bluuebunny/crossref_metadata_2025 huggingface dataset:
The effects of a hydrogen ambient on the interface state energy distribution of gamma irradiated and charge injected metal-oxide-semiconductor structures fabricated on germanium/boron doped silicon
Authors: Hashemipour O. , Ang S. S. , Brown W. D. , Yeargan J. R. , West L.
Published: July 2002

The effects of a hydrogen ambient on the change in interface state energy distribution of germanium/boron (Ge/B) counterdoped silicon substrate metal-oxide-silicon capacitor structures during and following gamma irradiation were investigated. The presence of hydrogen during irradiation produces both a significantly larger increase in interface state density throughout the band gap and a higher peak value at approximately 0.7 electron volts above the valence band (0.7-eV peak) than is obtained for devices irradiated in air. Additionally, the time for the 0.7-eV peak to reach its maximum value is much shorter as is the time lapse for the initiation of growth of a trap peak at 0.35 electron volts above the valence band (0.35-eV peak). Devices irradiated in air and then stored in hydrogen exhibit similar behavior as those irradiated in hydrogen except that the interface state buildup with time is much slower. Pretreating devices by exposure to hydrogen prior to Fowler–Nordheim (F-N) injection or avalanche electron injection (AEI) is shown to inhibit interface state generation including a significant reduction in the 0.7-eV peak height. This result suggests that hydrogen created by F-N injection and AEI passivate dangling bonds associated with germanium in the silicon.
Mixed methyl-(2,4-dinitrophenyl)-aminopropanoate: 2-methyl-4-nitroaniline crystal−a new nonlinear optical material
Authors: Rao S. M. , Batra A. K. , Lal R. B. , Evans R. A. , Loo B. H. , Metzger R. M. , Lee W. J.
Published: July 2002

Single crystals of mixed methyl-(2,4-dinitrophenyl)-aminopropanoate (MAP): 2-methyl-4-nitroaniline (MNA) have been grown from solution using a 40:60 mixture of ethanol and ethyl acetate. Formation of an equimolar complex is confirmed by x-ray diffraction studies. The complex crystallizes in the monoclinic space group P21. The lattice parameters are a=6.9196(±0.005) Å; b=7.673(±0.008) Å; c=18.554(±0.001) Å; and β=92.547(±0.006)0. b axis is the unique twofold axis and there are two molecules per unit cell. The molecules stack along the b axis with MAP atop MNA atop MAP. Details of the molecular orientation are given. The Raman spectra also indicate that the parent molecules are not disturbed in the process of crystallization. The second harmonic generation efficiency of the new material for 1064 nm laser radiation is observed to be the same as that of MAP but lower than that of MNA. The latter is attributed to the absorption at 532 nm in the mixed crystal. The melting point determined by differential scanning calorimetry is 104.30 °C.
Permanent magnet sources for extended interaction klystrons
Authors: Leupold H. A. , Kosa L. , McLane G. , Tilak A. S. , Potenziani E.
Published: July 2002

A major part of the bulk and mass of an extended interaction amplifier resides in its magnetic field source. Both can be considerably reduced by application of novel designs made possible by high-energy product permanent magnet materials such as the samarium–cobalts and neodymium–iron–borides. We compare a structure derived from a hemispherical source design (‘‘magic igloo’’) with several more conventional permanent magnet and electromagnet configurations and find that mass reductions of 50% to 80% are often realized when the hemispherical source replaces any of the others. Such reductions afford considerable performance improvement in radars, radios, and jammers, especially in airborne and ballistic devices. Further compaction results from closer packing of field-sensitive instruments afforded by leakage field reduction in the hemispherical source. Other advantages of the latter as well as drawbacks, are discussed.
Multivibrator transistor switch module using a high-temperature superconducting core
Authors: Uchiyama T. , Shibata T. , Makino M. , Mohri K.
Published: July 2002

A new high-speed transistor switch module is presented using YBa2Cu3O7−x high-temperature superconductor (HTcSC) bulk cores in liquid nitrogen. A Royer oscillator-type magnetic multivibrator is constructed using two switching transistors combined with four windings tightly set around a HTcSC disk core. Oscillation of the multivibrator is initiated by applying an ON signal pulse current (ION) to a coil, and terminated by applying an OFF signal pulse current (IOFF) to a small transistor which shorts another coil of the HTcSC core. The multivibrator transistor module switched with a turn-on time tON of about 0.2 μs and a turn-off time tOFF of about 0.2 μs for 1-A transistors. The values of tON and tOFF are about 1/10 and 1/100 that of the multivibrator transistor module using an amorphous ferromagnetic core. This quick switching property can be attributed to the Meissner effect or zero inductance effect. These high-speed transistor switching modules have applications in control of small motors with a high rotation speed.
Magnetic properties of sintered Nd-Fe-B magnets from hydrided materials
Authors: Kim Andrew S.
Published: July 2002

Hydrogen decrepitation is a very effective method of pulverizing Nd-Fe-B-type alloys for sintered magnets. Jet-mill response of hydrided materials is much better than that of unhydrided materials, i.e., shorter milling time is required and finer particle size is obtained for hydrided materials. The sinterability of hydrided powder is also improved from that of unhydrided powder. The minimum sintering temperature for obtaining full density (≳98% of theoretical density) magnets made from hydrided powders ranges between 900 and 950 °C, while that for magnets made from unhydrided powders ranges between 1050 and 1100 °C. Magnets made from hydrided powder sintered at low temperatures usually exhibit high coercivity comparable to or greater than that of the magnets made from unhydrided powder sintered at high temperatures followed by subsequent heat treatment. The enhancement of sinterability and the increase in coercivity of Nd-Fe-B magnets are discussed.
Polaronic absorption in uncompensated CaX:YIG
Authors: Thavendrarajah A. , Song Y. , Wigen P. E.
Published: July 2002

In charge uncompensated CaX:YIG, magnetic anomalies and a photoinduced absorption spectrum have been reported. The temperature dependence of the photoinduced peak observed at 1.85 eV (width 0.5 eV) has the same temperature dependence as the low-temperature anomalies observed in the magnetizations. A model to explain the low-temperature magnetic anomaly assumes that a non-S state iron ion, Fe4+, is created in the tetrahedral site that is ferromagnetically coupled to the octahedral Fe3+ ion. An alternative model recently proposed assumes that O− ions are formed which would influence the magnetic properties in similar fashion. The broad nature of the absorption at 1.85 eV is characteristic of a resonant transition of photocarriers. In this paper the absorption band is discussed in terms of a polaron hopping model.
Kerr rotation and perpendicular magnetic anisotropy of CoCr films with Al ultrathin interlayers and single-layer CoCr films
Authors: Hirata Toyoaki , Takahashi Takakazu , Hoshi Youichi , Naoe Masahiko
Published: July 2002

The Co81Cr19/Al multilayered films were prepared by using the plasma-free sputtering apparatus. The specimen films with the thicknesses of Co81Cr19 and Al layers lCo-Cr and lAl of 50–170 and 7–14 Å, respectively, were investigated for the Kerr rotation angle θK and the reflectance R of the multilayered films with total thickness of 1500 Å. Films with lCo-Cr and lAl of 138 and 7 Å, respectively, had a θK of 0.21° and R of 0.7 which is larger than Co81Cr19 single-layer films prepared by conventional sputtering where θK and R are 0.036° and 0.4–0.5, respectively. These results indicate that the films were entirely homogeneous, that is, the surface and interior of the films may be almost the same for composition, microstructure and magnetic properties. Consequently, the Co81Cr19 thin films with Al ultrathin interlayers may be useful for microcrystalline magneto-optical media with a high C/N ratio.
Improvement of magneto-optical characteristics of TbFeCo/Al bilayered films at a short wavelength of 400 nm
Authors: Song K. , Naoe M. , Ito Hiroshi
Published: July 2002

Bilayered films composed of amorphous TbFeCo alloy and Al layers were deposited successively on glass slide substrates without plasma exposure using the facing targets sputtering system. The specimen films with the magneto-optical (MO) layer thickness tMO of 14 nm have a Kerr rotation angle θK and figure of merit fM as large as about 0.36° and 0.25, respectively, at the wavelength λ as short as about 400 nm. These values of θK and fM are considerably larger than those of the bilayered films with larger tMO in the conventional MO media. Normally, the bilayered films with tMO above 50 nm have θK of about 0.25° and fM below 0.2 at λ of 400 nm.
Magnetic transitions in Pr2NiO4 single crystal
Authors: Obradors X. , Martinez B. , Batlle X. , Rodriguez-Carvajal J. , Fernandez-Diaz M. T. , Martinez J. L. , Odier P.
Published: July 2002

The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a first-order character and involves both the out-of-plane and the in-plane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a first-order character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the out-of-phase component of χac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two field-induced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the high-field anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4.
High reliability magneto-optic media
Authors: Hatwar T. K.
Published: July 2002

Amorphous alloys of rare earths and transition metals used for magneto-optic (MO) recording are extremely reactive to oxygen and water resulting in severe oxidation and corrosion pitting of the media making it unrecordable. We have shown earlier [D. Majumdar and T. K. Hatwar, J. Vac. Sci. Technol. A 7, 2673 (1989)] that the addition of an easily oxidized element such as Zr is effective in suppressing pitting corrosion, while the addition of a noble element such as Pt is effective in retarding the surface oxidation of the film. We found that when both Zr and Pd were added to TbFeCo, the synergetic effect was such that better corrosion and oxidation resistances were obtained at lower concentrations of the alloying elements. This was accomplished with no decrease in carrier-to-noise ratio and a 30% decrease in optimum record power, relative to undoped TbFeCo.
Magneto-optical properties of Ca-substituted Bi-YIG sputtered films
Authors: Hirano Teruyoshi , Namikawa Tatsuru , Yamazaki Yohtaro
Published: July 2002

Ca-substituted Bi-YIG films (Bi2YFe5−xCaxO12, x=0.0, 0.3, 0.5, 1.0) were prepared by rf sputtering, and their magnetic and magneto-optical properties in the visible wavelength region were investigated. The films were deposited on Corning No. 7059 glass substrates at 400 °C. Sputter gas was Ar, 6.7 Pa, deposition rate 3.3 nm/min, rf-power density 2.5 W/cm2. The films were annealed in air at 650 °C for 4 h to convert to polycrystalline films. Prepared films were examined by x-ray analysis and determined to be as garnet phase. The x-ray diffraction peaks of the films shifted to lower diffraction angle with increasing Ca content. The saturation magnetization of the films did not decrease with increasing Ca content. These results suggest that Ca2+ ions were partially substituted for c-site ions. The Faraday rotation of the films have maximum values at 470 nm. The magnitude of the Faraday rotation and absorption coefficients in the wavelength region from 450 to 550 nm decreased with increasing Ca content. And the absorption edge was shifted to short wavelength region with increasing Ca content. The reduction in the absorption coefficient was remarkable.
Structural and magnetic studies of metastable Gd–Cr alloys over the entire composition range
Authors: Hsu J. H. , Alten B. G. , Childress J. R. , Gavrin A. , Chien C. L.
Published: July 2002

Under equilibrium conditions, Gd and Cr are immiscible. However, by vapor quenching using sputtering techniques, metastable GdxCr1−x alloys can be prepared over the entire composition range. Since pure Gd and Cr are ferromagnetic and antiferromagnetic, respectively, such metastable alloys reveal the evolution from ferromagnetic ordering to antiferromagnetic ordering. X-ray diffraction shows that the structure for samples with x<0.20 is body-centered-cubic, the same as that of Cr, whereas for x≳0.20 it is hexagonal-close-packed (hcp), as is Gd. Surprisingly, no single phase amorphous state has been found. Pure Gd is ferromagnetic with Tc near room temperature. With the addition of Cr, Tc decreases slowly to about 226 K at x=0.60, and decreases rapidly as the Gd concentration is further reduced. For samples with x<0.50, magnetization at 5 K cannot be saturated even in an external field of 50 kOe, due to competing ferromagnetic and antiferromagnetic interactions. A magnetic phase diagram has been determined.
Mixed magnetic phase in Co-rich metallic glasses
Authors: Das A. , Majumdar A. K.
Published: July 2002

ac susceptibility (χ) and magnetoresistance (MR) have been measured in Co-rich amorphous ferromagnets [Fe5Co50Ni17−xCrx(BSi)28 (x=0,5,10,15), Fe5Co50Mn17(BSi)28, and Fe7.8Co31.2Ni24Mn15(BSi)22] to understand the changes in magnetic phase brought by the addition of Cr/Mn. The Tc decreases monotonically with Cr/Mn addition. Samples with x=0–15 remain ferromagnetic down to 16 K. But for the Mn substituted samples below 80 K, χ decreases drastically and at 16 K it is only 5% of its value at 80 K indicating spin freezing. dc magnetization data show a peak around 30 K as well as a time dependence in the Mn17 sample. Presence of significant ferromagnetic anisotropy of resistance (FAR) even at the lowest temperature for both Mn-containing samples indicates that ferromagnetism persists even when transverse spin freezing is present. With lowering of temperature FAR increases, and below 30 K it should have decreased considerably if the lowest temperature phase was purely spin-glass-like but here it remains almost constant. The high-field slopes for T < Tc in both orientations are negative. In particular, for Mn15 sample the slope has a broad maximum decreasing on both sides as T → Tc and T<80 K. This behavior is interpreted in terms of a transverse spin freezing. We conclude that the lowest temperature (<30 K) phase in samples containing Mn is a mixture of spin-glass and long-range ferromagnetism. Samples containing Cr show a large positive slope for T < Tc and a very low FAR. The ratio Δρ∥/Δρ⊥ (≂0) indicates that the domain magnetization lies in the ribbon plane and is parallel to the ribbon axis.
Magnetic properties of ferrimagnetic exchange-coupled rare earth-iron multilayered films
Authors: Aylesworth K. D. , Harford M. Z. , Lubitz P. , Koon N. C.
Published: July 2002

We report the results of magnetic and structural studies of sputtered and evaporated multilayers of Gd0.15Fe0.85 and Gd0.40Fe0.60 alloys. Using these alloy compositions, the net moment in each layer is antiferromagnetically aligned with the adjacent layers. The magnetic measurements were made at 300 K in a SQUID magnetometer with a maximum field of 55 kOe. The structures of the films were studied by large-angle x-ray diffraction and the compositions were checked with energy-dispersive x-ray fluorescence and Auger depth profiling. To first order, the magnetic behavior of these films can be described by a semiclassical model that treats the magnetization of each layer as uniform with an antiferromagnetic coupling to the adjacent layers. The necessity of using different parameters to fit the in-plane and perpendicular data indicates the existence of an anisotropy that is not included in this simple model. Spin-flop transitions, similar to those reported for multilayers of pure Gd and Fe are observed and are attributed to the antiferromagnetic exchange coupling at the interfaces. These films also display novel magnetic behavior that can be explained by enhancement of magnetostatic effects.
Temperature and layer-thickness dependencies of Kerr rotation in Dy/Co multilayers
Authors: Shan Z. S. , Shen J. X. , Kirby R. D. , Liou S. H. , Sellmyer D. J.
Published: July 2002

Amorphous Dy/Co multilayers with the individual layer thickness between 4 and 10 Å in steps of 0.5 to 1 Å were studied systematically over the temperature range 100–300 K. The Kerr rotation increases as temperature decreases, or as Co content increases and the coercivity increases both in the low temperature region and in the vicinity of the compensation point. These characteristics were analyzed in terms of simple models of the magnetic structure and magnetization reversal.
Characterization of the structural and magnetic ordering of Fe3O4/NiO superlattices grown by oxygen-plasma-assisted molecular-beam epitaxy
Authors: Lind D. M. , Berry S. D. , Chern G. , Mathias H. , Testardi L. R.
Published: July 2002

Oriented single-crystalline thin films of NiO and Fe3O4 and Fe3O4/NiO superlattices have been grown on cleaved and polished substrates of MgO(001) using oxygen-plasma-assisted molecular-beam epitaxy (MBE). We report the growth mode and structural characterization of these films using in situ RHEED and ex situ scanning electron microscopy and x-ray diffraction, and their magnetic characterization using SQUID magnetometry. Also reported are preliminary results of magnetotransport measurements. MgO has a very small lattice mismatch to the cubic rocksalt structure of NiO and to the half-unit-cell dimension of the spinel structure of Fe3O4. Pseudomorphic growth of superlattices consisting of alternating layers of NiO and Fe3O4 with repeat wavelength down to 17 Å and of single thick layers of either of these materials are readily obtained. The grown films exhibit cubic single-crystalline symmetry in registry with the substrate, with sharp interfaces and strongly layer-thickness-dependent strain. RHEED pattern evolution in situ during growth indicates development of the rocksalt-like NiO and spinel Fe3O4 crystalline symmetries as each of the alternating layers of the superlattices are deposited. Magnetic hysteresis curves for superlattices with repeat wavelength less than 75 Å exhibit a crossover from the conventional ferrimagnetic response expected of Fe3O4 found in longer wavelength superlattices and bulklike thick films to a more linear magnetic response that correlates strongly to the layer thickness dependence of lattice distortions and strain. Large magnetoresistance (6% at 55 kOe) is also observed for these short-wavelength superlattices, with the observed change in resistance nearly linear with field and with small hysteresis.
Rotational spin-glass properties of amorphous (Fe1−xMnx)75P16B6Al3
Authors: Goeckner H. P. , Kouvel J. S.
Published: July 2002

For the x=0.235 alloy, the bifurcation of the magnetization (M) versus temperature curves for zero-field cooling and field cooling indicates a spin-glass freezing point (Tg) near 60 K. Above Tg, the alloy is ferromagneticlike (with Tc ≊ 300 K) but its behavior at low fields (<100 Oe) shows that the bulk spontaneous M is zero, which is consistent with the existence of microdomains with randomly oriented spontaneous moments. From rotational measurements of M as a vector in a fixed field (H) at 4.2 K, the anisotropy field (HK) produced by field cooling to saturation is found to turn rigidly with the sample but only for H<100 Oe. At higher H, HK rotates up to some angle relative to H, where it remains as the sample continues to turn, thus exhibiting a frictional rotation relative to the sample. The magnitude of the rotating HK at 4.2 K is determined to be 310 Oe. Comparisons are made with the results of similar magnetization-vector measurements on other spin-glass systems.
Electron paramagnetic resonance of Ce3+ in La(CF3SO3)3⋅9H2O
Authors: Petasis D. T. , Bellesis G. H. , VanderVen N. S. , Friedberg S. A.
Published: July 2002

The rare-earth trifluoromethanesulfonates R(CF3SO3)3⋅9H2O (RTFMS) have a hexagonal structure similar to the ethylsulfates R(C2H5SO4)3⋅9H2O (RES). The lowest Kramers doublet in CeTFMS is ‖J=5/2; Jz = ±5/2〉 separated from an excited doublet ‖J=5/2; Jz= ±1/2〉 by Δ/k≂21 K. We have measured electron paramagnetic resonance on 1% and 2% Ce3+ in LaTFMS crystals at 9.214 GHz. At 4.2 K, absorption by two doublets is found yielding g∥(5/2) = 3.813 ± 0.001; g⊥(5/2) = 0.262 ± 0.002; g∥(1/2) = 0.977 ± 0.003; g⊥(1/2) = 2.151 ± 0.002. The temperature variation of intensities shows the ground doublet to be ‖±5/2〉, as in concentrated CeTFMS, but Δ/k=9.6±0.7 K. The doublet order is opposite to that for dilute Ce:LaES but the same as in CeES or Ce:YES and Ce:EuES. In 2% Ce:LaTFMS, satellite resonances due to coupled nn Ce–Ce pairs are also observed. Analysis of the pair data indicates that virtual phonon exchange, while the dominant nondipolar contribution to intrapair coupling, is smaller than for Ce pairs in CeES. This is consistent with the fact that Δ is larger in CeTFMS or Ce:LaTFMS than in CeES.
Magnetic phase diagrams of spin-frustrated stacked triangular antiferromagnets: Application to ABX3 compounds (invited)
Authors: Plumer M. L. , Caillé A.
Published: July 2002

A review is given of our recent results on magnetically ordered states occuring in geometrically frustrated stacked triangular (simple hexagonal) antiferromagnets based on phenomenological Landau theory, molecular field theory, and Monte Carlo simulations. Novel multicritical behavior in magnetic field-temperature phase diagrams is demonstrated for systems with axial and planar anisotropy (weak and strong). Crossover effects are discussed in terms of the new chiral universality classes recently proposed by Kawamura. Comparison of theory and simulation results with the observed magnetic ordering and phase diagrams in a wide variety of ABX3 compounds is made.
Concurrent enhancement of Kerr rotation and antiferromagnetic coupling in epitaxial Fe/Cu/Fe structures (abstract)
Authors: Bennett W. R. , Schwarzacher W. , Egelhoff W. F.
Published: July 2002

We have used the polar Kerr effect to investigate the magnetic coupling between two 3-monolayer (ML)-thick ferromagnetic fcc-Fe films separated by a Cu spacer layer in epitaxial Cu60Fe3CuxFe3Cu(100) structures specially prepared to have atomically sharp interfaces. We have observed concurrent oscillations in the interlayer exchange coupling strength and the zero-ellipticity polar Kerr rotation as a function of x. The oscillation in exchange coupling covers at least four periods (x=0–30 ML), and has a phase consistent with antiferromagnetic coupling in the limit of zero-Cu-spacer-layer thickness. Preliminary evidence indicates that the oscillation in exchange coupling is not only in magnitude, but may involve alternation between antiferromagnetic and ferromagnetic states. We note the similarity between two distinct magnetic/nonmagnetic layered systems: Co/Ru1 and our Fe/Cu/Fe, both of which exhibit oscillations in the magnitude of the exchange coupling with periods of ∼7 ML, and have phases consistent with antiferromagnetic coupling in the limit of zero-spacer-layer thickness. Finally, our samples exhibit a major enhancement in the Kerr rotation over that expected on the basis of plasma resonant absorption. Close correlation of this enhancement with the strength of the interlayer magnetic coupling suggests a common mechanism for these effects.
Mössbauer effect spectroscopy analysis of FeCx prepared by thermal decomposition of Fe(CO)5 containing exfoliated MoS2
Authors: Schurer P. J. , LaCombe J. L. , Templeton T. L. , Arrott A. S. , Frindt R. F.
Published: July 2002

Mössbauer effect spectroscopy (MES) and x-ray diffraction are used to characterize the various phases present in samples of FeCx prepared by thermal decomposition of a solution of Fe(CO)5 containing exfoliated monolayers of MoS2. MES is used to determine how these phases change their structural and magnetic properties with thermal annealing.
Modifications of weak localization contributions to magnetoresistance due to magnetic impurities in amorphous DyxY1−xNi alloys (invited)
Authors: Barbara B. , Amaral V. S. , Filippi J. , Jansen A. G. M. , Sousa J. B. , Moreira J. M.
Published: July 2002

High-field magnetoresistance experiments performed on thick amorphous alloys offer a simple way to study three-dimensional (3D) weak localization of conduction electrons. After the precursor work of Fert et al. on nonmagnetic amorphous alloys, we found it interesting to study how these effects would disappear under substitutions of magnetic impurities (1% to 10% of Dy) in a nonmagnetic amorphous alloy (YNi). The experiments, performed between 1.5 to 50 K and in magnetic fields up to 20 T, showed (i) in YNi, characteristic features of the magnetoresistance due to weak localization under strong spin-orbit scattering and (ii) in DyxY1−xNi, a coexistence of weak localization effects with the classical contribution of spin alignment by the applied magnetic field, saturating at negative values. This last contribution dominates the behavior of Dy-richer samples whereas weak localization is clearly observed for x≤3%. In all the samples a dramatic increase of the initial magnetoresistance slope Δρ/ρH2 (where ρ=resistivity and H=applied field) is observed when magnetic impurity concentrations increase. We explain this increase of weak localization effects, in the framework of the available weak localization theory, by an enhancement of the Zeeman spin splitting due to interactions between localized (4f ) and delocalized (d, s) electronic states.
The influence of local magnetization of magnetotransport in Cd0.92Mn0.08Te:In (invited) (abstract)
Authors: Terry I. , Becla P. , Penney T.
Published: July 2002

The magnetically tuned insulator-metal transition in magnetic semiconductors is one of the most striking demonstrations of the interaction between localized magnetic moments and carriers.1 On the metallic side of the transition, it is believed that magnetotransport arises from electron-electron interactions in the presence of a giant Zeeman splitting of the conduction band. However, magnetotransport in the insulating phase is not well understood. The discovery of persistent photoconductivity in Cd1−xMnxTe:In2 has made it possible to control carrier concentration by illumination. This enables us to make transport measurements over a wide range of carrier concentrations in one single sample, ultimately leading to the phototuning of the Mott–Anderson transition in this magnetic semiconductor. In the present work, we report an investigation of magnetotransport and permittivity as a function of carrier concentration in Cd0.92Mn0.08Te:In(2×1018 cm−3). In the insulating phase, the resistivity shows an exp[(T0/T)1/2] temperature dependence, characteristic of variable range hopping in the presence of electron-electron interactions.3 Our results imply that magnetotransport is governed by local magnetization characteristic of the relevant length scale for the transport process.
Study of infrared divergence for the magnetic relaxation in amorphous Fe80B20 alloy
Authors: Jin Zhun-Zhi , Zou Xian-Wu
Published: July 2002

Using the two level model and unified theory of low frequency fluctuation, dissipation, and relaxation processes for condensed matter, the magnetic aftereffect spectrum has been calculated for the amorphous Fe80B20 alloy after annealing treatment for 4 h at 540 K. The calculated infrared divergence exponent n=0.14, characteristic relaxation time τ∞ = 2 × 10−17 s and apparent activation energy EA, at the temperature of the relaxation peak, is 1.7 eV. They are in good agreement with experiments (τ∞ = 2.2 × 10−17 s, EA = 1.66 eV, from Arrhenius plot, and EA = 1.87 eV, from the crystallization). The calculations support that for this amorphous alloy, the magnetic aftereffects are caused by the local rearrangement of the alignment of atom pairs Fe-B as a result of jumps of B atoms. Calculated energy EA increases as the temperature rises, indicating that the larger free volumes vanish faster than the smaller ones, as the temperature increases.
Applications of the overcompensated n-channel Kondo problem
Authors: Sacramento P. D. , Schlottmann P.
Published: July 2002

We discuss three applications of the overcompensated n-channel Kondo problem for an impurity of spin S =1/2 in the context of the exact numerical solution (as a function of field, temperature, and number of channels) of the Bethe-ansatz equations. The entropy has an essential singularity as H and T tend to zero, giving rise to critical behavior in the susceptibility and the specific heat. The specific heat in a small constant (nonzero) field shows a double peak structure, arising from the singularity in the entropy. This gives rise to giant γ values of the specific heat. The applications discussed here are (i) the quadrupolar Kondo effect, (ii) the low temperature properties of a two-level system interacting with conduction electrons, and (iii) a S =1/2 magnetic impurity embedded in a SU(2)-invariant Heisenberg chain of arbitrary spin (Takhtajan–Babujian model). The divergent quadrupolar susceptibility in (i) gives rise to a local tetragonal distortion of the lattice below a critical temperature Tc, which depends on the strength of the spin-lattice coupling. The same divergence destabilizes the low-temperature strong-coupling fixed point of (ii) for a symmetric double well.
Giant magnetoresistance effects in UNiGa
Authors: Sechovsky V. , Havela L. , Jirman L. , Ye W. , Takabatake T. , Fujii H. , Brück E. , de Boer F. R. , Nakotte H.
Published: July 2002

Magnetic transitions in UNiGa were found to cause pronounced anomalies in temperature and magnetic-field dependencies of the electrical resistivity. The resistivity values are drastically reduced by the field induced transition from an antiferromagnetic to a ferromagnetic phase. The giant magnetoresistance (as large as Δρ/ρ=−87% at T=1.2 K for i∥c) observed in UNiGa is typical for materials characterized by a strong coupling of conduction electrons with highly correlated f electrons. The observed anisotropy of transport properties is closely connected with the strong uniaxial magnetic anisotropy of this compound.
Room-temperature optical nonlinearities of GaInAs/AlInAs and GaAlInAs/AlInAs multiple quantum wells and integrated-mirror etalons at 1.3 μm
Authors: Hsu C. C. , McGinnis B. P. , Sokoloff J. P. , Khitrova G. , Gibbs H. M. , Peyghambarian N. , Johns S. T. , Krol M. F.
Published: July 2002

The room-temperature nonlinear absorption spectra of a 40 Å GaInAs/AlInAs and a 90 Å GaAlInAs/AlInAs multiple quantum well (MQW) were measured near 1.3 μm using a pump and probe technique. Saturation carrier densities at the heavy-hole exciton peak were determined to be 1.2×1018 and 1.0×1018 cm−3 with carrier recovery times of ≂5 ns and ≂750 ps for the two samples, respectively. These measured saturation carrier densities are close to the reported values for GaAs/AlGaAs MQWs. Fabry–Perot etalons with integrated mirrors grown by molecular beam epitaxy with GaInAs/AlInAs MQWs as spacer layers were also fabricated as optical logic gate devices. We demonstrate a 125-ps recovery time for the etalon switching device at room temperature.
Magnetic interparticle correlations in ferroxdure, magnetic pigments, and micrometer Ni and Fe particles, studied with neutron depolarization
Authors: Rosman R. , Rekveldt M. Th.
Published: July 2002

Neutron-depolarization (ND) measurements on ferroxdure compact and slurry, several magnetic pigments, and compacts of micrometer Ni and Fe particles in different remanent magnetic states are discussed. The pigments involve CrO2, Co-modified γ-Fe2O3, CoZn ferrite, Fe, and mixtures of p-wt % Co-modified γ-Fe2O3 and (100−p)-wt % α-Fe2O3 (1≤p≤100). The depolarization yields information about the local magnetization during the magnetization reversal process and about particle orientational correlations and density variations within the particulate media. From the ND measurements on ferroxdure it follows that at most weak correlations exist in the compact while strong correlations exist in the slurry. The magnetization reversal process in the slurry takes place by a coherent rotation of groups of particles. The correlation length in the virgin pigments ζ0v is in agreement with magnetically uncorrelated single-domain particles. Its value in the state of maximum remanence ζm is more than tenfold ζ0v, being due to the presence of clusters of particles in which the mean particle density and/or orientation differs from the mean particle orientation and/or density within the pigments. The dependence of ζm on p in the Co-modified γ-Fe2O3/α-Fe2O3 mixtures shows that the Co-modified γ-Fe2O3 and the α-Fe2O3 particles are badly dispersed. The magnetic correlation length in the compacts of micrometer Ni and Fe particles is in agreement with magnetically uncorrelated (quasi-) single-domain particles. The maximum value of the remanent reduced magnetization of the compacts is relatively low. This is a consequence of the fact that the demagnetizing fields of a particle magnetized to saturation highly exceed the coercive field of these relatively soft materials. From ND measurements on the pigments after dc or ac demagnetization it follows that most of the switched volumes are elongated along the direction of the applied field. The switched volumes are the magnetic inhomogeneities in the demagnetized remanent state. The switched volumes after dc demagnetization are much larger than those after ac demagnetization. The latter only slightly exceeds the particle volume.
Optical properties of InAs/InP strained single quantum wells grown by organometallic vapor-phase epitaxy
Authors: Schneider R. P. , Wessels B. W.
Published: July 2002

The optical emission characteristics of highly strained InAs/InP single quantum wells prepared using atmospheric-pressure organometallic vapor-phase epitaxy have been studied. For well thicknesses of one to three monolayers (ML), the photoluminescence (PL) spectra exhibited intense emission in the energy range 1.15–1.3 eV, with typical full width at half maximum of 8–14 meV. The dependence of PL emission energy on well thickness for 1–5-ML-thick wells was compared with the results of a finite-well calculation, taking into account the effects of strain on the band structure. Good agreement between experiment and theory was obtained for a valence-band offset of 270 meV, consistent with recent reports for the InAs/InP system.
The dielectric response of boron carbide due to hopping conduction
Authors: Zuppiroli L. , Papandreou N. , Kormann R.
Published: July 2002

We present a quantitative analysis of ac electrical conductivity and dielectric constant measurements that were performed on boron carbide samples. This material is crystalline but disordered on the atomic scale because of the existence of a large stoichiometry range from B9C to B4C. The polaronic character of the hopping process is well recognized in this disordered solid. Moreover, the boron carbides of this study are also ceramic materials with grain sizes ranging from 1 to 100 μm, depending on the sample. After having considered most of the models which account for the dielectric response, we have applied Dyre’s model which assumes a uniform distribution of activation energies between two extreme values and uses an effective-medium-type calculation. It permits an elegant interpolation between dc and ac results and a reasonable estimate of the density of localized states (∼1018/cm3 K). The microstructure has little or no influence on the dielectric response in the explored frequency and temperature ranges (10 kHz to 1 GHz and 4 to 100 K). However, when T≳100 K microstructural effects were visible at higher frequencies in a preliminary experiment performed at 9 GHz.
Superconductivity of Bi2Sr2Can−1CunOy (n=2, 3, 4, and 5) thin films prepared in situ by molecular-beam epitaxy technique
Authors: Nakayama Y. , Tsukada I. , Uchinokura K.
Published: July 2002

Preparation techniques and properties of Bi-Sr-Ca-Cu-O thin films are reported. The optimization of the substrate temperature in ozone atmosphere and the shuttering of molecular beams on an atomic scale enabled us to prepare thin films of Bi2Sr2Can−1CunOy (n=1, 2, 3, 4, and 5). These films showed superconducting transition with the exception of the n=1 phase and resistivities showed strong temperature dependence without heat treatment after deposition. Reflection high-energy electron diffraction showed sharp streaks, which suggest the growth of films with flat surface on an atomic scale.
A study of laser-ion-deposited carbon films on tungsten by x-ray diffraction, field ion microscopy, and electron spectroscopy
Authors: Wagal S. S. , Adhi K. P. , Joag D. S. , Sharma A. K. , Abhyankar Neelima , Kulkarni S. K.
Published: July 2002

Thin carbon films have been deposited on polycrystalline tungsten foil as well as field ion microscope (FIM) tips by laser-ion deposition in a high-vacuum environment with an ion extraction voltage of −2 kV. Structural characterization of these films has been carried out by using low-angle x-ray diffraction (XRD) and FIM. The low-angle XRD reveals the formation of an interfacial α-W2C phase. The FIM image indicates the formation of the α-W2C phase on the tungsten tip. X-ray photoelectron spectroscopy has been utilized to reveal that the bonding character in the film is sp3. Further, x-ray-excited Auger electron spectroscopy has also supported the diamondlike nature of the films. The results are discussed, and a sequence of layers deposited on tungsten is suggested in view of the structural match.
Measurement of a multicomponent granular system using acoustic slow waves
Authors: Stearns Richard G.
Published: July 2002

In an air-filled porous medium, the slow acoustic mode may potentially be a useful probe of the geometrical properties of the rigid frame. Use of the acoustic slow wave for probing a two-component granular system is studied. In a mixture of two granule types of disparate size, experimental results show that the slow wave velocity depends sensitively upon the relative concentrations of the two types of granules. This may be reasonably interpreted as resulting from the varying tortuosity associated with the pores of the granular mixture, with change in relative concentration. From the experiment described, it is concluded that the acoustic slow wave measurement may be used as a sensitive, noninvasive probe of relative concentration in such a multicomponent system.
Thermal-noise-limited performance in single-chip superconducting quantum interference devices
Authors: Fujimaki N. , Gotoh K. , Imamura T. , Hasuo S.
Published: July 2002

The single-chip superconducting quantum interference device (SQUID) we made using niobium Josephson junctions integrates a two-junction SQUID and a superconducting feedback circuit. The switching probability, its derivative to input flux, and the flux spectral density measured by varying the sinusoidal bias frequency and amplitude agree well with thermal noise theory. The best sensitivity and cutoff frequency were 6.2×10−6 Φ0■√Hz and 1.1 kHz for a bias frequency at 10 MHz. The dynamic range was 2.5×105. These data show the SQUID to be suited to practical biomagnetic application.
Phase formation in the interfacial reactions of ultrahigh vacuum deposited titanium thin films on (111)Si
Authors: Wang M. H. , Chen L. J.
Published: July 2002

Phase formation in the interfacial reactions of ultrahigh vacuum deposited Ti thin films on (111)Si has been studied by in situ reflected high energy electron diffraction (RHEED) and transmission electron microscopy (TEM). In situ RHEED and high resolution TEM data showed unambiguously that there is considerable intermixing of Ti and Si atoms during Ti deposition. Ti5Si3 was found to be the first nucleated phase followed by Ti5Si4 and TiSi. Ti5Si3, Ti5Si4, TiSi, and C49-TiSi2 along with a-interlayer were found to form in samples annealed at 475 °C for 30 and 60 min. Ti5Si4 was the first silicide phase to disappear followed by Ti5Si3 then TiSi. In samples annealed at 700 °C for 10 min, C54-TiSi2 started to appear. To understand the thermodynamic origin of the phase formation, metastable free energy diagrams at 450–600 °C have been constructed. Ti5Si3 and C49-TiSi2 were found to have the highest and lowest driving force, respectively. On the other hand, Ti5Si4 has the lowest interface energy. The sequence of formation and disappearance of silicide phases is discussed accordingly.
The deposition of BaFe12O19 by metalorganic chemical vapor deposition
Authors: Donahue Edward J. , Schleich Donald M.
Published: July 2002

We report the first successful use of metalorganic chemical vapor deposition (MOCVD) in the fabrication of ferromagnetic oxide thin films. A substituted acetylacetonate barium complex has been used with ferrocene in a horizontal hot wall MOCVD system to deposit both polycrystalline and c-axis preferred films of BaFe12O19. These films were examined for crystallinity, morphology, composition, and magnetic properties. Their magnetic properties were found to be consistent with films prepared by other techniques.
Performance characteristics of a high-pressure pulsed singlet oxygen generator
Authors: Endo M. , Kodama K. , Handa Y. , Uchiyama T.
Published: July 2002

The characteristics of a high-pressure pulsed singlet oxygen generator is investigated. This generator was designed for the energy source of new pulsed chemical lasers. A ceramic porous pipe is devised in it to attain the high-pressure pulsed operation. New methods were developed to measure a water vapor pressure and a reaction efficiency in the pulsed singlet oxygen generator system. The maximum singlet oxygen pressure obtained was 35 Torr, and the estimated initial excitation efficiency was 70%–80%
Parametric analysis of a surface electrical discharge model
Authors: Kadish A.
Published: July 2002

A transmission line model for electric surface flashovers is analyzed. The model features an empirical scaling of resistance with current: The local resistance per unit length is taken to be inversely proportional to the magnitude of the local current. This scaling results in thresholds for discharge current onset and local abrupt termination of discharge current, as well as other observed properties of transient electrical discharges. The model predicts secondary current propagation: The current may terminate in a portion of the current channel and be restarted there at a later time. This results from the wave dynamics of voltage gradients in the channel. Conditions for current reversal and current termination are derived, and the dynamics of the free boundary between current-carrying and current-free regions of the current channel is studied.
Size effect of the alkali cation on the structure and the molecular mobility of solid polymeric electrolytes
Authors: Di Marco G. , Bartolotta A. , Carini G.
Published: July 2002

Polymeric electrolytes of the poly(ethylene) oxide (PEO) potassium thiocyanate system have been studied by analyzing the temperature dependence of the low-frequency (0.3–30 Hz) mechanical characteristics. It is found that the local and cooperative molecular motions characterizing the host polymer are strongly influenced by the addition of the salt, as a result of strong interactions between the polymeric chains and the alkaline salt molecules, which ensure the formation of homogeneous complexes. In the low-concentration region (molar fraction X≤0.08) a relevant enhancement of the noncrystallinity and a considerable shift of the glass-to-rubber transition toward higher temperatures characterize the blends. Further salt addition stiffens the material, this peculiarity being connected to the gradual formation of a crystalline complex with a fixed stoichiometry. It is believed that the observed inversion in the structural trend arises from an increase of the interchain interactions via the polarizing effects of the salt molecules. At low temperatures the single γ relaxation, observed in pure PEO and in complexes of the same kind with smaller cations, is modified by the presence of the salt; two close peaks appear, whose strength follows behavior closely correlated to that of the primary relaxation. A quantitative interpretation in terms of a parallel relaxation permits the microscopic origin to be discussed and the underlying local motions to be ascribed to the amorphous fraction of the polymer.
Luminescence characteristics of the (GaP)n(GaAs)n/GaAs atomic layer short-period superlattices
Authors: Takanohashi Tsugunori , Ozeki Masashi
Published: July 2002

We investigated luminescence characteristics of the (GaP)n(GaAs)n atomic layer superlattices grown on the [001] GaAs substrate by pulsed jet epitaxy. In the single monolayer superlattice (n=1), only direct transition was observed at the whole range of measurement temperature and photoexcitation intensity. Both direct Γ-type and indirect X-type emissions were observed in the indirect transition multimonolayer superlattices (n=2–4). These transitions were competitive. The direct emission was stronger at room temperature; however, the indirect emission was intense at low temperatures. This could be explained in terms of oscillator strength and thermal distribution of photoexcitation electrons into the Γ and X levels in the conduction band. Under high excitation, the direct emission increased superlinearly with the excitation intensity, but the indirect emission showed a saturation for its longer radiative recombination lifetime. We also observed the band-to-acceptor transition at the lower energy side of the indirect band-to-band transition. This acceptor was expected to be incorporated into the GaAs well layer during growth.