bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic Excitations in Nickel
Authors: Lowde R. D. , Windsor C. G.
Published: February 2004

In continuation of our* earlier studies on this topic, the generalized susceptibility of nickel has now been measured and calculated over a wide range of temperatures covering the ferromagnetic, the paramagnetic, and the ``critical'' regions. Experimental results (from thermal neutron inelastic scattering) are expressed by means of contour maps showing the Van Hove correlation function S(K,ω) and the related imaginary part Imχ(K,ω) of the susceptibility. The whole Brillouin zone of wave-vectors is explored over a range of energies ℏω up to about ⅛ eV from 0.47Tc–1.9Tc. An absolute comparison is made with calculations based on direct evaluation of the Lindhard expression for generalized susceptibility followed by exchange enhancement of the spin part in random-phase approximation as per Izuyama, Kim, and Kubo. Tight-binding-approximation band structures have been employed, based on Slater-Koster interpolation of the bands obtained by Fletcher, by Yamashita and Wakoh, and by Dalton and Hubbard. It has been possible to give a credible account of the observed susceptibility over essentially the whole temperature range from the spin-wave region upwards. At low temperatures, the collective or spin-wave mode is the most prominent feature, the Stoner mode states being so strongly mixed with the spin-wave states by the interaction that little trace is left of the ``band of Stoner modes'' normally drawn in simple accounts of this phenomenon. With increase of temperature, a severe broadening of the spin-wave and the characteristic evolution of the susceptibility function through the critical temperature are correctly predicted. The agreement with experiment is improved by assuming that the effective Coulomb interaction parameter Ieff(K) falls off some 25% between K=0 and Kmax. An attempt to estimate the orbital susceptibility will be described.
Search for Noncollinear Spin Density in Hexagonal Co
Authors: Moon R. M. , Koehler W. C.
Published: February 2004

The interpretation of the magnetic form factors for the 3d metals has been based on the assumption that the magnetic moment is everywhere collinear, so that a scalar density function may be used. Blume1 has pointed out that if the spin density is noncollinear, the form factor measurements should be reinterpreted. Using the new technique of neutron polarization analysis, we have seen no evidence for scattering due to a noncollinear component of the spin density in hexagonal cobalt. Upper limits on the magnitude of such scattering have been established for the (002) and (110) reflections. Further, it can be shown that the polarized beam technique used in the form-factor measurements is not very sensitive to a small noncollinear spin component. Ignoring the presence of noncollinear spin scattering, of magnitude given by the upper limit of the polarization analysis experiment, produces an error in the Co form factor smaller than the experimental error due to other effects. For the cases of Fe and Ni, the noncollinear spin density would have to be much larger than the upper limit in Co to have any influence on the form-factor measurements. We conclude that the noncollinear spin density in hexagonal cobalt is zero within our limits of detection and, further, that it is highly improbable that a noncollinear spin density could exist in Fe or Ni large enough to affect any of the conclusions which have been drawn from the form-factor measurements.
Some Magnetic and Structural Properties of Epitaxial Garnet Films Prepared by rf Sputtering
Authors: Sawatzky E. , Kay E.
Published: February 2004

Epitaxial garnet films have been prepared by rf sputtering from a polycrystalline gadolinium iron garnet target onto single-crystal substrates of YAG. Epitaxy is achieved on all orientations of the substrate, either in situ when depositing at ≥500°C, or during post-deposition crystallization of amorphous or partially crystallized films deposited at <500°C. The compensation temperature in epitaxial films depends strongly on the deposition temperature. The lowest compensation temperatures are found in films prepared at the highest deposition temperatures. Flims prepared on glass substrates at elevated deposition temperatures are iron deficient and crystallize in at least two phases, Gd3Fe5O12 and GdFeO3. The iron deficiency in these films is assumed to be caused by a variation with temperature in the net cation sticking coefficients. No GdFeO3 phase is found in epitaxial films prepared at the same elevated deposition temperatures, although these films should also be iron deficient. It is believed that they are, in fact, iron deficient, but that they crystallize with the garnet structure with iron vacancies in one of the iron sublattices, giving rise to some of the unexpected magnetic and structural properties of the films.
Optical Measurements in Erbium Iron and Erbium Gallium Garnet
Authors: Orlich E. , Hüfner S.
Published: February 2004

Optical absorption spectra of erbium iron garnet (ErIG) have been measured at 4.2°, 20°, and 62°K with external magnetic fields up to 25 kOe. From these measurements, the magnetic moment of the groundstate for the two magnetically inequivalent sites can be deduced as μ1=7.0 μB and μ2=4.6 μB yielding an average moment of μ=5.4 μB in the [100]-direction. In addition, we have measured the G-tensor of one excited state (4I9/2) as Gx=(10.0±0.5) cm−1, Gy=(15.0±0.5) cm−1, and Gz=(10.0±0.5) cm−1. The comparison with the g-tensor of this state in the gallium garnet (gx=3.1, gy=3.1, gz=3.9) shows a large anisotropic exchange. Crystal-field energy levels and g-factors of erbium gallium garnet and erbium aluminium garnet have been measured and a good fit to a set of nine crystal-field parameters Akq〈rk〉 has been obtained.
Scattering of Infrared Radiation by Two Magnon Processes in YIG
Authors: Gall H. Le , Jamet J. P. , Cagan V.
Published: February 2004

Experimental results providing evidence of interaction between spin waves and a laser beam in YIG are reported. When coherent magnons, parametrically excited by parallel pumping at 9.4 GHz in a YIG single-crystal rod, are irradiated by infrared photons produced by a Nd laser at 1.06 μm, the transmitted power through a measurement microwave cavity is considerably modified. If the amplitude h of the rf field is lower than the spin-wave threshold hcrit, we detect a microwave absorption when a laser pulse is applied to the YIG sample. It seems that this absorption is due to Stokes scattering process which produce optically excited magnons with k and −k wave numbers. These Stokes magnons lead to an apparent increase in the rate at which magnons with the same wave number are parametrically excited by microwaves. The light then gives, as far as the microwaves are concerned, an apparent decrease in the magnon damping, i.e., a decrease of the parallel pump threshold. In that case (Pi<Pcrit) the microwave absorption increases with Pi. In that case, with hhcrit, the parallel susceptibility decreases when the infrared pulse irradiates the sample. This decrease is attributed to anti-Stokes scattering processes with the parametric magnons, which define an additional damping of the spin-wave system. The respective amplitudes of the Stokes and anti-Stokes processes are presented as a function of rf field.
Ferromagnetic Resonance in Epitaxial Garnet Thin Films
Authors: Sparks M. , Tittmann B. R. , Mee J. E. , Newkirk C.
Published: February 2004

The chemical vapor deposition process for growing epitaxial yttrium iron garnet (YIG) has matured to the point where films of varying geometry and thickness from ½ to as great as 40 μ can be readily grown on several orientations of gadolinium gallium garnet, yttrium aluminum garnet, and YIG, itself. As a measure of the quality of the films, line widths (at 9.4 GHz and room temperature) as small as 0.6 and 1.4 Oe were obtained in samples with substrates removed and intact, respectively, without a major effort toward improvement. Magnetostatic modes, exchange modes, and ferromagnetic normal modes having comparable exchange and demagnetization energies were observed in these films and in thin bulk disks and were explained in detail by a new theory which applies not only to films, but also to samples of arbitrary shape and size, and includes the effects of exchange and demagnetization energies. The frequencies of the general normal modes were calculated by casting the linearized equation of motion of the magnetization into the form of an eigenvalue equation and solving this equation by a variational method. In addition to explaining the experiments in detail, the theory affords a new method of studying pinning from the observation of magnetostatic modes, has implications concerning the main-resonance position in finite films, and, together with the experiments, further verifies Portis' mode-spacing theory. The behavior of the magnetostatic modes in the epitaxial films and thin bulk samples, when interpreted according to the present theory, gives direct information about the pinning conditions which the magnetization obeys at the edges and surfaces of these ferrite samples. For a single-crystal bulk sample 0.005 in. thick and for a single-crystal film 12.5 μ thick, the magnetization was pinned at the small edges of the film and was unpinned at the large faces, indicating that the surface-anisotropy pinning mechanism is not effective in these samples. It is well known, that the frequency of the main resonance in a finite film is shifted slightly from the frequency of the uniform precession mode (k=0) in an infinite film. The present theory indicates that this shift is a consequence of the volume microwave demagnetization field [arising from ∇·≠0 in the volume, as opposed to the edges, of the sample]. The shift is not based on a spatial average of the static internal field Hi as previously thought. The average 〈Hi〉 should be a weighted average, and the weighting factor mx2+my2 is small at the small edges of the film where Hi differs from its infinite-film value.
Characteristics of Magnetic Properties of Grain-Oriented Silicon Iron with High Permeability
Authors: Taguchi Satoru , Sakakura Akira
Published: February 2004

Grain-oriented silicon iron with high permeability is prepared using aluminum-nitride needles to restrain normal grain growth. The improvements in hysteresis loss of specimens with a permeability of 1920 are about 0.120 W/kg at 15 kG and 0.160 W/kg at 17 kG as compared to the commerical product with a permeability of 1820. That is the reason why the crystal orientation has a strong influence on total loss, especially at B = 17 kG. The effect of grain size on total loss is important also. The detrimental aluminum nitride and aluminum oxide are not present in the base metal after final annealing. For example, aluminum oxide is removed from the base metal and absorbed into a glass film. Glass-type coating plays a dominant role, not only for the magnetostriction, but also for total loss of these specimens with high permeability, especially in thin-gauge specimens.
Heat Capacity Singularity for a Ferromagnet in a Finite Applied Field
Authors: Griffiths Robert B.
Published: February 2004

The heat capacity of a ferromagnet in a magnetic field is of value in establishing thermodynamic properties near the Curie point, but certain complications arise due to demagnetizing effects. These should lead, in particular, to a singularity (nonanalytic behavior, not a divergence) in the heat capacity C as a function of temperature when the latter reaches the value where the spontaneous magnetization Ms equals the external field divided by the demagnetizing factor D. If the initial susceptibility χ at this temperature is finite, C decreases by an amount DT(dMs/dT)2/(1+Dχ). If χ is infinite, C is continuous but will possess a discontinuous slope or other stronger (or weaker) singularity which can be related to the dependence of magnetization on field at fixed temperature. There is no reason (in general) to expect this singularity to be the same as that observed at the critical temperature in zero applied field.
Some Critical Properties of Quantum-Mechanical Heisenberg Ferro-and Antiferromagnets
Authors: Stanley H. E.
Published: February 2004

High-temperature expansions of the staggered susceptibility for the Heisenberg antiferromagnet are analyzed for two-dimensional lattices. The evidence favoring a divergent staggered susceptibility χ̃ is found to be essentially as strong as that presented earlier in support of a divergent susceptibility χ for the two-dimensional Heisenberg ferromagnet. Thus the very recent experimental and theoretical findings which favor a phase transition in various ``planar antiferromagnets'' (or materials with the K2NiF4 structure in which the intraplanar interactions may be as much as 105 times stronger than any interplanar interactions) would appear to be supported by the series expansions. We also consider the question of the dependence of the susceptibility exponent γ[χ∼(T-Tc)−γ] upon spin quantum number S for three-dimensional lattices. The previously proposed behavior must definitely be revised in the light of additional terms in the expansions and the availability of more refined extrapolation procedures. Here we argue (a) that γ(∞)=1.38±0.01 and (b) that γ(S) does not appear to jump, for S finite, to 1.43 (its presumed value for S=½).
Ferrimagnetic Resonance in Cu2+-Doped YIG
Authors: Krishnan R.
Published: February 2004

By doping an yttrium iron garnet (YIG) single crystal with certain paramagnetic ions, it is possible to get some information about their properties by ferrimagnetic resonance, as was shown by us before for Ni2+ ions. Here we describe the properties of YIG containing 1 and 2 wt% of CuO. Cu2+ ions contribute to the anisotropic energy, their per ion contribution to K being −20×10−2 cm−1 at 4.2°K. K2 in these crystals is smaller and is of the order of −104 ergs/cm8 at 4.2°K.
Single-Ion Magnetic Interactions in Diamagnetic Spinels
Authors: Kahan H. , Kestigian M.
Published: February 2004

The continuing interest in the magnetic properties of the spinel structure has prompted us to explore in detail the single-ion interactions of magnetic ions in diamagnetic spinel host crystals. In particular we report ESR and optical absorption measurements on Cr3+ and Fe3+ ions in crystals of ZnGa2O4 and ZnAl2O4. The characteristics of the ESR and optical spectra are extremely sensitive to the thermal history of the crystals. The Cr3+ measurements are explained in a fairly satisfactory way by MacFarlane's theory. However, some difficulties in the interpretation of the optical spectrum are noted.
Theory of Anisotropic Spontaneous Magnetization and Anisotropy Energy in Orthorhombic Ferromagnets at 0°K
Authors: Rado George T.
Published: February 2004

A new mechanism of anisotropic spontaneous magnetization is proposed. It consists of the mutual admixture, due to perturbations caused by one-ion anisotropy, of appropriate eigenstates of the Hamiltonian describing the molecular field energy. A self-consistent method is developed in which the anticipated anisotropic part of the spontaneous magnetization contributes to the molecular field, and the direction of the spontaneous magnetization is fixed by an applied field. Using the quadratic parts of the one-ion spin Hamiltonian as the perturbation, the first- and second-order portions of the anisotropic spontaneous magnetization and anisotropy energy are calculated for an orthorhombic ferromagnet at 0°K. It is pointed out, however, that the present method applies to other perturbations also. The calculation shows, among other things, that higher-order perturbations do usually contribute to first-order anisotropy constants. This result is contrary to accepted conclusions contained in the literature. The predictions of the present theory are compared with those of others, and its possible generalizations and applicability to experiments are discussed. Finally, the first-order portion of the anisotropic spontaneous magnetization and anisotropy energy arising from an anisotropic g factor are calculated and compared to experimental data on gallium iron oxide. The full text of this paper will appear in The Physical Review. [Note added in proof: See, G. T. Rado, Phys. Rev. 176, 644 (1968).]
Influence of Sulfur on Initial Permeability of Commercial 49% Ni–Fe Alloys
Authors: Colling D. A. , Aspden R. G.
Published: February 2004

Attainment of high initial permeability in commercial 49% Ni–Fe alloys is shown to be a function of the sulfur content and grain size. Low sulfur material was obtained by desulfurization during melting or by strip annealing. μ40 of the low sulfur material was significantly higher than that of normal commercial material at annealing temperatures below 1200°C. When annealed at 1200°C, μ40 for the initially low sulfur material was slightly lower because of a smaller grain size or a higher final sulfur level. Analysis of sulfur content of annealed material by the methylene-blue technique allowed a direct correlation between μ40 and sulfur content in secondary recrystallized structures. In general, μ40 values in excess of 10 000 cannot be obtained unless final sulfur content is less than 12 ppm. Highest initial permeabilities result from a coarse-grained secondary recrystallized structure and low impurity contents.
Magnetic Symmetry and Ferroelectricity Induced by Antiferromagnetic Transitions
Authors: Goshen S. , Mukamel D. , Shaked H. , Shtrikman S.
Published: February 2004

Ferroelectricity is forbidden in centrosymmetric crystals. An antiferromagnetic transition in such crystals which results in a magnetic cell different from the chemical one may lead to noncentrosymmetric magnetic symmetry. It is therefore expected that ferroelectricity may be induced by such a transition. Neutron-diffraction experiments in CaMn2O4, TbCrO3, BiMn2O5 show that the magnetic transitions in these compounds belong to this category. To illustrate this effect from a symmetry point of view an antiferromagnetic transition in a structure belonging to the space group Pmma is considered.
Analysis by Representation Theory of Helical Spin Structures. Application to Dy and MnP
Authors: Bertaut E. F.
Published: February 2004

We have extended representation analysis of magnetic structures to arbitrary wave vectors k by defining ``k components of spins'' which transform like Bloch waves. The theory provides all possible isotropic and anisotropic couplings which are compatible with k. Applied to the helical structure of Dy the theory shows that antisymmetric coupling cannot be ruled out. For MnP the possible presence of antisymmetric coupling provides an explanation which is different from that recently forwarded by Takeuchi and Motizuki.
Magnetism and Medicine
Authors: Frei E. H.
Published: February 2004

The use of magnetic fields as a tool for control and measurement has important medical applications. Present devices include magnets to remove objects from the eye and to facilitate the withdrawal of swallowed safety pins and similar objects, ensuring that the sharp end points backwards. In more sophisticated applications magnetic materials are purposely introduced into the body to accomplish various tasks. Magnetically guided catheters have been used to negotiate the tortuous passageways of blood vessels. Ferrite powders may replace barium sulfate for gastrointestinal x-ray diagnosis in some applications. Magnetic fields have also been used for various measurements, employing not only fields due to small amounts of material artificially introduced into the body but also the extremely weak fields produced by the electrical or electromagnetic activity of the body itself.
Magnetoresistance in Chalcogenide Spinels
Authors: Bongers P. F. , Haas C. , van Run A. M. J. G. , Zanmarchi G.
Published: February 2004

The consequences of a simple type of exchange interaction between charge carriers in a broad energy band and localized magnetic moments will be discussed for a ferromagnetic semiconductor. The interaction causes (1) a splitting of the energy band into two bands for the different spin directions, (2) spin disorder scattering of the charge carriers. The calculated temperature and field dependence of the magnetoresistance are compared with experimental data of CdCr2Se4. The magnetoresistance of compounds MeCr2S4, Me=Fe, Co, Cd was measured. For n-type CdCr2S4 and for p-type FeCr2S4 and p-type CoCr2S4, the magnetoresistance −Δρ/ρ0 was found to be 0.78, 0.05, and 0.05, respectively, at 12 kOe near Tc, whereas n-type FeCr2S4 and CoCr2S4 show no effect. The magnetoresistance of the system Fe1−xCdxCr2S4 was also measured. The temperature dependence of the polar magneto-optical Kerr effect of CdCr2Se4 is discussed in connection with the described model for the energy bands.
Magneto-Optic Hologram
Authors: Fan G. , Pennington K. , Greiner J. H.
Published: February 2004

In this paper the first experiment of thermomagnetic writing of a hologram on a thin film of EuO and the reconstruction of the hologram is reported. Writing of the hologram is achieved by using a giant pulsed ruby laser as a source for thermomagnetization. Erasing of the hologram is accomplished by a small applied magnetic field. The ruby laser is used because of its ability to provide the energy required for writing in a time that is short compared with the thermo-relaxation time. A hologram with shallow angle (10 μ resolution) was recorded and the reconstruction of the hologram was done using a He–Ne laser as a source. The formation of the hologram is based on the fact that the index of refraction of the material is a function of magnetization. Thus, the different state of magnetization induced by the laser pulse acts as a phase-hologram for polarized light. Polarizing elements were used to observe the reconstructed images from the magnetic holograms. Results were also obtained for the diffraction intensity in the different orders of a simple thermo-magnetic hologram. An analysis is presented to interpret the observed phenomena and gives the calculated efficiency that can be obtained under ideal conditions.
Magnon-Assisted Chromium Emission in YCrO3, LuCrO3, and GdCrO3
Authors: van der Ziel J. P. , Van Uitert L. G.
Published: February 2004

Measurements have been made of the absorption and fluorescence spectrum of Cr3+ in orthorhombic YCrO3, LuCrO3, and GdCrO3. We observe several very sharp emission lines at energies about 100 cm−1 lower than the intrinsic 4A2 to 2E Cr3+ absorption, and attribute the emission to impurity-perturbed Cr3+ ions. Intrinsic emission was not observed. Magnon sideband fluorescence is found at lower energy and results in the simultaneous optical de-excitation of a perturbed Cr3+ ion and the creation of a magnon on a neighboring lattice site. The sideband is spin allowed if the photon and magnon are created on the same sublattice, and has electric dipole polarization. Analysis of the sideband line shape shows the nearest-neighbor exchange interaction between pairs along the c axis, and between pairs in the plane perpendicular to the c axis are not equal. The value of J averaged over the six nearest neighbors is 22.1, 17.6, and 25.5 cm−1 for the Y, Lu, and Gd chromites, respectively. These results are in good agreement with J values computed from magnetic data.
Self-Consistent Ultrasonic Method for the Determination of the Equation of State of Liquids at Very High Pressures
Authors: Heydemann Peter L. M. , Houck James C.
Published: February 2004

Ultrasonic measurements on liquids at very high pressures and an accurate self-consistent ultrasonic method for the determination of the equation of state of liquids at very high pressures are described. The liquid sample is contained in a polyethylene-lined piston and cylinder device. The method described requires only the measurement of the transit time of an ultrasonic pulse through the liquid as a function of pressure. The difficulties and errors connected with a direct volumetric determination are thus avoided and a high accuracy is achieved. A computation of the uncertainties in the determination of the bulk moduli, the density and the velocity of sound is presented. Experimental results are given for water at 22°C and pressures up to 12.6 kbar. Data for the density of water reported by P. W. Bridgman and by L. H. Adams, and for the velocity of sound reported by Holton et al. are found to be in good agreement with the present work.
Ultrasonic Amplification in CdS with Continuous Drift Field at 20°C
Authors: Krischer Charles
Published: February 2004

Measurement results on CdS ultrasonic amplifiers which were operated with continuous drift fields at room temperature are presented. A material gain of 26 dB was obtained for a 200-MHz transverse-wave signal. Comparisons are made with pulsed-field data obtained from the same samples. The presence of saturation-level ultrasonic flux significantly reduces the maximum gain obtainable under continuous operation.
Shapes of Nuclei of Evaporated fcc Metals
Authors: Fukano Y. , Wayman C. M.
Published: February 2004

A simple analysis assuming hard-sphere atomic behavior and considering surface and volume energies has been applied to estimate the stability of very small fcc crystals having different external shapes. Such crystals or particles are commonly produced by vacuum evaporation, or evaporation in a reduced pressure inert gas atmosphere. The analysis has been extended to small crystals (or nuclei) of fcc metals condensed on substrates by assuming that the metal-substrate interfacial energy is comparable to that of a large angle metallic grain boundary. Icosahedral, pentagonal decahedral, tetrahedral, octahedral, and pyramidal particles were considered, and the relative stability predictions are generally in accord with observations.
Study of Certain Strain Centers in Copper Crystals by Etch-Pit and X-Ray Techniques
Authors: Bertocci U. , Bertocci C. , Young F. W.
Published: February 2004

Strain centers with attendant prismatic dislocation loops have been studied in copper crystals of low dislocation density by use of Borrmann x-ray topograph and etch-pit techniques. Direct comparison of these defects with the two techniques showed an exact correlation. While the nature of the centers of the strain could not be certainly determined, there was evidence that the loops were of the interstitial type.
Diffraction by fcc Crystals with Intrinsic and Extrinsic Faults
Authors: Holloway H. , Klamkin M. S.
Published: February 2004

A model is given for diffraction by an fcc lattice which contains a randomly arranged mixture of intrinsic and extrinsic stacking faults. The expression for the distribution of intensity in reciprocal space contains, as special cases, the equations derived for intrinsic and extrinsic faulting by Paterson and by Johnson, respectively. The result and its derivation clarify a controversy which arises from Sabine's model for faulting by condensation of interstitials; some comments on this problem are given in an appendix.
Theory of X-Ray Diffraction by a Vibrating Crystal
Authors: Kuriyama Masao , Miyakawa Toru
Published: February 2004

The scattering amplitude for x-ray diffraction by a vibrating crystal is obtained. The Bragg diffracted propagators for the perfect crystal are replaced by new propagators in which the Fourier transform of the polarizability of the crystal, v(H), is multiplied by the Bessel function of zero order, J0(H·Aq), of the product of the vibrational amplitude Aq and a reciprocal lattice vector H, in a similar fashion to the correction by the Debye-Waller factor in the thermal problem. The scattering amplitude contains both dynamical and kinematical contribution. The case of ideal thickness vibration is studied in detail in the Laue geometry. The real part of the perturbed scattering amplitude is proportional to v(H) sin (H·Aq). This theory elucidates the transition from dynamical to kinematical scattering, that is, a decrease in extinction, as a function of the vibration amplitude.
Growth of Grain-Boundary Precipitates
Authors: Brailsford A. D. , Aaron H. B.
Published: February 2004

A model descriptive of the growth of precipitates which nucleate at grain boundaries and grow pre ferentially along them (grain-boundary allotriomorphs) is studied. The sequential processes during growth are presumed to be volume diffusion of solute to the grain boundary, transport along the grain boundary to the precipitate, and interfacial diffusion and deposition over the surface of the growing allotriomorph. An analysis of this mode of growth is applied to the results of recent observations of the lengthening and the thickening rates of θ-allotriomorphs in an Al-4% Cu alloy. On the basis of these measurements the diffusivity within the grain boundary Db is found to be 0.1 exp (−18480/RT) cm2sec−1, while the diffusivity within the allotriomorph-matrix interface, D8, is D8=1.8 exp (−22020/RT) cm2 sec−1 when the mole fraction of solute in this interface is taken to be the average of that in θ and in α at the boundary, or D8=0.06 exp (−12740/RT) cm2 sec−1 when this surface concentration is assumed to be simply that in α at the boundary. The latter value of D8 is in good agreement with that obtained in a previous analysis where the same value was used for the interface concentration.
High-Temperature-Air Microwave Breakdown
Authors: Light G. C.
Published: February 2004

The test section of an electric-arc shock tube was used to explore the dependence of microwave breakdown behavior on elevated gas temperature. Special diagnostic techniques included microwave interferometry and a flat-plate ion density probe. Results for air, nitrogen, and argon are in agreement with the proposed theory, and the importance of the nitrogen molecule in explaining the air data is established in the context of the theoretical model.
Photoemission from Al–Al2O3 Films in the Vacuum Ultraviolet Region
Authors: Pong W.
Published: February 2004

Measurements of quantum yield of Al–Al2O3 films were made at photon energies between 7 and 22 eV. The photoelectric threshold for intrinsic photoemission from amorphous Al2O3 appeared to be about 8 eV. Approximately 1 eV was estimated for the electron affinity of Al2O3. The yield of anodic oxide film on aluminum at 10.2 eV decreased with increasing oxide thickness while the yield at 21.2 eV increased. The decrease in yield at 10.2 eV was a tributed to photoelectrons originating from the aluminum film beneath the oxide layer. Attempts were made to study the escape probability function for electrons moving through the oxide into the vacuum. An attenuation length of about 130 Å for the photoelectrons in the oxide was obtained by using the exponential-function approximation. The maximum initial energy of the electrons was estimated to be 7.8 eV above the vacuum level. The electron mean free path for electron-phonon collisions in the oxide was estimated to be approximately 16 Å. Key words: Photoelectric effect, vacuum ultraviolet, photoelectric threshold, Al2O3 films, thin films, electron attenuation length, electron mean free path.
Estimates of the Efficiencies of Production and Detection of Electron-Excited Auger Emission
Authors: Bishop H. E. , Rivière J. C.
Published: February 2004

The efficiency of production of electrons emitted by the Auger process from the surface of a solid irradiated with a beam of electrons in the energy region below 2 keV is estimated. It is shown that the probability of photon production is negligible and that to a first approximation the optimum primary energy required for the ionization of an inner atomic level is between 3 and 3.5 times the critical ionization potential of that level. Calculation of the Auger current is made for a chosen system, that of oxygen adsorbed on a copper surface, and is found to be about 2×10−11 A in the group of oxygen peaks around 500 eV, for a complete monolayer, detected on a spherical screen of the size typically found in LEED display systems. Consideration of the two methods, the retarding field and the electrostatic deflection, that have been used for detection of Auger currents, leads to the conclusions that, in the retarding field analyzer, the current in the second derivative increases as the square of the amplitude of modulation up to an amplitude about one-half that of the rms width of the Auger peak, and thereafter tends to a constant value, whereas in the electrostatic-deflection analyzer, the current in the second derivative increases linearly with amplitude of modulation up to an amplitude about one-third that of the rms width, passes through a maximum, and, beyond an amplitude about six times the rms width, decreases again as the inverse square root of the amplitude.
Anode Voltage Drop and Anode Spot Formation in dc Vacuum Arcs
Authors: Kimblin C. W.
Published: February 2004

Factors influencing the formation of anode spots in vacuum arcs have been investigated. For anode diameters of 1.25 and 5 cm, copper vapor arcs have been drawn to electrode spacings of 2.5 cm, and spot formation threshold currents of 400 and 2100 A have been determined for the small- and large-diameter anodes, respectively. Comparison of the arc voltage vs current characteristics, together with probe observations of potential distribution, reveals that an anode voltage drop is established prior to spot formation. The magnitude of this drop increases with both electrode spacing and arc current, and decreases with increase of anode area. By analogy with phenomena in mercury arcs, anode voltage drop formation is associated with vapor starvation. Photographs reveal that, even in the presence of an anode spot, high-voltage arcs can exist with a multiplicity of cathode spots and a diffuse arc column. Delays in the onset of the spot phenomena indicate that a temperature rise of the anode surface to the melting temperature precipitates the vapor release necessary for spot formation.
AC Field-Induced Flux Jumps in Nb3Sn
Authors: Gandolfo David A. , Dubeck Leroy , Rothwarf Frederick
Published: February 2004

An ac magnetic field was applied parallel to the surface of a tubular sample of superconducting Nb3Sn. The frequency and amplitude of the ac field were used to calculate the largest hysteresis power loss per unit surface area, Ws, which could be applied without flux jumping. The values of Ws increased with increasing background dc field.
Measurements of Energy Losses in Pulsed Superconducting Magnets
Authors: Morgan G. H. , Dahl P. F. , Sampson W. B. , Britton R. B.
Published: February 2004

Measurements have been made of ac power losses in a number of small superconducting magnets. The losses were determined by the volume of helium vaporized. The magnets had inductances from 1.4 to 47 mH and were wound with commercially available, half-inch wide, partially stabilized Nb3Sn ribbon. They were powered by sawtooth current pulses of 1 to 1/20 Hz, similar to those of a proton synchrotron and also by sinusoidal current waveforms of 60 Hz. Peak currents ranged from 1–1100 A and the corresponding peak fields from about 500–50 000 G. It is found that at low frequencies, loss per cycle is independent of frequency. It is also found that at low frequencies and moderate fields, the loss depends only on the total field change and is largely independent of current or of steady components of field. Losses measured at 60 Hz and low fields are in agreement with these low-frequency results where the data overlap. Losses obtained from magnets of different geometries can be correlated on the basis of the average component of peak field normal to the ribbon surface. Losses at high fields are quite dependent on winding density, which is attributed to a diamagnetic or field-exclusion effect. They increase with Nb3Sb content at high fields, but tend to be independent of Nb3Sb content at low fields. Losses vary approximately as the 2.5 power of the change in field. The ratio of energy stored to energy dissipated is about the same in all magnets in moderate to high fields. The results are compared with theory and with those of other experimenters.
Determination of Magnetic Anisotropy and Porosity from the Approach to Saturation of Polycrystalline Ferrites
Authors: Dionne Gerald F.
Published: February 2004

From measurements of magnetization in the approach to saturation region of the hysteresis loop, a semiempirical method has been developed for determining reasonably accurate values of | K1 |/Ms and peff, the fraction of nonmagnetic volume of polycrystalline ferrimagnetic materials. The technique involves the precise measurement of magnetization at five values of applied magnetic field, and the subsequent solution of four linear simultaneous equations. In combination with a measurement of the ferrimagnetic resonance linewidth, it is also shown how the intrinsic single-crystal linewidth may be estimated. The results for a variety of garnet and ferrite compositions are reported, and selected aspects of their behavior are discussed in terms of magnetic anisotropy, porosity, and linewidth. In particular, materials containing manganese appear to have large intensic linewidths, and it is considered to be highly probable that octahedral-site Mn3+ ions with short spin-lattice relaxation times are responsible for this effect.
Magnetic Domain Nucleation and Propagation in Fine Wires
Authors: Turner P. A. , Stockbridge C. D. , Theuerer H. C.
Published: February 2004

The effect of material composition and structure on the magnetic domain nucleation field (Hn) and the domain wall propagation field (Hp) in fine cold-drawn Permalloy wires is reported. Binary Ni/Fe wires (Fe>20%) were reentrant (Hn>Hp) when cold drawn to less than 250 μm diam. Alloying additions inhibited stress recovery during drawing and annealing and thereby increased and stabilized Hn. Variations in Hn along the wire length correlated with defects in the wire surface. Sputter coating the wire improved the surface condition and reduced variations in Hn. A magnetostrictive model of re-entrancy is proposed and discussed.
Applicability of Vegard's Law to the PbxSn1−xTe Alloy System
Authors: Bis R. F. , Dixon J. R.
Published: February 2004

The relationship between lattice constant a0, alloy fraction x, and stoichiometry has been studied for the PbxSn1−xTe alloy system. Using the hole concentration p as a measure of the deviation from stoichiometry, it is shown that a0 for SnTe varies with p according to a0(SnTe)=−1.7×10−23 p+6.327, where a0 is measured in Å, and p in cm−3. For small deviations from stoichiometry it is shown that the alloys obey Vegard's law. Evidence is presented that Vegard's law holds in general providing fixed deviations from stoichiometry are considered. The functional relationship between a0, x, and p may be expressed as a0(alloy)=0.133x+a0(SnTe), where a0(SnTe) is given by the expression presented above. On the basis of this equation it is shown that previously reported deviations from Vegard's law arise in a natural way from the varying deviations from stoichiometry which characterize these alloys.
Preparation of Superconducting Compounds by the Co-condensation in Vacuum of the Elements
Authors: Hammond R. H. , Snowden D. P. , Meyer C. H. , Pereue J. H. , Kelly G. M. , Stern M. O.
Published: February 2004

The method of co-condensation of the elements from separate sources in vacuum to form intermetallic compounds has been used to make high current density superconducting ribbons. Evaporation rate monitors and control loops have been developed that allow control of the compound composition. The technique has been used to make continuous lengths of ribbon, and a number of compounds have been tested, Nb3Sn in particular. The condensation conditions are suitable for the fabrication of a microstructure consisting of alternate layers of compound superconductors and other materials, including normal metals and dielectrics. The microstructure assists in the pinning of transport currents in the presence of transverse magnetic fields, and in some apparent inherent stability against flux jumps.
Measurements of the Kinetic Inductance of Superconducting Linear Structures
Authors: Meservey R. , Tedrow P. M.
Published: February 2004

The kinetic inductance Lk is associated with the inertial mass of the current carriers. For a long superconductor of length l and very small cross-sectional area σ, Lk = (m* / nse*2) (l/σ) and is the main contribution to the temperature-dependent inductance LT. We have measured LT in superconducting tin wires and thin film meander lines by a technique which uses a counter to determine frequency changes of a 15 MHz tunnel-diode oscillator whose tank circuit contains LT. The measured frequency changes are proportional to z = [λ(t)/λ(0)]BCS for wires and thick films; for thin films the frequency changes are proportional to z2. This result agrees with calculations of LT from the London theory and the values of λ(0) agree well with the values expected for these samples. This technique allows measurement of the carrier concentration or penetration depth in thin films over the whole H, T plane of the superconducting state and can be independent of the penetration law assumed. Frequency shifts observed with thin film alumimum meander lines have been used to detect changes of temperature of 5∼10−7 °K, changes of magnetic field of 10−5 G, and changes of current associated with one quantum in a flux quantum magnetometer.
Slow-Wave Structures Utilizing Superconducting Thin-Film Transmission Lines
Authors: Mason P. V. , Gould R. W.
Published: February 2004

Slow-wave propagation of electromagnetic waves in transmission lines formed of thin-film superconductors has been studied theoretically and experimentally. Previous theoretical analyses have been extended to include nonlocal theories. Strong dependence of phase velocity is found on film thickness and interfilm spacing when these become less than a few penetration depths. Velocity is also modified by coherence length, mean free path, nature of reflection of electrons at the film surfaces, and by temperature and magnetic field. Experimental measurements were made to verify the dependence on thickness, spacing, and temperature by means of a resonance technique. Agreement with theory was excellent in the case of temperature. Data taken for varying thickness and spacing verified the general trend of theoretical predictions. They indicate a nonlocal behavior with some specular reflection, but scatter of the data taken for different films prevents precise comparison of theory and experiment. Estimates of bulk penetration depths were made for indium, λIn = 648±130 Å. For tantalum a rough estimate could be made of λTa = 580 Å. Data were consistent with the estimate of coherence length for indium of ξ0 ≈ 3000 Å. Velocity was found to be independent of frequency in the range 50–500 MHz, while losses increased as the square. Pulse measurements indicated that delays of several microseconds and storage of several thousand pulses on a single line are feasible.
Tuning Superconducting Microwave Cavities
Authors: Smith T. I.
Published: February 2004

The extremely high Q of superconducting microwave cavities makes them very attractive for use in many applications. In some of these applications the resonant frequency of the superconducting cavity must be adjustable while the cavity is at liquid-helium temperatures. Because of the requirement that the losses in the tuning mechanism be very low, in order that the Q of the cavity not be degraded, the usual methods of tuning microwave cavities are not always applicable to superconducting devices. One example of a case in which new techniques have had to be developed is the superconducting linear accelerator which is being constructed at Stanford University. In this accelerator, 25 superconducting sections, each 20 ft long, will have to resonate at the same frequency to within about 1 part in 108. The sections are being constructed to be tunable at liquid-helium temperatures over a range of 1 part in 104, with a resolution of 1 part in 104.
AC Energy Losses Above and Below H
c1 in Niobium and Niobium-25 At.% Zirconium
Authors: Beall W. T. , Meyerhoff R. W.
Published: February 2004

Calorimetrically determined energy losses for a series of niobium and niobium-25 at.% zirconium samples carrying an af transport current agreed with those calculated from hysteresis loops determined by dc magnetization measurements. The results of this study showed that the energy losses in ultra-high-purity annealed Nb are less than those for any of the other samples studied at magnetic fields less than 1500 G. The magnetic field dependence of the energy losses in all of the samples studied is given by EL = E12hm, where EL is the energy dissipated per unit surface area per cycle and is independent of frequency, E12 is a constant which depends on the material and surface finish, and h = Hp/Hc1, where Hp is the peak ac field amplitude, and Hc1 is the lower critical magnetic field; m = n1 when h≤1, and m = n2 when h≥1. For both highly reversible and highly irreversible samples, n1 ≈ 3. For the least reversible samples, n2 ≈ 4, increasing to ∼8 for the most reversible samples studied. E12 generally increased with decreasing reversibility and was strongly dependent on the surface finish of the sample, increasing as the surface roughness increased. The power loss PL per unit surface area is given by PL = (EL)f. This linear frequency dependence is consistent with a hysteretic loss mechanism as is the agreement between the calorimetrically determined losses and those calculated from the measured hysteresis loops.
Comparison Between Measured Critical Currents and a Model of Transiently Stabilized Nb3Sn Magnets
Authors: Schrader E. R.
Published: February 2004

A previously proposed model used to predict transient stabilization in high-current superconductive solenoids is assessed in the light of results on actual test magnets made with a variety of fabrication parameters. The possibilities for the use of the model is discussed with reference to the missing elements and when considering special ``short-sample'' measurements on thermal propogation characteristics and magnetization characteristics. Results of recovery-current tests are presented.
Frequency and Time Domain Analysis of a Superconductive Coaxial Line Using the BCS Theory
Authors: McCaa W. D. , Nahman N. S.
Published: February 2004

A miniature superconductive lead-teflon-niobium coaxial line has been analyzed in terms of the complete BCS theory of superconductivity. The surface impedance of both superconductors, the line attenuation, and the picosecond step responses have been calculated for a temperature of 4.24°K. The dielectric has been assumed lossless. In addition, the frequency and time domain responses obtained with the BCS theory are compared with those predicted in 1968 by McCaa and Nahman for the same line parameters by the two fluid model.
Persistent Current Magnetometer Utilizing a Vibrating Superconducting Plane to Shuttle Flux
Authors: Opfer James E.
Published: February 2004

A technique that has been developed to fabricate niobium thin film microcircuits is described. This fabrication technique has been employed in making a 125-cm long superconducting inductance element that is 10 μ wide and is contained in an area 0.5 cm on a side. A superconducting niobium ground plane vibrating near the inductance element at a frequency of 100 kHz has been used to shuttle magnetic flux into and out of a superconducting loop connected in series with the inductance element. The suitability of this method of flux shuttling to the measurement of very small, steady magnetic fields is considered in detail.
Maximum Tunneling Supercurrents through Josephson Barriers
Authors: Schroen Walter , Pritchard J. Paul
Published: February 2004

The maximum tunneling supercurrent through Josephson barriers as a function of barrier dimensions and external magnetic fields has been investigated experimentally. A versatile sample preparation technique allows a two-dimensional distribution of the tunneling current. The results confirm earlier theoretical predictions qualitatively. At zero applied field the maximum tunneling supercurrent initially increases linearly with the barrier length, then saturates due to self-limiting. At nonzero applied field the maximum tunneling supercurrent decreases linearly with increasing magnetic field. It is shown that this holds also for structures in which the field is generated by crossed-film control supercurrents. The Josephson penetration depth of the samples has been 9–22 μm.