1Fe5O12. The value of the FR for the crystal with x=0.25 was about −205°/cm at λ=1.5 μm. We have also found empirically that between FR coefficient θF(x) and Bi-concentration x, there is a simple relationship given by θF(x)=−1540x+180°/cm. By using these crystals as a Faraday rotator, we have developed wide band optical isolators for wavelength of 1.5 μm. Forward loss of 0.5 dB and backward loss of 33 dB were obtained at the wavelength region from 1.5 to 1.7 μm. The size of the isolator is 6 mm diam×6 mm long and its weight is 0.4 g. Through an optical fiber transmission system with the isolator, we could transmit a stable and clear picture for the lights of 1.5–1.7 μm wavelength.
Electronic structure and Fe moment distribution in a-Fe1−xBx glass by first-principles calculations
Authors: Ching W. Y. , Xu Yong-Nian
Published: July 2002

The electronic structure and the magnetic properties of a-Fe80B20 glass have been studied by a first-principles approach. A large periodic model containing 160 Fe atoms and 40 B atoms is constructed by the Monte Carlo method. The spin polarized electronic structure of this glass model is calculated within the local density approximation using the orthogonalized linear combination of atomic orbitals method. The potential used is obtained from the self-consistent band structure calculation of ferromagnetic Fe3B. We find that the presence of B has not only reduced the average Fe moment per site, but has also significantly changed the shape of the distribution curve of the moment. The density of states for the spin-up and spin-down bands at the Fermi level in a-Fe80B20 are comparable, in sharp contrast to that of pure a-Fe. The B atoms are found to be slightly polarized in opposite direction to the moments of Fe as in the crystalline Fe–B compounds.
Extended x-ray absorption fine structure studies of IBS Fe–Tb alloy films
Authors: Harris V. G. , Aylesworth K. D. , Kim K. H. , Elam W. T. , Koon N. C.
Published: July 2002

We have employed extended x-ray absorption fine structure (EXAFS) analysis to study the compositional dependence of the atomic structure in Fe–Tb alloy films. Fourier transforms of EXAFS data, relative to both the Fe K and the Tb LIII absorption edges, provide information about the local atomic environments relative to each atom. Results indicate the Fe EXAFS data to be dominated by Fe–Fe correlations, and consists of contributions from two Fe atomic shells at radial distances near 2.47 and 2.66 Å and a Tb shell near 2.91 Å. The coordination number of the Fe shells are measured to increase, while radial distances decrease, with increased Fe content. The Tb EXAFS data was found to have an atomic shells of Fe and Tb at 2.91 and 3.47 Å, respectively. Analysis suggests that the Fe shell is very disordered and is comprised of approximately 9.5 atoms while the Tb shell has ≊3 atoms.
Structural and soft magnetic properties of Fe/Fe–Hf–C multilayered films with the high B
s and thermal stability
Authors: Hasegawa Naoya , Kataoka Noriyuki , Fujimori Hiroyasu , Saito Masamichi
Published: July 2002

Fe/nanocrystalline–Fe–Hf–C multilayers were prepared by annealing sputter-deposited Fe/amorphous–Fe–Hf–C multilayers. The grain size of the Fe layer is also refined by the multilayer effect. The columnar growth is interrupted even by a very thin Fe–Hf–C layer, when epitaxial growth of the crystalline phase on the Fe layer in the early stage of the amorphous Fe–Hf–C layer deposition is prevented. The annealed multilayers have good soft magnetic properties (‖μ‖1 MHz=6000 max., Hc = 9.6 A/m min.) as well as high-saturation flux density (Bs = 1.9–2.1 T). Furthermore, the soft magnetic properties were maintained even after annealing at 1031 K. This high thermal stability is probably due to the extremely fine and stable HfC particle dispersed in the Fe–Hf–C layer.
Dependence of interface anisotropy on rare earth in R/Mo (R=Dy,Er) multilayers
Authors: Perera P. , O’Shea M. J.
Published: July 2002

We report on the magnetic properties of Er/Mo multilayers and compare to our earlier work on Dy80Ni20/Mo multilayers. The magnetic order in these multilayers is not collinear and most likely is speromagnetic, much like a spin glass. Magnetic fields as high as 55 kOe do not saturate the magnetization. Both the interface anisotropy Ks and the bulk anisotropy are determined. We find a qualitative correlation between Ks and the single ion anisotropy (calculated from the bulk anisotropy) with Dy multilayers where the single ion anisotropy is larger having a larger value of Ks than Er multilayers.
New magnetic phase in the quasi-one-dimensional hexagonal antiferromagnet CsMnBr3
Authors: Kotyuzhanskii B. Ya. , Nikiforov D. V.
Published: July 2002

The static magnetic properties of the easy-plane antiferromagnet CsMnBr3 were studied by means of a three-coordinate vibrating sample magnetometer in fields up to 75 kOe and in the temperature range 1.7–80 K. It was revealed that in some field interval, H⊥c, a magnetization component M∥c appears in the crystal at low temperatures. The results obtained allow us to conclude that between a triangular phase and a spin flop one discovered earlier there exists an intermediate magnetic phase with spins deviating from the basal plane. Consideration of the static magnetic properties based on a thermodynamic potential Φ, which contains all second-order magnetic terms allowed by crystal symmetry, has shown that the observed phase diagram can take place at definite ratios of the uniaxial anisotropy and the weak exchange within the basal plane.
Origin of coercivity in Co/Pt sputtered multilayers
Authors: Honda S. , Tanimoto H. , Ago J. , Nawate M. , Kusuda T.
Published: July 2002

Co/Pt multilayers have been prepared by rf magnetron sputtering onto glass substrates with negative bias voltage. For 0-V bias, the film consists of the columns grown perpendicular to the substrate and the convex interfaces between layers. The column boundaries and the convex interfaces act as the pinning center for the domain wall motion resulting in the high perpendicular coercivity, Hc(⊥ ). In the −30-V biased multilayers, the columns are fine and close packed, and thereby the layer interfaces are smooth and sharp. So that Hc(⊥ ) is very small in these multilayers. Higher bias voltage of −70 V causes the obscure interfaces and decreases the anisotropy energy. In the Co/Pt multilayer deposited on 0 V-Pt underlayer, the coercivity Hc(⊥ ) increases with thickness of 0 V-Pt underlayer and reaches 4 kOe at 1000 Å thickness. On the other hand, for −30 to −50-V Pt underlayer, the layer interfaces are smooth and sharp resulting in small Hc(⊥ ). Thus, Hc(⊥ ) is strongly affected by the film morphology such as the column boundaries or the layer interfaces.
The Heisenberg antiferromagnet with a low concentration of static defects
Authors: Wan C. C. , Harris A. B.
Published: July 2002

The static and dynamic response associated with a low concentration, x, of static defects in a Heisenberg antiferromagnet at zero temperature is analyzed within linearized spin-wave theory. We obtain the dispersion relation for long-wavelength spin waves in the form ω(q)= c(x)q + iγ(x)qτ. Our results for c(x) agree with previous work, and in particular give c(x) = c(0)[1 + αx + O(x2)] where the coefficient α, which can be related to the helicity modulus and the uniform perpendicular susceptibility, diverges in the limit d→2, where d is the spatial dimensionality. One major new result is that τ=d−1 for defects whose spin, S′, is different from that (S) of the host lattice and τ=d+1 when S′=S.
Sm2Fe17Nx: Site and valence of the interstitial nitrogen (abstract)
Authors: Mishra R. K. , Capehart T. W. , Pinkerton F. E.
Published: July 2002

The discovery that nitriding Sm2Fe17 greatly enhances its Curie temperatures and the magnetic anisotropy, making it potentially attractive for permanent magnet applications, has intensified research efforts to understand the physical and chemical properties of this material. In this work we have combined x-ray diffraction, x-ray absorption near edge structure (XANES), and extended x-ray absorption fine structure (EXAFS) to determine the position and valence of the nitrogen atoms in the expanded Sm2Fe17 lattice following nitriding. The XANES from the Sm L3 edge and Fe K edge of Sm2Fe17 before and after nitriding reveals that the valences of both Sm and Fe are unchanged. This implies that the nitrogen is incorporated covalently into this material, i.e., there is no net charge transfer between the N atoms and the Sm or Fe. From the observed changes in the Sm EXAFS, we determine the changes in the local Sm environment produced by nitriding. It is determined that in Sm2Fe17Nx there are 2.8±1.0 nitrogen atoms at a distance of 0.254 nm from the Sm site. X-ray diffraction patterns of the Sm2Fe17 before and after nitriding show the original Th2Zn17 lattice expands such that c and a lattice parameters change from 1.2435 and 0.8548 nm to 1.2639 and 0.8731 nm, respectively. The above valence, coordination number, Sm-N distance, and lattice parameters are consistent with N atoms occupying the 9(e) interstitial sites of the expanded lattice.
Magnetic phase transition and magnetocrystalline anisotropy of Sm2Fe17CxNy
Authors: Kou X. C. , Grössinger R. , Li X. , Liu J. P. , de Boer F. R. , Katter M. , Wecker J. , Schultz L. , Jacobs T. H. , Buschow K. H. J.
Published: July 2002

Alternating current (ac) susceptibility and high magnetic field measurements were performed to investigate the magnetic phase transitions and the magnetic anisotropy fields in Sm2Fe17CxNy with x=0, 0.4, 0.7, and 0.9. An unidentified magnetic phase transition is observed in all Sm2Fe17CxNy compounds, which is neither due to a spin reorientation nor due to a first-order magnetization process (FOMP) transition. Samples of Sm2Fe17CxNy (x≳0) have even higher anisotropy fields than are found in Sm2Fe17Ny.
Effects of surface adsorption on the properties of α-Fe2O3 fine particles
Authors: Wang G. H. , Zhong X. P. , Wang Q. , Luo H. L. , Hou D. L.
Published: July 2002

The properties of fine particles of α-Fe2O3 (sample A), and α-Fe2O3 coated with cobalt (sample B) and adsorbed with pyridine (sample C) have been investigated by means of magnetic measurements, Mössbauer spectra and x-ray photoelectron spectroscopy (XPS). The size of these fine particles is about 500 Å. The Morin transition temperatures Tm for samples A, B, and C are 191, <80, and 224 K, respectively. The Mössbauer spectra of the samples were measured in a temperature range from 80 to 300 K and the angles between 〈111〉 axis and the spin of iron ions for these samples have been calculated from these spectra. The XPS results show that the Fe 2p and 3d peaks of sample C shift to lower energy regions compared with those of sample A. It seems to be that some electrons may move from pyridine molecules to the iron atoms in sample C. The reasons for the Tm change of samples B and C were discussed on the basis of the experimental results.
Magnetic properties of ultrathin films of Ni/Cu(001) probed by soft-x-ray magnetic circular dichroism (abstract)
Authors: Tjeng L. H. , Idzerda Y. U. , Rudolf P. , Sette F. , Chen C. T.
Published: July 2002

The magnetic properties of MBE-grown ultrathin Ni films on a Cu(001) substrate are investigated using soft-x-ray magnetic circular dichroism (SXMCD) spectroscopic technique. These measurements are carried out by recording Ni L2,3 absorption spectra with highly circularly polarized soft x rays recently obtained at the AT&T Bell Laboratories Dragon synchrotron radiation beamline.1 For low Ni coverages, no MCD effect along the 〈100〉 direction is observed at room temperature, while for intermediate coverages, the MCD effect becomes appreciable and increases to its bulk value at ∼25 ML. With only the assumption that the unoccupied density of states of Ni lie mainly in the minority bands, the amount of the MCD effect can be related to the magnetic moment. The thickness and temperature dependence of the magnetic moment obtained from our SXMCD measurements are compared with those of Ni/Cu(111) obtained from magneto-optical data.2 The advantages and differences between the SXMCD and MOKE techniques are discussed. The novel SXMCD technique allows for element specific magnetic characterization of a variety of interesting structural architectures including 2 D films, dilute impurities, and disordered matrices.
Cluster relaxation in iron-rich amorphous FeZr alloys near T
c
Authors: Ren Hong , Ryan D. H.
Published: July 2002

A cluster relaxation model has been used to fit the Mössbauer spectra of iron-rich amorphous Fe–Zr alloys near their ordering temperatures. While the results are consistent with the presence of relaxation, deduced cluster sizes are an order of magnitude smaller than expected. Incorporating static magnetic and structural disorder into the model extends the temperature range over which the data may be fitted.
Current transport in Y1Ba2Cu3−xMxO7−δ (M≡Fe, Zn) epitaxial thin films (abstract)
Authors: Ogale S. B. , Bendre S. T. , Guptasarma Prasenjit , Multani M.
Published: July 2002

The critical current density (Jc) and the nature of its temperature dependence have been studied for epitaxial thin films of Y1Ba2Cu3−xMxO7−δ (M≡Fe, Zn). These films were deposited on (100) oriented Y-stabilized ZrO2 substrates by pulsed excimer laser ablation and were characterized by four probe resistivity, x-ray diffraction, and scanning electron microscopy. The critical currents were measured on a 20 μm wide and 200 μm long laser patterned microbridges. In addition to their direct interaction with the superconducting state the impurities also lead to changes in the growth features of the films which in turn affect their critical properties. The problems which have been addressed in this context of transport property systematics include effect of impurity scattering on Cooper pair condensation amplitude, the effects of impurity induced twin boundary layers on transport and the flux pinning effects at such planar defect centers, as well as the growth related grain texture features.
Epitaxial strain, metastable structure, and magnetic anisotropy in Co-based superlattices (invited)
Authors: Clarke Roy , Elagoz S. , Vavra W. , Schuler E. , Uher C.
Published: July 2002

We explore the relationship between interface structure and magnetic anisotropy in three types of Co-based superlattices: Cohcp-Au; Cofcc-Cu; and Cohcp-Cr, grown epitaxially on GaAs(110). For very thin layers of Co, Co-Au, and Co-Cu superlattices exhibit a perpendicular easy axis due to magnetoelastic contributions to the anisotropy energy. The magnetic anisotropy in Co-Cr is found to be strongly dependent on growth conditions. At slow deposition rates of Co the interface between Co and Cr becomes diffuse as is evidenced by a low saturation moment and a shift toward perpendicular anisotropy whereas samples with abrupt interfaces show predominantly parallel anisotropy. The Cr layers grow in a metastable hcp phase which appears to be paramagnetic. The results illustrate the influence of the heterointerface on magnetic properties.
Vapor-deposition-induced magnetic anisotropy in amorphous RE-TM alloys (invited) (abstract)
Authors: Hellman F.
Published: July 2002

Macroscopic anisotropy may be induced in amorphous materials by either preparation or subsequent annealing under appropriate conditions. Large, macroscopic magnetic anisotropy is observed in amorphous rare earth–transition metal (a-RE-TM) alloys produced by vapor deposition. The vapor deposition process inherently contains characteristic directions about which an anisotropy can occur. These include the substrate, incident atomic beams, and magnetic or electric fields at the surface during growth. These intrinsic directions combine to produce effects such as columnar microstructure, stress and strains, pair ordering in alloys, and nonperpendicular film growth directions. All of these structural effects potentially can produce magnetic anisotropy. The presence of the localized nonspherical f-electron nature of the RE ions causes a large local magnetic anisotropy; a macroscopic structural anisotropy can then cause a coherence in the directions of the local anisotropy axes, thereby producing a large macroscopic magnetic anisotropy. When the induced structural anisotropy is local in nature, it can be subtle and difficult to resolve, even though the resultant magnetic anisotropy itself is easily measured. We have used the magnetic anisotropy directly to monitor the effects of the important parameters of vapor deposition growth (e.g., substrate temperature, alloy composition, state of stress during growth, incident atomic beam angles) in order to understand what fundamental growth process is causing the anisotropy. Both the local anisotropy (as reflected by the high-field magnetic susceptibility) and the macroscopic anisotropy has been analyzed. I will present results which show that the dominant contribution to the macroscopic anisotropy in a-Tb-Fe is an amorphous phase texturing at the surface during the growth, due to minimization of the free energy of the surface. This effect is analogous to well-known effects in growth of crystalline materials. A secondary contribution due to stress plus magnetostriction is also observed.
Low temperature magnetic properties of U1−xPrxAl2
Authors: Kuwai T. , Miyako Y.
Published: July 2002

The low temperature properties of U1−xPrxAl2 (0<x<0.15) were studied by susceptibility and specific heat measurements down to 100 mK, focusing attention on the mechanisms of spin fluctuations. The low temperature behavior of C/T has been explained by the paramagnon theory for low energy excitations with long wavelengths. However, the spin fluctuations in wider temperature ranges in UAl2 seems to be more complicted as suggested by a recent nuclear magnetic resonance experiment. In UAl2, the susceptibility and the specific heat show two different temperature ranges, above and below about 50 K where there is a crossover from a single impurity Kondo regime to a low temperature coherent Kondo state. On the other hand, the enormous increase of γ with x in U1−xPrxAl2 seems to be a consequence of the static f and d-band structures near the Fermi level.
Bulk c-axis texture in Ho-Ba-Cu-O material produced by magnetic annealing
Authors: Holloway A.
Published: July 2002

A novel technique, consisting of the application of a magnetic field during grain growth process for materials having an anisotropy of the normal state paramagnetic susceptibility, has been tested on the example of HoBa2Cu3O7−δ (HoBCO) superconducting compound. It has been experimentally observed that the application of an 8-T magnetic field during sintering results in a directional growth of microcrystals and considerable bulk degree of grain alignment. A combination of x-ray, magnetic and optical measurements gives evidence that the degree of c-axis texture, produced by this technique, can approach 100%.
Photoconductivity of Cd0.7Mn0.3Se
Authors: Jiang S. , Ma K. J. , Shen S. C.
Published: July 2002

Photoconductivity spectra of Cd0.7Mn0.3Se have been measured between 50 and 300 K and in the wavelength range from 550 to 800 nm. The energy gap of Cd0.7Mn0.3Se changed linearly with temperature with a temperature coefficient dEg/dT of about −7×10−4 eV/K. A photoconductivity peak and a photoconductivity valley, associated with Mn2+, have been found. The possible transition mechanisms in Cd1−xMnxSe are discussed.
Silane adsorption on Si(001)2×1
Authors: Hirose Fumihiko , Suemitsu Maki , Miyamoto Nobuo
Published: July 2002

Surface hydrogen coverage and the surface reconstruction of a silane-saturated Si(001)2×1 surface were investigated using the thermal-desorption-spectroscopy (TDS) and the reflection-high-energy-electron-diffraction measurements. The TDS spectrum mainly presented a single β1 peak around 520 °C, indicating the predominance of a Si monohydride phase on this surface. This observation agreed with the surface hydrogen coverage (H/Si=1.1–1.4) obtained from the integrated peak area of the TDS spectrum. By a repeated silane-saturation/thermal-desorption experiment, it was also clarified that all the hydrogen atoms in the silane molecules adsorb at this room temperature exposure. Adsorption mechanisms of silane molecules onto Si(001) surfaces are discussed and a model is presented based on the result.
Surface resistance studies of laser-deposited superconducting Tl2Ba2CaCu2O8 films
Authors: Eddy M. M. , Sun J. Z. , Hammond R. D. , Drabeck L. , Ferreira I. B. , Holczer K. , Grüner G.
Published: July 2002

We have measured the surface resistance Rs of laser-deposited Tl2Ba2CaCu2O8 thin films on LaAlO3 substrates. After correcting for leakage of radiation through the films, we find that the normalized surface resistance Rs/RN (with RN the normal-state surface resistance at T ≥ Tc) is comparable to YBa2Cu3O7 (123) films. The higher transition temperature of the Tl films leads to superior microwave performance in particular at temperatures around T=77 K and above.
Electron lifetime at the surface of insulators: α-S, red HgI2, and p-chloranil
Authors: López-Cruz Elías
Published: July 2002

An analysis using the Simhony and Gorelik model of some experimental results reported in the literature concerning the transit time measurements in α-S, red HgI2, and p-chloranil is made. Values for the electron lifetime at the surface of these materials are obtained. For sulfur single crystals this lifetime ranges from 1 to 6 ms, for HgI2 it is around 10 ns, and for p-chloranil this lifetime is 1.3 μs. For the case of α-S crystals, it is concluded that the processes influencing the lifetime are of the bulk type in the sense that the lifetime at the surface is of the same order of magnitude as in the bulk.
Growth and self-pumped phase conjugation of Ce-doped KNa(Sr0.61Ba0.39)0.9Nb2O6 crystal
Authors: Sun Daliang , Chen Jun , Lu Xinliang , Song Yongyan , Shao Zongshu , Chen Huanchu , Xu Jingjun , Wu Yuanqing , Liu Simin , Zhang Guangyin
Published: July 2002

Self-pumped phase conjugate in trapezoid-cut Ce-doped KNa(Sr0.61Ba0.39)0.9Nb2O6 crystal was demonstrated without external reflection mirrors and applied field, phase conjugation reflectivities as high as 25% were measured. The threshold power was lower than 0.03 W/cm2. The device operated by one-region four-wave mixing using the photorefractive effect.
Investigation of strain in metalorganic vapor-phase epitaxy grown ZnTe layers by optical methods
Authors: Leiderer H. , Jahn G. , Silberbauer M. , Kuhn W. , Wagner H. P. , Limmer W. , Gebhardt W.
Published: July 2002

We have studied the strain in ZnTe epilayers grown by atmospheric-pressure metalorganic vapor-phase epitaxy on (001) GaAs and GaSb substrates. Reflectivity and absorption measurements are performed at 2 K using single-crystalline layers with thicknesses of 0.2–2 μm. The biaxial strain in the samples caused by the lattice mismatch of layer and substrate is deduced from the splitting of the degenerate heavy- and light-hole exciton. A polariton model is used to describe the reflectivity structure at the E0 gap and to determine the transverse exciton energies. The deformation potentials obtained from an analysis of the absorption structures are a=−5.5 eV and b=−1.4 eV. The critical thickness for ZnTe/GaSb is lower than 0.8 μm near thermodynamic equilibrium. It also slightly depends on growth temperature which has its optimum at 345 °C.
Radiation damage and optical properties of Ar+-implanted GaP
Authors: Wendler E. , Wesch W. , Götz G.
Published: July 2002

The defect production in GaP during Ar+ ion implantation at 300 and 80 K is investigated by means of Rutherford backscattering spectrometry and optical-absorption spectroscopy. It is shown that the transformation to the amorphous state in the medium dose region is different for the two temperatures and differs remarkably from the behavior observed in GaAs. An attempt is made to understand the amorphization process in the framework of previously predicted overlap damage models. Contrary to the situation in GaAs, in GaP the number of overlaps of primarily produced clusters necessary to amorphize a certain region increases with decreasing temperature connected with an increasing area damaged by one ion. A possible explanation is a modified change of the primarily produced damage regions in GaP compared to GaAs. Generally, however, the primarily damaged regions are more stable in GaP than in GaAs.
A numerical simulation of the R-curve behavior in microcracking materials
Authors: Murat Michael , Englman Robert , Jaeger Zeev
Published: July 2002

We propose several models for crack propagation in ceramic materials with microcrack formation and coalescence. The models differ in the relative time scales involved in the internal relaxation processes and the external loading rate. We find that when the external loading rate is very high, a rising R-curve is obtained. For slow external loading, the results depend upon the relative rate of the different internal relaxation processes and upon the distribution of the critical stresses needed to form the microcracks. In this regime, the existence of a rising R-curve behavior is found to be primarily due to microcrack shielding.
ac capacitance and conductance measurements of two-terminal metal-oxide-semiconductor-oxide-semiconductor capacitors on silicon-on-insulator substrates
Authors: Flandre D. , Campabadal F. , Esteve J. , Lora-Tamayo E. , Van de Wiele F.
Published: July 2002

Results of combined ac capacitance and conductance measurements on two-terminal metal-oxide-semiconductor-oxide-semiconductor capacitors fabricated in a silicon-on-insulator substrate formed by oxygen implantation are presented for the first time. We demonstrate the efficiency of the technique to investigate the presence of bulk traps presumably caused by metallic contaminants introduced during the high-energy oxygen implantation, as well as to determine the interface trap density at the buried oxide/Si substrate interface.
Transition from elastic to dissipative motion in a magnet–high-T
c superconductor system
Authors: Johansen T. H. , Bratsberg H. , Yang Z. J. , Guo S. J. , Loberg B.
Published: July 2002

Using quasistatic harmonic excitations, we have measured the lateral force between a permanent magnet and a Bi-based ceramic high-Tc superconductor (HTSC) for lateral displacement amplitudes down to 1 μm. We find clear evidence for a transition from elastic (nondissipative) to inelastic interaction, and attribute the effect to flux pinning. The crossover amplitude can easily reach several micrometers, with the consequence that the lateral disturbance of a levitated magnet will decay to this amplitude, whereas further damping will be extremely slow. For applications of HTSCs in magnetic bearing systems this can be a very relevant aspect of the interaction, and it can set the limit for precision positioning of levitated objects.
Carrier-induced optical nonlinear effects in semiconductor quantum well wire structure
Authors: Ando Hiroaki , Oohashi Hiromi , Kanbe Hiroshi
Published: July 2002

Carrier-induced optical nonlinearities in a GaAs quantum well wire structure are calculated within the framework of the effective mass approximation to clarify the nonlinear features of the one-dimensional (1D) system. In the calculation, the excitonic nonlinearity due to phase-space-filling, band-gap renormalization, and Coulomb screening effects is estimated on the basis of an effective 1D Coulomb potential. The 1D potential is derived, taking wire cross section into account. The change in the optical absorption for the band-to-band transition is evaluated, considering the band-filling effect and the changes in the Sommerfeld factor caused by the screening effects. The numerical results show that a blue shift of the excitonic absorption peak occurs at the absorption edge because the blue shift caused by the screening effect exceeds the red shift due to the band-gap shrinkage. In the spectral region above the band edge, where the band-to-band transition determines the optical properties, a decrease in optical absorption caused by band filling is compensated by increasing absorption, which stems from recovery of the Sommerfeld factor because of the Coulomb screening. The effect of the Coulomb interaction in a quantum wire laser is also discussed in relation to the gain spectrum in the high excitation regime. These results show that exact inclusion of the Coulomb interaction is crucial in estimating both linear and nonlinear optical properties in the 1D system.
Effect of guideway discontinuities on magnetic levitation and drag forces
Authors: Rossing Thomas D. , Korte Rodney , Hull John R.
Published: July 2002

Transients in the lift and drag forces on a NdFeB permanent magnet were observed as the magnet passed over various discontinuities in a rotating aluminum disk at velocities of 4 to 25 m/s. For full cuts in the disk, the amplitude of the lift and drag transients and the wave form of the drag transient depend on the width, and the amplitudes are much larger than for partial cuts. The use of a backing plate to join two cut segments is ineffective.
Tunable (Al)GaAs lasers using impurity-free partial interdiffusion
Authors: O’Brien S. , Shealy J. R. , Chambers F. A. , Devane G.
Published: July 2002

Impurity-free interdiffusion has been used to fabricate single mode quantum well ridge lasers from the same (Al)GaAs epitaxial material which differ in emission wavelength by as much as 11.7 nm. This represents a shift of approximately 80 laser linewidths, as measured under pulsed conditions. Threshold currents for the interdiffused and nondiffused lasers are nearly identical. However, the differential quantum efficiencies of the most interdiffused lasers are a factor of 2 lower than nondiffused lasers. The interdiffusion coefficients for the quantum well laser structure are approximately a factor of 6 smaller than those observed for intrinsic GaAs multiple quantum wells.
Photoacoustic investigation of low optical absorption in several nonlinear-optical crystals
Authors: Wang Guifen , Ma Genyuan , Zhang Guangyin , Zhang Gefeng , Li Xue Qian
Published: July 2002

We use developed high-sensitivity photoacoustic cell to determine the absorption coefficient in crystals of pure LiNbO3, Mg-doped LiNbO3, Fe-doped LiNbO3, KH2PO4, and KD2PO4 at the wavelengths of 1060.0, 632.8, 514.5, 488.0 nm. The experimental results are in good agreement with those measured by the laser calorimetric method.
Metalorganic chemical vapor deposition of PbTe films on GaAs substrates
Authors: Pollard K. T. , Erbil A. , Sudharsanan R. , Perkowitz S.
Published: July 2002

PbTe films on GaAs substrates were grown by metalorganic chemical vapor deposition for the first time, at substrate temperatures as low as 300 °C. Films grown directly on GaAs below a substrate temperature of 400 °C showed (111) orientation on (100) GaAs and (100) orientation on (111) GaAs. However, above 400 °C on GaAs and on CdTe buffer layers, the films followed the orientation of GaAs and CdTe, respectively. Hall measurements showed n-type conductivity in PbTe films grown directly on GaAs regardless of the Te/Pb input gas ratio and substrate temperature. The conductivity type of PbTe films grown on CdTe buffer layers depended on the input gas ratio of Te/Pb. The resistivity measured by Hall measurement was higher than that determined from infrared data, suggesting microcracks in the films.