bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of the electrical resistivity and thermopower of U2Ni2In and Nd2Ni2Sn
Authors: Pinto R. P. , Amado M. M. , Braga M. E. , Sousa J. B. , Chevalier B. , Laffargue D. , Étourneau J.
Published: July 2002

Accurate measurements of the electrical resistivity (ρ,dρ/dT) and of the thermoelectric power (S,dS/dT) were performed in the ternary compounds U2Ni2In and Nd2Ni2Sn from 4 K to 250 K, and the results are compared with those previously obtained in U2Ni2Sn. The pair U2Ni2In/U2Ni2Sn enables us to study the influence of the p-electron character (and other band effects related to the elements In/Sn) on the transport properties, whereas the pair Nd2Ni2Sn/U2Ni2Sn provides direct information on the role played by the 4f/5f electrons. All these compounds order initially in the antiferromagnetic state, exhibiting a characteristic minimum in dρ/dT at TN. In the U2Ni2In compound a drastic reduction is observed in ρ(T) slightly below TN, suggesting the coexistence of the antiferromagnetic state with a coherent Kondo effect when T≤0.8 TN. In Nd2Ni2Sn we observe two distinct phase transitions below TN, of first-order character and likely associated with order-order magnetic transitions. The anomalous behavior of ρ(T) in the paramagnetic phase of the Nd2Ni2Sn compound can be satisfactorily described in terms of crystal field effects associated with the 4f-electron levels.
Theory of the temperature dependence of giant magnetoresistance (invited)
Authors: Hasegawa Hideo
Published: July 2002

The temperature dependence of the giant magnetoresistance (GMR) for currents parallel and perpendicular to the multilayer plane is discussed by taking account of the random exchange potentials, phonon scatterings, and spin fluctuations. The effect of spin fluctuations, which plays an important role at finite temperatures, is included by means of the static functional-integral method developed previously by the present author. Our model calculations well explain the observed features of the parallel and perpendicular GMR of Fe/Cr and Co/Cu multilayers recently reported by Gijs et al. [Phys. Rev. Lett. 70, 3343 (1993); Phys. Rev. B 50, 16 733 (1994)].
Spin polarized tunneling in half-metallic ferromagnets (abstract)
Authors: Tanaka C. T. , Moodera J. S.
Published: July 2002

We present a study of spin polarized tunneling in tunnel junctions in which one of the conducting layers is a half-metallic ferromagnet (HMF). HMF are unique in that the Fermi level of these materials intersect the majority spin electron band, while the minority band has an energy gap near the Fermi level. Hence, HMF simultaneously have both metallic and semiconducting characteristics, and theory predicts that the conduction electrons are 100% spin polarized. As a result, the magnetoresistance in magnetic multilayers or trilayer tunnel junctions is expected to be significantly higher than with conventional ferromagnetic materials. Two important parameters affecting the performance of these junctions are the smoothness of the HMF surface and its surface composition. Tunnel junctions consisting of a layer of NiMnSb, a barrier layer of Al2O3, and a layer of aluminum were prepared and studied for their tunneling properties. Surface analysis of HMF films was done using Auger depth profiling and AFM.
Thermal effects in magnetization, anisotropy, and interface width in Fe/Cu multilayers (abstract)
Authors: Pechan Michael J. , Fullerton Eric E. , Schuller Ivan K.
Published: July 2002

Magnetic properties of Fe/Cu multilayers have been investigated as a function of temperature using dc magnetization and ferromagnetic resonance techniques. The samples were prepared via dc sputtering resulting in equal thickness elemental layers ranging in modulation wavelength from 1 to 30 nm. The interface anisotropy remains nearly constant at 0.32 erg/cm2 through the 4–300 K temperature range, which is in stark contrast to large thermal effects observed in Ni-based systems. Relative increases in magnetization with decreasing temperature range from 3% in the 30 nm sample to 27% in the 1.0 nm sample. This indicates an interface region in which the magnetization is more strongly temperature dependent than the interior of an Fe layer. By plotting magnetization as a function of the inverse layer thickness, we obtain the temperature dependence of the width of this interfacial region.
Occurrence of large perpendicular magnetic anisotropy in bilayered films with nanometer-thick TbFeCo and Al layers
Authors: Ito H. , Song K. , Naoe M.
Published: July 2002

For Tb-Fe-Co(MO)/Al bilayered films, dependence of perpendicular magnetic anisotropy constant Ku⊥ on MO layer thickness tMO of 3–300 nm and Al layer thickness tAl of 5 and 100 nm has been investigated. For either tAl, easy magnetization direction was normal to film plane at tMO above 4 nm. Ku⊥ of films with tAl of 5 nm increased drastically with increasing tMO for tMO<10 nm and then assumed a constant value of 4×106 erg/cm3 which is two times larger than that with tAl of 100 nm. This difference in Ku⊥ between them may be attributed to stress induced anisotropy. Estimation of rotational hysteresis loss Wr and its integral R suggests that the mechanism of magnetization reversal may be attributed to incoherent curling mode.
Freezing of the extended state in the random local field theory of Ising spin glasses with long range interactions
Authors: Vugmeister B. E. , Nowakowski D. , Huber D. L.
Published: July 2002

The onset of spin glass freezing in dilute Ising systems with long range interactions is investigated within the framework of a random local field approach (previously developed for disordered ferromagnets and ferroelectrics) with the use of numerical simulations. The problem reduces to the diagonalization of an N×N random matrix, N being the number of spins in the simulation, whose elements depend on the spin–spin interaction and temperature. We identify the onset of spin glass freezing with the temperature at which this boundary eigenvalue separating localized and extended states is equal to one. Numerical simulations give a reasonable value of the freezing temperature for dilute RKKY spin glasses and reproduce its linear concentration dependence in agreement with the scaling relation.
High sensitivity in magnetoresistance of epitaxial NiFe/Cu/Co (/Cu) (100) superlattices
Authors: Kawawake Y. , Sakakima H. , Irie Y. , Satomi M.
Published: July 2002

Noncoupled-type [NiFe/Cu/Co(/Cu)] single-crystal-like superlattices were prepared on Si/Cu(100) substrates using ultrahigh vacuum evaporation. Si/Cu (5 nm)/[Co (3 nm)/Cu (6 nm)/NiFe (3 nm)/Cu (6 nm)]×10 multilayers showed 6.1% magnetoresistance (MR) change with 2 kA/m of applied field, which is much higher sensitivity than those of poly-crystalline films. Furthermore, placing a thin Ag layer (0.2 nm) at the midpoint of a Cu layer improved the MR sensitivity of the epitaxial film. Sandwich-type Si/Cu (5 nm)/NiFe (10 nm)/Cu (2.4 nm)/Ag (0.2 nm)/Cu (2.4 nm)/Co (10 nm) superlattice showed sharp switching characteristics of the MR curve between lower and 4.3% higher resistivity. These epitaxial films showed biaxial anisotropy in the film plane with easy axes of [011] and [01̄1] directions. The higher MR sensitivity in the epitaxial films may originate in the square-shaped magnetization curves along the easy axes [011] and [01̄1] of the Co layers. Flatness of interface was also effective to improve the MR sensitivity of the films.
Correlation between the Barkhausen noise power and the total power losses in 3% Si–Fe
Authors: Birsan M. , Szpunar J. A. , Krause T. W. , Atherton D. L.
Published: July 2002

We present results on Barkhausen noise power in grain oriented 3% Si–Fe samples of different quality (power losses). Barkhausen noise is measured at constant values of the magnetic field applied in the rolling direction of the sheet. The measurements indicate a strong correlation between the Barkhausen noise power and total power losses. A model is proposed to interpret the results according to which the Barkhausen noise power is related to the density of pinning centers per unit coercive field, an intrinsic parameter of the material. It is shown that this parameter characterizes the ability of the external field to increase the number of moving walls. The model and the experiment demonstrate that the power losses in grain oriented 3% Si–Fe are improved either by dynamic homogenization of magnetization, or by decreasing the domain wall spacing in static conditions.
Temperature and field dependence of domain wall dynamics up to the Curie point of EuO
Authors: Flosdorff A. , Görlitz D. , Kötzler J.
Published: July 2002

Using a broadband vectorial microwave reflectometer, the dynamic susceptibility has been measured on a EuO sphere in the range from 0.3 to 20 GHz. The data taken at zero field between 4.2 K and the Curie temperature Tc=69.5 K were analyzed using a phenomenological model of a damped harmonic oscillator for the domain walls, that has been successfully applied to the early investigations on ferrites. Based on the micromagnetic Landau–Lifshitz (LL) model, the temperature variation of the intrinsic resonance and damping frequencies are explained up to T=0.98 Tc. The resulting LL-kinetic coefficient, L⊥=1.1×1011 s−1, remains independent of temperature and is related to the dipolar induced relaxation rate of the longitudinal magnetization fluctuations. In fields along the easy [111] axis and the symmetric [100] direction this wall process prevails for magnetizations smaller than Ms/4 and Ms/√3, respectively. Above these thresholds, the characteristic frequencies of the dominant process decrease with field, possibly due to rotational dynamics, and for H∥[111] a second dynamics associated with the paraprocess is contributing.
Second harmonic investigation of ac magnetized in-plane anisotropic garnet film
Authors: Lebedev Sergey V.
Published: July 2002

A magneto-optical method is applied to investigate the influence of external magnetizing on second harmonic signal of magnetic reversal of in-plane anisotropic epitaxial garnet film. The dependencies of the width, slope and amplitude of curves recorded in ‘‘amplitude-easy axis magnetic field strength’’ coordinates versus magnitude of ac longitudinal and dc transversal magnetic fields are obtained and analyzed. It is shown that the slope of transfer function reaches its maximum at 0.7–0.8 Oe ac magnetizing with very slow dependence from transversal magnetic field. The ‘‘amplitude vs dc transversal field’’ dependence appears to have two regions with rapid and slow signal changing corresponded to various magnetization mechanisms.
Systematic variation of T
N(Pr) for the two-CuO2-layer cuprate m212 (m=1, 2, 3) systems
Authors: Ku H. C. , Yang C. L. , Chou C. H. , Hsu Y. Y. , You Y. B. , Shieh J. H.
Published: July 2002

The systematic variation of anomalous Pr antiferromagnetic ordering temperature TN(Pr) for the two-CuO2-layer cuprate system MA2PrCu2Oy (M=Cu, Hg, Tl, Pb; A=Ba, Sr) was verified through the observation of a new Hg-1212 HgSr2PrCu2O6+δ tetragonal compound with a TN(Pr) of 6 K and a Pr—O bond length of 2.492 Å between Pr and oxygen in the adjacent CuO2 layers. For the tetragonal/orthorhombic 1212 compounds including the PrBa2Cu3O7−y system, TN(Pr) was found to decrease monotonically with increasing Pr—O distance. The importance of Pr-O-Pr superexchange magnetic coupling is expected for all two-CuO2-layer MmA2PrCu2Oy m212-type (m=1, 2, 3) compounds. A similar Pr anomaly in the 2212-type system was first observed on the new tetragonal (Pb0.5Cu0.5)2(Ba0.5Sr0.5)2PrCu2O8 compound with a high TN(Pr) of 9 K.
Magnetic structure and ordering of Nd ions in Ga substituted NdBa2Cu3O7 (abstract)
Authors: Niraimathi A. M. , Gmelin E. , Allenspach P. , Ritter C.
Published: July 2002

Magnetic ordering of Nd3+ ion changes with oxygen concentration in NdBa2Cu3O7−x. In the oxygen-rich sample NdBa2Cu3O6.9, Nd3+ ion orders antiferromagnetically with 2D-Ising type of ordering. Whereas, in the oxygen reduced compound NdBa2Cu3O6.2 Nd ion orders three dimensionally at 1.73 K. Similar effects has been observed by specific heat measurements performed on NdBa2Cu3−yGayO7 (0<y<0.5). In these samples, a small amount of Ga substitution changes the magnetic ordering of Nd ions from 2D Ising (y=0) to 1D or magnetic clusters (y=0.01, 0.03). With higher Ga substitution, the third magnetic ordering is restored. The magnetic ordering seen in NdBa2Cu3−yGayO7 (y=0.5) is more isotropic in nature compared to NdBa2Cu3O6.2. The critical exponent derived from the specific data show a 3D spherical type of ordering. Magnetic neutron diffraction are performed at low and high temperatures for y=0.5 to deduce the magnetic structure, which are compared with the unsubstituted compounds and to verify the isotropic nature. The second-order CEF parameter A02 decreases for NdBa2Cu3−yGayO7 (y=0.5) which in turn reduces the magnetic anisotropy constant, consistent with magnetic specific heat results.
Wear studies of contact recording interface with a microfabricated head
Authors: Bhushan Bharat , Li Youlin
Published: July 2002

In contact recording devices, a head is designed to remain in continuous contact with the disk surface. Friction/stiction and wear issues in continuous contact are critical to interface reliability. Objective of the present research is to study the wear mechanisms responsible for the wear of a microfabricated a-C:H contact pad sliding against a smooth disk surface. Wear rate of the contact pad was measured using a faceted technique. Disk wear track was measured using an atomic force microscope. After initial run-in, wear rate increased slightly with sliding distance. Wear rate at contact pressure when the pad is in complete contact with the disk is about 0.015 nm/h. Optical microscopy showed small accumulation of wear debris at the leading edge of the pad. The dominant wear mechanism initially is adhesive followed by some contribution by three-body abrasive wear caused by fine debris produced at the interface. Particulate contamination in the environment significantly increased the wear rate. Tests in various gaseous environments showed that wear rates in argon and in nitrogen are lower than that in ambient air, suggesting that tribochemical oxidation of carbon plays an important role in wear of carbon pad.
Orientation and structure dependence of interface magnetocrystalline anisotropy of Co/Cu overlayers and superlattices
Authors: Zhong Lieping , Kim Miyoung , Wang Xindong , Wang Dingsheng , Freeman A. J.
Published: July 2002

The full potential linearized augmented plane wave method and atomic force approach are employed for the theoretical determination of interface magnetocrystalline anisotropy (MCA) for superlattice systems of Co/Cu in (001), (110), and (111) orientations, and overlayer systems of the monolayer Co on Cu (111) substrate adsorbed by different further coverage of Cu. It is found in superlattices that the interface MCA is sensitive to the geometry arising from different orientations. In good agreement with experiment, the interface MCA with Cu overlayers is found to peak at 1 monolayer of Cu-coated Co/Cu(111) and then to decrease with further Cu deposition. In addition to the hybridization of electronic states at the Co/Cu interface, the interaction between the interface layers and the next-to-interface layers in superlattices and structure relaxation in overlayers may have a significant influence on the MCA of the Co layer.
Spin-resolved electron spectroscopies of epitaxial magnetite (001) (abstract)
Authors: Shaw Kimberly A. , Lochner Eric , Lind David M. , DiBari Rebecca C. , Stoyanov Plamen , Singer Brian
Published: July 2002

We will present the first spin-resolving electron spectroscopic studies of a magnetite (Fe3O4)(001) surface. Magnetite is a semimetal with a high density of states in the minority band, but a large band gap in the majority states at the Fermi energy. The polarization of the secondary emission cascade is measured using spin-resolved secondary electron emission spectroscopy (SRSEES), and reflects the semimetallic spin structure of Fe3O4. The polarization plateau of spin-resolved secondary emission (29.8%) matches the average 3D band polarization of stoichiometric Fe3O4 as determined from spin-resolved band structure calculations (34.2%). An enhancement of the polarization of the secondary electrons at lowest energies will also be discussed. Spin-resolved Auger emission spectroscopy (SRAES) of the Fe3O4 films have been measured and show correlation effects in the valence-valence Auger transitions. Suppressed intensity and polarization of M23M45M45 Auger emission relative to M1M45M45 Auger emission is observed, as well as strong resonant emission with shake-up. Conversely, no spin polarization is detected in the spin-resolved oxygen LMM Auger features, although oxygen Auger emission (in which we can distinguish between adsorbed and bonded oxygen) is used to verify surface cleanliness of the samples.

The synthesis of Fe3O4 films grown on magnesium oxide (001) substrates using oxygen plasma-assisted molecular beam epitaxy will be discussed, as will thin-film characterization using SQUID magnetometry and x-ray and electron diffraction. A unique angle-, energy-, and spin-resolved electron spectrometer has been designed and built for the study of magnetic surfaces, and these studies represent its’ first use. That spectrometer is based on a tandem configuration of an energy-dispersive energy analyzer and Mott spin polarimeter.
Autotuning of a servowriter head positioning system with minimum positioning error
Authors: Huang Y. H. , Weerasooriya S. , Low T. S.
Published: July 2002

This paper proposes a servopositioning system of a servowriter with autotuning capability. The position feedback is obtained through an interferometry-based laser positioning system. The retroreflector is mounted directly on the disk drive arm for hard coupling. The dynamics of the actuator are now included in the servopositioning loop. Since drive dynamics vary from drive to drive, a fixed controller may not provide optimal settling performance for a series of drives. In the proposed control method, the actuator dynamics are identified by injecting a pseudorandom binary sequence into a coarsely tuned feedback servoloop. The resulting frequency domain identification results are used to fine tune the compensator to enable faster settling and better following. Using this information in the servodesign, settling times of 9 ms and a following accuracy of 1 μin. seem achievable using existing hardware.
Exchange coupling in rare-earth/transition-metal multilayers for magnetic super-resolution
Authors: Ayres A. M. , Marinero E. E.
Published: July 2002

We study exchange coupling in rare-earth/transition-metal multilayers in which one layer has perpendicular magnetization and a second layer has in-plane magnetization at room temperature. The exchange coupling between these layers is mediated by a thinner intermediate layer, and we investigate the dependence of the exchange on the magnetic properties of this intermediate layer from room temperature through the compensation and Curie temperatures of the layers. The structures of interest consist of a 400 Å TbFeCo layer with perpendicular magnetization, a 400 Å GdFeCo layer with in-plane magnetization, and a GdFe intermediate layer of variable composition and thickness. The results indicate that the structures are well-described by either a two-layer or a three-layer coherent rotation model (depending on intermediate-layer thickness) and that the influence of the intermediate layer is a function of its anisotropy energy, its thickness, and the identity of its dominant atomic moment.
A dynamic study of domain formation mechanism during thermomagnetic recording based on micro-Hall effect measurements
Authors: Hsieh Yung-Chieh , Gadetsky S. N. , Mansuripur M. , Takahashi M.
Published: July 2002

A method for analyzing the dynamics of domain formation during the thermomagnetic recording process has been developed based on the extraordinary Hall effect. A magnetic domain is written at the center of a cross-shaped magneto-optical sample having an area of 5×5 μm2, and the Hall voltage is monitored during the recording process. As far as domain nucleation is concerned, we find that the temperature gradient around the transition region (i.e., the region whose temperature is between the critical temperature for magnetization reversal and the Curie point) is very important. Under the conditions of high power and short pulse-width laser, a domain can form only during the cooling period. However, it is possible for a domain to form during the heating cycle under a low power, long pulse laser beam.
Nonlinear magneto-acoustic waves in ferromagnets
Authors: Adamashvili G. T. , Maradudin A. A.
Published: July 2002

The nonlinear coherent interaction of a high-frequency acoustic wave pulse with a magnetically ordered medium is studied theoretically. We consider both microscopic and phenomenological theories of the formation of nonlinear magneto-acoustic waves in a ferromagnet. A self-consistent determination of the dispersion law for nonlinear magneto-acoustic waves in a ferromagnet is carried out. It is shown that in both of these theories nonlinear excitations arise which can be of the form of a periodic nonlinear wave, a soliton, or a kink. The nature of the change of polarization of transverse linearly polarized magneto-acoustic waves is also investigated.
Effects of slot closure by soft magnetic powder wedge material in axial-field permanent magnet brushless machines
Authors: Gair S. , Eastham J. F. , Canova A.
Published: July 2002

The article reports on a study of the effects of slot closure in axial-field permanent magnet brushless machines by a two-dimensional finite element method (2D FEM) of analysis. The closure of the slots is made by using soft magnetic powder wedge material. Parameter values and machine performance for the open and closed slot configuration are computed. In order to test the 2D FEM model, calculated results are compared with measurements and favorable agreement is shown.
Magnetic properties of fcc-Fe multilayer (invited) (abstract)
Authors: Matsui M. , Doi M. , Kida A. , Yamada Y.
Published: July 2002

Recently, magnetic properties of fcc-Fe ultrathin films have been extensively investigated. The results, however, are complicated and inconsistent with one another. These are antiferromagnetism, ferromagnetism, different Fe magnetic moment for different atomic sites in film or a rearrangement of surface Fe atoms etc. In this article the recent results for fcc-Fe multilayers will be reported and discussed to obtain a unified picture for magnetism of fcc-Fe ultrathin films. Epitaxially grown multilayers of fcc-Fe with Cu or Cu-M (M=Au,Ni) spacer on Cu(001) and Cu(111) substrates were prepared by molecular beam epitaxy method. The Cu-M spacer was adopted to change the mean interatomic distance between Fe atoms at interfaces by changing the composition of M (=Au,Ni), where Au was used to increase the distance and Ni to decrease it. Fe layers on a pure Cu spacer were fcc and the magnetic moment was at most 2.0μB. Fe layers on a Cu-Au spacer, however, were fct with c/a<1 and those on Cu-Ni with c/a≳1, where a is the in-plane lattice constant and c is the out of plane one. Then a mean magnetic moment of Fe atoms (μ) as a function of Fe atomic volume (V) was obtained. The moment μ continuously increases with increasing V by the so-called magnetovolume effect. The maximum μ value is 2.7μB for about ΔV=4% (ΔV is a change of V from pure Cu volume). And μ=0 for ΔV=−2%. The Mössbauer effect and EXAFS were also measured.

The measurements revealed that the samples for the ΔV≤0 region contain two kinds of Fe (mixture state), high spin state with 2.0μB and nonmagnetic low spin state. Thus it is concluded that the high to low spin state transition is the first kind as theoretically suggested by Moruzzi et al. We also performed band calculations for fcc-Fe/Cu multilayers changing V. Next, the reason for the mixture state is discussed. It is found that a well defined smooth Cu substrate surface promotes a rearrangment of surface Fe atoms due to the surface diffusion, preferring the low spin state and the rough surface obstructs the rearrangement and provides the fcc circumstance by surrounding Cu atoms, which results in the high spin state. Recent inconsistent results for fcc-Fe ultra thin films could be explained by the two spin state model, the surface Fe atomic rearrangement, and the magnetovolume effect.
Comparison of experimental and theoretical initial permeability taking into account anistropy dispersion in CoNbZr thin films (abstract)
Authors: Contreras M. C. , Rivas M. , Calleja J. F. , Iglesias I. , Guyot M. , Cagan V. , Krishnan R.
Published: July 2002

For thin films with very well defined uniaxial anisotropy, initial permeability spectra in the hard direction may be well described by Landau–Lifshitz theory that, in the low frequency range, is reduced to μi=4πMs/Hk. But the switching process of the magnetization becomes more complex when it is dominated by local variations of the anisotropy, what is known as magnetization ripple. Then, initial permeability at low frequency may be expressed as μi=4πMs/Heff. Here, effective field Heff is, from ripple theory: Heff=Hk(1+b+c), the terms b and c being additional anisotropy fields due to short and long range magnetization fluctuations. Measurements of Hk, b, and c can be obtained from the transverse bias permeability that has been measured along easy and hard axis by transverse Kerr effect. We obtain values of b+c between 0.04 and 0.30, what imply a reduction of permeability between 4% and 30% due to the dispersion of the anisotropy. Saturation magnetization was measured using a VSM and also with a computer aided hysteresismeter. Then, initial permeability was measured along hard axis using an inductive system previously described.
Rapidly solidified Fe–6.5%Si alloy powders for high frequency use (abstract)
Authors: Duk Choi Seung , Jin Yang Choong
Published: July 2002

Fe–(3∼6.5%) Si alloy powders having a high magnetic induction (Bs) and a low core loss value for high frequency use were obtained by an extractive melt spinning as well as a centrifugal atomization technique. Sintered core rings made by the rapidly solidified Fe–6.5% Si powders exhibited the high frequency electromagnetic properties: saturated induction (B8) of 1.23 T, coercivity (Hc) of 9.5 A/m, relative permeability (μa) of 6321, and core loss (W10/50) of 1.27 W/kg from the rings of 1.1 mm thick. The saturated induction values were found to be almost identical to those of nonoriented Fe–3% Si steel sheet and 6.5% Si sheet prepared by the CVD technique. The high frequency core loss values were measured not to be changed much up to 10 kHz (W1/10k=55 W/kg) in applied ac frequency.
Unusual magnetic behavior in Nd–Fe–B alloy powder compacts
Authors: Wan H. , Parker F. T. , Putris F. F. , Berkowitz A. E.
Published: July 2002

We report an unusual magnetic behavior in powders prepared by spark-eroding (in liquid Ar) alloy electrodes containing approximately equal weights of Fe and Nd2Fe14B in an effort to prepare composite permanent magnets. Magnetization exhibits reproducible thermal hysteresis, peaking in all applied fields near 520 °C when warming, but increasing monotonically when cooling to room temperature from 700 °C and above. Mössbauer spectroscopy was used to show that the behavior is due to the metastability of Fe1−xO produced in the powders by partial oxidation in Ar gas flow. This compound, paramagnetic at room temperature, decomposes only slowly below 570 °C into ferromagnetic Fe and ferrimagnetic Fe3O4. The reverse reaction occurs readily at higher temperatures.
Magnetic exchange coupling mediated by bound states
Authors: Sá de Melo C. A. R.
Published: July 2002

Usually indirect exchange coupling is mediated by unbound, noncorrelated intermediate states (RKKY-like mechanisms) or by unbound, correlated intermediate states (superexchange-like mechanisms). Here we investigate the possibility of indirect magnetic exchange coupling mediated by bound, correlated intermediate states. As a concrete example we study the magnetic coupling between two magnetic impurities embedded in a semiconductor matrix. The importance of long ranged attractive Coulomb interactions between electrons and holes is emphasized. This attraction leads to exciton bound states which act as mediators of the effective exchange interaction between the two impurities. The resulting exchange interaction presents strong temperature dependence and can be analyzed in terms of the symmetry of the internal wavefunction of the exciton bound states. Possible applications of these results may include recent experimental results on ferromagnetic metal-semiconductor multilayers.
Phenomenological description of the magnetization relaxation (abstract)
Authors: Sobolev V. L. , Huang Huei Li
Published: July 2002

The dynamics and relaxation of magnetization M(r,t) is usually described phenomenologically by the Landau–Lifshitz (LL) equation with relaxation term M=−γ[M×F]+R, where γ is the gyromagnetic ratio (γ≳0), F is the effective field, and R describes the relaxation of the magnetization toward its equilibrium value. It is already well known that the relaxation term of either the LL or Gilbert form does not give a correct description of the dependence of spin-wave damping on the wave vector at small wave vectors for the models of an isotropic and easy plane ferromagnet. The correct behavior of the spin wave damping may be obtained by taking into account the spatial dispersion of the relaxation caused by the exchange interaction and correct symmetry structure of the relativistic part of the relaxation term which now has to be represented as a sum of the exchange and relativistic parts by R=Rex+Rτ. A review of the recent results on the relaxation obtained in the frames of the LL equation with the relaxation term suggested by Bar’yakhtar is presented. Comparison of the spin wave damping calculated in the frames of the phenomenological and microscopic approaches has been carried out. In particular it is shown that, in the case of yttrium iron garnet or substituted garnets, the estimation of the value of phenomenological constant of the exchange relaxation has to be carried out by properly taking into account the many sublattice magnetic structure of these magnets. Numerical estimations of the phenomenological relaxation constants for the yttrium iron garnet are given.
Atomic magnetic moments and spin notion
Authors: Oudet Xavier
Published: July 2002

It is shown that there is an energy associated with the spin. Then the electron is considered as a fluid mass and the wave function as a quantification of the mechanical action. This approach enables us to understand why in Dirac’s theory just the total angular momentum is a constant of the motion and allows an explanation of the g factor. Then an interpretation of Dirac’s equation is proposed putting in view the trajectory of the electron.
Study of the proximity effect in Nb/Au, Bi2Sr2CaCu2O8+y/Au, and HgBa2CuO4/Ag using a scanning tunneling microscope
Authors: Zasadzinski Roberta , Vandervoort K. G. , Crabtree G. W.
Published: July 2002

Proximity effect gaps were observed consistently in Nb/Au, Bi2Sr2CaCu2O8+y/Au, and HgBa2CuO4/Ag bilayers using a low temperature scanning tunneling microscope. The Nb/Au conductance curves [dI(V)/dV vs V] were fit by the Arnold model, using the accepted energy gap value of Nb, 1.55 meV. The application of gold and silver overlayers allowed vacuum tunneling and topographic images of BSCCO single crystals and the HgBa2CuO4 polycrystalline sample. Cleavage steps of varying sizes (up to 1500 Å) in the BSCCO/Au sample were observed, and gaplike features were found on such steps. Consistent, reproducible energy gaps were also found in the Hg-compound sample.
Mercury vacancy equilibria in Hg1−xCdxTe
Authors: Neubert M. , Jacobs K. , Krause-Rehberg R. , Abgarjan Th. , Gille P. , Hoerstel W.
Published: July 2002

Experimental data of mercury vacancy equilibria in Hg1−xCdxTe for the compositions x=0.2 and x=0.3 are presented. In addition to common Hall-effect measurements, the composition x=0.2 is also studied by the positron annihilation technique. The results obtained are in good agreement with data published in the early eighties. A systematic discussion of the change of vacancy equilibria as well as conduction type for the composition interval x=0.2 to x=0.4 is given.
Poisson’s ratio of fiber-reinforced composites
Authors: Christiansson Henrik , Helsing Johan
Published: July 2002

Poisson’s ratio flow diagrams, that is, the Poisson’s ratio versus the fiber fraction, are obtained numerically for hexagonal arrays of elastic circular fibers in an elastic matrix. High numerical accuracy is achieved through the use of an interface integral equation method. Questions concerning fixed point theorems and the validity of existing asymptotic relations are investigated and partially resolved. Our findings for the transverse effective Poisson’s ratio, together with earlier results for random systems by other authors, make it possible to formulate a general statement for Poisson’s ratio flow diagrams: For composites with circular fibers and where the phase Poisson’s ratios are equal to 1/3, the system with the lowest stiffness ratio has the highest Poisson’s ratio. For other choices of the elastic moduli for the phases, no simple statement can be made.
Degeneracy of diffusion paths in ternary, two-phase diffusion couples
Authors: Maugis P. , Hopfe W. D. , Morral J. E. , Kirkaldy J. S.
Published: July 2002

Ternary, two-phase infinite diffusion couples exhibit one, two, or three planar interface solutions of the Fick equations, depending on the relative magnitudes of the four diagonal diffusion coefficients, the phase diagrams and the terminal compositions, thus conflicting with a long-standing consensus that solutions are unique. Supersaturated regions on both sides of couples attend the maximal degeneracy of all three different interface velocities and profiles implying instability and a combined problem of mode selection and wavenumber selection. A doublet of unstable solutions marks the transition by terminal composition change to singlet solutions of stable or unstable character. Such considerations are relevant to diffusion layering of multicomponent devices.
Quantum-well strain and thickness characterization by degree of polarization
Authors: Lakshmi B. , Cassidy Daniel T. , Robinson B. J.
Published: July 2002

InGaAsP/InP single quantum wells grown by gas-source molecular-beam epitaxy have been characterized for their strain and thickness using degree of polarization (DOP) of the edge emission photoluminescence at room temperature. The DOP is a measure of the relative strengths of TE- and TM-polarized e–hh and e–lh transitions. The value of the DOP increases with a decrease in thickness of the well and as the strain varies from tension to compression. For example, we observe a variation of DOP from 20% to 79% when the strain varies from 1% tension to 1% compression for a 30 Å layer and from 27% to 62% when the thickness of a lattice-matched well is varied from 100 to 30 Å. A simple theoretical model is used to predict this trend. We show that this technique provides a sensitive measure of the variations in the strain and thickness of quantum wells.
Planar disorder- and native-oxide-defined photopumped AlAs–GaAs superlattice minidisk lasers
Authors: Chen E. I. , Holonyak N. , Ries M. J.
Published: July 2002

Data are presented on the photopumped laser operation of planar AlAs–GaAs superlattice (SL) minidisk lasers. The SL minidisk (70 Å AlAs, 30 Å GaAs; 100 periods; ∼37 μm diameter) is defined by impurity-induced layer disordering (IILD), followed by wet oxidation (N2+H2O vapor, 400 °C which surrounds the minidisk with a low-refractive-index AlGaAs oxide. The planar minidisks exhibit laser operation at λ∼7540 Å, with wider mode separation (Δλ∼13 Å) than disks defined by only IILD (a smaller refractive index step) and cleaved sample edges. The mode separation of Δλ∼13 Å corresponds to disk modes that utilize the perimeter of the oxide-defined disks. In the fabrication of the SL minidisks, IILD forms a structural and doping difference beyond the disk perimeter that acts, in effect, as a p–n junction during etching or wet oxidation. Etch profiles are shown demonstrating this behavior.
Spectral hole burning and uniaxial stress study of radiation-induced defects in diamond
Authors: Osvet A. , Palm V. , Sildos I.
Published: July 2002

A persistent spectral hole-burning technique has been employed to investigate the impact of uniaxial stress on the defects created by neutron irradiation in natural Ia-type diamond. Shifts of the spectral holes in the 649, 681, and 774 nm zero-phonon lines (ZPLs) have been measured. The values of the shifts, 2.2, 1.2, and 0.65 cm−1/kbar, obtained from these measurements have been compared with the stress shifts of the lines of defects with well-known structure. It is suggested that vacancies are present in the defect responsible for the 681 nm ZPL. A combination of the results of polarized luminescence and uniaxial stress experiments leads to the conclusion of rhombic I symmetry of the defects that give rise to the 649 and 681 nm ZPLs.
Analysis of the impact of dislocation distribution on the breakdown voltage of GaAs-based power varactor diodes
Authors: McNally Patrick J. , Herbert P. A. F. , Tuomi T. , Karilahti M. , Higgins J. A.
Published: July 2002

A synchrotron x-ray topography analysis of the impact of the distribution of defects/dislocations on the electrical performance of GaAs power varactor diodes was carried out. Diodes fabricated on or near Liquid Encapsulated Czochralski cellular dislocation networks in the substrate, which are also known to be rich in As precipitates near these cell walls, were observed to have reduced breakdown voltages (VBR). This is consistent with the possibility that the presence of space-charge cylinders surrounding these dislocations gives rise to reduced VBR if they thread a p-n junction; it is also in accord with the possibility that the As precipitates themselves can act as sites for local field enhancement, thus promoting premature avalanche breakdown.
A method of investigating well width fluctuations in diluted magnetic semiconductor superlattices using spin-flip Raman spectroscopy
Authors: Roberts R. G. , Hagston W. E. , Harrison P.
Published: July 2002

The possibility of using spin-flip Raman spectroscopy as a tool for determining accurately the disorder in magnetic superlattice (SL) microstructures is examined theoretically. It is shown that the spin-flip Raman spectra for a weakly coupled SL, disordered in units of monolayers, will consist of a number of peaks equal to the number of possible values of the disorder (m) in a given quantum well associated with a given value of the magnitude of the disorder (M). The various peaks can be associated with a particular value of m from observation of their relative intensities. © 1996 American Institute of Physics.
Significant densification of sol-gel derived amorphous silica films by vacuum ultraviolet irradiation
Authors: Imai Hiroaki , Yasumori Masaru , Hirashima Hiroshi , Awazu Koichi , Onuki Hideo
Published: July 2002

Structural changes in sol-gel derived amorphous silica films were investigated using vacuum ultraviolet light emitted from an undulator. A significant increase in the refractive index and a decrease in the thickness for the silica films were induced by photons with energies above ∼9 eV. The frequency shift of ω4 (transverse optical mode) of infrared absorption to a lower wave number with the irradiation indicates a decrease in the average Si–O–Si bridging angle. These results suggest that the silica films were densified by the irradiation with the energetic photons. The densification of silica gel films dried at 50 °C was accompanied by a decrease of OH groups in a manner similar to thermal treatments. On the other hand, the structure of the irradiated silica was similar to that of silica glass densified under high pressures. Although silica gel films annealed at 600–1000 °C were also densified by the irradiation, the structural changes were relatively small for the films annealed at temperatures above 700 °C. The significant densification induced by the irradiation is presumed to be due to the electronic excitation of the distorted structure as well as residual OH groups in sol-gel derived amorphous silica.
Curvature of multilayer epitaxial films
Authors: Marcus P. M.
Published: July 2002

An evaluation is made of the curvature produced by epitaxial strain on a multilayer structure formed by repetition of a film–substrate pair n times. When the substrate has thickness comparable to the film thickness, the curvature is increased by many orders of magnitude over the usual ultrathin film epitaxial on a much thicker crystalline substrate. Although the curvature is also shown to be reduced by a factor of n2 in the multilayer compared to a single film–substrate pair, a large net increase is possible. Explicit formulas are obtained for the dependence of the curvature on n, on the misfit, on the ratio of film to substrate thickness, and on the ratio of a modified Young’s modulus for the film to that of the substrate. The curvature is calculated for a range of these parameters and asymptotic formulas for large n are given.
Growth and photorefractive properties of Mn-doped (KNa)0.1(Sr0.6Ba0.4)0.9Nb2O6 crystals
Authors: Zhang Y. L. , Xie X. S. , Mo D. , Pun E. Y. B. , Shi L. P.
Published: July 2002

(KNa)0.1(Sr0.6Ba0.4)0.9Nb2O6, or KNSBN, crystals have been grown with two different Mn concentrations: 0.02 wt % and 0.04 wt %. Two-wave mixing and self-pumped phase conjugation experiments were carried out on these crystals at 632.8 nm wavelength. The measured two-wave coupling gains for the 0.02 wt % and 0.04 wt % crystals are larger than 7 cm−1 and 9 cm−1, respectively, and the two-wave coupling time response is faster for the 0.4 wt % crystal. In addition, the self-pumped phase conjugation reflectivity of the 0.04 wt % crystal is measured to be as high as 70%. It is shown that the Mn dopant can increase effectively the absorption coefficient and enhance the photorefractive properties of KNSBN.
Twin screw extrusion of a metal particle dispersion
Authors: Göktürk H. , Maki K.
Published: July 2002

Twin screw extruder is employed as a mechanical dispersion tool in the processing of particulate media incorporating high specific surface area (SSA) pigments. In this study, the effectiveness of a typical screw configuration used in the processing of formulations containing acicular iron particles of SSA=50 m2/g was experimentally investigated. During a steady state mixing operation, the extruder was brought to a dead stop, the screws were pulled out and material samples were collected from nine different screw sections. These samples were analyzed to determine the progress of the mixing process as the dispersion flows from section to section. Using SEM micrographs of the material samples, magnetic particle distributions corresponding to different screw sections were identified. The microstructure was observed to change from a granular appearance of aggregates and voids to a coagulated but relatively more uniform distribution of magnetic particles. The state of the dispersion was also magnetically evaluated by probing the magnetostatic interaction in the dispersion samples. Both hysteresis curves and remanence curves of the samples were utilized for this purpose. Interaction parameters associated with the hysteresis curves, such as coercivity, did not exhibit clear trends from section to section. The remanence curves were found to be more appropriate to analyze the state of the dispersion.

Peak ΔM values calculated from the remanence curves followed the changes that were observed in the microstructure. It was concluded from the overall results that the extrusion process could be further improved by changes in the screw configuration.
A numerical study of the accuracy of single-electron current standards
Authors: Fonseca L. R. C. , Korotkov A. N. , Likharev K. K.
Published: July 2002

We have developed an algorithm for the analysis of single-electron standards of dc current. The algorithm is based on numerical solution of the master equation describing the time evolution of the probabilities of the electric charge states of the system, with iterative refinement of the operational set of states. To illustrate the method we have analyzed several standards of dc current. We have shown that the accuracy of the single-electron pump may be improved dramatically at lower frequencies and temperatures by replacing the traditional triangular drive wave forms with a special step-like drive. We have also shown that the M-junction turnstile does not achieve the accuracy of the 5-junction pump with the same values of capacitances and resistances even at M=8. However, a hybrid M-junction pump/turnstile system which is easier to control than the 5-junction pump, exhibits a comparable accuracy already at M=6.
Photoluminescence and Raman study of porous SiGe
Authors: Schoisswohl M. , Rosenbauer M. , Cantin J. L. , Lebib S. , von Bardeleben H. J. , Fave J. L. , Cernogora J. , Amato G. , Rossi A.
Published: July 2002

The photoluminescence bands in the visible and in the infrared of porous SiGe layers with initial Ge contents of 5% and 20%, prepared by anodical etching, are investigated at different temperatures. At room temperature the photoluminescence spectra are dominated by an intense red photoluminescence band centered at 1.8 eV. A weaker blue photoluminescence band is also observed which becomes more important at low temperatures. Raman measurements indicate only a low degree of confinement in our porous SiGe layers. The occurrence of blue–green luminescence seems to originate from the oxygen inclusions in the crystalline porous structure. Infrared photoluminescence spectroscopy on the porous layer shows that the broad infrared band known from porous Si is missing in the porous SiGe layers, but transitions via defects in the crystalline structure of the porous layers are observed.
Coupling effects observed in the intersubband photocurrent of photovoltaic double-barrier quantum-well infrared detectors
Authors: Ehret S. , Schneider H.
Published: July 2002

The intersubband photocurrent in photovoltaic n-type GaAs/AlAs/Al0.3Ga0.7As double-barrier quantum-well infrared detectors is studied as a function of the applied external voltage and the incident wavelength. The photoresponse shows a significant photovoltaic behavior, resulting from a dopant segregation process during the growth. For an externally applied bias voltage, which compensates the built-in field, we find a multiple sign change of the photocurrent for varying incident wavelengths. A simulation of the wave functions indicates that this multiple sign change can be attributed to the coupling between the second subband in the GaAs quantum well and the subbands which are localized in the Al0.3Ga0.7As region.
Photoacoustic thermal characterization of spark-processed porous silicon
Authors: Cruz-Orea A. , Delgadillo I. , Vargas H. , Gudiño-Martínez A. , Marín E. , Vázquez-López C. , Calderón A. , Alvarado-Gil J. J.
Published: July 2002

A non-separation approach to determine the spark-processed porous silicon thermal parameters is presented. This thermal characterization was performed through application of the photoacoustic technique, in combination with compositional models for spark-processed porous silicon samples. The thermal parameters obtained are in agreement with existing studies about the composition of this material. This approach opens the possibility of performing the thermal characterization of other porous semiconductors and analogous materials.
Interaction between Ni90Ti10 alloy thin film and Si single crystal
Authors: Levit M. , Grimberg I. , Weiss B.-Z. , Eizenberg M.
Published: July 2002

Interfacial reactions in annealed Ni90Ti10 alloy thin films deposited on Si(100) were studied. The investigation was carried out by Auger electron spectroscopy, x-ray diffraction, and transmission electron microscopy combined with energy dispersive x-ray spectroscopy. Well defined phase separation was observed in the reaction zone. The first layer, adjacent to the Si substrate, contains Ni silicide, the second layer is most probably composed of the ternary Ni-Ti-Si compound.