bluuebunny/crossref_metadata_2025 huggingface dataset:
Fast cathode processes in vacuum discharge development
Authors: Mazurek B. , Cross J. D.
Published: July 2002

Results of high-speed streak photography of the initial stages of a discharge in technical vacuum are presented. The roles of electrode material, electrode spacing, gas pressure, and gas composition were investigated as well as the influence of cathode temperature. The results show that the visible light in the initial stages of the discharge oscillates in intensity with a period of between 4 and 5 ns. The period is little changed by any of the parameters studied. The results are examined in the light of three models for the process; explosive emission, oscillations in layers of solid adsorbate on the cathode, and oscillations in the space charges in the gap. The results definitely exclude explosive emission and do not fit well with the requirements of oscillations in cathode surface layers. It is concluded that space-charge oscillations are responsible for observed oscillations in visible light intensity and gap current.
A model for damage release in ion-implanted silicon
Authors: Cerofolini G. F. , Meda L. , Volpones C.
Published: July 2002

A model to describe the amorphization in ion-implanted silicon is presented, based on the idea that a transferred energy threshold (around 5 keV) is necessary to produce a sufficiently branched and energetic cascade. This amorphizing cascade is seen first as energetic two body collisions, then as the branching of the primary skeleton and, at the final stages, as a hot cloud in which the primary energy is equipartitioned, and remains adiabatically confined before cloud quenching. This view allows some quantitative estimations: the dimension of the cloud, the threshold energy for their formation, and the number of the involved atoms. Original experimental evidence is presented to support this framework. In principle, the behavior of boron implanted in (100) silicon, at a constant fluence (9×1015 cm−2) and at a variable impinging energy (in the range 20–45 keV), puts in evidence the possibility to amorphize only above a critical threshold. The position of the amorphized regions also suggests that amorphization can take place only where sufficiently energetic recoils are produced. The existence of point defects out of the amorphized regions and their progressive involvement causes superlinear amorphization effects. This mechanism is viewed, in the framework of this model, as a decreasing of the amorphization threshold energy, because of the releasing of the residual energy by the involved point defects. The predictions of the model are compared with the experimental data referring to N+ implanted into (111) silicon at 40 keV, and the resulting agreement is quite satisfactory.
Seed shape dependence of Si solid-phase epitaxy: Preferential facet growth
Authors: Murakami Eiichi , Moniwa Masahiro , Kusukawa Kikuo , Miyao Masanobu , Warabisako Terunori , Wada Yasuo
Published: July 2002

A new facet formation mode in Si lateral solid-phase epitaxy (L-SPE) of amorphous Si on SiO2 is presented. This mode occurs in the case of L-SPE using a two-dimensional seed area. Depending on whether the seed shape is convex or concave, a {111} facet with a slow SPE rate or a {110} facet with a fast SPE rate dominates in the case of {100} Si substrate. This preferential facet growth is explained using a microscopic model of crystallization, and seems to be generalized to other crystal growth and etching processes. Similar results are obtained for seeds surrounded by an SiO2 layer and for seeds surrounding an SiO2 island. These results must be considered in practical device design.
Rh4Si5 formation in the multilayer geometry: Explosive reaction versus nucleation-controlled kinetics
Authors: Floro Jerrold A.
Published: July 2002

Rh4Si5 formation occurs by two different mechanisms, depending on the film geometry. Thermal annealing of a Rh film on a silicon substrate results in the formation of Rh4Si5 at about 850 °C by a process described as ‘‘nucleation controlled.’’ Alternatively, when the film consists of thin Rh/Si multilayers, Rh4Si5 can form at room temperature in a very rapid, self-sustaining fashion that is the chemical-reaction analog of the explosive crystallization process. This mechanism of explosive silicide formation, reported recently for the first time [J. A. Floro, J. Vac. Sci. Technol. A 4, 631 (1986)], apparently occurs in the multilayer geometry due to the large driving forces and reduced diffusion lengths associated with this configuration.
Broad-area semiconductor lasers with gain-length variation for lateral mode control: The bow-tie geometry laser
Authors: Sheem S. K. , Vojak B. A.
Published: July 2002

A novel idea is proposed to control the spatial mode profile of broad-area semiconductor lasers in which the cumulative optical gain in the direction of light propagation is tailored by varying the effective length of the contact electrode across the lateral direction. Special interest is on a structure with the highest gain near the center of the device so as to favor the lower-order lateral modes of oscillation. Preliminary experiments indicate that the concept works as intended.
Low-temperature annealing of As-implanted Ge
Authors: Hattangady S. V. , Fountain G. G. , Nicollian E. H. , Markunas R. J.
Published: July 2002

Furnace annealing (FA) and rapid thermal anealing (RTA) of As75-implanted Ge is studied and contrasted. Activation has been observed in furnace-annealed samples at 500 °C. Rapid thermally annealed samples show activation at 575 °C and thereafter. Diffusion effects are significant during FA above 575 °C, while RTA is accompanied with very little dopant diffusion. Damage annealing is best in the FA samples as indicated by the mobility profiles. A dual process such as a 430 °C-FA/650 °C-RTA offers best results for activation, especially in the case of low-dose implants (∼97%). Carrier concentration profiles resemble theoretical implant profiles except near the surface where a region of high concentration is observed.
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u observation of interactions between Bloch walls and local crystalline defects
Authors: Hartmann U.
Published: July 2002

The influence of local crystalline defects upon the magnetization reversal process of single-crystalline iron whiskers has been investigated by direct observation of the domain configuration and by inductive magnetization measurements. As an essential result, the investigations bring to evidence that only weak interactions are obtained between the dislocations and 180° Bloch walls. On the other hand, a strong pinning efficiency to 90° Bloch walls is observed which is due to the stress field created by the perturbation.
High dose Fe implantation of graphite at elevated temperature
Authors: Lusnikov A. , Ohana I. , Dresselhaus M. S. , Withrow S. P.
Published: July 2002

Highly oriented pyrolytic graphite samples have been implanted at elevated temperature (450 °C) with 57Fe species at high fluence (2×1017 cm−2) and a bombarding energy of 35 keV. Under these conditions of implantation, materials with ferromagnetic properties have been synthesized, as demonstrated by magnetization measurements using a superconducting quantum interference device magnetometer. It is further shown that the samples exhibit ferromagnetic, paramagnetic, and diamagnetic contributions to the total magnetization. The value of the measured coercive force and a comparison of scanning electron microscopy studies of the sample surface with the calculation of the saturation ferromagnetic moment suggest that fine iron particles are created on the surface. A mechanism is proposed to explain these observations. Complementary Rutherford backscattering spectroscopy and Raman measurements indicate that the lattice is not completely amorphized under these implantation conditions.
Two-dimensional effects in two-terminal n+-p-n+ devices fabricated by planar technology
Authors: Jain S. C. , Staszak Z. J. , Musallam A. , Mattson R. H.
Published: July 2002

I-V characteristics of two-terminal n+-p-n+ devices fabricated by planar technology are studied theoretically and experimentally. A quasi-two-dimensional (2D) model is used to interpret the experimental results. The results suggest that the 2D effects are a very likely candidate to explain the difference in our results and those obtained with the conventional 1D BARRIT diodes.
Environmental influence on the nuclear spin-lattice relaxation rate in CuxPt1−x
Authors: Banhart J. , Weinberger P. , Ebert H. , Voitländer J.
Published: July 2002

The influence of the surrounding atomic configuration on the nuclear spin-lattice relaxation rate of Pt in disordered CuxPt1−x alloys has been studied theoretically by calculating the electronic structure of CuPt clusters embedded in a coherent potential approximation medium. The resulting average nuclear spin-lattice relaxation rate is found to be in a very satisfying agreement with experiment.
The track-edge bias profile in shunt-biased magnetoresistive heads
Authors: Heim D. E.
Published: July 2002

We present a micromagnetic calculation of the variation in bias angle near the track edge in shunt-biased magnetoresistive heads. This track-edge bias profile shapes the head response at the track edge and hence determines the minimum track spacing based on considerations of adjacent track crosstalk. Our results show that the width of the bias profile at track edge can be substantial and that conditions for minimizing it conflict with those which maximize signal amplitude.
Behavior of thin-film magnetic recording heads and their application to micromagnetics (invited)
Authors: Walker I. R. , Arrott A. S.
Published: July 2002

The ac response at frequencies to 50 MHz of two thin-film magnetic recording heads placed in mirror symmetry with respect to one another is measured as a function of relative position and applied dc fields. One driven head can be used to sense the presence of a second head even when the second head is not driven. An array of wires with current in alternating directions from wire to wire is used to obtain the absolute response of thin-film heads. The efficiency of a head is found to be about 2/3.
Stoner theory of magnetic structure of alternate cubic phases of transition metals
Authors: Fry J. L. , Zhao Y. Z. , Pattnaik P. C. , Moruzzi V. L. , Papaconstantopoulos D. A.
Published: July 2002

Epitaxial growth of bcc Co on a (110) GaAs substrate has demonstrated the feasibility of producing thin films of transition metals with structures not usually stable at room temperature and pressure. Thus an entirely new group of magnetic materials may be possible with interesting and perhaps significant magnetic properties. In this paper the simple Stoner theory of ferromagnetism is employed, with exchange-correlation parameters obtained from Janak’s work and density of states at the Fermi level determined from self-consistent, paramagnetic energy-band calculations. The theory is applied to each of the 3d and 4d transition metals in both fcc and bcc phases. Ferromagnetism is obtained for bcc Fe, Co, and Mn and fcc Co and Ni, and strongly enhanced paramagnetism for bcc Sc, Ni, and Y and fcc Sc, Fe, and Pd. Comparisons are made with predictions of total energy and enhanced magnetic susceptibility calculations.
Antiferromagnetism in the high-T
c related compounds (invited)
Authors: Sinha S. K. , Moncton D. E. , Johnston D. C. , Vaknin D. , Shirane G. , Stassis C.
Published: July 2002

Magnetic susceptibility measurements on La2CuO4−y have revealed maxima in temperature indicative of antiferromagnetic ordering. Powder diffraction measurements have demonstrated the existence of superlattice peaks at temperatures below those corresponding to the susceptibility maxima, and polarized neutron measurements have confirmed that the peaks are indeed of magnetic origin. The deduced antiferromagnetic structure in the orthorhombic (Cmca) phase of La2CuO4−y consists of ferromagnetic sheets of Cu atoms in the b-c plane which are alternating in sign along the a axis [100]. The moment is parallel to the c axis. The Neel temperature is extremely sensitive to the oxygen deficiency y and decreases rapidly to zero as y goes from 0.03 to 0. The saturated moment is ≂0.4 μB/Cu atom for samples with the highest TN (290 K). Antiferromagnetic spin fluctuations have also been observed by polarized neutron scattering in the paramagnetic phase above TN. Recent results obtained on magnetic ordering in the YBa2Cu3O6−y family of compounds are also discussed, together with the possible implications for novel superconductivity mechanisms.
Spin-wave excitations in amorphous Fe78B13Si9
Authors: Yu S. C. , Lynn J. W. , Rhyne J. J. , Fish G. E.
Published: July 2002

Inelastic neutron scattering measurements have been used to study the long wavelength spin dynamics of the high Tc amorphous ferromagnetic alloy Fe78B13Si9 (Metglas 2605S2). Spin waves were observed over the accessible wave-vector range of 0.06 Å−1≤q≤0.12 Å−1, for temperatures between 473 K (0.67 Tc) and 705 K (0.99 Tc). The magnon dispersion curves exhibit the conventional quadratic relationship E=D(T)q2+Δ, typical of an isotropic ferromagnet, where the small energy gap Δ≊0.05 meV is attributed primarily to the dipole-dipole interaction. An estimate of the T=0 value of D≊156 meV Å2 was obtained from a plot of D(T) vs T extrapolated to low temperatures, while the Curie temperature of 710 K was obtained from the extrapolation of D(T) to zero at high temperatures. Spin-wave linewidth data for T/Tc≊0.95 showed the q4 wave-vector dependence expected for magnon-magnon interactions.
‘‘Zero’’ frequency spin relaxation in reentrant magnets
Authors: Jackson E. M. , Bhagat S. M. , Liao S. B. , Manheimer M. A.
Published: July 2002

We present the frequency and concentration dependence of Γ1, the parameter which characterizes the low-temperature increase in the magnetic resonance linewidth, for several reentrant magnets belonging to the alloy series (FepNi1−p)75G25 and (FepNi1−p)80G20. A comparison is made and it is shown that, near the spin-glass-reentrant transition concentration, the low-frequency dependence of Γ1 is dramatically different for these two apparently similar systems.
rf sputtered CoCr for magnetography
Authors: Jeanniot D. , Bouchand J. C. , Sache G.
Published: July 2002

In order to improve the resolution in the inking of the dots, vertical anisotropy media configuration is applied to magnetography. The magnetization transition zone length between the dot and the nonmagnetized drum is hoped to be shorter than in the longitudinal configuration. Some preliminary inking tests were successfully carried out on plane samples. A specially designed sputtering group was built for the deposition of CoCr layers. The substrates are full-size drums (diameter=10 cm, length=40 cm) and the Co80Cr20 target is a cylinder (diameter=20 cm, length=50 cm) around the drum. In this paper, the first results obtained with this group are described. The dispersion of the properties of the CoCr layer around the drum is less than 10%. The whole incident power range (up to 15 kW) was scanned. The deposition rate continuously increases with increasing incident power (Pw) up to 9 kW. This rate saturates after 9 kW to a value of 20 Å/s. As in the planar diode sputtering system, the vertical anisotropy of CoCr thin films is improved when high Pw values are used. For these reasons, the optimal incident power was determined as 9 kW. These results show, for the first time, that a dry process could be used for the deposition of a magnetographic medium on a drum.
The effect of Lorentz demagnetization field (LDF) on the saturation magnetization of SmCo5 magnets
Authors: Heh Shiang-Jiun , Chen Swe-Kai
Published: July 2002

In order to have a further understanding of the cause of the increase in the saturation magnetization, 4πMs, of SmCo5 magnets in sintering, saturation magnetization as a function of density, phase change and the grain sizes were measured in this study. While Tawara and Senno considered that the increasing in 4πMs of the ‘‘Ce-Co-Fe-Cu’’ system was due to the improvement of the magnetization alignment caused by increasing the grain size during the magnet sintering process [Y. Tawara and H. Senno, IEEE Trans. Magn. MAG-8, 560 (1972)], it was shown that the Lorentz demagnetization field (LDF) [S. Chikazumi, Physics of Magnetism (Wiley, New York, 1964), Sec. 2-2; B. D. Cullity, Introduction to Magnetic Materials (Addison-Wesley, Reading, MA, 1972), Sec. 4-3, p. 132] was the major factor in the 4πMs increment after the magnet sintering. As a result of calculation by the use of the LDF in the SmCo5 system, an accuracy of ±3% was readily obtained, while that of Tawara and Senno has an accuracy of 29.6% higher than its normalized (or expected) value in their system. But if LDF was involved in the ‘‘Ce-Co-Fe-Cu’’ system, a value of +6.5% can be obtained. Finally, it was thought that the phase change occurring in sintering, e.g., Sm2Co7 to SmCo5, may be a minor factor in favor of the increasing phenomena in the system by the observation of x-ray diffraction patterns.
Magnetic field dependence of the cyclotron effective mass in the Kondo lattice CeB6
Authors: Joss W. , van Ruitenbeek J. M. , Crabtree G. W. , Tholence J. L. , van Deursen A. P. J. , Fisk Z.
Published: July 2002

We report the first observation of a field-dependent mass in a hybridizing f-electron system. CeB6 is an ordered moment heavy fermion system with an electronic specific heat coefficient γ of order 225–300 mJ/mole K2. Using the de Haas–van Alphen effect at temperatures as low as 60 mK in steady magnetic fields as large as 22 T, we observe a cyclotron orbit of frequency 8680 T for fields along the [100] direction. The mass of this orbit was measured at eight fixed fields and found to decrease from 18me to 8me as the field increases from 12 to 22 T. The observed Fermi surface is very similar to that of LaB6, indicating that the f-electrons are largely local rather than itinerant in CeB6, a picture confirmed by bandstructure calculations. The observed field dependence of the cyclotron mass is consistent with the low-energy scale of the system as measured, for example, by the Kondo temperature. Our results are compared with Fermi surface observations in other heavy fermion systems.
Transport properties of CeAl3 under pressure
Authors: Fierz Ch. , Jaccard D. , Sierro J. , Flouquet J.
Published: July 2002

Transport experiments on CeAl3 samples under quasihydrostatic pressure up to 1.63 GPa are reported. Magnetoresistivity measurements in magnetic fields up to 7.3 T from 40 mK to 4 K were performed. Resistivity and thermopower were measured from 1.5 to 300 K. Taking the low-temperature resistivity as ρ=ρ0+AT2, A increases initially with pressure as expected from the zero pressure thermal dilatation. Above 0.2 GPa, both A and ρ0 decrease dramatically with pressure. It is, however, noteworthy that a T2 law of the resistivity can hardly be extracted from the data above 0.2 GPa. The positive low-temperature magnetoresistivity also varies strongly with pressure. At 0.82 GPa the low field bump has disappeared and the magnetoresistivity is still positive up to 2.5 K. The thermopower shows an even more drastic pressure dependence. The negative bump around 3.5 K at zero pressure becomes positive above 0.3 GPa and a complex temperature dependence develops. The temperature variation cannot be scaled by a unique pressure-dependent parameter.
New routes for the synthesis of iron oxide based magnetic materials
Authors: Joshi Sushama , Phase D. M. , Kanetkar S. M. , Ogale S. B.
Published: July 2002

New routes using laser and ion beam techniques are developed to synthesize iron oxide based magnetic materials. The first method, pulsed ruby laser treatment (pulse duration 30 ns, λ=0.694 μm) of thermally grown iron oxide (α-Fe2O3) under water, is shown to lead to the formation of FeO and Fe3O4 phases depending upon the choice of the treatment parameters. The other method, ion beam mixing to atomically mix a deposited overlayer of cobalt (400 Å thick) on an α-Fe2O3 substrate with 100-keV Ar+ at a dose higher than 1×1016 ions/cm2, leads to ferrite formation. The surface layer sensitive technique of conversion electron Mössbauer spectroscopy (CEMS) was employed for characterization.
Simplified Mie series calculation of dipole moments for artificial dielectric constituents
Authors: Bush Gary G.
Published: July 2002

Artificial dielectrics are composed of particles embedded in a binding material that fills all of the space between the particles. They have homogeneous macroscopic constitutive parameters but significantly inhomogeneous microscopic properties. The macroscopic electric and magnetic polarizations of such a composite material can often be expressed in terms of the effective dipole moments of its constituents. The dipole moments of spherical multilayered particles can be calculated exactly in terms of Mie series. Except for very simple cases, the calculation of dipole moments by Mie series requires the use of significant computer resources to evaluate complex argument special functions. To alleviate this situation, closed-form solutions for spherical particles have been derived in terms of elementary functions for the case of a semiconductive or reactive coating on a dielectric core. If k is the propagation constant and r is the radius of curvature, the product kr must be greater than 10 for the coating and less than (1)/(2) for the core and surrounding media. These solutions treat the important case of a coating which does not fully shield the core, such that the core properties must be included. The dipole moments calculated by these equations show very close agreement with exhaustive Mie series results. These equations have also proven useful in evaluating Mie series of large complex arguments. The equations are very easily implemented on a desktop computer and facilitate investigation of the mutual coupling between the imposed electric and magnetic fields and the resulting electric and magnetic dipole moments.
Ferromagnetic resonance measurements on epitaxial iron and cobalt films of ZnSe/GaAs substrates (abstract)
Authors: Oliver S. A. , Vittoria C. , Krebs J. J. , Prinz G. A.
Published: July 2002

Ferromagnetic resonance (FMR) measurements have been performed on single-crystal epitaxial Fe and Co films deposited by molecular-beam epitaxy onto {100} and {110} ZnSe/GaAs substrates. In-plane FMR results as a function of magnetic field H were obtained over the frequency range 0.01<f<20.0 GHz by means of a slot line technique. Additional in-plane measurements and measurements with H perpendicular to the plane of the film were performed using a 9.5-GHz microwave cavity. Zero-field results, as well as frequency versus magnetic field dispersions, were obtained with H in the field plane parallel to the 〈100〉 and 〈110〉 axes. The following parameters were deduced from the results on the {100} films: effective magnetization (Fe: 21.6 kOe; Co: 18.1 kOe), gyromagnetic ratio (Fe: 2.11; Co: 2.17), and cubic anisotropy field (K1/M) (Fe: 0.29 kOe; Co: 0.32 kOe). Higher-order terms in the anisotropy field were found to give a negligible contribution. No terms for uniaxial anisotropy energies were included in the fitting equations. These results are in reasonable agreement with published values. Spin-wave excitations were observed in iron films on both substrate orientations, and are found to obey the n2 law. The experimental exchange constant for the iron films was found to be A=(1.9±0.1)×10−6 erg/cm. Spin waves were excited in the {100} Co film, but not in the {110} Co film.
Spin-wave energies in ferromagnetic nickel alloys (abstract)
Authors: Muniz R. Bechara , d’Albuquerque e Castro J. , Edwards D. M.
Published: July 2002

An expression is derived for the spin-wave stiffness constant D in a ferromagnetic metallic alloy. The formalism is based on a multiorbital tight-binding model and the Hartree–Fock approximation. Both diagonal and off-diagonal disorder are taken into account. Results for Ni-Fe, Ni-Co, and Ni-Cu alloys are obtained within the coherent potential approximation (CPA) and compared with previous calculations and experimental data.
Reorientations, freezing, and plastic phase
Authors: Galam Serge
Published: July 2002

A physical mechanism for the production of random fields in plastic molecular crystals is proposed. To account for the reorientating processes, a rotational state is introduced in addition to the set of equivalent allowed orientations. Molecules in the rotational state act as static impurities which in turn generate random fields. A magneticlike Hamiltonian is presented. Experimental implications of the rotational state are discussed.
Effect of demagnetizing fields on Kerr effect in ferrites
Authors: Singh Prabhat K. , Singh P. K. , Prasad S.
Published: July 2002

The analysis of experimental data of magnetomicrowave Kerr effect in MgMn ferrite samples at 9.442 GHz has been carried out using external permeability tensor components which take care of demagnetizing fields more accurately in the theory and is based on the guided wave propagation of the microwave inside the sample. The present analysis gives a better agreement with the experimental data in comparison to our earlier analysis which was based on the free-wave propagation of the microwave. The importance of the image effect in the demagnetizing factor is also discussed.
Materials for magneto-optic data storage (invited) (abstract)
Authors: Bell Alan E.
Published: July 2002

The optimization of magneto-optic media for data storage applications involves a detailed consideration of the impact of material properties on the overall performance and reliability characteristics of the complete media design, which includes both the active layer structure and the substrate material. Currently, the most highly developed materials for the active layer are the amorphous rare earth-transition metal (RE-TM) alloys. The RE-TM composition determines both the Curie temperature and the compensation temperature, both of which parameters affect the sensitivity, and therefore the data rate which can be achieved using currently available diode lasers. The thermal and optical characteristics of the active layer thin-film structure affect the sensitivity, and also determine the degree of enhancement of the shot-noise-limited SNR during playback. Reliability of magneto-optic media is affected by uniform surface oxidation of RE-TM alloys, which leads to reduced Kerr rotation through the formation of an absorptive surface layer with only in-plane anisotropy. The proper choice of dielectric passivation layers and deposition process can significantly limit this and other degradation mechanisms. The choice of substrate material must be compatible with the coating process for the active layer structure, provide a stable mechanical interface to the drive mechanisms, and also possess good optical properties, most notably low birefringence. This talk will review the current directions in materials research on magneto-optic media, and relate the physical characteristics of the materials involved to the major requirements of the data storage application.
Substrate modified growth of epitaxial Fe films (abstract)
Authors: Batra S. , Wowchak A. M. , Cohen P. I. , Lottis D. K. , Dahlberg E. D.
Published: July 2002

It is now clear that both the atomic and magnetic properties of epitaxial Fe films can be controlled by the precise nature of the substrate which acts as the template for the growth. We have investigated the role of defects and strain arising from lattice mismatch on the growth and the magnetic properties of the Fe films. The films were grown by molecular-beam epitaxy (MBE) on three different well characterized surfaces: GaAs(100), In0.2Ga0.8As(100), and Fe(100). The first two surfaces were also prepared by MBE. This mole fraction of In was chosen to match the lattice constant of bulk α-Fe. The growth of the Fe film was characterized by using reflection high-energy electron diffraction (RHEED) and the results indicate that the morphology of the Fe films is smoother on the InGaAs surface than on the GaAs surface. Yet is is still much rougher than the growth on Fe(100) substrate. The magnetic properties were measured by using a SQUID magnetometer and coercivity values are comparable to that of the films grown on ZnSe.1
Photoemission study of 5f localization in UPd3−x(Pt,Rh)x
Authors: Arko A. J. , Koelling D. D. , Dunlap B. D. , Mitchell A. W. , Capasso C. , del Giudice M.
Published: July 2002

Photoemission measurements in the two systems UPd3−x(Pt,Rh)x show that the 5f spectra are consistent with localized 5f electrons (peak in spectral weight is below EF for all x within the double hexagonal DO24 phase) while at both phase transitions the 5f peaks lock in at EF consistent with intinerancy. A satellite 5f peak which we attribute to d screening is observed in both localized and itinerant systems.
Unstable valence rare earths ion implanted into transition metals: Valence variation studies
Authors: Lu F. , Gunapala S. , Croft M. , Stoffel N. G. , den Boer M. L.
Published: July 2002

The technique of ion implantation is applied to study the problem of valence instabilities of Ce and Eu in transition-metal hosts. L3 x-ray absorption spectroscopy measurements are used to probe the rare-earth valence state in these materials. Typical extreme valence variations are illustrated with Y, Ti, and Pd hosts. The important question of identifying extrinsic oxidation of the rare-earth implant is addressed in detail. The signatures of rare earth oxidation in the x-ray spectra of both bulk compound and implanted materials are discussed using specific examples.
Microstructure and stability of rf-diode sputtered GdTbFeCo thin films
Authors: Shieh H-P. D. , Hong M. , Nakahara S.
Published: July 2002

The correlation of deposition parameters, microstructure, and stability of rf-diode sputtered magneto-optical thin films has been studied by using transmission electron microscopy and a magneto-optical hysteresis loop tracer. Among the sputtering parameters, we found that both argon bleeding pressure and substrate bias have strong influences on microstructure and consequently the oxidation resistance of the films. Sputtering using moderate argon pressure and substrate bias produces dense and smooth films. The bare films are strongly resistant to oxidation. In contrast, low or high argon pressure and substrate table bias result in films of high porosity, columnar ‘‘islandlike’’ structure, or ‘‘spongelike’’ morphology. They degrade rapidly in air.
Characterization of amorphous TbFeCo films for magneto-optical media
Authors: Ruane M. F. , Calkins J.
Published: July 2002

Sputter-deposited amorphous TbFeCo films can exhibit magneto-optical Kerr rotation and are leading candidates for erasable optical storage media. Thirty-two TbFeCo samples, deposited on quartz substrates in four different sputtering runs, were characterized by RBS, VSM, Kerr loop tracing, and polarization microscope photographs. Both Tb- and Fe-rich samples were deposited, with room-temperature coercivities ranging from 1 kOe to over 6 kOe. Kerr rotations varied from 14 to 32 min of arc (film side) and from 19 to 21 min of arc (substrate side). Domain photographs, using fields just below coercivity to initiate reversal at inherent nucleation sites, show domain shapes that vary from irregular structures with internally striped reversal to rather circular, well-reversed domains. The shape of observed domains and their internal magnetization patterns are related to composition and compared with important magneto-optical properties such as coercive field Hc and Kerr rotation θK. Variability of these properties across different sputtering runs and within the same run are also discussed.
Crystal structure and magnetic properties of [Co(py)2(H2O)2Br2]
Authors: Carlin Richard L. , Shum Danny , Sinn Ekk
Published: July 2002

The crystal structure and magnetic properties of [Co(py)2(H2O)2Br2] are reported. The space group is P1̄, with one molecule in the unit cell. The cell dimensions are a=6.400(1) Å, b=6.789(2) Å, and c=8.638(1) Å; the angle α=109.06(2), β=103.01(2), and γ=97.50(3) deg. The magnetic susceptibilities of both the polycrystalline powder and single crystals have been measured. The data are quite anisotropic and may be analyzed by the Ising model with g∥=5.99(2) and J/kB=−0.99(2) K. The data show that the substance undergoes magnetic ordering at Tc=3.43(3) K, with the easy axis of spin alignment lying nearly parallel to the a axis. The likely superexchange paths are described.
Neutron inelastic experiments on actinide dioxides: Search for crystal-field levels in NpO2
Authors: Kern S. , Morris J. , Loong C. K. , Goodman G. L. , Lander G. H. , Cort B.
Published: July 2002

Early experiments at the pulsed neutron source at Argonne National Laboratory (ANL) showed that it was possible to observe crystal-field (CF) levels at energy transfers above 150 meV in UO2. More recent investigations at the UK pulsed source at Rutherford Laboratory have improved the resolution and seen a series of well-resolved peaks. Based on these measurements the CF parameters are V4=−130 meV and V6=+30 meV. Since these parameters should be independent of the precise An ion, there is interest in looking at the tetravalent oxides UO2, NpO2, and PuO2. We have now performed experiments at ANL on NpO2 at temperatures between 15 and 200 K. We find no sharp peak in the excitation spectrum up to energy transfers of about 250 meV. This result is contrary to the expected strong transition at about 53 meV using the above CF parameters in a complete intermediate coupling calculation. The observed phonon spectrum of NpO2, in comparison with that of ThO2, shows larger widths in all of the three vibrational bands above ∼30 meV, which may indicate strong interaction between CF levels with optic phonons in NpO2.
Determination of the absolute magnetic moment direction in Cr2O3 using generalized polarization analysis
Authors: Tasset F. , Brown P. J. , Forsyth J. B.
Published: July 2002

Generalized polarization analysis, in which all three components of polarization of both incident and scattered neutron beams are determined, can be used to probe fine details of magnetic structure. For instance, in certain antiferromagnetic crystals such as Cr2O3 neutrons scattered from reflections containing both nuclear and magnetic components have the direction of their polarization rotated in the plane containing the scattering vector as was demonstrated by Alperin [International Conference on Magnetism (Moscow, 1973)]. The sense of the rotation depends on whether the moments point towards the center of symmetry or away from it. The two corresponding configurations are distinguishable because the coordinating octahedra of oxygen ions are not centrosymmetric and should therefore have different energies. We have used a new technique which exploits the Meissner effect to obtain magnetic isolation to measure the rotation of the neutron polarization by the 102 and 1̄04 (hexagonal) reflections of Cr2O3 between 7 and 320 K. The results show that the lower-energy configuration is the one in which the chromium moments point towards the smaller of the two equilateral triangles of oxygen neighbors. The measured depolarizations were found not to be consistent with a simple domain model.
Magnetic and structural properties of MMNi4.15Fe0.85 and its hydride (abstract)
Authors: Oliver F. W. , Morgan W. , Hammond E. C. , Bang-Zheng L. , Meng-Zhao L. , Thorpe A.
Published: July 2002

Mischmetal (MM) has been considered as a possible substitute for the rare-earth component in both permanent magnets1,2 and hydrogen storage intermetallics. We have investigated the magnetic and structural properties of a mischmetal-nickel-iron intermetallic and its hydride using magnetization, Mössbauer, x-ray diffraction, and SEM measurements. Magnetic susceptibility measurements were made between room and liquid-nitrogen temperatures. There was a change in the magnetic character of the unhydrided sample at 140 K. Hydriding produced a large increase in the susceptibility with a corresponding change in the magnetization at low temperatures. The 57Fe Mössbauer spectra of the intermetallic yielded a doublet at room temperature and demonstrated a change in magnetic behavior similar to the magnetization results at low temperatures. Experimental data were used to interpret the hydrogen absorption mechanism acting in this intermetallic and the results are discussed with respect to previously proposed models for rare earth-transition metal compounds.3,4
Scaling behavior in the S=1 antiferromagnetic X
X
Z chain (abstract)
Authors: Bonner J. C. , Müller G. , Parkinson J. B.
Published: July 2002

A recent remarkable prediction by Haldane is the 1D integer spin antiferromagnets of XXZ type should show strikingly different T=0 phase behavior from their counterparts with half-integer spin. The consensus of a wide variety of numerical evidence is in support of the Haldane prediction. However, one aspect which has been particularly difficult to confirm has been the behavior in the vicinity of the critical point Δ=Δ2. The point Δ2 is predicted to be a second-order transition in the universality class of the transverse Ising model at its critical field. It has been numerically established that at Δ=Δ2∼1.18–1.20, the Haldane gap disappears and an excited SzT=0 state becomes degenerate with the SzT=0 ground state for Δ≥Δ2. The mapping to the transverse Ising model implies the existence in the limit N→∞ of an infinite continuum of scaling states quasi-degenerate with the ground state(s) at Δ=Δ2. Numerical calculations to determine the presence of these scaling states have been performed up to N=12 spins for the spin-1 XXZ model. The development of this scaling continuum is only apparent for large N, when a class of k=0, SzT=0 high-lying spectral excitations develop a minimum in the vicinity of Δ∼1.18 which intensifies with increasing N. These excitations extrapolate well below the lower edge of the triplet continuum, and we conclude these are the Haldane scaling states. This conclusion is reinforced by a detailed study of the corresponding excitations for the spin-1/2 transverse Ising model. However, we also find a class of k=0, ‖SzT‖=1 excitations which show similar scaling behavior in the vicinity of Δ2. These states were not included in the Haldane prediction. The implications for the behavior of the correlation functions at Δ2 are discussed.
Effects of electric and magnetic fields on the convective heat transfer in gaseous O2 and N2O
Authors: Haque M. F. , Arajs Sigurds , Moyer C. A. , Anderson E. E. , Blums E.
Published: July 2002

New studies of convective heat transfer in gaseous paramagnetic O2 and diamagnetic N2O have been carried in the presence of electric and magnetic fields. The corresponding heat transfer coefficients indicate a complicated development of heat convections which appear to be strongly interacting. These new results are presently not fully understood in terms of existing theories.
Radiation-induced changes in magnetic properties of Nd-Fe-B permanent magnets (abstract)
Authors: Brown R. D. , Cost J. R.
Published: July 2002

The high remanence and intrinsic coercivity of Nd-Fe-B magnets make them desirable for use in accelerators. Use in a radiation environment requires that the sensitivity of these magnets to radiation be determined and the mechanisms responsible be understood. We present results comparing the relative radiation sensitivity of a number of commercially available Nd-Fe-B magnets. Small samples of these magnets were irradiated to examine the decay of remanence. Our results show that the sensitivity to radiation varies widely among magnets from different vendors, indicating that microstructure and possibly composition play key roles in determining the loss of remanence. We examined optically the microstructure of several of the magnet materials and report on the correlation between microstructure and radiation-induced changes in magnetic properties. Samples were neutron irradiated at the Omega West Reactor in a neutron flux of 4×1012 n/cm2 s at a temperature of 350 K (77 °C). At various time intervals during the irradiation, samples were removed from the reactor and the open-circuit remanence measured. We also measured the quadrant II demagnetization curve of large samples before and after neutron irradiation to monitor changes in remanence and intrinsic coercivity. In general, these samples showed a decrease in the remanence and an increase in the intrinsic coercivity. We also present results comparing the relative decay of remanence for the large cylindrical samples having three different length-to-diameter ratios. This illustrates the effect of the demagnetizing field in determining the rate of remanence decay.
Quasi-one- and two-dimensional antiferromagnetism in α-FeC2O4⋅2H2O and α-MnC2O4⋅2H2O
Authors: Simizu Satoru , Friedberg S. A.
Published: July 2002

Magnetic heat capacities have been determined for the quasi-one-dimensional antiferromagnets α-FeC2O4⋅2H2O and α-MnC2O4⋅2H2O. In each case, two distinct heat capacity anomalies are found. The first one is a small λ-type anomaly which marks the antiferromagnetic ordering and is located at TN=11.7 K for α-FeC2O4⋅2H2O and 2.4 K for α-MnC2O4⋅2H2O. The second anomaly is a tiny kink which corresponds to a magnetic structure change. It is found at T2=9.5 K for α-Fe2O4⋅2H2O and 1.5 K for α-MnC2O4⋅2H2O. The heat capacity and magnetic susceptibility data well above TN can be reproduced in each case by a one-dimensional model assuming Ising-like anisotropy for the ferrous compound and Heisenberg isotropy for the manganous compound. The behavior near and below TN appears to be explained by a rectangular two-dimensional lattice model with the ratio of the two in-plane interactions, ‖J′/J‖, being ∼0.02.
Magnetic order in the Kondo-lattice compound CePdIn
Authors: Brück E. , van Sprang M. , Klaasse J. C. P. , de Boer F. R.
Published: July 2002

Out of a systematic research program on equiatomic ternary compounds RETM (RE is a rare earth, T a transition metal and M a metal out of the p block of the periodic table) crystallizing in the ZrNiAl structure some results on CePdIn are reported. Transport and magnetic properties and specific heat were investigated as functions of temperature. An antiferromagnetic transition at 1.65 K shows up in the specific heat as a maximum and in the resistivity as a distinct change of slope. Other salient features are the occurrence of two maxima in resistivity, a huge anomalous Hall effect and a high electronic-specific-heat coefficient of 123 mJ/mole K2, reminiscent of low Tsf spin-fluctuation systems. The isostructural compound LaPdIn becomes superconducting at 1.6 K.
Narrow bands and magnetic properties of heavy fermions (abstract)
Authors: Marabelli F. , Wachter P.
Published: July 2002

Far infrared measurements performed at low temperature on the heavy fermions UPt3 and CeCu6 give us evidence that an appropriate model describing these materials must consider two narrow quasiparticle bands in the vicinity of the Fermi level separated by a gap, or a pseudogap, in the order of a few meV. The bandwidth obtained by point contact spectroscopy is also in the meV range. With such a model and with the experimentally obtained values for the bandwidth and the gap we tried to calculated the temperature dependence of the resistivity and of the γ value of the specific heat and we found a very good agreement with the experiments. We want now to apply the model to calculate the magnetic properties of the heavy fermions. We show that a Pauli susceptibility including the effects of a strongly energy dependent density of states can account for the temperature dependence of the susceptibility in these materials. For temperatures above about 50 K we find a Curie law and for lower temperatures we compute a weak maximum. Furthermore, the magnetic saturation of UPt3 in very large fields (35 T) can be explained in this context as a splitting of the quasiparticle band at the Fermi level in two completely separated spin polarized subbands.
Magnetoelasticity and structure of Er/Y superlattices
Authors: Erwin R. W. , Rhyne J. J. , Borchers J. , Salamon M. B. , Du R. , Flynn C. P.
Published: July 2002

Single-crystal superlattices (Erx/Yy) have been grown by molecular-beam epitaxy techniques with the c axis perpendicular to the growth plane. The magnetic structure has been determined by neutron diffraction, and demonstrates long-range order of the c-axis Ising-like Er moments extending through the ‘‘magnetically dead’’ Y layers (TN superlattice ≊TN bulk), as was found for the X-Y superlattices (Dyx /Yy). The Er c-axis effective turn angles for x=13, 23, and 32 are all nearly temperature independent at ω≊2π/7, the high-temperature lock-in value in bulk Er (although it is difficult to determine if a lock-in transition occurs), with the ferromagnetic transition completely suppressed. This results from the fact that the magneto-elastic energy density, which causes ω to decrease in the bulk, is considerably reduced in the superlattices. As in bulk Er, the high-temperature sinusoidally modulated phase ‘‘squares up’’ upon reducing the temperature, and this squaring is enhanced at the basal-plane ordering transition, which is at roughly half the temperature found in bulk Er. The basal-plane turn angle tends towards the low-temperature value of bulk Er, ω≊π/4, which is surprisingly different from the c axis ω.
Forces in finite-element magnetostatics using co-energy related estimation of field accuracy
Authors: McFee S. , Lowther D. A. , Webb J. P.
Published: July 2002

Accuracy and consistency problems are often encountered when magnetostatic forces are estimated from imperfect finite-element solutions. Recently, these difficulties were attributed to localized field-error fluctuations characteristic of the co-energy extremization commonly used to solve finite-element systems. In this paper, a new scheme for finite-element based magnetostatic force calculation is introduced. The approach is based on the integration of a generalized Lorentz force density guided by a co-energy related field accuracy measure. Preliminary numerical results show that the co-energy based field assessment is a precise and stable measure for rating force integration regions. By extending the Lorentz force integration to the Maxwell stress method, the co-energy based field-error criterion is shown to be equally useful for evaluating integration surfaces.
Erasing magnetic recording media
Authors: Wiesen K. C. , Katti R. R. , Charap S. H. , Kryder M. H.
Published: July 2002

The erasure characteristics of longitudinal magnetic recording media have been explored by correlating measurements and the results of a magnetic recording simulation. The two-dimensional simulation uses the standard self-consistent method with a vector Preisach hysteresis model. Writing NRZI-All-1 patterns with a ferrite head at 1 MHz generates the unerased magnetization pattern. The simulated unerased voltage pulse shapes agree well with experiment. The Fourier harmonics of the readback waveform characterize the recorded signal, and are observed as a function of longitudinally applied dc and decaying ac erase fields. Results from experiment and simulation indicate that dc erase fields, of strength near Hc, enhance even Fourier harmonics of the NRZI waveform by destroying the antisymmetric relationship between successive pulses. The measured superiority of ac erasure is explained by both more rapid approach to demagnetization, and broadening of the transition.