bluuebunny/crossref_metadata_2025 huggingface dataset:
Selective field evaporation in field-ion microscopy for ordered alloys
Authors: Ge Xi-jin , Chen Nan-xian , Zhang Wen-qing , Zhu Feng-wu
Published: July 2002

Semiempirical pair potentials, obtained by applying the Chen-inversion technique to a cohesion equation of Rose et al. [Phys. Rev. B 29, 2963 (1984)], are employed to assess the bonding energies of surface atoms of intermetallic compounds. This provides a new calculational model of selective field evaporation in field-ion microscopy (FIM). Based on this model, a successful interpretation of FIM image contrasts for Fe3Al, PtCo, Pt3Co, Ni4Mo, Ni3Al, and Ni3Fe is given.
A theoretical model for acoustic emission sensing process in contact/near-contact interfaces of magnetic recording system
Authors: Sheng Gang , Liu Bo , Hua Wei , Chen Qisuo
Published: July 2002

This article presents a theoretical model to account for acoustic emission (AE) sensor dynamics in the measurement of head-disk contact/near-contact in magnetic recording system. The AE transducing system is modeled as a linear dynamic system excited by deterministic and random input. The impulse response method and power flow method are applied, respectively, to characterize the system responses. An analytical model is established to describe the dependence of AE output on system transfer functions, filter parameters, excitation forms, contact force, velocity, topographic and mechanical parameters, as well as lubricant thickness.
Effect of disk acceleration on the generation of acoustic emission signal at the head–disk interface
Authors: Ravikiran A. , Liew T. , Low T. S.
Published: July 2002

Generation of acoustic emission (AE) signal from head–disk interaction as a function of acceleration of the disk was studied using lubricated and unlubricated, mechanically textured and laser textured disks, with positive and negative pressure tripad sliders. Root mean square (rms) AE signal voltage (Vrms) generally increases, attains a peak value, and then decreases with increasing spindle speed during the initial period. In this period, changes in Vrms magnitude, speed at peak Vrms position, slope of ascending AE signal with speed (Vrms/speed) were observed with acceleration. Lubricated and unlubricated disks showed different response to disk acceleration. Possible explanations for these are provided. The present work shows that acceleration of the disk has an effect on the generation of AE signal at the head–disk interface.
Photothermal displacement detection and transient imaging of bump growth dynamics in laser zone texturing of Ni–P disk substrates
Authors: Chen Shaochen , Grigoropoulos Costas P. , Park Hee K. , Kerstens Pieter , Tam Andrew C.
Published: July 2002

A novel photothermal displacement method has been applied to probe the pulsed laser-induced feature formation of Ni–P hard disk substrates in the laser zone texturing process. The deflection signals of the reflected probing beam show the variation of the feature shape resulting from different pulse energies of the heating laser beam. A laser flash photography system is also developed to visualize the feature growth dynamics. This system has nanosecond time resolution and about one micron spatial resolution. Both techniques show clearly the transient melting and deformation process and the time scale of such deformation.
Charge localization and magnetic dynamics in ferromagnetic and charge-ordered manganites
Authors: Garcı́a-Muñoz J. L. , Llobet A. , Frontera C. , Ritter C.
Published: July 2002

The local spin dynamics in the paramagnetic phase of ferromagnetic and charge-ordered Mn perovskites has been investigated by muon spin relaxation. Significant differences are observed in the muon relaxation in ferromagnetic (F) and antiferromagnetic (AF) Ln1−xCaxMnO3 samples with x=1/3 and 1/2. A glassy like spin dynamics is found close to TC in the former which is completely suppressed in samples with dominant AF exchange interactions. The observation of a falloff in the muon relaxation rate at TCO in Ln1/2Ca1/2MnO3 is also discussed.
Femtosecond spin dynamics in the time domain
Authors: Zhang G. P. , Hübner W.
Published: July 2002

Recently magnetization dynamics on the time scale of 100 fs has been observed. In this article, we explain this ultrafast spin dynamics by a microscopic many-body theory. Our theory yields a speed limit of the order of 10 fs for the spin dynamics in Ni. The fundamental mechanism is the dephasing of the initial excited states. We investigate the dephasing effects in detail as a function of the number of electronic states and K points. We also calculate the density of states as a crosscheck of our theory, where the typical satellite structure of nickel is present. Moreover, the magnetic moment of the monolayer is found to be 0.88μB, which is also consistent with the experimental results. Finally, based on our Hamiltonian, we briefly discuss the origin of ferromagnetism in our model.
Sheath and presheath potentials for anode, cathode and floating plasma-facing surfaces
Authors: Stephens K. F. , Ordonez C. A.
Published: July 2002

Simple expressions for sheath and presheath potentials are provided for anode, cathode and electrically floating plasma-facing surfaces. The expressions are developed using a fully kinetic theory that includes the effects of secondary or thermionic electron emission from the plasma-facing surface. The emission can be space-charge limited and a criterion is provided for the onset of space-charge saturation.
Lattice distortion and uniaxial magnetic anisotropy in single domain epitaxial (110) films of SrRuO3
Authors: Gan Q. , Rao R. A. , Eom C. B. , Wu L. , Tsui F.
Published: July 2002

Single domain epitaxial (110) films of SrRuO3 exhibit uniaxial magnetic anisotropy instead of the biaxial anisotropy observed in the bulk material. The magnetic easy axis for the film is along the orthorhombic [010] direction below TC, and it rotates toward the [110] perpendicular direction as temperature decreases. The [100] direction, which is also magnetically “easy” in the bulk, becomes “hard” in the film. X-ray diffraction experiments show that this unique transformation of magnetic anisotropy is related to a distortion from the bulk orthorhombic lattice into a triclinic structure in the epitaxial film, such that the lattice along the [010] direction expands while its [100] counterpart contracts. The distortion appears to arise from rotation and tilt of RuO6 octahedra. The finding indicates that the magnetic anisotropy in epitaxial SrRuO3 films is rooted in the crystalline anisotropy influenced by strong spin–orbit interactions.
Magnetic properties of self-organized epitaxial cobalt structures
Authors: Demand M. , Hehn M. , Ounadjela K. , Kim Joo-Von , Vagov A. , Stamps R. L.
Published: July 2002

Structural and magnetic properties of ultrahigh vacuum-grown cobalt particles deposited on self-organized Ru buffer layers are presented and correlated. The Co structures formed this way present fascinating examples of how magnetism is influenced by mesoscopic geometrical constraints. Especially, the variation of the magnetic anisotropies at low temperature suggests effects of large interparticle interactions.
Transport and magnetic properties of Tl2Mn2−xRuxO7 diluted system
Authors: Senis R. , Martı́nez B. , Fontcuberta J. , Obradors X. , Cheikh-Rouhou W. , Strobel P. , Bougerol-Chaillout C. , Pernet M.
Published: July 2002

The magnetic and transport properties of Tl2Mn2−xRuxO7 pyrochlores are analyzed as a function of the substitution rate (0⩽x⩽2). For x⩽0.2, the system exhibits colossal magnetoresistance (CMR) and ferromagnetic ordering. For higher doping rates CMR is completely washed out and the transport properties mimics that of the Tl2Ru2O7 pure compound with a semiconductor-to-metal transition at a temperature that decreases as x increases. The magnetic frustration progressively increases signalling the increase of antiferromagnetic interactions and the geometrical frustration intrinsic to the pyrochlore structure.
Structure and magnetic properties of intermetallic compounds Gd3(Fe,Nb)29
Authors: Huang F. , Liang J. K. , Liu Q. L. , Chen X. L. , Hou G. Y.
Published: July 2002

The crystal structure and magnetic properties of intermetallic Gd3Fe29−xNbx compounds (0.9⩽x⩽1.3) have been investigated by x-ray powder diffraction and magnetic measurement. The lattice parameters of Gd3Fe29−xNbx with the Nd3Fe29−xTix-type structure are a=10.606–10.618 Å, b=8.529–8.552 Å, c=9.703–9.711 Å, and β=96.962–96.944°: Nb atoms show a strong preference to occupy 4g, 4i3, and 4i4 crystallographic positions. The Nb content dependence of the Curie temperature exhibits a maximum value of Tcmax=516 K when x=1.2. The saturation magnetization Ms linearly decreases with increasing Nb content at 1.5 K, from 36.39 μB/f.u. to 35.30 μB/f.u. The compound exhibits a planar anisotropy perpendicular to the [102] direction. The anisotropy field HA also linearly decreases with increasing Nb content at 1.5 K, from 6.9 to 5.0 T. In contrast to other Nd3Fe29−xTix-type iron-rich compounds, Gd3Fe29−xNbx compounds have higher Curie temperature, higher saturation magnetization, and room-temperature stability.
Modeling of magnetic properties and alternate current losses in high-Tc superconductors using a power law voltage–current characteristic
Authors: Mahdi A. E. , Tami A. , Stoll R. L.
Published: July 2002

This article presents numerical diffusion models which describe the electromagnetic properties and alternate current (ac) losses of a high-Tc superconducting tape carrying transport current. The models simulate the tape as a highly nonlinear conductor with equivalent resistivity being a function of J, the local current density, as deduced from a power law E–J characteristic of the material. The power law E–J expression can be adjusted by varying both the power, α, and the value of the critical current density, Jc, to accurately fit the current–voltage characteristic of a given high-Tc superconducting tape, as obtained from direct current measurements. Results of various field quantities, in the interior of the tape, and losses for a range of ac currents and frequencies are presented and discussed in conjunction with the critical state, flux creep, and flux flow theories of high-Tc materials.
X-ray photoemission and photoabsorption of organic electroluminescent materials
Authors: Treusch R. , Himpsel F. J. , Kakar S. , Terminello L. J. , Heske C. , van Buuren T. , Dinh V. V. , Lee H. W. , Pakbaz K. , Fox G. , Jiménez I.
Published: July 2002

Thin films of tris-(8, hydroxyquinoline) aluminum (Alq3) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) were measured using synchrotron radiation-based core and valence level photoemission and core level photoabsorption to elucidate the element-specific electronic structure of organic electroluminescent materials. The energy level alignment of an Alq3/TPD interface is given for both occupied and unoccupied states. A comparison of freshly evaporated films of Alq3 and TPD with films that have been exposed to intense radiation or oxidative conditions sheds light on possible damage mechanisms of the molecular solid.
Bandwidth enhancement of a shear-force-controlled distance regulation in near-field microscopy
Authors: Lippitz M. , Schüttler M. , Giessen H. , Born M. , Rühle W. W.
Published: July 2002

The distance between sample and probe in a scanning near-field optical microscope is regulated via tracing the shear-force on the tip which is glued to a tuning fork piezo. A lock-in technique is used. We demonstrate that the bandwidth of the control loop is increased if not only amplitude or phase, but a favorable combination of both is used as feedback signal. The enhancement of bandwidth is connected with a reduction of signal-to-noise ratio. The optimum combination of both, bandwidth and signal-to-noise ratio, can be adjusted purely electronically to the specific needs of an experiment. A theoretical model is developed that discloses the relation between the mechanical and electrical properties of the combination of tuning fork and fiber tip. The frequency response of the shear-force detection system is calculated with a numerical simulation based on this model. Experimental frequency response curves are well fitted by these simulations. Our results are especially important for low-temperature scanning microscopy, where the bandwidth enhancement is essential for obtaining a reasonable scanning speed.
Near band-edge optical properties of GaAs at interfaces of ZnSe/GaAs/GaAs by phase selection in photoreflectance
Authors: Constantino M. E. , Navarro-Contreras H. , Salazar-Hernández B. , Vidal M. A. , Lastras-Martı́nez A. , López-López M. , Hernández-Calderón I.
Published: July 2002

GaAs at the ZnSe/GaAs and GaAs/GaAs interfaces of ZnSe/GaAs/GaAs heterostructures is studied by phase selective photoreflectance (PR) spectroscopy. Four samples with ZnSe layers of various thickness were examined. We unambiguously determined the origin of two different features observed in the PR spectra by combining in phase and out of phase measurements, with PR measurements employing excitation lasers with different wavelengths. These two features are found to originate at different regions of the heterostructure. One contributing transition is a bulk-like signal, resembling that of bare GaAs, which originates in a region that encompasses the buffer layer/substrate GaAs homointerface. A second contributing signal is attributed to a strained region adjacent to the ZnSe/GaAs heterointerface. Both this second signal and the bulk-like signal show Franz–Keldysh oscillations that allow us to determine the electric field strength at the ZnSe/GaAs and GaAs/GaAs interfaces. It is found that the electric field strength at the heterointerface is larger than that of the homointerface. Reflectance difference measurements further support the existence of two spatially separated GaAs regions, which produce two independent overlapping optical modulated signals in the ZnSe/GaAs/GaAs heterostructures.
Hydrogen in undoped and heavily in situ phosphorus doped silicon films deposited using disilane and phosphine
Authors: Pejnefors J. , Zhang S.-L. , Grahn J. V. , Östling M. , Persson L. , Hult M.
Published: July 2002

The kinetics of hydrogen incorporation in amorphous silicon films were studied. The layers were deposited by low pressure chemical vapor deposition using disilane (Si2H6) and phosphine (PH3). The hydrogen concentration, determined by nuclear resonant reaction analysis, increased with decreasing substrate temperature. In accordance with the reported reduction of hydrogen adsorption in the presence of surface phosphorus, the addition of phosphine to disilane was observed to reduce the hydrogen film concentration. The results are discussed in terms of hydrogen adsorption/ desorption kinetics. The activation energy for hydrogen desorption in an undoped film was 1.8±0.2 eV, in good agreement with previously reported values obtained by surface analysis and desorption studies. When phosphine was added, an increase in activation energy was observed.
Effect of annealing on Y/Mo multilayers
Authors: Gu Xiaofeng , Hufnagel T. C.
Published: July 2002

We have studied the effect of annealing on sputter-deposited Y/Mo multilayer films with a wide range of bilayer periods, looking for evidence of solid-state amorphization. The as-deposited samples are layered, with the individual Y and Mo layers showing little or no crystallographic texture. Upon annealing at 350 °C, the Y and Mo x-ray diffraction peaks become sharper, suggesting slight grain growth, and not the consumption of Y and/or Mo by a solid-state reaction. The layered structure persists, with no change in the intensity of the low-angle superlattice lines that would suggest interdiffusion (which is a prerequisite for solid-state amorphization). Finally, analysis of the stress state of the Y and Mo layers shows essentially no change in unstrained lattice parameter, confirming that no interdiffusion occurs. We conclude that, despite recently published claims to the contrary, solid-state amorphization does not occur in the Y-Mo system. Our results are in accord with the conventional understanding that solid-state amorphization does not occur in systems with positive enthalpies of mixing.
Interaction between surface acoustic waves and resonant tunneling structures in GaAs
Authors: Talyanskii V. I. , Hutchinson A. B. , Batov I. E. , Ritchie D. A. , Linfield E. H.
Published: July 2002

We report the observation of interaction between a 1 GHz surface acoustic wave and vertical electron beams in a specially designed GaAs resonant tunneling structure. The interaction relies on the vertical component of the surface acoustic wave’s electric field to trigger a current through the structure. A theoretical analysis is presented that reveals the importance of both the spatial distribution of the surface acoustic wave potential and the nonlocality of the structure’s conductivity on the operation of the device. Possible applications of this interaction for signal processing and powerful microwave devices are discussed.
Thermodynamic studies on dissociation of metal iodides at high temperatures in metal halide lamps
Authors: Ozaki Tetsuya , Adachi Gin-ya
Published: July 2002

Thermodynamics of dissociation of metal iodides into metal and iodine atoms in metal halide arc discharges was investigated for ScI3/NaI/Hg, DyI3/Hg, CsI/Hg, and TlI/NaI/Hg systems at the temperature range of 1000–3000 K. Densities of the metal vapors calculated in the systems with the common thermodynamic quantities of metal atoms and iodides in vapor phases were compared with those experimentally determined from spectroscopic measurements or model calculations. For CsI and TlI, the experimental and calculated density values were closed to each other. On the other hand, the experimental values were much smaller than the calculated ones for sodium and rare-earth iodides because of diffusion of the iodides by chemical transport and convection in the discharges at high temperatures. Apparent Gibb’s energy changes of the dissociation in the lamps were expressed as first-order equations of temperature containing the terms of the increments of the average enthalpy and entropy changes, which were dependent on the molecular weight of the iodides. The densities of metal vapors derived with the obtained equations were comparable to those experimentally determined for all the systems under investigation. A series of equations were applicable to estimating the vapor density distribution of the other metal iodides, whose partial pressure distribution had never been experimentally estimated.
High-speed spin-polarized intersubband lasers
Authors: Vurgaftman I. , Meyer J. R. , Ram-Mohan L. R.
Published: July 2002

Spin-polarized intersubband lasers based on optically pumped type-II antimonide quantum wells in a magnetic field are proposed. Complete discretization of the electron energy spectrum is predicted to extend the electron lifetime considerably. Continuous-wave room-temperature operation is projected for both interband and intersubband pumping configurations lasing at λ=16–24 μm. Furthermore, the parasitic capacitances associated with electrical injection are eliminated, and the large differential gain and fast intrinsic time scale should give modulation bandwidths in excess of 100 GHz.
Irradiation effects in Cu/W multilayers: Ion beam mixing and structural evolution
Authors: Pacaud J. , Jaouen C. , Gladyszewski G.
Published: July 2002

Irradiation effects in the immiscible Cu/W system are investigated for helium 70 keV and krypton 340 keV irradiations at liquid-nitrogen temperature. Evolution of the [(3.1 nm Cu/4.2 nm W)×12] multilayer was characterized by x-ray diffraction measurements. Limits of solubility were observed in both matrixes for both irradiations. As expected, a strong thermal spike effect is evidenced in copper layers under krypton irradiation where almost no solubility is observed. For helium irradiations very similar limits of solubility are observed in both matrixes. These latter results can be analyzed by a model with two atomic fluxes: one due to ballistic displacement and the other due to chemically driven thermally activated transport. A structural study shows a transition to an amorphous state for heavy ion irradiation in the large cascades above a critical solubility threshold.
Reconstruction of diffusion length distributions in semiconductors by photocurrent measurements with the depletion region modulation
Authors: Feklisova O. V. , Yakimov E. B. , Boström O. , Pichaud B.
Published: July 2002

The photocurrent measurements in combination with a modulation of the depletion region is a method which has the potentiality to determine the diffusion length profile in semiconductors directly. In this article the method is applied to strongly compensated (Au-doped) silicon. The expressions for signal description are derived and the situations in which the reconstruction of the diffusion length profile can be obtained are presented. An example of diffusion length profile reconstruction is given for a structure highly compensated by Au and the profile obtained has been compared with that calculated in the frame of the kick-out mechanism with the help of secondary ion mass spectrometry experiments.
Unified model for breakdown in thin and ultrathin gate oxides (12–5 nm)
Authors: Kamoulakos George , Kelaidis Christine , Papadas Constantin , Vincent Emmanuel , Bruyere Sylvie , Ghibaudo Gerard , Pananakakis Georges , Mortini Patrick , Ghidini Gabriella
Published: July 2002

A unified model explaining the wear-out and the breakdown of thin and ultrathin films of silicon dioxide when subjected to electrical stressing is proposed. The suggested breakdown model is based on the ability of charge trapped inside the dielectric to increase locally the effective dielectric permittivity (ε) of the material by increasing the polarization locally. The impact of this local perturbation to the macroscopic characteristics of the oxide is investigated. Breakdown is correlated with the existence of a fixed amount of trapped charge inside the oxide. Moreover, the dependence of this bulk oxide trapped charge at breakdown with respect to the oxide thickness has been justified by the suggested mechanism and has been found to have excellent agreement with the experimental data.
Interdiffusion in NiMnSb/V/NiMnSb: X-ray and neutron reflectivity investigation of ion beam sputtered trilayer systems
Authors: Schlomka J.-P. , Tolan M. , Press W. , Fitzsimmons M. R. , Siebrecht R. , Schubert D. W. , Simon P.
Published: July 2002

The new trilayer system NiMnSb/V/NiMnSb on MgO(001) was investigated by means of vibrating sample magnetometry (VSM), x-ray, and neutron reflectivity. VSM revealed a coercive field Hc=23 Oe and a hysteresis loop similar to that of an uncoupled ferromagnet. The x-ray and neutron reflectivity data proved that interface roughnesses of 10 Å are present. A detailed analysis yielded significant interdiffusion at the NiMnSb/V interfaces which is one possible explanation for the weak magnetoresistance effect measured in similar NiMnSb-based multilayer systems.
Predicting effective thermal conductivities of unbonded and bonded silica sands
Authors: Park Sang Il , Hartley J. G.
Published: July 2002

A theoretical model to predict the effective thermal conductivities of unbonded and bonded sands is developed by considering the relevant heat transfer mechanisms and their relationship to the model geometry. Comparison between model results and experimental measurements indicates that the assumptions leading to the introduction of two model parameters (φef and δ) were justified. As a result, the model was proved to predict the effective thermal conductivities of unbonded and bonded silica sands within reasonable accuracy and to estimate the effects of important parameters reasonably well.
Molybdenum substitutional doping and its effects on phase transition properties in single crystalline vanadium dioxide thin film
Authors: Wu Z. P. , Miyashita A. , Yamamoto S. , Abe H. , Nashiyama I. , Narumi K. , Naramoto H.
Published: July 2002

Molybdenum (Mo) doped vanadium dioxide thin films were synthesized using a Mo striped vanadium (V) target during pulsed laser ablation process. The film structure was characterized by high resolution x-ray diffraction, x-ray rocking curve and Rutherford backscattering/channeling measurements. The results show that the full width at half magnitude of the x-ray rocking curve is as narrow as 0.0074°, comparable to that of the (0001) sapphire substrate, 0.0042°, in this study. The ratio of the aligned-to-random backscattered yield reaches 5%, implying that the growth is that of the single crystalline epitaxy. The result of angular scans for both V and Mo atomic channelings reveals that Mo atoms successfully take sites of the V sublattice as a substitutional dopant. It has been noted that the degradation of the phase transition properties of the film upon doping is closely related to the conductivity in the semiconductor phase.
Analysis of postirradiation annealing of n-channel power vertical double-diffused metal–oxide–semiconductor transistors
Authors: Ristić Goran S. , Pejović Momčilo M. , Jakšić Aleksandar B.
Published: July 2002

The behavior of densities of the oxide trapped charge and the interface traps in gamma-ray irradiated n-channel power vertical double-diffused metal–oxide–semiconductor transistors during annealing at different temperatures and gate biases has been investigated. The experimental results have revealed the existence of a latent interface trap buildup (LITB) process. By use of numerical modeling, based on the hydrogen–water (H–W) model, the LITB process has been successfully simulated. The interface trap densities have been determined by both the midgap and the charge pumping methods, and the results have shown good qualitative agreement between these two methods. Isochronal annealing and switching bias experiments have also been performed. The experimental results are consistent with the H–W model.
Elastic-moduli and ultrasonic-attenuation anomalies near antiferromagnetic phase transitions in La1−xCaxMnO3
Authors: Zhu Changfei , Zheng Renkui
Published: July 2002

Both the longitudinal and transverse ultrasonic sound velocities and attenuations in the single-phase polycrystalline compound La1−xCaxMnO3 (x=0.63,0.83) have been measured by a conventional pulse-echo-overlap technique at a frequency of 10 MHz, between 50 and 300 K. The variations with temperature of the Young modulus E, shear modulus G, bulk modulus B, Poisson’s ratio σ, and attenuation α have been determined. Anomalies in E, G, B, σ, and α were observed near the temperature of the charge-ordering transition TCO and antiferromagnetic (AFM) transition TN. The results imply a very strong spin–phonon interaction caused by linear magnetostriction near the AFM transition. The detailed form of the anomalies is different for different types of phase transitions.
Rotational loss separation in grain-oriented Fe–Si
Authors: Dupré L. R. , Fiorillo F. , Appino C. , Rietto A. M. , Melkebeek J.
Published: July 2002

Rotational and alternating magnetic losses have been investigated in high permeability grain-oriented laminations as a function of frequency (2.5 Hz⩽f⩽150 Hz) and peak magnetization (0.15 T⩽Ip⩽1.7 T). The results have been analyzed according to the concept of loss separation. The rotational hysteresis Whrot and excess Wexcrot energy loss components exhibit a similar increasing behavior with the magnetization Ip and reach a maximum value around 1.5–1.6 T. Wexcrot shows, in particular, a definite f1/2 dependence. The ratios between rotational and alternating hysteresis and excess losses (Rh=Whrot/Whalt, Rexc=Wexcrot/Wexcalt) follow similar trends with Ip, with their values depending, however, on the testing configuration adopted for the alternating loss measurements (single strip versus X-stacked strips). The 90° domain wall transitions between the in-plane [001] and the out-of-plane [100] and [01̄0] directions are the basic loss producing mechanism under rotational fields, but, on approaching the saturation, the magnetization can eventually rotate only through coherent spin rotations and the classical loss term Wclrot alone survives.
Relaxation to a bistable state in a quantum cell
Authors: Smirnov Anatoly Yu. , Horing Norman J. M. , Mourokh Lev G.
Published: July 2002

We analyze a simple model of a quantum cell composed of two Coulomb-coupled double dot systems with one electron in each double dot. The interaction of electrons with acoustic phonons is considered as the principal mechanism that relaxes the cell to the ground state. Non-Markovian stochastic equations for population differences and dipole moments of the constituent double dots are derived. It is found that in order for the bistable state of the quantum cell to exist, the effective energy of Coulomb repulsion between the double dot systems must exceed the individual tunnel splitting energy, as well as the phonon temperature. The behavior of the cell polarization near the critical temperature is described analytically. We calculate the damping rates determining cell relaxation to its minimum-energy state and discuss limitations on the speed of cell response to external fields caused by the finite relaxation time.
Electronic structure and Fe moment distribution in Nd5Fe17
Authors: Gu Zong-quan , Xu Yong-Nian , Ching W. Y.
Published: July 2002

The electronic structure and the Fe moment distribution in Nd5Fe17 are calculated using the first-principles orthogonalized linear combination of atomic orbitals method. It is shown that there is no obvious correlation of the Fe moment with the nearest neighbor Fe–Fe separations. There is a reasonable correlation with the number of nearest Fe neighbors, NNN except for NNN=12. When the averaged Fe moments within the same group of Fe atoms of same NNN are plotted against the average Fe–Fe distances, the results are in good agreement with similar data derived from neutron diffraction analysis. The calculated total moment per formula unit and the average Fe moment are also in good agreement with experimental values.
Two-dimensional Preisach model for inhomogeneous magnets
Authors: Ktena A. , Fotiadis D. I. , Massalas C. V.
Published: July 2002

Previous work on magnetic hysteresis modeling using the Preisach formalism forms the basis of this work which proposes a two-dimensional Preisach-type model for anisotropic inhomogeneous rare-earth–transition-metal magnets. The model deals with the two phases in a statistical sense and is not constrained by the specific geometry or the number of soft inclusions. The model is identified for a single-phase Sm2Fe14Ga3C2 sample and a two-phase exchange-coupled Sm2Fe14Ga3C2/α-Fe sample. Calculated major and first-order minor loops are compared against existing experimental data for the two samples.
Origin of the universality of long-time thermal relaxations in hysteretic systems
Authors: Freidlin M. I. , Mayergoyz I. D. , Huang M. W.
Published: July 2002

It is demonstrated that asymptotic temporal behavior of thermal noise is responsible for the universality of long-time thermal relaxations in hysteretic systems. By using the Preisach hysteresis model and by assuming stationary Gaussian noise, the ln t intermediate asymptotics for the thermal relaxations is derived along with a simple expression for the viscosity coefficient in terms of temperature and differential susceptibility. Experimental data supporting these analytical results are presented.
Thermal relaxation of clusters of interacting Stoner–Wohlfarth particles
Authors: Zhang Kezhao , Fredkin Donald R.
Published: July 2002

The relaxation of the total magnetization of a cluster of identical Stoner–Wohlfarth particles on a cubic lattice is studied by solving the Langevin Landau–Lifshitz equation which incorporates thermal fluctuations into the full dynamics of the magnetization. The resultant ordinary differential equations are numerically integrated with the stochastic Euler method. In the computation, the particles are located at nodal points of a 12×12×12 lattice (total 1728 nodes). All particles have uniaxial anisotropy and the easy axes are aligned in the same direction. The interaction between particles is dipolar. The relaxation of the magnetization is calculated. Initially, the magnetization in all particles is aligned in the up direction. Due to thermal fluctuations, the average magnetization decays. Simulations show that the speed of decay is faster when the interaction between particles is stronger. Furthermore, the magnetization decays exponentially without interaction. As the interaction between particles increases, the decay is no longer exponential and apparently there are two or more relaxation time scales present.
Annealing dependence of giant magnetoresistance in CuFeNi alloys
Authors: Martins C. S. , Missell F. P.
Published: July 2002

Giant magnetoresistance (GMR) in granular CuFeNi alloys is comparable in magnitude to that observed in CuCo. Here we study magnetization M and GMR (0<H<70 kOe; 4.2 K<T<300 K) in metal-spun and annealed Cu80Fe20−xNix (x=0, 2.5, 5, 10, and 15) as a function of annealing temperature Tan<500 °C, using a superconducting quantum interference device (SQUID) magnetometer. A wide variety of granular structures characterized by different average values of the particle sizes is obtained for different Fe/Ni ratios and annealing conditions. For Cu80Fe10Ni10, neither M nor GMR exhibit static hysteresis for T>50 K. At this temperature, the largest GMR value (19%) was obtained for a sample annealed at 400 °C for 2 h. In Cu80Fe5Ni15, on the other hand, the microstructure and magnetic properties of the alloy are much more sensitive to annealing. The magnetoresistence is strongly dependent upon both the annealing and the measuring temperatures. For Fe-rich Cu80Fe20−xNix, the magnetic properties other alloys show a weak dependence upon annealing temperature. Magnetization curves for both as-cast and annealed alloys indicate many large particles which saturate at low magnetic fields. GMR versus alloy composition is presented for two annealing temperatures.
Magnetic phase transitions in (Fe1−xTx)3Ga4 alloys
Authors: Al-Kanani H. J. , Booth J. G. , Ko K. Y.
Published: July 2002

The magnetic ordering in the series of transition metal alloys (Fe1−xTx)3Ga4 with T=Ni and Ti having values of x in the range 0.05⩽x⩽0.20 has been investigated at high fields and low temperatures. Magnetization measurements revealed that in contrast to other T substitutions, Ni and Ti induce completely different types of effects on the magnetic properties. In the case of Ni substitution a ferrimagnetic-like type of magnetic order is induced at low compositions and low temperatures and Ti produces a ferromagnetic ground state. For Ti alloys with low composition as the temperature is increased a field induced transition starts to appear at about 50 K which persists up to room temperature suggesting a phase transition from a ferromagnetic to an antiferromagnetic state. The phase diagrams of the Ni and Ti system are explainable in terms of the Moriya-Usami and Isoda theories. Preliminary neutron diffraction measurements on the Ni alloys carried out at ILL suggest the existence of short-range order over a certain temperature range. The behavior observed so far on the various substituted systems leads us to believe that there is a clear relation between the magnetic properties and the relative position of the substitution element with respect to Fe in the Periodic Table.
Anomalous itinerant magnetic properties of a Cr+2.2 at. % Al alloy single crystal
Authors: Smit P. , Alberts H. L. , Venter A. M.
Published: July 2002

Neutron diffraction and high-pressure elastic constant measurements are reported on a Cr+2.2 at. % Al crystal. This is a critical Al concentration, placing the crystal at a concentration very close to the triple point on the magnetic phase diagram. Anomalously strong volume-dependent magnetoelastic coupling effects are observed in the crystal at temperatures well into the paramagnetic phase.
Exchange anisotropy in epitaxial NiO(001)-fcc Co bilayers
Authors: Dubourg S. , Negre N. , Warot B. , Snoeck E. , Goiran M. , Ousset J. C. , Bobo J. F.
Published: July 2002

We have sputter deposited NiO–Co bilayers on MgO (001) substrates. NiO grows epitaxially on MgO at 800 °C and subsequently room temperature deposited Co films have a fcc crystal structure in epitaxy with the oxide underlayer. Bias fields higher than 700 Oe at 5 K were reported. Polar Kerr effect measurements evidence saturation fields for perpendicular magnetization up to 15 T at low temperature, proof for a strong NiO–Co exchange interaction. The formation of a domain wall in the Co layer is a possible mechanism for such large saturation fields.
Nonlinear spin wave resonance modes
Authors: Heide C. , Temiryazev A. G. , Tikhomirova M. P. , Zilberman P. E.
Published: July 2002

Nonlinear spin wave resonance modes of magnetic films are studied both theoretically and experimentally. In the theoretical analysis the normal modes with arbitrary amplitudes are studied for a film magnetized perpendicular to its plane. The film is assumed to be uniform apart from a layer close to the surfaces, where the spins may be pinned. The spin wave resonance mode of the lowest frequency (principal mode) is considered in detail for the film with arbitrary surface spin pinning. The mode profile changes with increasing amplitude trying to redistribute itself in a more uniform manner across the film thickness. The nonlinear shift of the resonance field depends critically on the pinning. In experiments, we use yttrium–iron–garnet films with a strong gradient in the anisotropy field across the film thickness. With increasing pump power, the principal mode shows a sublinear shift of the resonance field and superlinear growth in the absorption amplitude. These features are explained in terms of the spatial nonuniformity of the mode profile.
A technique for capturing the transition curvature and analysis of the write head characteristics
Authors: Hamaguchi Takehiko , Ichihara Takayuki , Tomiyama Futoshi , Hara Miki , Akagi Kyo , Takano Hisashi
Published: July 2002

Transition shift as a function of cross-track position and the remanent magnetization profile was obtained using a spin-stand with a reference clock head. This measurement method captures the written patterns from a GMR readback waveform in high resolution, and is useful for precise study of the head/media characteristics. Tracks recorded using a high Ms head or a high Hc disk revealed flat transition patterns across the track and showed good on-track performance. The write field affected the transition curvature, and the off-track margin could be improved by controlling the write current.
Resonant amplification of spin transferred across a GaAs/ZnSe interface
Authors: Malajovich I. , Kikkawa J. M. , Awschalom D. D. , Berry J. J. , Samarth N.
Published: July 2002

Time-resolved Kerr rotation and resonant spin amplification are used to study spin dynamics in ZnSe epilayers and across a GaAs/ZnSe interface. In accord with previous studies in GaAs we find that modest n doping of ZnSe epilayers enhances spin lifetimes over three orders of magnitude at low temperatures. Lifetimes reaching 60 ns are seen at low doping concentrations at temperatures from 5 to 50 K. A recently developed two-color technique of time-resolved Kerr rotation is used to excite electron spins in a GaAs substrate and to measure their arrival in an adjacent ZnSe epilayer. We find that resonant spin amplification may be used to increase the net spin transferred to the ZnSe, and discover that interlayer transport extends spin lifetimes 100-fold.
Observation of single-ion magnon bound states in the metamagnet FeI2
Authors: Katsumata K. , Hagiwara M. , Tokunaga M. , Yamaguchi H.
Published: July 2002

The results of electron spin resonance measurements at high frequencies are reported on the antiferromagnet FeI2 with strong uniaxial anisotropy. We have found new signals whose g value is 16.5 together with the signal with g=7.0 reported before [Fert et al., Solid State Commun. 26, 693 (1978)]. We interpret these electric spin resonance signals as originating from single-ion four magnon and two magnon bound states, respectively. Some of the exchange interaction constants are determined from the analysis of the results.
Phase transition in ultrathin magnetic dots
Authors: Marty F. , Vaterlaus A. , Maier U. , Pescia D.
Published: July 2002

We have measured the magnetization of in-plane magnetized ultrathin magnetic dots of Co on Cu(100) as a function of temperature and applied field by scanning Kerr microscopy. Co dots with lateral sizes down to 4 μm and a thickness of 1.8 atomic layers show a sharp magnetic phase transition which is indistinguishable from the one of extended films. The scaling plot suggests that these small particles belong to the Ising universality class.
First-order antiferromagnetic ordering and metamagnetic transition in (TbPd3)8T (T=Ga and Ge)
Authors: Mitra C. , Dhar S. K. , Ramakrishnan S.
Published: July 2002

( TbPd 3 ) 8 T ( T=Ga and Ge) compounds have the cubic, Pm3m symmetry and are a superstructure derivative of the cubic, AuCu3-type, TbPd3. The ternary compounds order antiferromagnetically and the Néel temperature is different from the binary compound TbPd3, which has an antiferromagnetic ordering at 3 K. Two transitions are observed in (TbPd3)8Ge, one at 4 K and the other at 5.2 K. The compound (TbPd3)8Ga has an antiferromagnetic ordering at 5.2 K. The resistivity exhibits hysteresis in the transition regime in both compounds. The magnetic transitions are first-order like and sharp metamagnetic transition is observed below the magnetic ordering temperature in both (TbPd3)8Ga and (TbPd3)8Ge.
Influence of electron relaxation on electron tunneling
Authors: Zhang Shufeng
Published: July 2002

It is shown that the current–voltage characteristics of a three-terminal tunnel junction depend on the energy relaxation time. If the electrodes are made of transition metals, one may extract the fundamental parameters on the spin dependence of the hot electron relaxation time from magnetoresistance measurement. Furthermore, an enhanced tunnel magnetoresistance is predicted.
Effects of heating rate and surface oxidation on magnetic induction of 3% Si–Fe strip
Authors: Heo N. H. , Chai K. H. , Kim C. Y. , Na J. G.
Published: July 2002

The effects of heating rate and surface oxidation on magnetic induction of thin-gauged 3% Si–Fe strips have been investigated. Magnetic induction increased with increasing final annealing temperature. Before and after 1150 °C, the planar orientation of grains shifted from (100)[011] to (110)[001], resulting in magnetic induction higher than 1.90 T. Magnetic induction after final annealing was higher in case of a faster heating rate. This is because enough time for growth of (110) grains is given in the case. Irrespective of transient annealing temperature, magnetic induction after final annealing was mostly higher than 1.90 T. A maximum, 1.97 T, was obtained at a transient annealing temperature of 800 °C. This is because iron oxide formed at 800 °C inhibited the surface-energy-induced selective growth of (100)[011] grains during heating up to 1200 °C.
Asymmetric giant magnetoimpedance in annealed amorphous (Co0.94Fe0.06)72.5Si12.5B15 wire under the circumferential field
Authors: Song Seon-Ho , Yu Seong-Cho , Kim Cheol Gi , Kim Ho Chul , Lim Woo Young
Published: July 2002

The permeability spectra and asymmetric giant magnetoimpedance (GMI) have been measured in annealed amorphous Co-based wire under the circumferential field produced by the bias current. The permeability from the wall motion of core domains is nearly constant; however, the permeability from rotational magnetization of shell domains increases with bias current. The asymmetric GMI profile is revealed for the bias current over 4.5 mA, due to the influence of unidirectional bias field on the wall motion in the circumferential direction, and maximum field sensitivity is observed for the bias current of 4.5 mA.
The effects of random bond breaking on diffusion limited cluster–cluster aggregation
Authors: Meakin Paul
Published: July 2002

The effects of random bond breaking on cluster–cluster aggregation have been investigated using both 2d and 3d simulations. A combination of random bond breaking with the normal cluster–cluster aggregation process produces structures which are more compact on short length scales and contain many loops. However, the fractal dimensionality of the aggregates is increased only slightly, if at all, by random bond breaking. Fast bond breaking rates have the largest effect on short length scale structure, but the effect on the structure at larger length scales is quite small. Fast bond breaking leads to the formation of many small loops but intermediate and slow bond breaking rates lead to the formation of larger loops.
Towards organic polymers with very small intrinsic band gaps. I. Electronic structure of polyisothianaphthene and derivatives
Authors: Brédas J. L. , Heeger A. J. , Wudl F.
Published: July 2002

We present valence effective Hamiltonian (VEH) band-structure calculations on polyisothianaphthene and some of its simple derivatives. Our goal is to rationalize the lowering of the band gap by about 1 eV which is experimentally observed in polyisothianaphthene (Eg=∼1 eV) relative to the parent polymer, polythiophene (Eg=∼2 eV). We show that the smaller band gap found in polyisothianaphthene can be understood on the basis of a relationship which can be established between the band gap energy and the importance of the effective quinoid contributions to the electronic structure. Our theoretical results also indicate that simple dimethyl, dimethoxy, or dicyano substitutions are not expected to affect the size of the band gap in any significant way.
Optical measurement of the Soret coefficient of ethanol/water solutions
Authors: Kolodner Paul , Williams Hugh , Moe Cheryl
Published: July 2002

We have employed a laser-beam-deflection technique to measure the Soret coefficient of ethanol/water solutions with ethanol weight fractions ranging from 0.02 to 0.39, over the temperature range 10 to 40 °C. This technique also allows precise determination of the mass-diffusion coefficient and of the index of refraction. These results have permitted us to obtain a quantitative understanding of recent experiments on oscillatory traveling-wave convection in binary fluid mixtures.
A thermodynamic theory of the Soret effect in a multicomponent liquid
Authors: Kempers L. J. T. M.
Published: July 2002

A phenomenological theory of the Soret effect in a multicomponent mixture is presented. The theory is applicable to mixtures in which the Soret effect is determined mainly by the energetic interaction between the molecules, such as liquids and dense gases, and it is based on the assumption that the steady state has a maximum number of possible microstates. The equations derived for the separation enable the thermal diffusion factor αT of each component to be calculated using only an equation of state for the mixture and the standard state enthalpy. With a sufficiently accurate equation of state, the calculated αT values correspond well with the data on measured αT values of liquids and dense gases and show a better correspondence than Haase’s theory. The method presented may be applicable to other coupled transport phenomena in the steady state.
Monte Carlo simulation of equilibrium chemical composition of molecular fluid mixtures in the Natoms PT ensemble
Authors: Shaw M. S.
Published: July 2002

A new Monte Carlo simulation method is introduced which gives the equilibrium chemical composition of a molecular fluid directly. The usual NPT ensemble (isothermal–isobaric)is implemented with N being the number of atoms instead of molecules. Changes in chemical composition are treated as correlated spatial moves of atoms.