bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructures of undercooled Si
Authors: Leung K. K. , Kui H. W.
Published: July 2002

Molten Si was allowed to solidify isothermally at initial bulk undercoolings ΔT from 5.7 to 157 K. It was found that in addition to the 〈211〉 twin dendrites two novel dendrites were found: the 〈110〉 twin and the 〈100〉 twin free dendrite. The 〈110〉 twin dendrite has multiple twin planes, and the orientation of the twin plane is {111}. These different dendrites exist at different initial bulk undercoolings, and the dendritic transition temperatures for 〈110〉 to 〈211〉 and 〈211〉 to 〈100〉 are ΔT=22 and 68 K, respectively. The dendritic transitions for 〈110〉 to 〈211〉 and 〈211〉 to 〈100〉 are edgewise to edgewise and edgewise to continuous normal growth, respectively.
Impurity effects on the thermoluminescence of Al2O3
Authors: Pokorny P. , Ibarra A.
Published: July 2002

The thermoluminescence of several Al2O3 samples doped with different impurities (mainly Cr and Ni) is studied. The main characteristics (emission spectra, dose dependence, peak parameters) of the observed thermoluminescence glow curve are determined. A very high peak is found at 335 °C for some concentrations of Ni. The possible use of this material for dosimetry applications is discussed.
A simple method for determining band-gap energies from inhomogeneous electric field electroreflection spectra applied to GaAs
Authors: Poras H. , Wang H. , Goldsmith G. J. , Pan N.
Published: July 2002

A new method for determining band-gap energies using modulation spectroscopy is described. In contrast to the usual method for extracting transition energies from modulation spectra, which assumes a constant electric field distribution, this method pertains to cases where an inhomogeneous field exists. The band-gap determination applied herein to GaAs epilayers between 100 and 380 K takes advantage of a modulation feature arising from an inhomogeneous distribution of the electric field. This approach is quick, applicable over a wide temperature range, and does not necessitate curve fitting.
Electrical characterization of ZnSe epitaxial layer reactive-ion-etched by a gas mixture of ethane and hydrogen
Authors: Ohtsuka K. , Imaizumi M. , Endoh Y. , Suita M. , Isu T. , Nunoshita M.
Published: July 2002

The sheet resistances of ZnSe epitaxial layers etched by reactive ion etching (RIE) with use of a gas mixture of ethane and hydrogen were measured. The results showed that a high resistivity region was introduced by RIE. Characterization was done by transforming the measured resistances to the electrically active thickness estimated from the resistivity of as-grown ZnSe layers. The estimated electrically active thicknesses were smaller than the thickness of ZnSe epitaxial layers remaining after RIE, for both p- and n-ZnSe. The influence of heat treatment and current injection on the high resistivity regions differed between p- and n-ZnSe.
Time-resolved thermal-emission electron microscopy of laser-pulsed metal surfaces
Authors: Schäfer B. , Knecht T. , Bostanjoglo O.
Published: July 2002

A time-resolving thermal-emission electron microscope was developed for tracking transient phenomena on pulse-laser-heated metal surfaces. Short-time exposures with a temporal/spatial resolution of 3 ns/1 μm were achieved by adding a high-gain image intensifier and a fast beam blanking unit to a conventional emission microscope. Fast surface heating was realized using a frequency-doubled neodymium yttrium aluminum garnet pulse laser (12 ns full width at half-maximum). Experiments with tungsten and tantalum are presented, showing the potential of this method for imaging fast changes of local surface parameters, e.g., temperature. The emission microscope is found to be a sensitive probe for incipient laser-induced modifications of the surface, even in the conventional mode. Apart from this some technical problems as well as the physical limitations of time-resolved thermal-emission microscopy are discussed.
Structural characterization of a bonded silicon-on-insulator layer with voids by micro-Raman spectroscopy
Authors: Usami Akira , Ichimura Masaya , Wada Takao , Ishigami Shun-ichiro
Published: July 2002

Crystalline quality in a void region of a bonded silicon-on-insulator (SOI) wafer is evaluated by micro-Raman spectroscopy. Downshifting and broadening of the Si optical-phonon peak are observed at the edge of the void, while spectra within the void are little different from those outside the void. Comparison with calculated results based on the theory of the phonon localization shows that both the shift and the broadening are mainly due to structural disorder and not strain. Electrical properties in a void region are also evaluated by a laser-microwave method. The lifetime of excess carriers has its minimum value at the void edge. Those results consistently show that the SOI layer is deformed plastically rather than elastically at the boundary of the void.
Depth profile of thermal donor in boron-doped Czochralski-grown silicon
Authors: Filangeri Edward M. , Nishida Toshikazu
Published: July 2002

Near-complete suppression of thermal donor formation is observed at the silicon surface independent of annealing time. Schottky barrier capacitance-voltage profiling of p-type Czochralski silicon following heat treatment at 200–450 °C in N2 indicates thermal donor depletion near the surface consistent with earlier reports. Analysis of the capacitance-voltage method indicates that series resistance effects on the depth profile are negligible. Possible mechanisms for the thermal donor depletion at the silicon surface are discussed.
High quantum efficiency for Pt2Si Schottky-barrier diodes in the vacuum ultraviolet
Authors: Hajsaid M. , Charlson E. J. , Charlson E. M. , Zhao G. , Meese J. M. , Stacy T. , Popovici G. , Prelas M. A.
Published: July 2002

Pt2Si Schottky-barrier diodes were found to exhibit excellent photoresponse in the wavelength region of 116.4–221.4 nm when they were operated in the front illumination mode. Quantum efficiencies as high as 220% were achieved, depending on the substrate resistivity used and the Pt2Si film thickness.
Observation of the Stark effect in coupled quantum wells by electroluminescence and circularly polarized photoluminescence excitation spectroscopy
Authors: Kato Y. , Takahashi Y. , Fukatsu S. , Shiraki Y. , Ito R.
Published: July 2002

Successful observation is reported of the quantum-confined Stark effect in electroluminescence (EL) under a ‘‘forward’’ electric-field condition, with a current injected into a p-i-n diode containing Al0.3Ga0.7As/GaAs symmetric coupled double quantum wells (CDQWs). The Stark shift observed in EL and (under a reverse bias) in photoluminescence (PL), in both of which the transitions involved are between the electron and heavy-hole lowest states, is found to be symmetrical with respect to the flatband bias, in agreement with calculations. The origin of the optical transitions between electron and light- or heavy-hole states in the CDQWs is also clearly identified by a combination of PL and circularly polarized photoluminescence excitation spectra.
Model for the dry etching of heavily doped n-type silicon by atomic fluorine in the absence of ion bombardment
Authors: Kojima Masahiko , Kato Hisao , Gatto Mitsuru
Published: July 2002

Etch rate equations for heavily doped n-type silicon in the absence of ion bombardment are derived from a diffusion-controlled model. The theoretical equations indicate that the etch rate for lightly doped silicon is independent of donor impurity concentration, whereas the etch rate for heavily doped silicon is increased with an increase in donor impurity concentration. Numerical calculations of the etch rate as a function of donor impurity concentration are made with some typical values of the parameters in the equations.
Structure variations of mechanically alloyed Al-Co powder during annealing
Authors: Sui H. X. , Zhu M. , Liu W. G. , Qi M.
Published: July 2002

Al-Co powder mixtures of several different compositions are mechanically alloyed by high-energy ball milling (HEBM) and then annealed. The structure analysis of x-ray diffraction and transmission electron microscopy revealed that the metastable phase obtained by HEBM decomposes to several stable phases during heating. The formed phases include decagonal quasicrystal (DQC), a new approximant of DQC, and several other known crystal phases that appear to depend on the composition of powder mixture. Convergent-beam electron-diffraction analysis has also been taken to determine the space group of the new phase. Finally, the structural relation between the known Al3Co and the new phase is also discussed in view of the crystal symmetry.
Dual arsenic and boron ion implantation in silicon
Authors: Yokota Katsuhiro , Okamoto Yoichi , Miyashita Fumiyoshi , Hirao Takashi , Watanabe Masanori , Sekine Kouhei , Ando Yasuo , Matsuda Kouji
Published: July 2002

Arsenic (As) and boron (B) ions were implanted into silicon (Si) at energies such that their projected ranges coincided. The implanted Si was annealed in argon gas at a temperature of 950 °C for 30 or 300 min. The activation efficiency of the implanted As atoms decreased with an increase in the implant dose of the B ions, and the diffusivity of the As atoms decreased. On the other hand, the diffusivity of the B atoms decreased with an increase in annealing time.
Magnetic field arising from current dipoles randomly distributed in a homogeneous spherical volume conductor
Authors: Lütkenhöner Bernd
Published: July 2002

Certain types of magnetic noise arising from physical as well as biological sources can be explained by the model of current dipoles randomly distributed in a volume conductor. The volume conductor investigated in the present study is the homogeneous sphere, which is the model commonly used in analyses of the magnetic field of the human brain (magnetoencephalogram). Uniform distributions of random dipoles in three different source spaces are considered: a spherical surface, a spherical shell, and a sphere. The main emphasis is put on the radial component of the magnetic field. It is shown that the expectation value for the product of magnetic fields simultaneously measured at two locations, the covariance, depends on only two parameters of the measurement locations: the angular distance and the product of the radii. The formulas derived for the covariance can be expressed in terms of elliptic integrals of the first and the second kind, so that a very efficient numerical calculation is possible. For the special case of the variance (two identical measurement locations) these formulas reduce to expressions composed of elementary functions. Numerical examples show that the noise produced by random dipoles in a sphere is similar to the noise generated by random dipoles on a spherical surface having a slightly smaller radius.
Observation of small electron temperature variations in a nitrogen plasma by laser-induced fluorescence
Authors: Fukuchi T. , Wuerker R. F. , Wong A. Y.
Published: July 2002

Variations in electron temperature in a pulsed radio-frequency nitrogen plasma have been investigated with a floating double-Langmuir probe and by laser-induced fluorescence. Observations suggest that the N2(B 3Πg) level is populated by excitation from the metastable N2(A 3Σ+u) level by low-energy electrons.
Enzymatic oxidation-reduction processes under magnetic fields up to 8 T (abstract)
Authors: Iwasaka M. , Ueno S.
Published: July 2002

The question of whether magnetic fields affect enzymatic activities or not is of considerable interest in biochemistry and in biomagnetics. Xanthine oxidase, contained in liver, lungs, intestine, and other organs, catalyzes the degradation of hypoxanthine to xanthine, and xanthine to uric acid, which is the terminal waste of purine nucleotides in mammals. During the oxidation of xanthine, the enzyme releases superoxide anion radicals as intermediates which reduce ferricytochrome c (Fe3+). Superoxide anion, as well as any type of free radical, is also paramagnetic. The study is focused on whether these magnetically related enzymatic activities can be affected by magnetic fields. There is a possibility that free radicals, as intermediates, can be modified by magnetic fields of specific intensities. In our previously reported study, we examined a possible effect of magnetic fields up to 1.0 T on biochemical reaction catalyzed by xanthine oxidase, and obtained negative results. In the present abstract, we examine the effect of magnetic fields up to 8 T on this oxidation-reduction process. Reduced cytochrome c (Fe2+) has an absorption maximum at 550 nm which can be detected by a spectrophotometer. Xanthine oxidase was assayed by superoxide dismutase—inhibitable reduction of cytochrome c. We measured optical absorbance of reduced cytochrome c by superoxide anion which was produced by the reaction catalyzed by xanthine oxidase. The absorbances of the mixture exposed to an 8 T magnetic field at 25 °C were higher than control samples in the re-oxidation proces of cytochrome c. The results show that the 8 T magnetic fields altered the rate of reduction of cytochrome c by superoxide anion which was produced by the reaction catalyzed by xanthine oxidase. It may conclude that the electron transfer from xanthine to molecular oxygen or the transfer from superoxide anion to cytochrome c, can be affected by the magnetic fields up to 8 T.
Longitudinal and transverse magnetoresistance and magnetoresistive anisotropy in ternary (Pd1−xFex)95Mn5 alloys
Authors: Wang Z. , Kunkel H. P. , Williams Gwyn
Published: July 2002

A summary of detailed measurements of the longitudinal and transverse magnetoresistance, and of the anisotropy in the magnetoresistance of ternary (Pd1−xFex)95Mn5 samples, is presented. While neither the longitudinal nor the transverse magnetoresistance saturate, these data suggest that the anisotropy (i.e., the difference) might become field independent in quite moderate fields. This feature enables a comparison to be made between this ternary system and binary PdFe, as a function of composition and temperature
Antiferromagnetism of metastable Cr-rich Cr-Gd alloys
Authors: Hsu Jen Hwa , Fu Y. W.
Published: July 2002

Cr is an antiferromagnet with a Néel temperature of 311 K. The addition of impurities will alter the antiferromagnetic properties of materials. Studies of the effect of magnetic Gd atoms on the Cr-rich CrxGd1−x alloys are reported. The metastable CrxGd1−x alloy films (1.00≥x≥0.96) have been prepared by the rapid-quenching method. It has been found from x-ray diffraction that the alloy films with 1.00≥x≥0.97 have body-centered-cubic structure, the same as Cr. For higher Gd concentration, the alloy film contains both body-centered cubic and amorphous phases. The variation of the Néel temperature TN as well as the spin-flip temperature TF of metastable Cr-Gd alloys has been investigated by resistiviity measurements between 10 and 300 K. It has been found that TN decreases slowly from 311 K for pure Cr to about 290 K for Cr0.97Gd0.03. These results seem to indicate that magnetic Gd atoms show little effect on the antiferromagnetism of Cr. Furthermore, there is a broad resistivity minimum at low temperatures. This broad minimum occurs at higher temperature as Gd concentration is increasing. Thus, it suggests that a different mechanism accounts for this resistivity minimum. A detailed discussion is reported.
Direct measurement of spin dependent mean free paths in metals (abstract)
Authors: Gurney Bruce A. , Speriosu Virgil S. , Lefakis Harry , Wilhoit Dennis R. , Need Omar U.
Published: July 2002

We describe results obtained using a new spin valve structure that is specifically designed to measure the spin up and spin down mean free paths in ferromagnetic metals. We report how these mean free paths can be measured more directly and with greater accuracy than previous methods, which were based mostly on indirect evidence from magnetoresistance in ternary alloys. The technique is based on the ‘‘backed’’ spin valve layered structure: substrate/seed/80 Å FeMn/50 Å NiFe/23 Å Cu/20 Å NiFe/t Å b/50 Å Ta where the back layer of material b with thickness t is probed by the rest of the structure, which forms a spin polarized conduction electron source. As t is varied the majority carrier mean free path λ+b in the layer b is obtained directly from the form taken by the change in film conductance between parallel and antiparallel magnetization states, ΔG, whose solution of the Boltzmann transport equation shows is well approximated by the form ΔG=ΔG0+ΔGb{1−exp([t−tx]/βλ+b)}, where ΔG0 arises from the 20 Å NiFe layer, tx is the high resistivity region of intermixing at the b layer interfaces, and β≂1 from observations with b=Cu. The minority carrier mean free path in a ferromagnetic layer b, λb, is obtained by comparing ΔG and λ+b with ΔG′, and λb′+, of a nonmagnetic b′ layer of similar resistivity to b; it is a less direct measurement than that of λ+ since it relies on the connection between conductivity and mean free path for the minority subband. We have obtained room temperature results for Ni80Fe20 (λ+=46±3 Å, 0<λ<6 Å), Fe (λ+=15±2 Å, λ=21±5 Å), and Co (λ+=55±4 Å, 0<λ<8 Å). The connection of spin dependent conductivity and mean free paths in ferromagnetic metals is crucial in exploring the mechanism of giant magnetoresistance and, more broadly, is central in all theories of transport in magnetic metals; this new technique should prove a powerful tool in measuring these fundamental quantities.
Giant magetoresistance in epitaxial sputtered Fe/Cr(211) superlattices (abstract)
Authors: Conover M. J. , Fullerton Eric E. , Mattson J. E. , Sowers C. H. , Bader S. D.
Published: July 2002

We report giant magnetoresistance (GMR) as high as 150% at 4.2 K in epitaxial Fe/Cr superlattices. Samples prepared by magnetron sputtering on MgO(100) produced the expected Fe/Cr(100) orientation; however, deposition on MgO(110) produced a novel (211) low-miller-index orientation. The crystallographic orientations and superlattice nature of the samples were confirmed by both low- and high-angle x-ray diffraction. The Fe/Cr(211) superlattices have an in-plane uniaxial anisotropy (KU=9×105 ergs/cm3) along the Fe[01̄1] which give the sample a rich magnetic behavior. The GMR with the current along the [01̄1] is ≊20% higher than that along the [1̄11] direction of the (211) plane. The GMR for the (211) superlattices shows four oscillations with Cr thickness. Both orientations shows the expected long-period oscillation of 18 Å Cr and a GMR increase by a factor a 4 when cooled from room temperature to 4.2 K.
X-ray-absorption fine structure of selected R2Fe17 nitrides
Authors: Capehart T. W. , Mishra R. K. , Pinkerton F. E.
Published: July 2002

X-ray-absorption fine structure from the rare-earth L3 edges in R2Fe17N3−x (R=Sm, Pr, or Nd), x∼0.2, establishes that these nitrides have 2.6±0.6 nitrogen atoms bonded 2.54–2.57 Å from the rare-earth site. The observed enhancement of the nitrogen backscattering from magnetically oriented powders of Sm2Fe17N3−x establishes that between 80% and 100% of the nitrogen atoms occupy 9(e) sites of the Sm2Fe17N3−x. No evidence is found for partial nitrogen occupancy of the 18(g) sites in Sm2Fe17N3−x.
A metastable ternary Nd-Fe-B compound
Authors: Gu B. X. , Shen B. G. , Zhai H. R.
Published: July 2002

The metastable compound Nd6Fe77B17 has been obtained by the crystallization from the amorphous phase in the Nd-Fe-B system. Its crystal structure is similar to that of the compound Nd2Fe23B3. At high temperature this metastable compound decomposes into α-Fe and Nd1.1Fe4B4 phases. The room temperature magnetization and coercivity are 154 A m2/kg and 12 kA/m, respectively.
The structure and spin dynamics of lanthanide-bearing silicate glasses
Authors: Ellison A. J. G. , Loong C.-K. , Wagner J.
Published: July 2002

The structure of 3Na2O⋅R2O3⋅6SiO2 (R=Nd, Yb and Lu) glasses has been studied by time-of-flight neutron diffraction. The average Si-O and O-O nearest-neighbor coordination is found to be similar to those of the SiO4 tetrahedral units in vitreous SiO2 and other silicate glasses. The rare-earth (RE) ions coordinate with no more than seven nearest-neighbor oxygen atoms. The dynamic and static response of these glasses and their isochemical crystalline analogs has been investigated by inelastic neutron scattering and magnetic susceptibility measurements. The magnetic excitation spectra were analyzed by a ligand-field model using a method of descending symmetry. The correlation between short-range atomic order in a network glass and magnetic interactions of the RE ions with the local environment is discussed.
Hysteresis and magnetic aftereffect in amorphous CoZrDy films: Trends and variations versus Dy content
Authors: Suran G. , Roky K.
Published: July 2002

The magnetization process was studied in amorphous (Co95Zr5)100−xDyx thin films, where 4≤x≤26, between 6 and 300 K. The films exhibit a well-defined in-plane uniaxial anisotropy Ku. The temperature dependence of the coercive field Hc could be related to the variations of the local anisotropy Kloc, Ku and local defects. The thermally activated magnetic relaxation was detected for T≤40 K, and it follows with time the law ln(M/Ms)=at. The constant a changes rapidly with the applied field H. The results are explained by the sharp distribution of the energy barriers. The as deduced activation volume is about the order of the ferromagnetically correlated volume.
Observations of magnetization reversal and magnetic clusters in copper ferrite films
Authors: Ibrahim M. M. , Seehra M. S. , Srinivasan G.
Published: July 2002

The observation of a magnetization reversal and a superparamagnetic behavior are reported for films of copper ferrite sputtered in an oxygen atmosphere. X-ray diffraction studies indicate the presence of 14-nm nanoscale crystals of CuFe2O4 in the films. Data on the temperature dependence of the film magnetization M for zero-field-cooled (ZFC) and field-cooled (FC) show that ZFC and FC curves separate at a temperature Tf and that Tf decreases with increasing static field strength. For T<Tf, M vs T data for low fields indicate the presence of a compensation point in virgin samples. For T≳Tf, a superparamagnetic character is evident form the high-field magnetization data. An average cluster size of 16 nm obtained from such data is in excellent agreement with the results of x-ray diffraction studies.
Magnetostriction in RE-Co amorphous alloy films (abstract)
Authors: Uchiyama S. , Yoshino S. , Takahashi H. , Tomi-ita K. , Mori T. , Itakura A. , Iwata S. , Tsunashima S.
Published: July 2002

Rare earth-transition metal (RE-TM) amorphous films have been used as magneto-optical recording media. One of the most important properties is the perpendicular magnetic anisotropy, the main origin of which is explained by one ion anisotropy of rare earth ion. However, it is also noted that the anisotropy contribution due to the magnetostriction depends in a very similar manner on the RE species as in the case of the main anisotropy. Thus it was suggested that the main origin of the magnetostriction might be the one ion anisotropy of RE. The present work reports the saturation magnetostriction constant λs of various RE-Co amorphous alloy films with RE=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er in the range of RE concentration of 0%–40%. In most cases, λs increases with RE concentration, then decreases after showing a peak and disappears altogether with ferromagnetism. For an RE species with a small one ion anisotropy, λs is small with a value of less than 2×10−5, and is found to be contributed primarily from Co. For RE with larger one ion anisotropy, namely for RE=Sm, Pr, Tb, and Dy, λs has values of the order of 10−4 and is estimated to be contributed mainly from RE one ion anisotropy.
Improving ferrite MIG head read-back distortions caused by domain walls and granularity (abstract)
Authors: Argyle B. E. , Schäfer R. , Trouilloud P. L. , Re M. E. , Praino A. P. , Takayama S. , Dingley D.
Published: July 2002

Ferrite MiG heads intended for narrow track (≲10 μm) digital recording were recently investigated in the critical pole-tip region at the air-bearing-surface using micro-ellipsometry, Kerr microscopy, and electron back-scatter diffraction from individual grains,1 and using magnetic force microscopy to detect air-gap remanent fields.2 Comparison of these direct observations with readback-after-write waveforms from written test tracks, and consideration of granularity influences on bulk permeability and domain size, indicate that waveform instability and asymmetry from polycrystalline ferrite (PCF) heads would be diminished by suitable size and orientation of the grains.1 The use of single-crystal ferrite3 (SCF) for advanced laser enhanced etch definition3 of narrow pole MiGs can avoid this type of distortion. However, secondary signals4 often appear as weak pulses separated in time from the main gap pulse. We have associated this effect with a zig-zag shaped wall seen nucleated and propagated from the pole tips by a write pulse.4 This wall and its underlying domains lie remanent in the stressed ABS material and evidently react to the bit fields during the read cycle. The secondary read-back response, though similar to the pseudo-gap effect, differs in origin. Its timing depends on the distance of the zig-zag wall to the gap, not the fixed position of the sendust-ferrite interface. Our results indicate that suitable grain oriented ferrite would reduce PCF MiG head read-back asymmetry and instability. For SCF heads, a method for electrically removing zig-zag walls is possible and secondary pulse removal has now been demonstrated on a spin test strand.
57Fe and 119Sn Mössbauer studies on La1.25Nd0.6Sr0.15CuO4: Evidence for local magnetic ordering below ≂32 K (abstract)
Authors: Breuer M. , Büchner B. , Micklitz H. , Baggio-Saitovitch E. , Souza Azevedo I. , Scorzelli R. B. , Abd-Elmeguid M. M.
Published: July 2002

57Fe and 119Sn Mössbauer effect studies on 0.5 at. % 57Fe or 119Sn doped nonsuperconducting La1.25Nd0.6Sr0.15CuO4 in the temperature region 300 K≥T≥4.2 K reveal an onset of local magnetic ordering occurring at TM≂32 K for both 57Fe as well as 119Sn-doped samples. The local magnetic ordering shows up in the presence of a very large transferred hyperfine field of Bthf≂11.0(5) T at the 119Sn nuclei. Since such a large field is present in neither antiferromagnetic La2CuO4 superconducting La1.85Sr0.15CuO4 nor overdoped La2−xSrxCuO4 (x=0.4), the occurrence of such local spin correlations seems to be a signature of the nonsuperconducting low temperature tetragonal phase of La1.25Nd0.6Br0.15CuO4.
Damping of spin waves in a square-lattice quantum antiferromagnet due to spin-phonon interaction (abstract)
Authors: Saenger Dirk Uwe
Published: July 2002

The damping of spin waves due to spin-phonon interaction in a spin-1/2-Heisenberg antiferromagnet on a square lattice is calculated. The theory applies to the regimes where the sound velocities vs are small compared with magnon velocities vm, which is the case in antiferromagnetic high-Tc superconductor basis materials like YaBa2Cu3O6, and kξ ≫1, where k and ξ are wave vector and correlation length, respectively. It is concluded that the broad spectral feature of the two-magnon line observed in layered perovskites can be explained qualitatively as a consequence of spin-phonon interaction.
Investigation of the microstructure of granular Ag-Fe and Ag-Co thin films by TEM and STEM (abstract)
Authors: Li Z. G. , Wan H. , Liu J. , Tsoukatos T. , Hadjipanayis G. C.
Published: July 2002

Giant magnetoresistance (GMR) at room temperature was found in granular thin films such as Co-Cu, Fe-Ag, Co-Ag, etc., which has stimulated intense research activity. It is still unclear whether the as-deposited Ag-rich (75%-90%) thin films are an alloy or are two separate phases. We have investigated the microstructure of granular Ag-Fe and Ag-Co thin films by atomic resolution transmission electron microscopy (TEM) and nanochemical analysis scanning transmission electron microscopy (STEM). Transition metal particles with a size of less than 1.5 nm in the as-deposited granular Ag-rich thin films were directly observed and were separated by the nanosize of Ag particles. The structure of the as-deposited thin films was metastable, and it changed by annealing. The GMR strongly depends on the nanostructure in the films, and their relationship will be discussed.
Magnetocaloric dependence of magnetic viscosity measurements in NdFeB
Authors: Folks L. , Street R. , Woodward R. , McCormick P. G.
Published: July 2002

Measurements of the time dependence of magnetization may be analyzed by means of a constitutive equation to give information regarding the fundamental nature of magnetization reversal processes. The quantity Λ (≡ the fluctuation field Hf) may be measured and then related to the activation volumes involved in the magnetization reversal processes. These measurements are commonly made at or near room temperature for hard permanent magnet materials, since this is the usual operating temperature, and the system is assumed to be isothermal. However, the experimental work described shows for these materials the magnetocaloric effect (MCE) cannot be neglected at these temperatures. Magnetic viscosity measurements have been made in two ways which lead to different thermal behavior in the sample. Thermometric measurements were made concurrently with the magnetic viscosity measurements in a sample of sintered NdFeB. The measurements show that the calculated value of Λ can be changed by as much as 10% when isothermal conditions do not hold. This effect is particularly evident near room temperature in NdFeB-type materials because of the strong dependence of coercivity on temperature and the proximity to the Curie point (Tc∼620 K) where reversible and irreversible components of MCE peak.
Magneto-optical and structural properties of BiAlDyIG/Fe multilayers
Authors: Shen J. X. , Wierman K. W. , Zhang Y. B. , Kirby R. D. , Woollam J. A. , Sellmyer D. J.
Published: July 2002

Bi- and Al-doped DyIG(y Å)/Fe(x Å) (y=50 to 130 Å and x=5 to 15 Å) multilayer thin films were made by magnetron sputtering onto Si(111) substrates. Rapid thermal annealing was used to crystallize these garnet multilayers. The crystallized multilayer samples possess well-defined interfaces. Atomic force microscopy showed that the annealed samples had smooth surfaces, with small grain sizes (∼30 to 80 nm). All of the samples had perpendicular magnetic anisotropy with very square hysteresis loops. The coercivity was found to vary from 600 Oe to 2 kOe, depending on Fe thickness and annealing conditions. The measured figure of merit √R(θ2K+ε2K) at 420 nm was bigger than 0.4, suggesting that Bi- and Al-doped DyIG/Fe multilayers are promising candidates for blue light magneto-optical recording applications.
Magnetism in metastable bcc and fcc iron-copper alloys
Authors: Serena P. A. , García N.
Published: July 2002

We present linearized augmented plane-wave calculations for FeCu fcc and bcc metastable phases using the local density approximation. Curves for the density of states (without including the spin-polarization effects) at several Fe concentrations are given, noticing that the density of states at the Fermi level for determinated fcc alloys suffers significant large increases in relation to the fcc Fe. This fact provides an explanation to the magnetic character found in some fcc FeCu alloys. The magnetization is calculated at different compositions by applying the Stoner criteria, assuming a simple behavior for the magnetic exchange integral, and including the effect of randomness in a simple way. We find that the magnetic moment per Fe atom is similar to that of bcc Fe for some fcc FeCu alloys.
Neutron diffraction study of the magnetic ordering of BaCuO2+x (abstract)
Authors: Wang Xun-Li , Fernandez-Baca J. A. , Wang Z. R. , Vaknin D. , Johnston D. C.
Published: July 2002

Neutron diffraction experiments have been performed in order to study the magnetic ordering of BaCuO2+x. In these measurements unpolarized neutrons and polarized neutrons with full polarization analysis have been utilized. From these measurements it has been determined that BaCuO2+x orders antiferromagnetically below TN=15.0±0.5 K with a magnetic propagating vector κ=[1 1 1]. This structure is commonly referred to as a [πππ] or a G structure. The Cu atoms in the Cu6 ring clusters, located at the (1/4, 1/4, 1/4) positions, order ferromagnetically within these clusters while the clusters themselves order antiferromagnetically. The ordered magnetic moment of each of these Cu atoms is 0.89±0.05 μB at T=4.2 K. No evidence of long-range magnetic ordering of the Cu atoms in the Cu18 clusters, located at the (0, 0, 0) and (1/2, 1/2, 1/2) positions, was found down to a temperature T=2.5 K. The work at Oak Ridge National Laboratory (ORNL) was sponsored by the Division of Materials Sciences, U.S. Department of Energy (DOE). ORNL is managed for the U.S. DOE by Martin Marietta Energy Systems, Inc., under Contract No. DE-AC05-84OR21400. Ames Laboratory is operated for the U.S. DOE under Contract No. W-7405-Eng-82. The work at Ames was supported by the Director for Energy Research, Office of Basic Energy Sciences.
Structure and magnetism of heat-treated nanocrystalline Cu80Co20 powders prepared via chemical means
Authors: Harris V. G. , Kaatz F. H. , Browning V. , Gillespie D. J. , Everett R. K. , Ervin A. M. , Elam W. T. , Edelstein A. S.
Published: July 2002

Nanocrystalline Cu80Co20 powders were fabricated using a chemical precipitation technique. As-prepared powders consist of metal-oxides having an average particle size of 100 Å. When appropriately heat treated, these powders reduce to form a CuCo face-centered cubic alloy; heat treatments at higher temperatures (≥400 °C) facilitate the phase separation of the alloy to its elemental components. Concurrent with an increase in annealing temperature, the coercive field decreases while the magnetization increases, reflecting particle coarsening and the relative amount of free cobalt in the mixture.
Nonoscillatory behavior in the magnetoresistance of Cu/Ni superlattice (abstract)
Authors: Abdul-Razzaq W.
Published: July 2002

It was reported that in many magnetic/nonmagnetic metallic multilayered systems, the interlayer-coupling oscillates between antiferromagnetic and ferromagnetic upon increasing the thickness of the nonmagnetic layer. This was evident by the oscillation of the magnetoresistance (MR) in these materials. Recently however, Harp, Parkin et al.1 found that the MR and coupling strength change monotonically with increasing Cu thickness in Co/Cu multilayers deposited by MBE, contradicting results on similar samples made by sputtering in which the MR was oscillatory. In this study, we show that in the Cu/Ni superlattice made by sputtering, the MR varies monotonically with increasing Cu thickness. This nonoscillatory behavior was observed at room temperature and at 77 K and, regardless of the direction of the magnetic field in relation to the direction of the current. The resistivity at zero magnetic field as a function of temperature also changes systematically with reducing the Cu layer thickness. The nature of the magnetic state in Cu/Ni superlattice is discussed in light of the transport property measurements.
Comparison of giant magnetoresistance in multilayer systems and uranium compounds
Authors: Nakotte H. , Prokeš K. , Brück E. , de Châtel P. F. , de Boer F. R. , Sechovsky V. , Havela L. , Fujii H.
Published: July 2002

The enormous magnetoresistance effect at low temperatures in various uranium-based intermetallic compounds, which exceeds by far the effect found in magnetic-multilayer systems, is a consequence of the strong 5f conduction electron hybridization. The consequences of spin-dependent scattering for the giant magnetoresistance (GMR) effect in uranium intermetallics will be discussed and compared with the behavior in multilayer systems. Possible changes of the model parameters to achieve giant magnetoresistance (GMR) effects at higher temperatures, which are desirable for practical applications, are considered.
Magnetic anisotropy in epitaxial Ni/Cu(001) thin films: Effects of misfit strain on perpendicular magnetic anisotropy (abstract)
Authors: Bochi G. , Balentine C. A. , Inglefield H. E. , Bogomolov S. S. , Thompson C. V. , O’Handley R. C.
Published: July 2002

Several epitaxial magnetic systems, notably Fe/Ag(001) and Fe/Cu(001), exhibit perpendicular magnetic anisotropy (PMA) over a relatively narrow thickness range, approximately 2–6 ML. Recently, it has been shown1,2 that a similar region of perpendicular anisotropy exists for Ni/Cu(001), but over a much broader range of film thicknesses extending up to approximately 40 ML. The effective anisotropy energy density can be written as Keff = −2πM2s + 2B1ε(t)+ (2KN/t), which includes magnetostatic (MS), magnetoelastic (ME), and Néel (averaged over two surfaces) terms. The effective anisotropy vanishes at the perpendicular to in-plane transition thickness of about 60 Å. If we assume bulk values for MS and B1, the ME coupling coefficient, and use our measured strain values ε(t), we find that the Néel term is an order of magnitude smaller than the ME and MS terms. These large competing terms therefore control the PMA in this system. We have extended our studies of PMA in Ni/Cu(001) to include Cu1−xNix (0%<x<50%) alloy substrates for which the misfit strain with the Ni film is reduced. We find that the perpendicular-to-in-plane transition, at t≊40 ML for Ni/Cu(001), shifts to thinner nickel layers as the misfit strain is decreased by deposition on Cu1−xNix substrates. This further confirms the important role of ME energy in PMA for this system.
The effects of closure domains on flux conduction in thin film recording head
Authors: Guo Zhenzhou , Della Torre Edward
Published: July 2002

The magnetic flux conductivity in the tip of a recording head was examined for various applied field and closure domain configurations using a two-dimensional micromagnetic model developed to obtain the domain patterns in soft magnetic thin films. A permalloy thin film sample was selected to simulate a monopole head configuration, and an external field was applied to various closure domain ground states that had zero net remanence magnetization. It was found that for small fields the magnetization process is linear and reversible, and both the susceptibility and the effective track width increase as the size of the closure domains is reduced. For large fields, however, the variation of the flux conduction is much less sensitive to the size of the zero-field closure domains, since the sample is nearly saturated. Small jumps were observed in the magnetization curves corresponding to the domain wall nucleation and annihilation, which can be a source of head noise. It was also observed that the effective center of magnetic pole shifts with increasing field.
Performance of GaxIn1−xP/GaAs heterojunctions grown by metal-organic molecular-beam epitaxy and metal-organic vapor-phase epitaxy
Authors: Ginoudi A. , Paloura E. C. , Frangis N.
Published: July 2002

The electrical and structural characteristics of GaxIn1−xP/GaAs heterostructures grown by metal-organic molecular-beam epitaxy (MOMBE) at 480–560 °C and metal-organic vapor-phase epitaxy (MOVPE) at 700 °C are comparatively studied. For the lattice-matched composition (x=0.51) transport in Schottky diodes fabricated on MOVPE material is almost thermionic and the barrier height is 0.96±0.05 eV. Deviation from the ideal thermionic behavior is observed in diodes on MOMBE material and the deviation is stronger as the growth temperature decreases. The undoped MOMBE material, grown at TG≤520 °C, as well as the MOVPE material, are characterized by a deep electron trap with an activation energy of 0.80±0.05 meV. The formation of this trap in the MOMBE and MOVPE material is suppressed by doping with shallow impurities, at doping levels higher than 1017 cm−3. However, doping of MOMBE Ga0.51In0.49P (hereafter called GaInP) with S and Si introduces another trap with an activation energy of about 300 meV, which has some of the properties of the DX centers, while Se doping of MOVPE GaInP does not introduce any donor-related traps. Rapid thermal annealing of the MOMBE material, at temperatures up to 600 °C, improves the Schottky barrier characteristics and reduces the deep trap concentration. An additional trap with an activation energy of about 500 meV, that has been identified in lattice-matched MOMBE Ga0.51In0.49P epilayers, is attributed to the electrical activity of dislocations, which have been identified with transmission electron microscopy (TEM) studies. Furthermore, the TEM analysis reveals the presence of compositional separation phenomena (spinodal decomposition) which are strong in the MOMBE material and less extended in the MOVPE material.
On the influence of collision mass and interaction potential to the energy transfer in small thermal gas phase clusters
Authors: Schulte J. , Marlow W. H. , Lucchese R. R.
Published: July 2002

We have studied the collisional energy transfer, and the sticking probability, between molybdenum clusters and the rare-gas atoms Ne, Ar, and Xe. We have chosen these systems as nontrivial models of the thermalization process of metal clusters in a background gas. The mean energy transfer cross sections and the energy transfer rate constants, and the sticking probability of molybdenum clusters (Mo4,9,14) with rare-gas atoms are computed as functions of relative collision energy (gas temperature) and reduced mass. The dynamics of gas phase molybdenum clusters are simulated by molecular dynamics trajectories whose initial conditions are sampled from a distribution appropriate to thermal collisions. The simulation shows that the energy transfer rate constants are dominated by the collision frequency. The mean energy transfer cross sections are coupled to the collision mass as well as to the actual interaction force. The coupling is nonlinear, and there is some evidence that in the energy transfer, for small clusters, complex collisions are involved. The sticking probability at equilibrium temperature is far below 1.
High current density electron beam generation from field emission tip cathodes
Authors: Song Y. , Garate E. , Rostoker N.
Published: July 2002

Electron beams with current up to 1.2 A and current density over 107 A/cm2 have been generated from a pulsed field emission tip cathode etched from commercial grade tungsten wire. Electron beams with current up to 0.8 A have also been generated from a molybdenum tip. Tip radius was 0.1–1 μm. Applied tip voltage was up to 50 kV with pulse duration of 0.3–1.2 μs. Depending on brightness, these types of electron beams could be suitable for channeling radiation x-ray lasers and might considerably decrease the operating wavelength of free-electron lasers at moderate beam energies.
Electronic structure of barium titanate studies by the extended Hückel tight-binding method
Authors: Holma M. , Kitamura M. , Chen Haydn
Published: July 2002

The electronic structure of barium titanate is determined according to the extended Hückel tight-binding method using atomic data based on a self-consistent-field calculation. The effects of the phase transition on the partial density of states are studied. The electronic contribution to the phase stability of the compound is calculated from the total density of states. The tetragonal phase is found to be stable as compared to the cubic phase. The density of states is compared with available x-ray photoemission spectra and is found to be in agreement with some features in experimental results.
Structural defects in glow discharge a-Si:C:H investigated by positron annihilation spectroscopy
Authors: Dusane R. O. , Rajarshi S. V. , Goyal D. J. , Bhide V. G. , Nagrajan T.
Published: July 2002

Glow discharge deposited hydrogenated amorphous silicon carbon alloy films with varying carbon content are studied for structural defects by positron lifetime spectroscopy. Measurements of optical properties like refractive index (n) and band gap (Eg) which indirectly reflect the varying carbon content in the films have also been made. The increased defect density in these films with increasing carbon content is very clearly indicated by the positron lifetime data. The nature of these defects and their relative concentrations are determined from the positron lifetime and intensity values. Interestingly a continuous transformation from one type of defect to the other with increasing carbon is indicated by these results.
Combined thermodynamic and kinetic model for the kinetics of M/ABu reactions
Authors: Russell S. W. , Wang S. Q.
Published: July 2002

We propose a model for the reaction kinetics of metal/compound systems in which growth occurs in the form of a phase MAv of fixed composition growing in between ABu and a phase of variable composition, MBw. An example of this is the Ti/SiO2 system, where Ti5Si3 grows between SiO2 and an oxide phase, TiOw. The model takes into account the slowing and eventual cessation of the reaction upon MBw reaching a limiting, equilibrium composition of MBz. We use the Gösele and Tu kinetic model but subject it to our thermodynamic constraint. The original kinetic model yields a simple parabolic relation, x2=2kt, for diffusion-controlled growth. Our model modifies this relationship, predicting a growth law of the form kt= x2/2+ax2f∑∞n=3(1/n)(x/xf)n, with x the thickness and xf the final thickness of the MAv phase. k is the reaction rate constant for diffusion-controlled growth and a∼1. xf depends on the initial metal film thickness and the stoichiometries of all phases involved in the reaction. The model provides an explanation for previous results that observed an increasing reaction rate with increasing initial metal layer thickness.
Adhesion and fracture analysis of metal/polyimide fine line structures
Authors: Poon Tze Wing , Leu Jihperng , Kasthurirangan Jaishankar , Ho Paul S.
Published: July 2002

Finite element analysis has been employed to study the adhesion and fracture behavior observed for metal/polyimide line structures formed by depositing Au/Cr lines on biphenylenetetracarboxylic dianhydride-phenylenediamine as a function of line width and line thickness. The key features can be attributed to the various mechanical environment induced at the interface. The results distinguish conditions where the edge effect becomes significant for line widths less than the substrate film thickness, while the bulk effect becomes dominant for line thicknesses larger than 0.25 of the substrate film thickness. Both effects reduce interfacial strain and deformation energy induced in the metal lines, leading to metal line delamination of the corresponding structures occurring at a higher applied strain. Due to the plastic yield of the metal, the interfacial stress behavior is found to be relatively insensitive as a function of the line dimension, and it cannot account for the delamination behavior observed. In contrast, the deformation energy induced in the metal shows a clear dependence on the applied strain. It continues to increase with applied strain, even after the maximum stress level is reached. Correlating with experiments, the energy in the metal line is about 2–10 J/m2 at the onset of metal line delamination for all structures with different widths and thicknesses. This value is comparable to the total bond energy between the metal and polyimide atoms along the interface, suggesting a correlation between the adhesion energy and the chemical bond strength of the interface. The deformation energy stored in the polyimide is an order-of-magnitude higher than that in the metal, so it dominates the energetics of the fracture process. In addition, the energy profile along the interface shows that maximum deformation occurs in the vicinity of the metal line edge where fracture occurs. These results emphasize the important role of the energetics in controlling the delamination and fracture of the interface.