bluuebunny/crossref_metadata_2025 huggingface dataset:
Motion of domain walls in helical magnets (abstract)
Authors: Fairbairn W. M.
Published: July 2002

The magnetically ordered structure of many helical magnets takes the form of planes of ferromagnetically ordered atoms for which the direction of ordering alters by rotation through a fixed angle γ on passing from one plane to the next. This ordered state can be characterized by the direction of the normal to the set of planes (the axis of a helix) and the angle γ (the turn angle of the helix). For most helical materials there can exist domains of two chiralities, those with turn-angle +γ and those with turn-angle −γ. The size and structure of the region between two such domains (a domain wall) has been investigated in the static regime. When a magnetic field is applied the direction of the magnetic moments may change. The equations of motion are set up in the continuum approximation and the resulting effects on the magnetic structure of the domains and the domain walls are considered in the two cases when the applied field is within or perpendicular to the easy plane of magnetization.
Magnetoelastic ‘‘gain’’ associated with saw propagation in magnetic garnet films
Authors: Hanna S. M. , Murphy G. P.
Published: July 2002

The magnetic field dependence of the surface acoustic wave attenuation in magnetic garnet films has been experimentally investigated. SAW delay lines have been fabricated on magnetic garnet films. Aluminum interdigital transducers on piezoelectric layer (zinc-oxide film sputtered on top of the garnet film) have been used for acoustic wave generation and detection. The variation in the SAW attenuation as a function of a dc magnetic field applied in different in-plane and out-of-plane orientations has been investigated. One of the interesting observations we obtained was that at certain applied field range the SAW attenuation was found to be reduced below its zero-bias value. This negative change in attenuation or ‘‘gain’’ in the SAW signal motivated us to investigate different loss mechanisms associated with SAW propagation in magnetic films. The use of lossy garnet films helped reveal different loss mechanisms dominating in different magnetic field ranges.
Thin-film ferrite materials for microwave and millimeter wave integrated circuits (invited) (abstract)
Authors: Glass H. L.
Published: July 2002

Conventional microwave and millimeter wave systems use ferrites in a variety of components. In general, each application imposes specific requirements on the magnetic properties of the ferrite. In addition, some devices require the ferrite to be in single-crystal form. In order to produce ferrite components that are compatible with microwave and millimeter wave integrated circuits, additional constraints are imposed on the ferrite, especially if fully monolithic integration is required. In general, the ferrites must be prepared in thin film form on substrates that are chosen primarily to meet requirements other than compatibility with the ferrite film. Moreover, as is well known, the magnetic properties of ferrites are quite sensitive to the conditions that exist during their preparation. Thus, ferrites prepared by thin film deposition techniques may have properties different from bulk ferrites. This talk will review progress on methods for preparing polycrystalline ferrite films by techniques such as sputter deposition and laser-assisted deposition and methods for preparing single-crystal ferrite films in forms that may be compatible with epitaxial deposition of compound semiconductors.
Spin-wave properties of a model magnetic superlattice
Authors: Leung K. M.
Published: July 2002

Theoretical studies by Hinchey and Mills [J. Appl. Phys. 57, 3687 (1985)] on the basic properties of magnetic superlattice structures formed from alternating layers of ferromagnetic and antiferromagnetic materials have revealed a number of fascinating results. In their model, the antiferromagnet consists of sheets parallel to each interface within which the spins are aligned ferromagnetically. A localized spin model with only nearest-neighbor interaction is assumed. In this work we work with the same model, but in the absence of an applied magnetic field. The low temperature magnetic properties are investigated theoretically within the spin-wave approximation. For arbitrary thicknesses of ferromagnetic and antiferromagnetic layers, an expression which determines the dispersion relation for the spin-wave modes is obtained. The transformation from spin deviations variables to these modes is also computed explicitly.
Preparation and structure of magnetic superlattices (invited) (abstract)
Authors: Schuller Ivan K.
Published: July 2002

Magnetic multilayers that exhibit long-range structural coherence perpendicular to the layers (i.e., superlattices) have been prepared using sputtering and molecular-beam epitaxy. The structure was characterized using x-ray diffraction and electron microscopy. A newly developed microcleavage technique has been applied to directly image the transverse cross section of the layers. The low-angle x-ray diffraction is directly related to the chemical modulation of the multilayer. The high-angle x-ray diffraction peaks due to the superlattice modulation are found to be different depending on the presence or absence of large lattice spacing modulation. The in-plane structure, the sharpness of the interface, and the mosaic spread are determined by lattice matching and/or the binary thermodynamic equilibrium phase diagram. A possible relation between the epitaxial growth temperature and characteristic temperatures of the phase diagram will also be discussed. If time permits, a comparison between the chemical and the magnetic modulation in Ni/Mo superlattices will also be presented.
Magnetic studies of Fe70Al30 alloys containing transition metal substitutions for Al
Authors: Okpalugo D. E. , Booth J. G. , Kĕpa H. , Hicks T. J.
Published: July 2002

It is well known that Fe-Al alloys show remarkable magnetic properties in a small range of composition near Fe70Al30. A number of different techniques have suggested that a progression from paramagnetism to ferromagnetism to superparamagnetism to mictomagnetism occurs as the temperature is lowered. We report here the influence on this progression of transition metal substitution for Al using low- and moderate-field magnetization measurements as well as neutron diffraction and SANS techniques to study polycrystalline samples quenched from 830 °C. The systems investigated may be written as Fe70Al30−x Tx, where T=Cr, Mn, or Co and x=1, 3, or 10. The neutron diffraction data showed that all the alloys crystallized in the doubly ordered L21 structure. For the alloys with x=1 the intrinsic magnetization deduced from measurements in moderate fields has a maximum near the temperature at which the reentrant transition to superparamagnetism is suspected together with an up-turn at a lower temperature. These features do not occur for the alloys having x=3 or 10. Low-field (∼5 Oe) measurements indicate, however, that a freezing temperature can be associated with alloys having x<4. A Q-dependent peak in the SANS data occurs near the ferro- to superpara-magnetic transition temperature suggesting an interpretation in terms of the freezing of clusters of different sizes. The results are interpreted to suggest that the main parameter determining the overall magnetic properties is the Al concentration.
Experimental demonstration of one-sided flux in magnetic recording tape
Authors: Flanders P. J.
Published: July 2002

It is shown that a magnetic tape can be magnetized so that the fringing field exists on only one side of the tape, a behavior predicted several years ago but never previously confirmed by direct experiment. This one-sided flux requires a tape specially produced to have equal remanence in any direction normal to the transverse axis.
The absence of gap loss in erased-media noise floors
Authors: Finkelstein B. I. , Otter E. L.
Published: July 2002

As part of a study into the nature and structure of noise in magnetic recording, the effect of gap loss on the noise power spectrum was investigated. Whereas gap loss is a component in the modulation noise power spectrum, neither ac-erased media noise nor dc-erased media noise seems to exhibit any evidence of a gap loss function.
Exchange and CEF interactions in R2Fe14B compounds
Authors: Cadogan J. M. , Coey J. M. D. , Gavigan J. P. , Givord D. , Li H. S.
Published: July 2002

The magnetic properties of R ions in R2Fe14B compounds (R=rare earth, Th) are determined by a combination of R-Fe exchange and crystalline electric field interactions. A comparison of the ordering temperatures of all compounds in the series shows that the exchange interactions between R and Fe moments are not simply proportional to the spin moments but there is a decrease across the series, with interactions for compounds with light R elements enhanced by a factor of 2 compared to those with heavy R elements. This decrease is related to the decrease of 4f-5d exchange interactions within the R ions. The second-order and fourth-order terms of the crystalline electric field potential of the environment have been obtained in a number of compounds from analysis of their magnetic properties (Nd, Er, Tm, Yb). Scaling the values obtained for other compounds (Pr, Sm, Ho,...) results in a fairly good description of their magnetic properties.
Investigation of heavy quasiparticles in d and f electron systems via the dHvA effect (invited) (abstract)
Authors: Lonzarich G. G.
Published: July 2002

Recent studies of the quasiparticle spectra of narrow band metals and intermetallic compounds, via the de Haas–van Alphen (dHvA) effect, are reviewed. The most extensively investigated systems include a group of transition metals close to a magnetic instability at low temperatures, the light rare-earth metals Pr and Nd and, most recently, the heavy fermion superconductor UPt3.1–4 In UPt3 a large number of sheets of the Fermi surface with cyclotron masses of unprecedented magnitudes, ranging between 25 and 90 bare electron masses, are observed.4 These values are two orders of magnitude greater than in simple metals and between 5 and 10 times larger than in other strongly exchange enhanced metals which we have investigated previously. The relationship between the dHvA results and conventional bandstructure models, and the possible mechanisms for the large mass renormalization observed in some of these systems, will be discussed.
Three-dimensional analysis of a turbogenerator terminal box
Authors: Mishra Amitabh
Published: July 2002

This paper presents a method of analyzing magnetic fields in and around turbogenerator terminal boxes in three dimensions. The magnetic field is modelled in terms of a magnetic scalar potential under the assumption that the depth of penetration of the field is very small such that eddy currents are restricted to surface currents. The potential formulation and the associated boundary conditions are discussed. A box model which has been analyzed using the present formulation is given along with plot of equipotentials on a box wall.
Effect of cobalt-ferrite epitaxial layer on the coercivity of ZnFe-ferrite particles (abstract)
Authors: Du You-wei , Lu Huai-xian , Xue Rong-hua , Wang Tiang-xiang , Tang Huan , Qiu Zi-qiang , Walker J. C.
Published: July 2002

Zn0.2Fe2.8O4 particles were prepared and coated with different amounts of cobalt-ferrite. The coercivity increases with increasing cobalt-ferrite coating. In the low Co-ferrite coating regime, the increase is induced by an enhanced anisotropy of the interface layer, while in the high coating regime, it is due to the Co-ferrite surface layer. The full depth of the interface layer was found to be approximately 8 Å, and the temperature dependence of the change in coercivity in both low and high Co-ferrite coating regimes was discussed. Mechanisms leading to a greatly increased coercivity of acicular Co-γ-Fe2O3 were also discussed.
Synthesis and characterization of modified barium ferrite particles
Authors: Hong Y. K. , Paig Y. J. , Agresti D. G. , Shelfer T. D.
Published: July 2002

A low-temperature, ambient-pressure coprecipitation method for preparation of pure and modified barium ferrite particles BaFe3+12−2x Co2+xMxO19 is introduced; for this study, M=Ti4+ or Mo4+ and x=0.5, 0.9, 1.5, and 2.0. Hexagonal platelets, identified by SEM and TEM, varied in size (∼0.1 to 0.6 μm) and aspect ratio (∼2 to 7) with both heat treatment temperature and x value. Coercivity was controllable over a wide range from 500 to 1400 Oe. Line positions in x-ray powder diffraction spectra of pure and doped barium ferrite are in good agreement. The relative intensity of the (108) line to the (114) or the (107) line increases with Co/Mo doping, but does not change with Co/Ti doping. In the Co/Mo Mössbauer spectra, the relative intensities of the 12 K lines decrease monotonically from 50% (x=0) to ∼25% (x=2), suggesting a corresponding replacement of Fe3+ ions in the 12 K site. Both x-ray and Mössbauer measurements show a nonmagnetic phase in samples coprecipitated at 95 °C, not present in samples prepared at 55 °C. The saturation magnetization of doped particles was found to be independent of local crystalline order.
Digital recording performance of Ba-ferrite media
Authors: Speliotis D. E.
Published: July 2002

Barium ferrite media have created a lot of interest for digital and analog applications because of their potential to achieve high recording densities. To fully exploit this potential, it is necessary to develop media with good perpendicular orientation and to employ high saturation narrow gap heads. It is possible, however, to obtain very good recording performance utilizing Ba-ferrite media which are nonoriented or only partially oriented and ordinary ferrite heads for high-density digital recording applications. In this study, we investigated the performance of such media in rigid disk form using conventional and narrower gap ferrite heads, and obtained much better short wavelength response than that obtained from high-coercivity cobalt-modified iron oxide media. The overwrite modulation of these media, however, is only marginally acceptable. The modulation noise observed is much larger for longitudinally oriented than for perpendicularly oriented media. A simple model is proposed to explain the origin of this noise and its dependence on the direction of orientation.
Static and dynamic bifurcations in magnetoelastic ribbons (abstract)
Authors: Savage H. T. , Adler Charles , Antman Stuart S. , Melamud M.
Published: July 2002

The ΔE effect in certain field annealed amorphous ribbon is now about 10, i.e., Young’s modulus E, can be reversibly changed by a factor of 9 with the application of a field of less than 1 Oe. We have reported the field-induced buckling of a vertically oriented ribbon. The ribbon buckles under its own weight due to the reduction of E with field H. Critical buckling values of H were found to be in good agreement with the eigenvalues of the linearized version of the operator describing the process. Here we present: (a) holographic data where the gradient in the fringe spacing obtained from the hologram of the straight and buckled states is a measure of the curvature; and (b) a rigorous mathematical formalism for extracting from experiment the nonlinear constitutive relation between the curvature θ′(s) and the bending couple m(s) where s is the distance along the ribbon. This process must be carried out (with H as a parameter) from H=0, to values of H somewhat above the anisotropy field, to effect a complete description of the magnetoelastic behavior. In dynamic experiments where the ribbon is driven by H=H0+h sin ωt we observe a subharmonic bifurcation (parametric resonance) when ω is about twice the half-wavelength elastic resonance frequency and h exceeds a threshold value governed by the magnitude of ∂E/∂H. Other, more complicated bifurcations are seen as h is increased further. We show the nonlinear equation of motion with damping to explain the bifurcation structure.
Spin dynamics in the diluted antiferromagnet MnxZn1−xF2 (invited) (abstract)
Authors: Birgeneau R. J. , Uemura Y. J.
Published: July 2002

The problem of spin dynamics in disordered magnets has been a subject of research for the past two decades. Diluted systems are of special interest since they exhibit percolation effects for concentrations near the percolation threshold (xp). For concentrations x>xp the system should look uniform at long wavelengths and highly disordered at short wavelengths; the latter regime should exhibit fractal geometrical characteristics; specifically, as a function of decreasing wavelength, one might expect a crossover from magnon to ‘‘fracton’’ behavior. A recent high-resolution inelastic neutron scattering study on a large (8 cm3) single crystal of (Mn0.5Zn0.5)F2 shows just these effects. At T=5 K Mn0.5Zn0.5F2 exhibits very sharp spin waves at small q and ω which become rapidly broader with increasing q; further, the excitations are essentially localized in character at higher energies. In this case by ‘‘localized’’ we mean extremely broad both in energy at fixed wave vector and in wave vector at fixed energy; the crossover from ‘‘magnons’’ to ‘‘fractons’’ occurs when the wave vector is comparable to the inverse pair connectedness ξ−1 for the diluted system.
Theoretical study of magnetic excitations in Ni3Al
Authors: Cooke J. F. , Kübler J. , Liu S. H.
Published: July 2002

Inelastic neutron scattering studies of the transition metal alloy Ni3Al have revealed highly unusual spin-wave behavior, that spin waves have been observed only in a small region around the Brillouin zone center (q≂0). Results from calculations of the inelastic neutron scattering cross section based on itinerant electron theory for Ni3Al have led to a relatively simple explanation of this phenomenon. To our knowledge, this is the first calculation of this type for an alloy system. The calculations yield the well-defined Goldstone mode (spin wave) at q=0 but no spin-wave peaks were found for the smallest calculable nonzero momentum transfer, which was just outside the range of q where spin waves were observed experimentally. The reason is simply that the spin wave runs immediately into a region of high density of Stoner excitations (single-particle spin-flip excitations) as q is increased from zero. This system, therefore, represents the extreme limiting case of the itinerant electron theory prediction of spin waves disappearing into the Stoner continuum.
Neutron diffraction study of Y2(Fe1−xCox)14B (abstract)
Authors: Yang Ying-chang , Fu Shou-can , James W. J. , Yelon W. B. , Xie Dacai , Yang Jilian
Published: July 2002

The crystallographic and magnetic properties of Y2(Fe1−xCox)14B, x=0.00, 0.1, 0.2, 0.3, 0.4, and 0.5, were investigated by neutron diffraction and magnetic measurements at selected temperatures from 4.2 K to above the Curie temperature. The preferential occupation by Co atoms among the six nonequivalent crystallographic sites of Fe has been determined together with the mean magnetic moments of the Fe and Co atoms. The Fe atoms are found to favor the j2 site. The atomic ordering and its relationship to the magnetic properties are explained on the basis of the local site symmetries and the effect of the differing electronic behavior of the Fe and Co atoms with respect to their nearest neighbors.
Integrated magnetic spectroscopy of dilute magnetic semiconductors (invited)
Authors: Awschalom D. D. , Warnock J. , Rozen J. R. , Ketchen M. B.
Published: July 2002

An integrated dc SQUID magnetic spectrometer has been developed to obtain direct high-resolution measurements of optically induced magnetization in a 10-μm-diam sample of Cd0.8Mn0.2Te. The static and picosecond dynamics of the magnetic response have been studied and are seen to be strikingly dependent on the energy and polarization of the optical excitation. The entire sample magnetization changes upon illumination, and the disturbed spins relax via an efficient spin coupling to the lattice.
Equivalent circuit of signal line with magnetic layer
Authors: Rubin Barry J. , Kopcsay Gerard V.
Published: July 2002

A system of conductors having arbitrary permeability is analyzed through the generation of an inductance matrix that includes magnetic effects. This formulation, which makes use of both a magnetic scalar potential and preexisting inductance calculation algorithms based on vector potential, directly generates an equivalent circuit suitable for subsequent circuit analysis. The approach is applied to a two-dimensional signal line situated above a ground plane and composed of three conducting layers, the middle one being magnetic. Such lines find application in computer modules, where the use of conductor laminates may be required because of metallurgical considerations. Results show that at high frequency, current crowding into the upper layer gives rise to a significant increase in signal line resistance.
Government funding of research in magnetism: A panel discussion
Authors: Wolf Werner P.
Published: July 2002

A panel discussion between representatives of four major government funding agencies (Air Force Office of Scientific Research, Department of Energy, National Science Foundation, and Office of Naval Research) and the magnetism research community was held concerning the role of government research funding in the field of magnetism. The program consisted of presentations by each of the four agency representatives and a response by a representative of the university research community. The floor was then opened for comments and questions by the audience. This article summarizes both the prepared presentations and the response of the audience.
Metal-matrix magnets for new TWT applications
Authors: Strnat R. M. W. , Clarke J. P. , Leupold H. A. , Tauber A.
Published: July 2002

New design concepts for magnetic focusing circuits are being investigated by the U. S. Army for application to mm- and microwave devices. They require that the magnetic properties of the permanent magnet cores used in these devices be varied in a precise manner over a wide range. In a representative application, the remanence would have to cover the range from 1800 to 3600 G, while the intrinsic coercivity MHc (Oe) must be greater than the remanence Br (G). The demagnetization (recoil) curve must be essentially a straight line with recoil permeability near μ=1 throughout the second quadrant of B vs H. An array may consist of 30–50 magnets working at different, closely controlled flux levels. This parametric flux variation would be extremely costly and difficult to achieve with sintered magnets. A method of blending reground sintered Sm2TM17 magnet material with Cu powder and Sn-Pb powder to produce predictable flux levels was developed. The above criteria are satisfied by this approach. Empirical curves were generated to show the magnetic flux as a function of dilution, and these closely matched our earlier predictions. Large rings were pressed to demonstrate mechanical feasibility.
Origin of magnetic anisotropy in R2Fe14B (R=rare earth) compounds: Crystalline electric field model calculations (abstract)
Authors: Malik S. K. , Boltich E. B. , Wallace W. E.
Published: July 2002

The tetragonal R2Fe14B compounds have attracted a great deal of attention because of the permanent magnet applications of the Nd-Fe-B system. Nd2Fe14B shows a spin reorientation from an axial arrangement at 300 K to a conical-type structure below 140 K. In contrast, Pr2Fe14B remains axial down to 4.2 K. We show that this behavior in the Pr and Nd compounds results from an interplay of crystalline electric field (CEF) effects and the R-Fe exchange interaction. There are two inequivalent R sites ( f and g) in these compounds which are magnetically further split into f 1, f 2 and g1,g2 sites. The CEF parameters B02 and B22 for each site are taken from the 155Gd Mössbauer measurements in Gd2Fe14B and the R-Fe exchange field is estimated from the Gd magnetization.1 These parameters are suitably scaled for the rare-earth ion under consideration. It is shown that with the known values of B02, B22, and Hex and with B04 =0.118 K for Nd (which corresponds to B04 =0.35 K for Pr), the four-site calculation, employing these three CEF terms, predicts a spin reorientation in Nd2Fe14B at about 130 K but an axial behavior in Pr2Fe14B at all temperatures.
Spin-wave polarization in single domain uranium selenide
Authors: Hughes K. M. , Holden T. M. , Buyers W. J. L. , DuPlessis P. de V. , Collins M. F.
Published: July 2002

Spin waves in ferromagnetic USe have been measured by inelastic neutron scattering. Previous measurements on a multidomain USe sample show that the spin wave branch has a zone center frequency of ≊9 THz, and that the spin waves are intrinsically broad. New measurements on a single domain sample show that, in addition, the spin wave is unpolarized. This behavior is unlike that of a classical ferromagnet, in which the polarization is transverse, but similar to behavior observed in some singlet ground state systems.
Crystal-field effects in the magnetic and thermal properties of URu2Si2
Authors: Franse J.J.M. , Menovsky A.A. , de Visser A. , van den Berg J. , Nieuwenhuys G. J.
Published: July 2002

Specific-heat and thermal-expansion data for URu2Si2 are confronted with crystal-field calculations, using parameters based on a previous analysis of susceptibility data.
Magnetic order and structural transformations in Ce deuteride single crystals (abstract)
Authors: Heidel A. , Arons R. R. , Zinn W.
Published: July 2002

From single-crystal work we showed previously that, both in absence and at full occupation of the octahedral D atoms, Ce deuteride orders antiferromagnetically (AF).1,2 In the intermediate region the order is ferromagnetic (F). A complete magnetic phase diagram can be found in Ref. 3. Previous authors have measured a lattice distortion in the intermediate concentration region (see references in Ref. 3). However, their data disagree with each other. In order to clarify whether the F order is related to the distortion mentioned above, we have investigated the structural transformations in the region of F to AF around the metal-semiconductor transition at D/Ce≊2.7. Due to the extremely small paramagnetic region around D/Ce≊2.7, reliable results can only be obtained by using the same crystals for the different experiments. The results of x-ray, neutron diffraction, and DTA experiments will be compared with the magnetic phase diagram.
Electronic subbands for AlxGa1−xAs/GaAs multilayer and superlattice structures
Authors: Ruden P. P. , Engelhardt D. C. , Abrokwah J. K.
Published: July 2002

We have carried out calculations of the conduction bandstructure for GaAs/AlxGa1−xAs (0<x≤1) multilayer structures with very thin layers. We report results for the case of a small number of layers for which the quasi-continuous bandstructure of a large superlattice is a poor approximation. Using a multivalley effective mass approach which includes nonparabolicity, we find that for large aluminum concentrations and thin layers the lowest bound state is derived from the X valley and is localized mainly in the alloy layers. The structures investigated have a range of design parameters which is useful for superlattice modulation-doped field-effect transistors.
A coagulation equation with separable solutions
Authors: Simons S.
Published: July 2002

It is shown that particular solutions exist of the coagulation equation which are separable in particle volume and time if there is a source term present with time variation of the form (t0−t)−2. It is conjectured on the basis of the detailed solution for a constant coagulation kernel that all solutions tend to this separable form as t→t0, and some consequences of the truth of this conjecture are pointed out.
Hydrostatic pressure dependence of the exponential optical absorption in CdInGaS4
Authors: Toyoda T. , Nakanishi H. , Endo S. , Irie T.
Published: July 2002

The optical-absorption spectra in the semiconductor CdInGaS4 have been measured precisely at various hydrostatic pressures up to 20 kbar, and analyzed in the region below the fundamental absorption edges. It can be seen that the absorption coefficients plotted semilogarithmically vary linearly with photon energy below the fundamental absorption edges at each hydrostatic pressure. The steepness of the absorption edge is proportional to the inverse of hydrostatic pressure.
Observation of a new magnetic phase in the garnet Ca3Mn2Ge3O12
Authors: Plumier R. , Sougi M. , Lecomte M.
Published: July 2002

Detailed magnetization curves have been performed up to H=11 T on both sides of the reported Néel temperature (T1=13.85±0.05 K) of a powdered specimen of the antiferromagnetic garnet Ca3Mn2Ge3O12. A detailed analysis of these curves shows that important biquadratic interactions have to be taken into account in addition to the usual bilinear ones. A phase diagram has then been drawn which indicates the existence of a quadrupolar phase at T1<T<T2 (T2≂20 K) and up to critical magnetic field values dependent both on temperature and directions of applied field. In the same T range and with magnetic field values larger than the critical fields, an ordered magnetic configuration is induced, confirming previous neutron diffraction experiments. It is then concluded that the reported Néel temperature T1 corresponds to a first-order magnetic transition in good agreement with specific-heat measurements. Size effects of the crystallites emphasize the first-order character of the transition taking place at T1.
Magnetic characteristics of ultrafine Fe particles reduced from uniform iron oxide particles
Authors: Bridger K. , Watts J. , Tadros M. , Xiao Gang , Liou S. H. , Chien C. L.
Published: July 2002

Uniform, cubic 0.05-μm iron oxide particles were formed by forced hydrolysis of ferric perchlorate. These particles were reduced to α-Fe by heating in hydrogen at temperatures between 300 and 500 °C. The effect of reduction temperature and various prereduction treatments on the microstructure of the iron particles will be discussed. Complete reduction to α-Fe was established by 57Fe Mössbauer spectroscopy and x-ray diffraction. Magnetic measurements on epoxy and polyurethane films containing these particles with various mass fractions gave coercivities as high as 1000 Oe. The relationship between the magnetic measurements and the microstructure will be discussed. Na2SiO3 is found to be the best coating material for the process of reducing iron oxide particles to iron.
Anomalous aging characteristics of amorphous GdCo films
Authors: Hong M. , Bacon D. D. , Nakahara S.
Published: July 2002

Amorphous GdCo films with perpendicular anisotropy have been prepared by dc diode getter sputtering under argon or neon pressure. To study the aging behavior, the films with no protecting layer were used and exposed to air at room temperature. We have observed that the perpendicular anisotropy in these uncoated GdCo films significantly increases with time. Furthermore, the negative (in-plane) anisotropy which existed at the initial stage during aging actually decreases with time and finally disappears.
Magnetic and atomic short-range order in Ni-Mn base amorphous alloys (invited)
Authors: Wu Y. , Dmowski W. , Egami T. , Chen M. E.
Published: July 2002

Magnetic and compositional short-range correlations in amorphous (MnxNi1−x)75P16B6Al3 (x=0, 0.2, 0.3, and 0.4) were studied by neutron scattering. The spatial magnetic correlation in the alloy with x=0.4 was determined by spin polarized scattering, and it was shown that Mn spins are coupled either antiferromagnetically or ferromagnetically depending upon the separation. The composition dependence of the atomic pair distribution functions obtained by pulsed neutron scattering indicated that there is a strong compositional short-range ordering between Mn and Ni atoms, and that Mn atoms are separated from each other by more than a contact distance. We also present the first clear observation of the annealing effect on the compositional short-range order, in the alloy with x=0.4.
Reentrant Kondo-mixed valent-Kondo regime behavior with T in the 3d CeT2Si2 series
Authors: Liang G. , Croft M. , Neifeld R. , Qi B.
Published: July 2002

In intermetallic compounds of Ce involving late 3d, 4d, and 5d row transition metals (T), the Ce valence state increases with decreasing T-d-band electron count. Neifeld, et al. [Phys. Rev. B 32, 6928 (1985)] have recently shown (using extensive Ce-L3-edge measurements) that this trend is reversed for the Ce-3d row compounds earlier than Co. This somewhat striking behavior is nowhere more apparent than in the ThCr2Si2 crystal structure series CeT2Si2 with T=Cu, Ni, Co, Fe, Mn, and (MnxCr1−x). In this paper, we will discuss the following: (1) the extension of this crystal structure to its stability limit in the T=(Mn,Cr) substituted system; (2), how this extension permits restoration of the Ce-valence state to the Kondo local-moment regime; (3) low-temperature electrical resistivity measurements which support first the Kondo to mixed valent followed by mixed valent to Kondo regime passage with decreasing 3d electron count in this series; and (4) finally the apparent compatibility of the Ce-Kondo effect with strong 3d antiferromagnetism for some of these materials.
Effects of temperature and composition on the thermal and electrical conductivities of Ni3Al
Authors: Williams R. K. , Graves R. S. , Weaver F. J.
Published: July 2002

The results of a study of the thermal conductivity, electrical resistivity, and Seebeck coefficient of Ni3Al are described. The thermal and electrical conductivities are sensitive to composition and attain their maximum values in well-ordered, stoichiometric Ni3Al. Nonstoichiometry (antisite defects), and Fe (a substitutional solute) and B (an interstitial solute) contents are all about equally effective in reducing the transport properties. Even for stoichiometric Ni3Al, the temperature variation of the thermal and electrical conductivities resembles that of an alloy, and this is attributed to scattering associated with the loss of ferromagnetic order at ∼60 K. Experimental data and a theoretical analysis show that phonon conduction is an important part of the thermal conductivity and not very sensitive to composition or stoichiometry.
New characterization method of deep levels in semi-insulating GaAs wafers using microwave impedance measurement
Authors: Fujisaki Yoshihisa , Takano Yukio
Published: July 2002

A new contactless characterization technique based on microwave impedance measurement was developed and applied to the nondestructive measurement of deep level concentration in semi-insulating GaAs crystals. Using this new technique, the nonuniform distribution of EL2 was measured in GaAs wafers used for IC fabrication. Taking advantage of the nondestructive and contactless feature of this new technique, both the EL2 concentration and the threshold voltage (Vth) of metal semiconductor field effect transistors were measured in the same sample. By analyzing the relation of these two quantities, it is found that the activation efficiency of implanted Si atoms is lower in the EL2 richer region, and vice versa. A possible mechanism to explain the experimental data is discussed in relation with crystal nonstoichiometry.
Comment on ‘‘Rapid thermal annealing and regrowth of thermal donors’’ [J. Appl. Phys. 5
9, 3495 (1986)]
Authors: O’Mara W. C.
Published: July 2002

This paper investigates the annealing of thermal donors at different temperatures in order to determine an activation energy for the process. Previously published data are analyzed using an implicit assumption of first-order decay kinetics. This gives rise to a very different value for the activation energy. Assumption of first-order decay kinetics does not coincide with previous observations in the 500–700 °C temperature range.
Electrical and stoichiometric characteristics of CdTe films deposited by the hot-wall flash-evaporation technique
Authors: Félix-Valdéz J. , Falcony C. , Tufiño M. , Menezes C. , Dominguez J. M. , Garcia A.
Published: July 2002

The electrical and stoichiometric characteristics of polycrystalline CdTe films deposited by the hot-wall flash-evaporation technique are reported for different deposition parameters. The crystallites in these films grow in a columnar type of grain. The stoichiometry of the films is largely dependent on substrate (Ts) and wall temperatures (Tw) during deposition. At low values of Ts and Tw (∼92 and 425 °C, respectively) a large excess of Te is present (∼30 at. %). At Ts≂192 °C and Tw≂560 °C, nearly stoichiometric films were obtained. The electrical characteristics were strongly dependent on the amount of excess Te present in the samples. A change in the resistivity of up to seven orders of magnitude was measured between the samples with ∼30 at. % of excess Te and those with a stoichiometry close to 1:1. Also a large difference in the resistivity measurements was observed on the surface and across the samples for the different deposition conditions studied. The behavior of the resistivity with temperature in the 100–500 K range is also discussed.
Temperature effects on carbon and zinc incorporations in GaAs grown by low-pressure metalorganic chemical vapor deposition
Authors: Chen L. P. , Chang C. Y. , Wu C. H.
Published: July 2002

The carbon and zinc incorporations in GaAs grown by low-pressure metalorganic chemical vapor deposition using triethylgallium as the gallium source have been studied. Carbon and zinc incorporations are identified by photoluminescence spectrum and Hall measurement. The carbon incorporation in undoped GaAs decreases as the growth temperature increases and can be explained by the dissociation effect of triethylgallium at high growth temperature. The zinc incorporation in p-type GaAs doped with diethylzinc also decreases with increasing growth temperature. It can be explained by a vacancy control model.
Efficiency of photoconductive switches
Authors: Senitzky Benjamin , Schrimpf Ronald D. , Kerwin William J.
Published: July 2002

Theoretical expressions which relate the terminal characteristics of photoconductive switches to the incident light are derived. The integral of the product of the total current density (conduction and displacement) and the electric field in the absence of light is shown to be equal to the vi product at the terminals. Theoretical predictions based on these relations are compared to experimental measurements with continuous and pulsed light sources on a 2000-Ω cm silicon switch embedded in a microstrip geometry. When n+ contacts are used, the device exhibits a photoconductive gain of 3.6 at the anode and almost no photosensitivity at the cathode. Its behavior is consistent with the space-charge neutrality condition. When p+ contacts are used the photosensitivity is localized in the cathode region. In both cases the enhanced carrier recombination at the surface strongly affects the terminal characteristics. The pulsed light measurements are compared to the continuous illumination results.
Measurement of conduction band offsets in Ga0.94Al0.06As:Ga0.57Al0.43As heterojunctions by electrochemical C-V profiling
Authors: Furtado Mário T. , Loural Maria Salete S. , Sachs Antonio C.
Published: July 2002

We have undertaken electrochemical C-V measurements of Ga0.94Al0.06As:Ga0.57Al0.43As isotype n-n heterojunctions in order to determine the conduction band offset ΔEc. Samples were grown by liquid-phase epitaxy; carrier concentrations were determined to be, respectively, 1016 cm−3 for the small band-gap layer and 1016 cm−3 or 1017 cm−3 for the large band-gap layer. From results obtained with various different samples, we determine ΔEc/ΔEg≊0.54–0.64, quite close to the new accepted value of the GaAs:GaAlAs interface. Moreover, the density of fixed interface charges are estimated and found to be related to the monolayer doping density of the large band-gap layer.
Magnetic stabilization of a rotating glow discharge
Authors: Harry J. E. , Evans D. R.
Published: July 2002

A series of measurements have been made on a magnetically rotated glow discharge over a range of pressure of 24–53 mbars. An increase in the current at which the glow-to-arc transition occurred from 550 mA for a stationary discharge to 2.5 A at a magnetic flux density of 0.025 T for a rotating discharge, with a corresponding power increase at the glow-to-arc transition from 250 to 1540 W was measured. At magnetic flux densities of about 0.017 T a discontinuity in the discharge voltage was observed which was caused by the discharge rotating in its own wake. Results from other sources have indicated that a glow discharge can be maintained up to discharge currents of 10 A, however, the results obtained here under identical conditions indicate that the discharge was not a glow but an arc. The results have important implications to power limited glow-discharge processes such as CO2 lasers, ozone generation, and low-pressure chemical synthesis.
Computer simulations of the proton irradiated (AlGa)As-GaAs solar cells
Authors: Yeh C. S. , Li S. S. , Loo R. Y.
Published: July 2002

An improved numerical model for computing the displacement defect density, the damage constants for the minority-carrier diffusion lengths and the degradations of the short-circuit current Isc, open-circuit voltage Voc, and the conversion efficiency ηc in a proton irradiated (AlGa)As-GaAs solar cell is presented in this paper. The model assumed that the radiation-induced displacement defects form effective recombination centers which reduces the minority-carrier diffusion length and hence degrades the Isc, Voc, and ηc of the solar cell. Excellent agreement was obtained between our calculated values and the measured Isc, Voc, and ηc in the proton irradiated GaAs solar cells for proton energies varying from 100 keV to 10 MeV and fluences from 1010 to 1012 cm−2 under normal incidence condition.
A flaw distribution function for failure analysis of brittle materials
Authors: Phani K. K. , De A. K.
Published: July 2002

The fit of fracture strengths of as-received, abraded, and HF-etched borosilicate glass specimens to the normal, Weibull, and type-I extreme value distribution is compared. The study shows that the fit of these distributions depend on spread and scatter of strength data. A modification to the Weibull-type distribution function has been suggested for analyzing the strength data. The function provides an upper and a lower strength limit and is characterized by two shape parameters. The function provides good fit to the data on all the three types of specimens mentioned above and is consistent with the boundary condition of the physical phenomenon it represents.
High-power broad-area lasers fabricated by selective area growth
Authors: Yang J. J. , Jansen M. , Sergant M. , Ou S. S. , Wilcox J. Z.
Published: July 2002

High-power broad-area stripe lasers have been fabricated by using a one-step selective area growth technique in the metalorganic chemical vapor deposition system. This technique provides devices with excellent control both on electrical and optical confinements. cw output power of more than 350 mW/facet (700 mW total) was achieved, and a differential quantum efficiency of 0.625 mW/mA/facet (1.25 mW/mA total) was observed.
Image transfer in photorefractive GaAs
Authors: Cheng Li-Jen , Gheen Gregory , Rau Mann-Fu , Wang Faa-Ching
Published: July 2002

Image transfer from one beam to the other using counterpropagation beam coupling in GaAs was demonstrated. Good image quality was achieved. The results also reveal that local birefringence due to the residual stress/strain field in the crystal can degrade the image quality.
Photoconduction of the a-Si:H/a-C:H heterostructure
Authors: Noda Kenju , Nakatsugawa Masashi , Ninomiya Yutaka , Itoh Tadatsugu
Published: July 2002

Photoconductive properties of the a-Si:H/a-C:H structure formed by the rf glow discharge process were evaluated from I-V characteristics and the photocurrent spectra in visible light. The optical energy gaps of the a-Si:H layer and the a-C:H layer were 1.7 and 2.8 eV, respectively. The photocurrent was dominated by carriers generated in the a-Si:H layer, and the carriers were injected from the a-Si:H layer to the a-C:H layer. The dark current flowing in the a-Si:H/a-C:H structure depended on both the thickness of the a-C:H and the polarity of the bias voltage. At the bias voltage of 2 V, the photosensitivity of the a-Si:H (320 nm)/a-C:H (5 nm) heterostructure, which was defined as the ratio of the photocurrent to the dark current, was larger than that of the a-Si:H film by three orders of magnitude.
Optical response at 10.6 μm in tungsten silicide Schottky barrier diodes
Authors: Kumar Sandeep , Boyd Joseph T. , Jackson Howard E.
Published: July 2002

Optical response to radiation at a wavelength of 10.6 μm in tungsten silicide-silicon Schottky barrier diodes has been observed. Incident photons excite electrons by means of junction plasmon assisted inelastic electron tunneling. At 78 K a peak in the second derivative of current versus junction bias voltage was observed at a voltage corresponding to the energy of photons having a wavelength of 10.6 μm. This peak increased with increasing incident laser power, saturating at the highest laser powers investigated.
Effects of surface tension of substrate on annealing of evaporated stearic acid film
Authors: Tawata Masahiro , Morita Shinzo , Hattori Shuzo
Published: July 2002

Stearic acid layers in the form of microcrystalline grains were formed by a vacuum evaporation method on several kinds of substrates. By annealing, the layer of microcrystalline grains was reformed into a plane crystalline film on the hydrophobic surface of plasma-polymerized methane and evaporated gold with water contact angles larger than 30°. However, the crystalline film was not formed on the hydrophilic surface of oxygen plasma-treated plasma-polymerized methane, mica, and glass with water contact angles smaller than 18°.
Operational characteristics of a gas discharge stabilized by a rotating magnetic field
Authors: Willis R. J. , Seguin H. J. J. , Capjack C. E. , Nikumb S. K.
Published: July 2002

The operational characteristics of a gas discharge under the influence of a transverse rotating magnetic field are described. The terminal characteristics and measurements of gas-discharge temperature are presented. Experimental results indicate that a transverse rotating magnetic field can improve gas-discharge stability and useful power deposition into the plasma. A 25% increase in discharge power loading has been demonstrated by using this discharge-stabilization technique.
Polarization of the far-infrared laser oscillation in p-Ge in Faraday configuration
Authors: Komiyama Susumu , Kuroda Shinji , Yamamoto Toyoji
Published: July 2002

Polarization of far-infrared laser oscillation in p-Ge under crossed electric and magnetic fields is studied by use of a linear polarizer. The laser emission is found to be polarized with the primary axis nearly parallel to the current vector in the Ge laser rod, independent of the strength of electric and magnetic fields and the oscillation wavelength. An analysis of the resonance condition of an optical resonant cavity placed in Faraday configuration together with the experimental results make it probable that net Faraday rotation vanishes in the laser oscillation. To support this it is pointed out that both the free-carrier Faraday effect and the intervalence band Faraday effect are strongly affected by the strong radiations caused by laser oscillation. The relation of the anisotropy of carrier distribution in velocity space to the observed polarization is also discussed.
Application of sputtered SiO2 insulator to Nb/AlOx/Nb Josephson junctions
Authors: Hoko H. , Imamura T. , Ohara S. , Hasuo S.
Published: July 2002

Sputtered SiO2 films were applied as an insulator in all-refractory Josephson circuits. The sputtered SiO2 exhibited excellent insulating characteristics with respect to infrared absorption, breakdown voltage, and step coverage. The sputtered SiO2 was employed in the actual fabrication of Nb/AlOx/Nb Josephson circuits, and no deterioration in the junction quality or the critical current density was observed. An 8Kbit memory cell array was fabricated, and perfect chips with no failures were obtained. In these chips, the integrity of the insulation and continuity was verified for four-level Nb electrodes. This indicates the availability of sputtered SiO2 in all-refractory Josephson integrated circuits.