bluuebunny/crossref_metadata_2025 huggingface dataset:
A method of magnetic field synthesis for medical NMR imaging
Authors: Poltz J. , Kuffel E.
Published: July 2002

A method of calculating currents in an array surrounding an image plane of a nuclear magnetic resonance spectrometer has been developed. The currents are evaluated in order to create a system of orthogonal magnetic fields in the entire region of the image plane where a sample is to be located. In a two-step procedure, the independent source functions are calculated and next, by applying the Gram–Schmidt process, they are orthogonalized. The linear combination of the set can be used to adjust a proper correcting field which is superimposed with the main field. To calculate the coefficients of the linear combination, the difference between expected and present magnetic field distribution is developed in a series of orthogonal functions. This permits the real time computation of the correcting field which may insert the gradient as well as remove static field inhomogeneities. The current distributions required for generating orthogonal functions and for creating a gradient field has been computed.
Generalized prediction model of the induction motor (abstract)
Authors: Sarma Mulukutla S.
Published: July 2002

The aim of the project is to develop an electromagnetic-field model of a three-phase induction motor in order to determine the flux distribution under various conditions of load, calculate certain saturation factors, predict the performance characteristics of the motor, and be able to modify the design for optimization. A mathematical model, established under certain simplifying magnetostatic assumptions, is treated as a boundary-value problem governed by nonlinear partial differential equations of the elliptic type in terms of the magnetic vector potential. A combination of different rectangular and polar grid systems is used to follow closely the contours of the steel-laminated punchings of the induction motor. The nonlinearity of ferromagnetic materials is duly taken into account. Numerical analysis consisting of iterative procedures, relaxation techniques, and difference schemes is applied for the computer-aided solution. Computer programs in fortran iv language have been developed and run successfully for the case of a 6-hp, 220-V, 3450-rpm, 60-Hz, 3-phase, 2-pole squirrel-cage induction motor. Further work is under way in order to consider the nonisotropic material properties of grain-oriented sheet steels, include the eddy-current and three-dimensional effects, and also extend the methods for the analysis of single-phase induction motors.
Finite-element modeling of laser beam heating of magnetic films
Authors: Bhattacharyya Manoj. K. , Cendes Zoltan J.
Published: July 2002

Finite-element models of the two-dimensional transient heating in multilayered magneto-optical recording structures are reported. In this analysis, the laser energy dissipation is computed by considering reflection and absorption at every layer of the structure. The effects of varying thermal conductivity, absorption coefficient, and thickness of the magnetic film on heating have been investigated. Also, the effects of different input power levels, pulse durations, and of placing a reflecting layer next to the magnetic film have been studied.
Microstructure, relaxation, and print-through in γ-Fe2O3 particles
Authors: Berkowitz A. E. , Goehner R. P. , Hall E. L. , Flanders P. J.
Published: July 2002

Four samples of γ-Fe2O3 representing a range of Hc, particle size, and print-through behavior were examined for their structure. All particles were single crystals with symmetric cross sections and predominant [110] longitudinal axis orientation. The sizes obtained from x-ray line broadening measurements were shown to relate to average particle diameters rather than average crystallite sizes within a particle. Neither Hc nor print-through seemed to correlate with porosity as determined from TEM. Print-through (PT) was highest in the sample with the smallest average particle size. This suggests that print-through is strongest in samples with the largest fraction of particles small enough to show magnetization instability. The data for some samples were not completely consistent with this view. The sample with the highest print-through also had the largest concentration of anti-phase boundaries (APB). The manner in which APB’s can lower the threshold for magnetization instability, thereby increasing print-through, is discussed.
A simple inductive contactless switch
Authors: van Dover R. B. , Jin S.
Published: July 2002

When a small coil wound around a fine core of low coercivity square-loop Fe-6 wt. % Ni alloy wire is excited with a 100-kHz ac current it develops a peak voltage much greater than that obtained from a coreless coil. This effect can be used to sense motion of the core in and out of the coil or alternatively to sense motion of an external biasing magnet. Either way one has a position sensor which is extremely simple and which could be useful in a variety of applications, such as high-reliability keypad switches, proximity switches, and switches for use in hazardous or corrosive atmospheres.
Hysteretic losses in Nb-Ti superconductors
Authors: Goldfarb R. B. , Clark A. F.
Published: July 2002

When subjected to transient magnetic fields, superconductors exhibit losses. At low frequencies, most of the dissipation is hysteretic. Magnetization was measured in an axial field for eight multifilamentary Nb-Ti superconducting wires with different filament sizes and different ratios of copper to superconductor. The full-penetration field Hp was estimated from the high-field ends of the hysteresis loops. The estimate of Hp provides a method to assess the critical current density Jc. There was good agreement between measured losses and those predicted from Hp and the peak applied field.
Measurements of magnetic moments of metal atom clusters (abstract)
Authors: Cox D. M. , Trevor D. J. , Rohlfing E. A. , Kaldor A.
Published: July 2002

We report the first measurements of the magnetic moments of isolated metal atom clusters. In particular we have measured the magnetic moments of iron atom clusters ranging in size from 2 to 15 atoms and for aluminum atom clusters Al2 and Al3 and their corresponding mono-oxides Al2O, Al3O. We produce a supersonic beam of metal atom clusters by pulsed laser vaporization of a metal substrate within the narrow throat of a pulsed nozzle. The clusters are passed through a Stern–Gerlach magnet and the spatial profile of the magnetic deflection pattern is measured using UV laser photoionization combined with time-of-flight mass analysis of the resulting photoions. The magnetic moments of the iron clusters are found to increase linearly with the number of atoms in the cluster consistent with a net spin per atom of about 3.0 μB compared to the bulk value of 2.2 μB. These observations are consistent with theoretical predictions of the magnetic moments of iron clusters of 4, 9, and 15 atoms. For Al2 we observe Stern–Gerlach behavior and for Al3 we measure a maximum moment of approximately 1/3 μB. The implication of these findings regarding the electronic and structural properties of small metal clusters will be discussed.
Galvano-magnetic thin film sensors (abstract)
Authors: Hoffmann Horst
Published: July 2002

Magnetoresistance detectors are used to observe the magnetic state of microscopic parts of magnetized samples. The planar Hall effect can be used instead. Both effects can be applied to find out the logic ‘‘1’’ or ‘‘0’’ state of magnetic storage devices. They also can be used for a quantitative and sensitive measurement of magnetic fields as well as a magneto-elastic strain gauge. Soft magnetic permalloy films are appropriate for the field detection while amorphous Fe1−xBx films are suitable for the strain measurements. The sensor films have to show a uniaxial anisotropy. The strength of it determines the range of the detectable fields and strains. The sensitivity of the magnetoresistance emr=dΔU/dH=2( ρ⊥−ρ∥) 1/db JH/HK2(V/Oe) depends linearly on the magnetic field H applied along the hard axis. l, d, b are the film length, thickness and width, respectively. The sensitivity of the planar Hall effect, which is given by epl=dUpl/dH =( ρ∥−ρ⊥)s/db J(1/HK) is independent of the applied field along the hard axis as long as H≪HK. s is the separation of the Hall contacts. Both sensitivities are reciprocal to the uniaxial anisotropy, HK as long as the applied field, H is smaller than HK. The theoretically expected sensitivity usually is decreased by domain splitting and the influence of the magnetization ripple. The smaller HK is (the larger the sensitivity is) the stronger is the influence of the ripple. It gives an upper limit of the interval in which external fields or applied strains can be quantitatively measured. Double layered films can prevent the domain splitting. The sensor theory including the ripple and experimental data will be given.
Magnetic dead layer of 25 Å thickness in Fe3O4 (abstract)
Authors: Parkin S. S. P. , Sigsbee R. , Felici Roberto , Felcher G. P.
Published: July 2002

The technique of polarized neutron reflection can be used to give a depth profile of the magnetization in thin films. We have applied this novel technique to study the magnetization of a thin film of Fe3O4, 2600 Å thick, prepared by reactive sputter deposition from an iron target onto a highly polished substrate. Standard magnetic characterization studies indicate magnetic behavior typical of such films (Hc=570 Oe, 4πMr=2250 G, magnetization at 3 kOe, 4πMs=3760 G). Analysis of the neutron reflection measurements (made with the sample in a field of 2500 Oe) indicate that the magnetization is fairly uniform over the thickness of the film with a value close to that obtained by static magnetization studies. However, the measurement indicates that at the surface there is a magnetic dead layer which is as thick as ∼25Å: This may (or may not) be related to the surface roughness which is of the same order of magnitude.
Magnetic properties of compositionally modulated Co/Mn thin films
Authors: Sakakima H. , Krishnan R. , Tessier M.
Published: July 2002

Magnetic properties of multilayered Co/Mn films with layer thickness ranging from 7–70 Å have been studied. The films were prepared by alternately depositing Co and Mn layers in an ultrahigh vacuum (UHV) evaporator. X-ray θ-2θ scans with Cu-Kα radiation showed a main diffraction peak around 2θ=44° for all films and satellite peaks for some of them. The data imply the existence of α-Co (002)/α-Mn (330) stacking texture. Ferromagnetic resonance (FMR) study and vibrating-sample magnetometer (VSM) measurement show that the magnetization of Co layers decreases with decreasing Co thickness as if 1.5 atomic layers lost their magnetic moments at the interface. FMR spectra of the films show a peculiar temperature dependence in that the perpendicular resonance field starts to decrease below a certain temperature. The results are discussed.
Effect of an Ag or a Pd overlayer on the magnetic moment of a Fe film
Authors: Schröder K.
Published: July 2002

An Ag or a Pd overlayer about 2–4 nm thick was deposited on a Fe film. Fe film and overlayers were prepared at room temperature in a high vacuum system, and the magnetic moment was measured in situ for both the iron film, and the iron film with overlayer. The dipole moment of the iron film with overlayer was larger than the dipole moment of the iron film alone, and the effect of Ag and Pd could be described as if the iron film thickness increased by 0.14±0.05 nm or 0.36±0.15 nm, respectively.
Magnetic-field-induced spin reorientations in magnetic superlattice structures: Explicit examples
Authors: Hinchey L. L. , Mills D. L.
Published: July 2002

Superlattice structures may exhibit unique properties, distinctly different from those of the individual constituents from which the structure is synthesized. We illustrate this by exploring some properties of model superlattices fabricated from a ferromagnet and an antiferromagnet, the latter with sheets of ferromagnetically aligned spins each parallel to the interface. We find magnetic-field-induced spin reorientation transitions in such a structure. These may be viewed as the antiferromagnetic spin flop transition, modified by the presence of the intervening ferromagnetic layers. For the case where the anitferromagnetic constituent contains an even number of spin sheets, the spin flop field can be very much lower than that realized in the isolated antiferromagnet.
Surface contribution to the ferrimagnetic resonance linewidth of single-crystal YIG versus frequency and temperature
Authors: Urban M. , Warin D.
Published: July 2002

The knowledge of microwave intrinsic losses of ferrimagnetic single crystals is often obscured by the scattering process induced by surface defects. The existing data and the corresponding model dealing with this effect are far from sufficient to understand the phenomenon; in the millimetric range, the importance of the surface imperfection linewidth ΔHs is a major difficulty for obtaining the single-crystal intrinsic linewidth ΔHi from ΔH measurements. We have investigated on a pure YIG single crystal the full dependence of the surface effect as a function of the surface pit size, the frequency, and the temperature. An automatic linewidth spectrometer is used, which allows measurements in the frequency range 1–18 GHz and the temperature range −100 to 270 °C. The dc magnetic field is aligned with the 111] easy axis of the crystal. Three main conclusions are obtained: (1) For a constant sample diameter, the variation of the surface contribution ΔHs is found to be a nonlinear function of the pit size. (2) Whatever the surface roughness, ΔHs is a minimum when passing the top of the spin-wave manifold. This is similar to the behavior of the volume porosity line-broadening contribution observed in polycrystalline materials. (3) For a constant reduced internal field Ω (Ω=Hi/4πMs), the temperature dependence of ΔHs follows the temperature dependence of the saturation magnetization.
Evaluation of spin-orbit and dipole-dipole contributions to magnetic relaxation in iron near T
c
Authors: Heinrich B. , Arrott A. S.
Published: July 2002

The results for iron from measurements in the region near Tc of ferromagnetic resonance in high and low fields (GHz and 100 MHz) and of the lower-frequency ac susceptibility (kHz) are analyzed to extract the intrinsic damping parameters. The damping in excess of the contribution from spin-orbit coupling is attributed to dipole-dipole interactions. The theory of dipolar interactions developed by Raghavan and Huber for EuO is applied to iron.
Ferromagnetic resonance in layered magnetic materials
Authors: Vittoria C.
Published: July 2002

We show that the surface impedance of a layered structure may be expressed in terms of the four transfer function matrix elements corresponding to surface microwave fields instead of internal microwave fields corresponding to all the layers within the layered structure. The ferromagnetic resonance fields and lineshapes are calculated for a layered structure containing as many as 23 alternating iron metal and insulating dielectric layers. We find that spin-wave resonance may be excited in the layered structure, although no spin pinning was assumed at each iron-layer surface. Exchange coupling between iron layers enhances the intensity of the spin-wave resonance.
Specific heat of amorphous Ni80−xFexB16Si4 alloys
Authors: Regelsberger M. , Wernhardt R. , Rosenberg M.
Published: July 2002

The specific heat of amorphous Ni80−xFexB16Si4 alloys has been measured in the temperature range of 1.5<T≤25 K with 0≤x≤19. Assuming a Gaussian distribution of Schottky terms the specific heat was described satisfactorily in the whole temperature and iron concentration range. Anomalous behavior of θD and N(EF) was found near the critical concentration xc, for the onset of ferromagnetic ordering. The entropy Sm determined from the Schottky-like contribution of the specific heat shows a maximum at xc.
Bulk and surface electronic structure of ferromagnets at finite temperatures by spin-polarized photoemission (invited)
Authors: Gudat W.
Published: July 2002

We report on spin-, angle-, and energy-resolved photoemission with synchrotron radiation. This novel experimental method allows an investigation of the spin-dependent bulk and surface electronic structure of ferromagnets. We illustrate its application and experimental potential with a few selected examples. We discuss the determination of the band structure of iron at temperatures well below and close to the Curie temperature TC and we consider a magnetic reconstruction for the Heisenberg ferromagnet Gd.
Ferromagnetism in reentrant P
d FeMn (abstract)
Authors: Gist G. A. , Dodds S. A. , MacLaughlin D. E. , Cooke D. W. , Heffner R. H. , Hutson R. L. , Leon M. , Schillaci M. E. , Boekema C. , Mydosh J. A. , Nieuwenhuys G. J.
Published: July 2002

Muon spin relaxation in zero applied field has been used to study the ferromagnetism of the reentrant ternary alloy [PdFe(0.35 at. %)] Mn(5 at. %). Below the ferromagnetic transition at Tc, the zero field linewidth due to quasistatically frozen impurities becomes much larger than expected from quasistatic ordering of the Fe spins only. This microscopic measurement shows that Mn spins participate in the quasistatic ferromagnetic ordering below Tc. The muon spin-lattice relaxation rate peaks at the Curie temperature Tc=8.25 K. Above Tc, the zero-field relaxation rate scales with earlier measurements of the ferromagnetic transition in PdMn (2 at. %) and shows the same behavior in a longitudinal applied field. We conclude that the Mn spins dominate the zero field dynamics of the ferromagnetic transition. The ferromagnet-spin glass transition at Tg can be identified from a peak in the imaginary part of the low-field ac susceptibility. The zero field muon spin-lattice relaxation rate is approximately constant for temperatures just above Tg.
Electromagnetic shielding properties of amorphous alloy shields for cathode ray tubes
Authors: Huang Der-Ray , Dow Wen-Huu , Yao Pei-Chih , Hsu Shu-En
Published: July 2002

Because of the excellent soft magnetic properties of the amorphous alloy Fe40Ni38Mo4B18 (2826MB), we have made funnel-shaped shields for cathode ray tubes by combining the amorphous ribbons and ferromagnetic powders via resin transfer molding composite technology. The electromagnetic shielding properties of shields were improved by increasing the thickness of ribbons and the weight percentage of ferromagnetic powders. For a shield with six-layer ribbons, the maximum magnetic shielding effectiveness obtained near the applied magnetic field 2 Oe (dc and ac 60 Hz) was about 25–27 dB, and the magnetic shielding factors were improved to 27–30 dB by adding the ferromagnetic powders. After the field annealing of shields, the magnetic shielding effectiveness increased and the maximum values of 35–40 dB were shifted near the applied magnetic field 0.2 Oe. The electric field shielding effectiveness over the broadband spectrum 10 kHz–18 GHz was in the range 50–80 dB.
Spin dynamics of the amorphous Invar alloy Fe0.86B0.14
Authors: Fernandez-Baca J. A. , Lynn J. W. , Rhyne J. J. , Fish G. E.
Published: July 2002

High-resolution neutron scattering studies have been made of the long wavelength spin excitations in a ribbon sample of amorphous Fe0.86B0.14, which exhibits Invar properties. Spin waves were observed in the wave vector range 0.05 Å−1≤q≤0.12 Å−1 at temperatures between 300 K (0.54 Tc) and 500 K (0.90 Tc). The spin wave energies are well described by a dispersion relation E=Dq2+Δ. The small energy gap Δ of ≊0.04 meV is attributed primarily to the dipole-dipole interaction. The stiffness constant renormalizes with temperature as D=D(0)[1.0−0.86 (T/Tc) 5/2] in the range of temperatures under study, with D(0)=132 meV Å2. This value of D(0) is approximately twice as large as that calculated from the T3/2 coefficient of the magnetization, a discrepancy common to many Invar materials. Plots of the intrinsic linewidths against q4 and T2 reveal that the data are consistent with the Γ∝q4[Tln(kT/E)]2 dependence predicted for a Heisenberg ferromagnet. There are no anomalies in the spin-wave lifetimes at long wavelengths which appear to relate to the Invar effect seen in the FexB1−x system.
Observation and interpretation of a new transition observed via the field dependent ac susceptibility of the spin-glass Cu-Mn (abstract)
Authors: Youm Dojun , Schultz S.
Published: July 2002

Detailed measurements of the temperature dependence of the ac (30 Hz) susceptibility χ for the spin-glass Cu-Mnx (x=2, 5, 8 and 10 at. %) in dc fields H from 0 to 1.2 kG reveal that as the temperature is lowered χ monotonically deviates from a Curie law dependence, but slightly above TC (the zero field cusp temperature) becomes independent of temperature until below TC, by an amount ΔTC, at which point there is an abrupt change in slope. The locus of this discontinuity is a straight line given by: Δχ/χ=αΔTC/TC=b(H/H0)γ, where Δχ is the change from the value of χ at TC and zero field, and H0 is referenced at 1 kG. We find nearly universal behavior for many samples with γ=2/3, b=0.2, and α≂3. We fit the magnetization data to a Sherrington–Kirkpatrick model modified to include macroscopic clusters of spins and deduce the T and H dependence of the mean cluster size and exchange interaction. When we introduce the new postulate that the transition occurs at a condition of collective rotation of the clusters, we find excellent agreement with the observed transition line. We also find that χ(H,T) satisfies scaling laws above and below TC.
Low-temperature spin correlations and spin dynamics in diluted magnetic semiconductors
Authors: Giebultowicz T. M. , Rhyne J. J. , Ching W. Y. , Huber D. L.
Published: July 2002

Neutron scattering measurements of static and dynamic spin correlations in the semimagnetic semiconductor Cd0.35Mn0.65Te are reported and compared to computer simulations for a dilute Heisenberg fcc antiferromagnet that is the model analog of Cd1−xMnxTe.
Spin ordering in a random antiferromagnetic Heisenberg spin system: Numerical simulation
Authors: Ghazali A. , Diep Hung T.
Published: July 2002

We study by a Monte Carlo method, a three-dimensional classical antiferromagnetic random Heisenberg spin system with an exchange interaction which decreases exponentially with distance. We find no indication of a spin glass transition when only isotropic exchange exists. However, a gradual spin freezing is observed as T→0. In the presence of a strong enough Ising-type uniaxial magnetic anisotropy, we observe a peak in the specific heat and a stable order parameter. However, no true thermoremanent magnetization is observed.
Viscosity of the field-cooled magnetization in the Y0.99Er0.01 spin glass
Authors: Bouchiat H. , Mailly D.
Published: July 2002

Accurate dc magnetization measurements have been made on the Y0.99Er0.01 spin glass system. Long-time relaxation and influence of the cooling rate on the field-cooled state are mainly described and discussed here, in comparison with strain experiments in polymers.
Macroscopic anisotropy and demagnetization in reentrant magnetism
Authors: Senoussi S. , Öner Y.
Published: July 2002

We report magnetic measurements on eight different Ni79Mn21 ellipsoidal samples as a function of temperature fields and reciprocal demagnetization factor N (0.2<N<12). For each sample, we have successively plotted the m vs H loop (I) in the ferromagnetic phase at 40 K, (II) in the reentrant phase at 1.5 K after zero-field cooling, and (III) after cooling in H=10 KOe from Tf=35 to 1.5 K. It is observed that the shape of the zero-field-cooled (ZFC) hysteresis loop, the field-cooled (FC)-shifted cycle, and the thermoremanent magnetization mr all depend (in an unusual way) on the reciprocal demagnetization factor N. It should be stressed, however, that the unidirectional anisotropy field Ha and the spontaneous magnetization ms are seen to be fairly independent of N, whereas the ‘‘internal’’ initial susceptibility in the ZFC state (T=1.5 K) exhibits a noticeable dependence on N. Our results are consistent with a domain picture in which the structure of the domains would depend slightly on the shape of the sample.
Solitons in the antiferromagnetic X
Y chain
Authors: Staudinger M. , Gerling R. W. , Murty C. S. S. , Landau D. P.
Published: July 2002

An important sampling Monte Carlo method is used to study the classical, antiferromagnetic, XY chain in a symmetry breaking uniform magnetic field. From equilibrium configurations we extract values for the bulk properties as well as explicit pictures showing individual spin directions. An analysis of the angles which the spin makes with the field shows clearly the presence of both ‘‘π-like’’ and ‘‘2π-like’’ solitons. We also see two distinct types of ‘‘π-like’’ solitons which differ in their behavior as a function of field. The bulk properties are discussed in terms of both spin waves and soliton excitations.
Monte Carlo simulation of fcc Heisenberg antiferromagnet with nearest- and next-nearest-neighbor interactions
Authors: Giebultowicz T. M. , Furdyna J. K.
Published: July 2002

We report the results of Monte Carlo simulation of the ordering phenomena and phase transition in a classical Heisenberg fcc spin system (S=∞) with antiferromagnetic nearest- and next-nearest-neighbor interactions (JNNN=0.1 JNN). At low temperatures the system exhibits Type III collinear antiferromagnetic order. At T=0.47JNNkB there is a transition to disordered phase. Unlike the second-order transition in a ‘‘frustrated’’ fcc antiferromagnet (only JNN≠0), the transition in the system studied is of the first order, as indicated by discontinuities in system energy and relaxation time versus temperature. The system we investigated may be considered as a classical analog of the Type III antiferromagnet β-MnS. We compare the calculated results with the experimental data obtained for this compound.
Amorphous networks of magnetic bubbles: Statics and dynamics=an approach of 2d melting (abstract)
Authors: Arnaud L. , Levy J. C. S.
Published: July 2002

Direct observations and Fourier transforms of amorphous networks of magnetic bubbles in ion-implanted garnets and in pure films are shown with evidence for a short coherence size. This is interpretated on the basis of a model of interaction of bubbles via dipolar interactions where only short-range effects are considered because long-range effects are totally screened by the numerous domains included in this range. Moreover these dipolar forces which are proportional to both the magnetizations stabilize the occurrence of different radii of bubbles. These two considerations enable us to understand these amorphous networks as due to short-range interactions and random fields and thence to predict a coherence size of typically 50–300 bubbles in a hexagonal arrangement. Small rotating fields in the phase of the film do not strongly perturb the size distribution of bubbles, i.e., the transverse magnetic field distribution, but induce many local displacements of bubbles. These displacements can be coherent and thus collective if the excitation frequency is associated with a soft mode of this amorphous structure. Thus local cascades of hopping motion as well as collective modes are observed as a function of the excitation frequency. The practical soft modes are localized on the grain boundaries and lead either to a decrease of the coherence size, i.e., melting or to an increase, i.e., recrystallization. Theoretical investigations of the soft mode frequency and localization are given. Experimentally direct and Fourier transform observations as a function of frequency are recorded in a video tape.
Dynamic magnetic susceptibilities of valence-fluctuation Ce compounds
Authors: Cox D. L. , Bickers N. E. , Wilkins J. W.
Published: July 2002

We have computed dynamic susceptibilities of valence-fluctuating Ce compunds for temperatures between 0.01 and 40 T0, where T0 is the position of the many-body ‘‘Kondo resonance’’ in the 4f density of states. Our calculations reproduce several results seen in neutron scattering and nuclear-magnetic-resonance experiments: (i) nonmonotonic behavior of the magnetic relaxation rate, (ii) non-Lorentzian behavior of the dynamic susceptibility for low temperatures, and (iii) high-temperature T1/2 behavior of the magnetic relaxation rate. Most of the magnetic relaxation data for valence-fluctuating Ce compunds are consistent with the Kondo resonance picture which has been applied successfully previously to thermodynamic and high-energy spectroscopy data.
Characteristic energy for the magnetic configuration in α-Ce: A polarized neutron with polarization analysis study
Authors: Fillion G. , Galera R. M. , Givord D. , Pierre J. , Schweizer J. , Vettier C.
Published: July 2002

Inelastic neutron scattering experiments, up to energy transfers of 250 meV, were performed on α-Ce at 8 K using polarized neutrons with polarization analysis. The sample, weighting 300 g, was elaborated by cooling down to 77 K in a pressure cell. At low temperature, the pressure was released and the sample extracted from the cell. The magnetic signal, negligibly small below 200 meV, is significant at higher energies. The results show that the ground state in this system is a nonmagnetic singlet; they reveal a characteristic energy of about 250 meV. These properties are reminiscent of those observed in other intermediate valence systems, e.g., CeSn3 and CePd3. They are related to other physical properties such as bulk susceptibility and electronic specific heat within the framework of both intermediate valence and Kondo models.
Transport properties of Kondo lattice CeB6
Authors: Ali Naushad , Woods S. B.
Published: July 2002

We have measured the resistance, thermoelectric power (TEP), and magnetoresistance (MR) of CeB6 at liquid-helium temperatures. CeB6, a dense Kondo material, has a resistivity that varies as −log T from 4 K to a minimum near 150 K. Below this a maximum occurs at 3.5 K and the antiferromagnetic order below TN=2.4 K is seen as a sudden change in the slope of the resistivity at TN. In the TEP data an extraordinary, giant peak is observed at 9.5 K and a much smaller peak appears at 4.3 K. Again a change in slope of TEP at 2.4 K is associated with TN. The TEP becomes negative below 2.1 K. Recent theoretical investigations, based on the Kondo lattice model and s-f hybridization even without taking into account any long-range magnetic order, can explain qualitatively the behavior of the low-temperature resistivity (T<4 K) and the giant peak in the TEP data. The MR is found to be negative and has a field dependence of H2 in phase I with an inverse temperature dependence. At 4.2 K a phase change that has been observed previously is seen at 1.6 T in the MR. Above 4.2 K the transverse and the longitudinal MR are the same, which suggests that the system is isotropic in the high-temperature phase.
Muon spin relaxation in ErRh4B4
Authors: Noakes D. R. , Ansaldo E. J. , Brewer J. H. , Harshman D. R. , Huang C. Y. , Torikachvili M. S. , Lambert S. E. , Maple M. B.
Published: July 2002

Data on zero-field μSR in ErRh4B4 are presented for temperatures from 4 to 300 K. Due to the large erbium moment, most of the muons have a relaxation time of less than 10 ns for temperatures below about 50 K. The data have been analyzed in terms of two distinct muon sites in the material. The deduced erbium relative fluctuation rate versus temperature is related to the crystalline electric field levels of erbium in the RERh4B4 structure.
Hall effect and magnetic properties in GdFe and CoCr sputtered films
Authors: Honda S. , Nawate M. , Ohkoshi M. , Kusuda T.
Published: July 2002

Hall and ohmic resistivities of sputtered GdFe and CoCr films were measured at room temperature in the wide range of the film composition. It was suggested that the Hall effect is connected with the side-jump mechanism in both materials. Especially for ferrimagnetic GdFe films, the Hall effect was reasonably analyzed by assuming that the dominant contribution to the Hall effect is the Fe sublattice magnetic moment.
Some anomalous aspects of resistivity behavior in dilute chromium alloy systems
Authors: Yakhmi J. V. , Gopalakrishnan I. K. , Iyer R. M.
Published: July 2002

The addition of up to 0.3 at. % Mn to pure Cr leads to a fall in ρ at low temperatures resulting in a crossover of the ρ(T) curves of the alloys vís-à-vís that of pure Cr. The effect of vanadium impurities (0.1, 0.2, and 0.3 at. %) on the binary matrix Cr+0.5 at. % Ir is, however, different in nature in that there is a common crossover point (≊280 K) below which the resistivity of V-containing alloys is higher, and above which it is lower, than that of the host, and the magnitude of the effect increases with the rise in V content. Yet another type of anomalous behavior is observed for CrPd alloys where the overall resistivity curve rises with increase in Pd content from zero to 0.8 at. % but the ρ(T) curves register a monotonic fall with further rise in Pd content. All these complex features of resistivity are discussed in relation to the spin density wave (SDW) antiferromagnetism of chromium as influenced by the effects of impurity scattering, change in e/a ratio, and the possibility of local moment formation on impurity atoms.
Upper critical magnetic field of the superconducting heavy fermion system (U1−x Thx)Be13
Authors: Chen J. W. , Lambert S. E. , Maple M. B. , Naughton M. J. , Brooks J. S. , Fisk Z. , Smith J. L. , Ott H. R.
Published: July 2002

Measurements of the ac electrical resistance as a function of temperature and applied magnetic field R(T,H) were made for various compositions x≲0.03 in the mixed binary system (U1−xThx)Be13. All samples within this range of x exhibit heavy Fermion superconductivity with a superconducting transition temperature Tc that is rapidly suppressed by the substitution of Th for U. The shape of the upper critical field curve Hc2(T) determined from the R(T,H) data is similar for all compositions with a very large initial slope (−dHc2/dT)Tc.
Anomalous surface impedance in reentrant ferromagnetic superconductors
Authors: Huang C. Y. , Olsen C. E. , Kozlowski G. , Matsumoto H. , Umezawa H. , Mancini F. , Maple M. B. , Hamaker H. C. , Torikachvili M. S. , Whitehead J. P. , Wang F. E.
Published: July 2002

We report evidence which suggests surface ferromagnetism in a ferromagnetic superconductor arising from the competition between the Meissner current shielding effect and the fluctuations of the localized magnetic moments. We have measured the surface impedance penetration depth Λ of ErRh4B4 and Er0.5Ho0.5Rh4B4 by employing a spectrometer operated at 9 GHz. For ErRh4B4, owing to the critical spin fluctuations just above Ts (>Tc2), the critical temperature at which surface ferromagnetism appears, Λ−1 decreases smoothly as temperature decreases toward Ts. For Er0.5Ho0.5Rh4B4, the decrease of Λ−1 arising from spin fluctuations for T>Tc2 is very small, and Λ−1 decreases abruptly at Tc2. We will discuss the theory and show how the predicted values of Λ(T) compare well with the observed values.
Superconductivity and magnetism in the pseudoternary system Tm1−xGdxRh4B4 (abstract)
Authors: Umarji A. M. , Malik S. K. , Shenoy G. K.
Published: July 2002

The phase boundaries between the paramagnetic, superconducting, and magnetic states have been mapped out for the pseudoternary system Tm1−xGdxRh4B4 using ac magnetic susceptibility measurements. The compound TmRh4B4 exhibits coexistence of superconductivity and antiferromagnetism with superconducting transition temperature Tc=9.85 K and Neél temperature TN=0.4 K, while GdRh4B4 is a ferromagnet with Curie temperature TC=5.6 K. In Tm1−xGdxRh4B4 the three phases meet at a critical concentration of x=0.32. The initial slope of dTc/dx indicates the critical concentration to be 0.36 at which Tc→0 in the absence of magnetic order. The temperature dependence of upper critical fields (Hc2) has been analyzed in terms of WHH theory modified by Fischer for magnetic superconductors. The Hc2 data indicate that the antiferromagnetic ordering temperature of Tm ions is suppressed by dilution with Gd ions. The results of 155Gd Mössbauer measurements in these compounds are also reported. Details of this work will be published elsewhere.
Magnetic to nonmagnetic behavior in U(Sn,Pb)3
Authors: Lin C. L. , Zhou L. W. , Crow J. E. , Guertin R. P.
Published: July 2002

The magnetic to nonmagnetic transition in actinide alloys and intermetallic compounds is attributable to the delocalization of the 5f electrons due to an increase in the f-f overlap and/or f-spd hybridization. USn3 is paramagnetic at all temperatures and appears to be very near the border between magnetic and nonmagnetic behavior. In contrast, UPb3 has an antiferromagnetic transition at 31 K and the 5f electrons appear to be highly localized. Both of these systems have sufficiently large U-U separations to expect that f-spd hybridization is the major contributing factor to the delocalization of the 5f electrons. The room temperature lattice constant and the temperature dependence of magnetic susceptibility and electrical resistivity have been measured for the U(Sn1−xPbx)3 system to examine the salient features associated with the onset of long range magnetic behavior in this system. The room temperature lattice constant follows Vegard’s Law from USn3 to UPb3 and thus indicates no significant change in the degree of localization of the 5f electrons. Upon substitution of Sn for Pb in UPb3 (i.e., decreasing x), the Neel temperature TN initially increases to 38 K at x=0.7 then decreases and appears to go to zero between x=0.0 and x=0.1. Also, the spin fluctuation temperature Tsf for the USn3 rich alloys (i.e., x<0.60) decreases with increasing x. The pressure dependence of Tsf for USn3 (dTsf/dP=l.08 K/kbar) and TN for UPb3 (dTN/dP=0.26 K/kbar) has been measured. The x dependence of Tsf and TN along with the pressure dependence of these quantities is discussed in terms of the expected trend in f-spd hybridization.
NMR imaging (invited)
Authors: Kaufman Leon , Crooks Lawrence E.
Published: July 2002

Although the basic physics of the NMR process dates back about 40 years, and the demonstration that images could be formed from NMR signals is over a decade old, only recently has NMR imaging come to receive wide attention. Analysis of the hardware of a system shows that success depends less on stressing the state-of-the-art of individual components than on careful engineering design and thoughtful integration of existing technologies.
Research opportunities in magnetic materials (invited)
Authors: White R. M.
Published: July 2002

Earlier this year the National Materials Advisory Board established a Committee on Magnetic Materials to assess the adequacy of U. S. research in magnetic materials and to identify opportunities for future research. The findings and recommendations of this Committee are presented here.
Measurements of the magnetic excitations above T
c in iron and nickel (invited)
Authors: Mook H. A. , Lynn J. W.
Published: July 2002

High-resolution neutron scattering measurements, utilizing full polarization analysis techniques, have been carried out to explore the nature of the magnetic response above Tc in these itinerant ferromagnets. Large isotopically enriched single crystals were employed in order to obtain reliable determinations of the scattering function S(q,E). The scattering in Ni is found to evolve in a continuous fashion from the well-known spin diffusion response at small wave vectors q to a response with peaks centered at ±E at larger q, in agreement with our original work. The ratio of the observed widths of these spin waves to the spin-wave energies, as determined directly from measurements at constant q, is found to decrease with increasing ‖q‖ as expected. We also show that the recent measurements by Shirane and collaborators were taken with resolution which was too coarse to observe these spin-wave excitations.
Paramagnetic form factors from itinerant electron theory
Authors: Cooke J. F. , Liu S. H. , Liu A. J.
Published: July 2002

Elastic neutron scattering experiments performed over the past two decades have provided accurate information about the magnetic form factors of paramagnetic transition metals. These measurements have traditionally been analyzed in terms of an atomic-like theory. There are, however, some cases where this procedure does not work, and there remains the overall conceptual problem of using an atomistic theory for systems where the unpaired-spin electrons are itinerant. We have recently developed computer codes for efficiently evaluating the induced magnetic form factors of fcc and bcc itinerant electron paramagnets. Results for the orbital and spin contributions have been obtained for Cr, Nb, V, Mo, Pd, and Rh based on local density bands. By using calculated spin enhancement parameters, we find reasonable agreement between theory and neutron form factor data. In addition, these zero parameter calculations yield predictions for the bulk susceptibility on an absolute scale which are in reasonable agreement with experiment in all treated cases except palladium.
Proton NMR and susceptibility measurements in TiCoHx
Authors: Bowman R. C. , Craft B. D. , Tadlock W. E. , Venturini E. L. , Cantrell J. S.
Published: July 2002

The proton spin-lattice relaxation times (T1) and magnetic susceptibilities (χ) are used to study the effects of hydrogen on the electronic structure properties of the intermetallic compound TiCo. Hydrogen addition systematically decreases both χ(T) and the proton (T1T)−1/2 parameters, which are directly proportional to the local density of the Fermi-level electronic states. The behavior for the TiCoHx is completely opposite to that previously found for several other Ti-based metal hydrides where these parameters tend to increase with hydrogen content. These differences are attributed to the Fermi levels falling on alternate sides of peaks in the densities of electronic states. The present results for TiCoHx are compared with the magnetic properties and the theoretical band structures for the closely related hydride TiFeHx.
Magnetism in the system Ce1−xLaxRh3B2 (abstract)
Authors: Malik S. K. , Umarji A. M. , Shenoy G. K. , Aldred T. A. , Niarchos D. G.
Published: July 2002

The RRh3B2 (R=La to Gd) compounds form in the hexagonal CeCo3B2-type structure. The La containing compound in this series has been reported to be a superconductor while CeRh3B2 orders magnetically with Ce being in a nearly 3+ state. The magnetic ordering in CeRh3B2 is presumed to arise from the hybridization of nearly localized 4f electrons of Ce with sp conduction electrons or the 4d band of Rh or from the hybridization of the two. We have examined the magnetic behavior of the system Ce1−xLaxRh3B2 (x=0.0, 0.2, 0.4, 0.6, and 0.8). All these compounds (except LaRh3B2) also order magnetically. The Curie temperature decreases from about 115 K for pure Ce compound to 2.3 K for Ce0.2La0.8Rh3B2. The magnetic moments per formula unit remains nearly constant for 0<x<0.4 and then decreases with increasing x. From the values of the effective paramagnetic moments and the saturation moments we find that magnetism, having the same origin, persists in Ce1−xLaxRh3B2 for x ranging from 0.0 to 0.8. The La-rich side is being investigated for the occurrence of superconductivity. Details will be published elsewhere.
Anisotropic (triplet) superconductivity in UPt3 (invited)
Authors: Batlogg B. , Bishop D. J. , Bucher E. , Varma C. M. , Fisk Z. , Smith J. L.
Published: July 2002

Ultrasound experiments were performed on single crystals of the heavy Fermion superconductor UPt3. From the ultrasound attenuation below Tc, a quasiparticle density of states linear in energy is deduced, which we interpret as due to an anisotropic polar-like (triplet) superconducting state. This is further supported by the agreement of the only fitting parameter Δ0/kTc with the theoretical value for polar p-wave pairing. The attenuation in the normal state gives the electron-phonon interaction strength and the product of the deformation potential times the effective mass is found to be the same as for ordinary ‘‘light’’ Fermion systems. The observed variations of the sound velocity are discussed.
Effect of spin-orbit interaction on heterojunction band discontinuities
Authors: Chen Z. H. , Margalit S. , Yariv A. , Chiu L. C.
Published: July 2002

The effect of spin-orbit interaction is included in the linear combination of atomic orbitals calculation of heterojunction band discontinuities. It is found that spin-orbit interaction is not negligible when the atomic number of the constituent atoms exceeds about 40. The effect of spin-orbit interaction as well as some interesting observations and their implications are briefly discussed.
Hydrogenation and annealing kinetics of group-III acceptors in oxidized silicon
Authors: Sah Chih-Tang , Pan Samuel Cheng-Sheng , Hsu Charles Ching-Hsiang
Published: July 2002

Acceptor (B, Al, Ga, and In) density versus time curves during avalanche electron injection (AEI) and constant-temperature thermal annealing experiments obtained from metal-oxide-silicon capacitors (MOSCs) show two distinguishable phases. The time dependence of the acceptor density during AEI shows an initial delay due to electron-impact release of hydrogen trapped in the gate conductor and oxide layers and a long-time decay due to the thermal capture and electron-impact emission of the atomic hydrogen at the group-III acceptor centers in the silicon surface layer. Thermal anneal of hydrogenated acceptor begins at 50 °C for boron and 100 °C for Al, Ga, and In. The initial phase during thermal annealing of AEIed MOSCs follows a first order kinetics at higher annealing temperatures, reaching a steady-state acceptor density before the second phase begins. The long-time anneal follows strictly a second-order kinetics which is rate limited by the recombination of two hydrogen atoms to form a molecule. Incomplete anneal is observed at higher temperatures when the dissociation rate of the hydrogen molecule becomes comparable with the recombination rate of two hydrogen atoms. Analytical solutions are obtained which account for all the details of the observed hydrogenation and annealing curves. These solutions are used to evaluate the thermal capture and emission rates and electron-impact emission rates of hydrogen or proton at the group-III impurity centers and the bimolecular generation and recombination rates of hydrogen. A new concept of hydrogen or proton traps in analogy to the electronic hole or electron traps is introduced to analyze the kinetics and account for the observed chemical trends between thermal capture and emission rates, thermal activation energy and bond strength. Chemical trends are noted which are consistent with the trapped proton activation energy and hydrogen bond strength trend, B<Al<Ga<In.
Ion backscattering investigation of the etching of silicon with XeF2
Authors: Alexander R. B. , Heffelfinger D. M. , Padmanabhan K. R. , Buchholz J. C.
Published: July 2002

The etching of both amorphous and single-crystal Si with XeF2 vapor has been investigated by ion backscattering spectrometry. The etching rate of vapor-deposited Si measured both in situ during the etch and after completion of etching indicates a strong dependence on the partial pressure and thus the flow rate of XeF2. Single-crystal (100) Si exhibits almost twice the etching rate of amorphous Si, which can possibly be attributed to a preferred etching direction in single crystals.