bluuebunny/crossref_metadata_2025 huggingface dataset:
Acceptor and donor centers introduced by oxygen ionosorption at the a-Si:H film surface
Authors: Aoucher M. , Mohammed-Brahim T. , Fortin B.
Published: July 2002

The reversible interaction between molecular oxygen and hydrogenated amorphous silicon films is studied using conductance, photoconductance, and thermally stimulated current measurements versus temperature. The effect of the oxygen ionosorption on the electrical properties of the film has been found to be reversible. Using thermally stimulated desorption treatment, the electronic centers introduced at the film surface by the ionosorption of oxygen are highlighted. They can be described by a donor and acceptor behavior. The numerical analysis proposed corroborates the experimental interpretation. The oxygen may act equally as donor center or as acceptor center or simultaneously as acceptor and donor center localized at the a-Si:H film surface.
Electron dynamics in a free electron laser with a reversed axial guide field
Authors: Bourdier A. , Gardent D.
Published: July 2002

In order to interpret the experiment of Conde and Bekefi, electron trajectories are studied for a configuration with a reversed axial guide field. In the vicinity of ‘‘antiresonance,’’ electrons are shown to have chaotic trajectories when emitted far enough from the axis. Thus, the interaction efficiency is degraded for most particles. This explains the large dip in radiative power observed and predicted by different simulation codes. The very good efficiency observed in the experiment is explained by a compromise between a high amplitude perpendicular velocity and a regular enough axial momentum for the electrons. It is also explained by the fact that when photons are emitted, the axial kinetic energy is not too much affected. A new situation for which a small peak in efficiency should be observed is predicted.
Influence of the depolarization effect on third-harmonic generation in quantum wells
Authors: Załużny M. , Bondarenko V.
Published: July 2002

The influence of the depolarization effect on the third-harmonic generation due to intersubband transitions in the near-triply-resonant quantum well structures is studied theoretically. Calculations are performed in the framework of the perturbative density-matrix formulation with relaxation-time approximation. The results obtained show that the depolarization effect not only shifts the peak in the third-harmonic-generation spectrum but also, due to the coupling between different intersubband transitions, can enhance its maximum value.
Giant magnetoresistance induced by spin-correlation scattering in magnetic thin films and other compounds
Authors: Zou Liang-Jian , Gong X. G. , Zheng Qing-Qi , Pan C. Y.
Published: July 2002

We present the study of the giant magnetoresistance effect in ferromagnetically ordered thin film and bulk based on the Hund’s rule coupling between the mobile d electron and the core spin of Mn ions. It has been shown that the resistivity is proportional to the spin–spin correlation functions, a maximum resistivity appears near the critical point in absence of magnetic field and an applied field drives the resistivity peak to higher temperature and reduces the peak value, which is in agreement with the experiments. The giant magnetoresistance effect in thin film is attributed to the spin-correlation-dependent scattering and the low-dimensional character.
Giant magnetoresistive memory effect in Nd0.7Sr0.3MnOz (abstract)
Authors: Xiong G. C. , Li Q. , Ju H. L. , Greene R. L. , Venkatesan T. , Dominguez M. , Lofland S. E. , Bhagat S. M.
Published: July 2002

A novel giant magnetoresistance memory effect has been observed in epitaxial Nd0.7Sr0.3MnOz thin films which have previously been found to exhibit a linear increase in conductivity on first application of a magnetic field (B). Here we reported that the conductivity of the films depends not only on the applied field but also on the magnetic history. At T well below the temperature Tp where the zero-field resistivity has a peak, the film enters a high conductivity state [(ΔR/RB)≳103] upon application of a magnetic field which persists even when B is reduced to zero. The original ‘‘zero’’ field state is not recovered until the sample is warmed to T∼Tp. Surprisingly, the dc magnetization exhibits only a weak irreversibility while the magnetoconductivity is markedly hysteretic. That is, while the remanent magnetization is small the remanent magnetic resistivity is 10−3 times the initial zero-field-cooled resistivity. A possible explanation based on a two-component model of semiconducting matrix with embedded shunting paths of ferromagnetic material will be presented.
Superparamagnetic behavior and giant magnetoresistance in oxygen deficient R0.67Sr0.33MnOz (R=Nd, Pr) epitaxial films (abstract)
Authors: Ju H. L. , Li Qi , Xiong G. C. , Venkatesan T. , Greene R. L.
Published: July 2002

We have studied the magnetic and magnetotransport properties of oxygen deficient R0.67Sr0.33MnOz (R=Nd, Pr) films and have observed superparamagnetic behavior in these films. The superparamagnetism is indicated by (i) the increasing spread between zero field cooled (ZFC) and field cooled (FC) magnetization curves as temperature decreases, (ii) a sharp drop of the ZFC magnetization at low temperature, (iii) enhanced Curie–Weiss constant above the ferromagnetic temperature, and (iv) at low temperature a sharp drop of coercivity (Hc) with increasing temperature. These findings strongly suggest that all oxygen deficient manganites are magnetically inhomogeneous; i.e., ferromagnetic clusters exist. We estimate the spin cluster diameter to be 7–10 nm at low temperature (T<30 K). We discuss a possible origin of the giant magnetoresistance effect based on the existence of spin clusters.
Long range order in 2-D arrays of nanometer-sized Fe islands on CaF2/Si(111) (invited) (abstract)
Authors: Scheinfein M. R. , Schmidt K. E. , Heim K. R. , Hembree G. G.
Published: July 2002

The effective magnetic moment was measured as a function of Fe island size during the initial stages of Fe growth on CaF2/Si(111) in an ultrahigh vacuum scanning electron microscope equipped with an in situ SMOKE analysis chamber. This substrate was selected for its wide bandgap, chemical inertness, viability of integration into an Si based technology and the availability of nanopatterning using electron beams for producing devices. Fe grown at room temperature initially nucleates into a monodisperse distribution of 3-D islands at very high nucleation densities (8×1012/cm2). Increased Fe coverages lead to 2-D island growth. A room temperature superparamagnetic to ferromagnetic phase transition occurs as a function of Fe island radius. Mean field and Monte Carlo calculations illustrate that three distinct magnetic phases exist as a function of island diameter. Ferromagnetic order is present at room temperature when r≳3 nm, superparamagnetism is favored when 2 nm < r <3 nm, and a frustrated random antiferromagnetic phase exists when r<2 nm. Further depositions of Ag on superparamagnetic Fe island arrays produce Ag islands which couple the covered Fe island moments in-plane, implying that the Ag mediates the magnetic exchange between individual Fe islands within an Ag island. Implications for 2-D giant magnetoresistance devices will be discussed.
Multilevel master equation for bicrystal clusters
Authors: Klik I. , Yao Y. D. , Chang C. R.
Published: July 2002

A bicrystal film is modeled as an ensemble of clusters of four particles which interact via short-range exchange coupling. Their easy axes are oriented pairwise along two randomly chosen orthogonal directions and the relaxational dynamics of the resultant clusters are described in terms of a multilevel master equation. The randomly deposited bicrystal film is compared with a randomly deposited film of uniaxial clusters. The relaxational properties of the entire array are determined by the easy axes arrangement and by the nature of coupling within the cluster. In particular, for ferromagnetic coupling the occurrence probability of fully magnetized bicrystal cluster states is high and we propose that in conjunction with the highly isotropic coercivity this property leads to a sharp transition between film regions magnetized in opposite directions.
Rotatable anisotropy in radio frequency diode sputtered iron thin films
Authors: Win W. , Yun E. J. , Walser R. M.
Published: July 2002

Iron films deposited by rf diode sputtering exhibited a robust, rotatable hysteresis loops with nearly perfect squareness; i.e., ∼100% of Ms rotates to any in-plane direction x, following the application of Hx≳Hc, and exhibits a stable uniaxial hysteresis for further variations in Hx. The rotatable anisotropy (RA) observed is highly reproducible on various substrates (glass, silicon, and alumina), and is unaffected by variations in; substrate temperature (to 300 °C), deposition rate (15–300 Å/min), in-plane orienting magnetic fields (to 200 Oe), and annealing at temperatures to 370 °C in a large magnetic field (to 1.3 kOe). RA was absent only in films deposited in higher Ar pressures.
Dynamic hysteresis of two-dimensional magnetic islands with uniaxial anisotropy
Authors: Luse C. N. , Zangwill A.
Published: July 2002

Frequency-dependent hysteresis for two-dimensional magnetic islands with uniaxial anisotropy is studied with a nearest-neighbor spin-one Ising model in the Bethe–Peierls approximation. A characteristic frequency for magnetization reversal is identified that depends on the island morphology only through the ratio θ/p of the coverage to the mean island perimeter roughness as measured by the density of unsaturated near-neighbor bonds. As a result, dissimilar hysteresis is predicted for large versus small and compact versus fractal islands.
Thickness dependence of magneto-optical properties in face-centered-cubic Co/Cu(001) ultrathin films
Authors: Nakajima K. , Miyazaki T.
Published: July 2002

Magneto-optical Kerr spectra of ultrathin Co films grown on Cu(001) surfaces have been measured in situ. The growth mode and the crystal structure have been investigating by reflection high-energy electron diffraction observation. A 20-Å-thick fcc Co grown on Cu(001) had a lateral lattice constant of 3.59±0.01 Å, which was about 1.4% expanded compared with that of the bulk fcc Co. There was a remarkable difference above 4 eV in ωσyz spectra between 20- and 1000-Å-thick films. ωσyz spectra for 20-Å-thick Co film showed a resonance-type structure at around 5 eV. It is considered that the structure is caused by the lower energy shift of the 1→6 interband transitions due to the narrowing of the 3d bands.
Local density approximation of magnetocrystalline anisotropy: An approach for narrowband materials
Authors: Erickson R. P.
Published: July 2002

We exploit the purely magnetocrystalline response associated with the Goldstone excitation of a spontaneously magnetized system in order to describe, within a local density approach, the ground state interaction between a local magnetic moment and its surroundings, whose origin is in strong short-range (on-site) electron interactions and the hybridization of localized states. The idea is to consider an applicable form for the ground state energy, and therefore the magnetic anisotropy energy, beyond the local spin density approximation, which might be inferred from existing models of narrowband materials.
Specific heat and magnetization of Co2−xSc1+xSn
Authors: Chen Z. W. , Lin C. L. , Mihalisin T. , Wang J. T. , Wang X. Q.
Published: July 2002

Co2ScSn crystallizes with the cubic L21 Heusler structure and is an itinerant ferromagnet with Tc=238 K. X-ray diffraction measurements show that Co2−xSc1+xSn retains the L21 crystal structure with an essentially unchanged lattice constant for 0≤x≤0.15. However, Tc determined from the magnetization measurements decreases monotonically with increasing Sc concentration from 238 K for x=0 to 40 K for x=0.11, and then remains at 40 K from x=0.11 to 0.14. The electronic specific heat coefficient γ is enhanced from 13 mJ/mole-K2 for Co2ScSn (x=0) to 30 mJ/mole-K2 for x=0.13 (Co1.87Sc1.13Sn). In addition, the C/T versus T2 plot shows a leveling-off behavior at low temperatures for the x=0.13 sample. The γ enhancement and level-off behavior observed when a system approaches a magnetic instability are discussed in terms of the self-consistent renormalization theory of spin fluctuations for weak itinerant ferromagnets and nearly magnetic systems.
Microstructure of flash and conventionally annealed melt spun NdFeB (abstract)
Authors: Altoé M. V. P. , Chandramouli M. , Thomas G.
Published: July 2002

Spring magnets consist of a mixture of exchanged coupled, hard and soft magnetic phases and exhibit improved energy products and remanence. In this study, magnets containing a mixture of Nd2Fe14B and Fe3B have been fabricated using melt spinning under either Ar or He atmospheres. In addition, samples have been heat treated using conventional (furnace) annealing and flash annealing techniques. Magnetic measurements using VSM confirm good spring magnet behavior for all the conventionally annealed samples and for samples flash annealed for t<30 s and T<750 °C. X-ray diffraction reveals that α–Fe, Fe3B, and Nd2Fe14B are present in different amounts depending on the annealing conditions. Transmission electron microscopy shows differences in grain size, crystallinity, and phase distribution. Lorentz electron microscopy reveals the influence of the microstructure on the magnetic domain structure. These results are correlated with the observed magnetic properties.
The archival stability of metal evaporated tape for consumer digital VCRs
Authors: Onodera Seiichi , Takeda Tsutomu , Kawana Takahiro
Published: July 2002

A new metal evaporated (ME) tape was released for consumer digital VCRs. It has greatly improved its recording characteristics and durability compared with conventional Hi-8ME tape. Demagnetization of the new ME tape has been studied using accelerated and ambient conditions. Demagnetization of saturation magnetization follows the cubic law, and it was found that the estimated archival lifetime is about 60 years. Moreover, we have investigated characteristics such as dropout, carrier-to-noise ratio, and error rate, and we confirmed that the new ME tape has enough stability for practical applications.
Plastic substrate preembossed 100 kbpi, 3.4 μm track width magnetic disk (abstract)
Authors: Ishizaki O. , Ohnuki T. , Adachi K. , Akagi K. , Momiji H. , Komoda O. , Ohta N.
Published: July 2002

To achieve higher areal recording density and at the same time very low cost magnetic media, a preembossed disk is proposed in which servo marks, header signals, and ROM data are formed on a plastic substrate by an injection method. Those media give very attractive features to reduce track pitch comparable to optical disks. A very expensive and low throughput servo writer is not necessary by using such preembossed servo marks. To realize the preembossed disk, a magnetic layer should be prepared by low-temperature deposition. In this paper, we studied the sputtering process and suitable magnetic materials such as CoCrPt/Cr film on plastic substrate. Plastic substrate was made by amorphous polyolefin (APO). The under layer (Cr), magnetic layer (CoCrPt), protective layer (C) are formed by magnetron sputtering without substrate heating. Effects of Pt content, thicknesses of magnetic layer, and under layer were investigated. Results showed that coercivity was achieved to 1900 Oe at Pt about 20 at. %, 30-nm-thick magnetic layer and 200-nm-thick under layer. Recording characteristics of the disk were measured by an MR head. The head has a track width of 3.4 μm and a shield distance of 0.35 μm. The results were D50=100 kfci, C/N=47 dB (at 75kfci), and a resulting density of 0.7 Gb/in2. The signal output from the prepit shows 80% maximum value compared with conventional magnetic bits.
Analysis of extremely low frequency brain magnetic fields associated with short-term memory and recognition processes (abstract)
Authors: Yoshida Hideki , Ueno Shoogo , Cheyne Douglas , Weinberg Harold
Published: July 2002

One of the advantages of neural magnetic measurement using a dc-SQUID machine is to measure magnetic fields associated with the direct current or the extremely low frequencial components of neural electric activities. In this study, the order of four tones is memorized by the subject and compared with that of another four tones. The extremely slow magnetic fields were observed by using this paradigm. Two kinds of tones (two octaves apart: the 1046.5 Hz tone and the 261.6 Hz tone) were used. A male subject memorized the order of four tones chosen randomly. After 1.4 s rest, the other four tones were presented. The subject discriminated the order of the first tones from that of the last tones in the forward order. When another long (1 s) tone was presented 2.4 s later, the subject was to push either a true or a false button. Pushing the true button meant the order of the first four tones was the same as that of the last in the forward order. Pushing the false button meant the order was not the same. The whole-cortex 64 channels of MEG data were digitized with a sampling frequency of 125 Hz, and filtered (zero phase shift, low pass digital filter with a cutoff at 20 Hz). In comparison with the control wave, the wave during memorizing clearly includes very low frequency components. We observed that the intensities of the power spectral components at 0.5 Hz during memorizing and recognition is higher than that of the control power spectral components, and tried to make the topographies using the differences between the power spectral components at 0.5 Hz during memory and recognition and the control power spectral components. Because the memory processes affect the very low frequency components of MEG, the current dipole source models are made from the data filtered below 0.5 Hz. We observed that the memory processes affected the very low frequency components of MEG. We estimated the current dipole sources using MEG data below 0.5 Hz.
Measurement of texture in magnetic recording media using a biaxial vibrating sample magnetometer
Authors: Schmidlin F. , Bissell P. R. , Gotaas J. A.
Published: July 2002

A method is described for the determination of the three dimensional easy axis distribution in recording media using a biaxial vibrating sample magnetometer and compensation for out-of-plane demagnetization field effects. Comparison of Lorentzian function fits to the data and measurement of orientation ratio with theoretical calculations indicates that the method gives a reasonable measure of the easy axis distribution. Examination of the theoretical relationship indicates that low values of orientation ratio are insensitive to distribution width indicating that it is necessary to make a full measure of easy axis distribution in-plane. However, out-of-plane distributions are generally narrower and it is often possible to estimate distribution width from a simple orientation ratio measurement.
Magnetic properties and magnetization reversal in Co/Au multilayers (abstract)
Authors: Shan Z. S. , Shen J. X. , Kirby R. D. , Sellmyer D. J.
Published: July 2002

We report studies of magnetic properties in CoX Å/Au60 Å multilayers (X=5,6,7,8,12,20 Å), which were prepared by sputtering on Si(111) substrates, with emphasis on the magnetization reversal. Magnetization reversal was investigated by measurements of initial magnetization curves, minor loops, coercivity as a function of the maximum field of minor loops, temperature dependence of magnetic properties using Kronmüller’s model, time decay of Kerr rotation angle ΘK, and the field-sweep speed dependence of coercivity Hc(dHa/dt) at room and/or low temperature. It is found that (1) the thermal activation volumes determined by Hc(dHa/dt) increase from ∼2.0×10−17 to ∼9.9×10−17 cm3 as X varies from 5 to 20 Å, which corresponds to a cylindrical activation volume with ∼800 Å diameter. (2) A Kronmüller analysis together with the initial magnetization curves, etc., for a Co5 Å/Au60 Å sample at room and low temperature indicates that wall pinning with small pinning sites is the major coercivity mechanism. The interaction between grains was studied with the so-called ΔM method: samples with thin Co layers (X=5,6,7 Å), which show perpendicular anistropy, exhibit negative ΔM or dipolar interactions, while samples with a thick Co layer (e.g., X=20 Å), which show in-plane anisotropy, exhibit positive ΔM or ferromagnetic exchange interactions.
Spin-resolved density of states of 3d magnets
Authors: See A. K. , Klebanoff L. E.
Published: July 2002

Spin-resolved x-ray photoelectron spectroscopy and high-resolution x-ray photoelectron spectroscopy studies of the valence electronic structure of ferromagnetic Fe, Co, Co66Fe4Ni1B14Si15, and Ni are reported, and compared with theoretical densities of states. Agreement between theory and experiment is very good for ferromagnetic Fe and Co66Fe4Ni1B14Si15. For metallic Co, experiment agrees poorly with theory incorporating a 1.5 eV exchange splitting, suggesting a reduced (1.2 eV) exchange splitting attributable to valence electron correlation in Co metal. Ferromagnetic Ni shows poor agreement between experiment and theory, particularly in the ↑-spin channel.
Critical dynamics at the spin-density-wave transition of chromium
Authors: Sternlieb B. J. , Hill J. P. , Inami T. , Shirane G. , Lee W.-T. , Werner S. A. , Fawcett Eric
Published: July 2002

Neutron scattering measurements of the critical fluctuations associated with the spin-density-wave transition at the Néel temperature, TN≊312 K, in a single-Q chromium crystal are reported. Critical fluctuations are observed emanating from allowed magnetic satellite positions and from satellite positions, corresponding to absent magnetic domains, at which no elastic scattering occurs. The inelastic scattering from these ‘‘silent satellites’’ grows rapidly with increasing temperature, becoming equal to the allowed single-Q satellite scattering at TN. The line shapes of the inelastic scattering at both the allowed and silent satellites are found to be intrinsically asymmetric, reflecting the underlying nesting geometry of the Fermi surface. Energy scans at the silent satellite positions reveal a weakly temperature dependent, characteristic energy below which the observed scattering intensity falls rapidly to zero.
Dynamics of quantum spin systems in dimer and valence-bond-solid ground states stabilized by competing interactions
Authors: Yu Yongmin , Müller Gerhard
Published: July 2002

For special coupling ratios, the one-dimensional (1D) s=1/2 Heisenberg model with antiferromagnetic nearest and next-nearest neighbor interactions has a pure dimer ground state, and the 1D s=1 Heisenberg model with antiferromagnetic bilinear and biquadratic interactions has an exact valence-bond-solid ground state. The recursion method is used to calculate the T=0 spin dynamic structure factor for both models and, for the s=1/2 model, also the dimer dynamic structure factor. New results for line shapes and dynamically relevant dispersions are obtained.
Spin correlation functions in random-exchange s=1/2 XXZ chains
Authors: Röder Heinrich , Stolze Joachim , Silver Richard N. , Müller Gerhard
Published: July 2002

The decay of (disorder-averaged) static spin correlation functions at T=0 for the one-dimensional spin-1/2 XXZ antiferromagnet with uniform longitudinal coupling JΔ and random transverse coupling Jλi is investigated by numerical calculations for ensembles of finite chains. At Δ=0 (XX model) the calculation is based on the Jordan-Wigner mapping to free lattice fermions for chains with up to N=100 sites. At Δ≠0 Lanczos diagonalizations are carried out for chains with up to N=22 sites. The longitudinal correlation function 〈Sz0Szr〉 is found to exhibit a power-law decay with an exponent that varies with Δ and, for nonzero Δ, also with the width of the λi-distribution. The results for the transverse correlation function 〈Sx0Sxr〉 show a crossover from power-law decay to exponential decay as the exchange disorder is turned on.
Magnetovolume effects of MnAs1−xSbx
Authors: Nashima O. , Suzuki T. , Ido H. , Kamishima K. , Goto T.
Published: July 2002

Thermal expansion, high-field magnetization, and susceptibility measurements have been made for MnAs0.7Sb0.3. The thermal expansion measurements show that the spontaneous magnetostriction is large and anisotropic below Tc. The spontaneous magnetostriction at 0 K is estimated to be 1.2×10−2 for a axis, −0.8×10−2 for c axis, and 1.6×10−2 for the volume. The spontaneous magnetization decreases sharply in the narrow temperature region below Tc=230 K. All the spontaneous magnetostrictions are found to be proportional to the square of spontaneous magnetization. The magnetization curves measured in a high field up to 40 T show the metamagnetic-like transitions at temperatures above Tc, and they are almost perfectly reproduced by an equation H=A1σ+A2σ3+A3σ5, where H is the applied field and σ is the relative magnetization.
Perfect conductivity approximation: Modification for polyphase systems
Authors: Namjoshi K. V. , Lavers J. D. , Biringer P. P.
Published: July 2002

In this paper, we use an analytical model to investigate voltage drop across the conductors of a three-phase parallel wire planar transmission line carrying balanced currents. The expressions derived for the voltage drops are identical to those given by the perfect conductor model except for a term arising due to the propagation of fields inside the conductors. The voltage drops thus calculated yield different values for the two end conductors even when they are symmetrically placed with respect to the central conductor. The calculations show that the imaginary part of the correction term is of more significance than the real part. The results obtained show agreement with the measured values.
Solution of induction heating problems involving media with hysteresis
Authors: Adly A. A.
Published: July 2002

Induction heating is widely used in various industrial processes. In this paper, the Crank–Nicolson finite-difference approach is employed in the analysis of induction heating problems involving relatively long rods that exhibit hysteresis. The analysis is, thus, reduced to solving a 2D space-time electromagnetic field problem and exact B-H characteristics of rods are taken into account by utilizing Preisach-type models of hysteresis. A detailed numerical example is given in the paper and some sample simulation results covering different excitation frequencies and amplitudes are reported.
An efficient three-dimensional demagnetizing field calculation scheme (abstract)
Authors: Kazmi A. S. , Giles R. C.
Published: July 2002

We present an efficient three-dimensional demagnetizing field calculation scheme for micromagnetic simulations of the domain walls in garnet-like materials. We take advantage of the fact that the domain wall is nearly planar and distribute computational power nonuniformly between the bulk of the material and the wall regions. We use fast Fourier convolutions along one dimension (approximately parallel to the domain wall) and a two-dimensional variant of Greengard’s adaptive fast multipole method in the other dimensions. Even a 2D nonadaptive multipole method has the time complexity of the order of N, for N points in contrast to full Fourier convolution (Nlog N) or direct methods (N2). The adaptive method is based on maintaining a point-to-point error bound which allows us to work at high resolution in the region of the domain wall and coarser resolution in the bulk material. In this case, the calculation scales proportional to the domain wall size rather than the sample volume. The error bound calculation is important for our method and it depends on the Gegenbauer’s addition theorem for the modified Bessel functions. Due to the complex nature of this addition theorem, we have relied on numerical methods, comparing our method to the direct calculations for small size samples. We have implemented this scheme on an SGI workstation and we are reporting the outcome of the 2D adaptive fast multipole method implementation. We have studied the relationship between error bounds, domain wall size, and actual time. We have also compared our results with that of M. Redjdal’s method.
Low temperature magnetic properties of a molecular compound (abstract)
Authors: Tholence J. L. , Novak M. , Luneau D. , Rey P. , Belorisky E.
Published: July 2002

In molecular compounds, the magnetic properties depend on the coupling between the magnetic moments of transition metal ions, rare earth ions, and/or free radicals. This coupling is often antiferromagnetic, but can give a high spin moment in some particular cases. When organic molecules form crystals, again the coupling between molecules can be antiferromagnetic or ferromagnetic like. In a few cases the ferromagnetism, only due to nitroxide radicals, has been observed at low temperature. We present here a more general case of a manganese(II) derivative of an ‘‘imidazole substituted nitronyl nitroxide’’ with complex behavior. The high temperature properties can be explained by ferrimagnetic coupling between the Mn and nitroxide spins. At low temperature a weak intermolecular coupling appears below 10 K, as well as a weak ferromagnetic component below 4.5 K. These low temperature properties might be due to the canting of antiferromagnetically coupled spins.
Domain structure and anisotropy of exchange coupled Co/Cr/Fe multilayers (abstract)
Authors: Kleinefeld Th. , Figge M. , Theis-Bröhl K. , Scheidt R. , Zeidler Th. , Mathieu Ch. , Hillebrands B.
Published: July 2002

We have studied the magnetic properties of Co/Cr/Fe multilayers by means of magneto-optic methods. These multilayers are of special interest because they combine different anisotropies and exchange coupling through the nonmagnetic Cr-spacer layer which was found to oscillate from ferromagnetic to antiferromagnetic coupling. The samples were grown by MBE methods on MgO(100) substrates at temperatures of about 300 °C. The fourfold crystalline symmetry is also obtained in the magnetic easy axis structure of Fe and Co. From the results of magneto-optic Kerr hysteresis loops we were able to distinguish between crystalline anisotropy and exchange coupling contributions. Additional monitoring of the domain structure by applying longitudinal Kerr microscopy was necessary to identify the regime were magnetization reversal occurs via domain wall motion or coherent rotation of the magnetization of the entire layer, respectively. From these results we were able to perform numerical simulations of the micromagnetic properties of trilayer samples. We discuss the results in terms of bilinear and biquadratic coupling.
The effect of an oxygen plasma exposure on the reliability of a Ti/TiN contact metallization
Authors: Ouellet L. , Tremblay Y. , Gagnon G. , Caron M. , Currie J. F. , Gujrathi S. C. , Biberger M. , Reynolds R.
Published: July 2002

This article demonstrates that the exposure of a TiN barrier to an ex situ oxygen plasma results in a more stable TiN/AlSiCu interface up to temperatures of 600 °C as shown by the time-of-flight elastic recoil detection measurements. A quaternary phase diagram of the Al–Ti–O–N system was calculated in the range of temperatures between 450 and 550 °C and suggests that the stabilization of the TiN/AlSiCu interface is possible since oxidized TiN reacts with Al to form AlN, TiAl3 and Al2O3 at the interface. A Ti/TiN/(oxygen plasma exposure)/AlSiCu/TiN contact metallization in 1.2-μm-diam and 1.4-μm-deep straight wall contacts to 0.2-μm-deep N+ and P+ diffusions, to gate polysilicon as well as to capacitor polysilicon shows stable electrical results even after a [(450 °C, 60 min)+(500 °C, 60 min)+(550 °C, 60 min)] combined thermal stress.
Calculation of a Al–Ti–O–N quaternary isotherm diagram for the prediction of stable phases in TiN/Al alloy contact metallization
Authors: Caron Mario , Gagnon Gilles , Fortin Vincent , Currie John F. , Ouellet Luc , Tremblay Yves , Biberger Max , Reynolds Reese
Published: July 2002

We have calculated a quaternary phase diagram of the Al–Ti–O–N system, valid in the range of temperatures between 450 and 550 °C. In particular, the respective composition ranges of all the binary compounds were taken into account and are listed along with their composition and structure. The four simplified ternary diagrams (Ti–Al–O, Ti–N–O, Ti–Al–N, and Al–N–O) were first calculated to constitute the four faces of the tetrahedron representing the quaternary phase diagram. The interior tie lines of the quaternary diagram were calculated with the same proposed procedure that was used to obtain the ternary diagram. Two of these tie lines are important, one of which links TiN and Al2O3 and one which links AlN and TiO2. The utility of this diagram is illustrated by predicting the phases that are likely to occur during the reaction between Al and TiN layers, the latter compound being oxydized or not.
Signal processing for 10 GB/in.2 magnetic disk recording and beyond (invited)
Authors: Fisher Kevin D. , Modlin Cory S.
Published: July 2002

After providing an overview of the read channel function in a magnetic disk recording system, we describe a range of possible partial-response maximum-likelihood (PRML) detection schemes. Improvements upon the current industry-standard PRML system can potentially provide substantial increases in areal density, as is shown by means of computer simulation of channel and detector.
Mechanical properties of Al–Y–Ni amorphous ribbons
Authors: Freitag J. M. , Koknaev R. G. , Sabet-Sharghi R. , Koknaeva M. , Altounian Z.
Published: July 2002

Mechanical properties for two series of amorphous Al–Y–Ni ternary alloys were measured. Both series of alloys can be divided into two distinct groups. Those whose primary crystallization proceeds by growth of quenched-in Al nuclei and those which show a clear glass transition and their crystallization product is more complex. The Young’s modulus, ranging from 82.3 to 45.5 GPa in these series of alloys, is found to be correlated to the crystallization temperature and thermal behavior as well as the crystallization product implying that the mechanical properties depend on the short-range order in the glass.
Redistribution of in situ doped or ion-implanted nitrogen in polysilicon
Authors: Nakayama Satoshi , Sakai Tetsushi
Published: July 2002

This article reports a secondary ion mass spectroscopy analysis of the redistribution of in situ doped or implanted nitrogen in polysilicon and the segregation of nitrogen at the polysilicon/SiO2 interfaces during heat treatment at 700–1000 °C. When nitrogen-doped polysilicon is subjected to heat treatment at a temperature above 800 °C, nitrogen diffuses to the poly-Si/SiO2 interface and surface, and piles up there. Some of the nitrogen is immobile when the concentration is above a particular threshold concentration. This immobile nitrogen becomes mobile and diffuses during annealing. The threshold concentration for nitrogen diffusion depends on the grain size. There is a limit to how much nitrogen segregates to the interface. The limit depends not on the initial amount of nitrogen in the polysilicon, but only on the annealing temperature. A comparison of data for polysilicon films with data for bulk silicon suggests that the redistribution of nitrogen in the polysilicon films is limited by the transformation process by which immobile nitrogen becomes mobile.
EXAFS and magnetic characterization of inverted ball milled zinc ferrite powders (abstract)
Authors: Oliver S. A. , Harris V. G. , Hamdeh H. H. , Ho J. C.
Published: July 2002

Divalent zinc is commonly used as a replacement cation in ferrites, as it increases the material magnetization when substituted in moderate amounts. However, pure zinc ferrite (ZnFe2O4) is a normal spinel, as zinc has a strong preference for the tetrahedral (A) site in the spinel structure, with Fe occupying the octahedral (B) sites. Then antiferromagnetic B–B coupling dominates the magnetic behavior yielding a material with a Neel temperature near 10 K. It is typically difficult to produce inverted zinc ferrite due to this strong site preference, but recently we produced fine zinc ferrite powders having an inversion parameter of 0.20 using an aerogel process. Ball milling these powders increased the inversion parameter to 0.55, and the median particle size from 14 to 40 nm. To confirm the x-ray diffraction inversion parameters, extended x-ray absorption fine structure (EXAFS) measurements were taken on these powders and ZnFe2O4 standards for both the Fe Kα and Zn Kα absorption edges. Fourier transformed results were fingerprinted against theroretical radial distrubtion functions generated from multiple-scattering FEFF codes for scattering atoms located on the tetrahedral and octahedral sites. Qualitatively, the zinc ferrite standard appears as a normal spinel, while both the iron and zinc radial distributions for the ball milled sample showed large site occupation of both A and B sites. This sample shows superparamagnetic behavior at room temperature, with some ferrimagnetic particles exceeding the critical volume, as indicated by a 15 Oe coercive field. The saturation magnetization of the metastable ball milled powder exceeds 2100 G.
Dual-complementary variational methods in eddy currents (abstract)
Authors: Lois P. , Baldomir D. , Rivas J.
Published: July 2002

Dual-complementary variational principles and their expressions in static electric and stationary magnetic fields fit quite well into computational methods. We have got a very efficient computer graphics package exploiting these methods. But the main advantage with respect to other methods such as finite elements is that they are associated with the intuitive picture of lines of force for the fields. This enables the designer to use his physical intuition to lead him to practical improvements in the achievement need much less memory and CPU of a computer for most of the usual practical works. Actually our computer package was made on a cheap desk-top computer. Unfortunately these variational methods fail in eddy-current systems. The main reason is that eddy-current equations are equivalent to a diffusion equation which is not a self-adjoint differential operator. Nevertheless we have got to remove this difficulty by changing the geometry associated to the conductor. Besides, to employ the exterior algebra of differential forms we get the best understanding of the basics of the mathematical problem. Finally, the paper gives a brief description of the new software that we introduce into this applied mathematical methods.
Pseudo-negative photocurrent spectroscopy in GaAs-AlAs superlattices
Authors: Schrottke L. , Grahn H. T. , Fujiwara K.
Published: July 2002

Pseudo-negative photocurrent spectra, i.e., the appearance of a minimum in photocurrent at an absorption maximum, are experimentally studied at different temperatures and excitation intensities in GaAs-AlAs superlattices on GaAs substrates. Superlattice and substrate are isolated by a thick Al0.3Ga0.7As barrier, but electrically connected through penetrating contacts. A simple model is proposed for the analysis of the conditions which can lead to pseudo-negative photocurrent in this sample configuration. The radiative recombination of the carriers in the superlattice was found to be the main process determining the sign of the photocurrent at an absorption maximum.
A model for magnetomechanical hysteresis and losses in magnetostrictive materials
Authors: Bergqvist A. , Engdahl G.
Published: July 2002

A phenomenological hysteresis model for the magnetomechanical constitutive law in highly magnetostrictive materials is proposed. Magnetization and strain are represented using a set of noninteracting particles. Any change in magnetization or strain in individual particles is assumed to be obstructed by a counterforce analogous to mechanical dry friction. Comparisons have been made with experiments performed on Terfenol-D under varying field and/or stress with satisfactory results.
Crystallographic and magnetic structure of UCu1.5Sn2
Authors: Purwanto A. , Robinson R. A. , Nakotte H. , Swainson I. P. , Torikachvili M. S.
Published: July 2002

We report on the crystallographic and magnetic structures of the antiferromagnet UCu1.5Sn2, as determined by x-ray and neutron powder diffraction. It crystallizes in the tetragonal CaBe2Ge2 structure type, with space group P/4nmm, and we find no site disorder between two different Sn 2c sites, in contrast with a previous report. UCu1.5Sn2 orders antiferromagnetically with a Néel temperature of about 110 K. This is unusually high among uranium intermetallics. The uranium moments align along the c axis in a collinear arrangement but alternating along the c axis. The low-temperature uranium moment is 2.01μB.
Magnetic and magnetotransport properties of Ni/Fe multilayers
Authors: Cai M. , Veres T. , Morel R. , Cochrane R. W.
Published: July 2002

We present a study of sputtered Ni/Fe multilayers in order to examine the effect of interfacial mixing and interdiffusion on the magnetic and transport properties. The multilayer structure has been examined by low-angle x-ray reflectivity, which reveals interface mixing of about two monolayers. The magnetization measurements show differences in the anisotropy of multilayers for which the thickness ratio of nickel to iron is changed from 1:1 to 3:1. With the 1:1 composition, multilayers show high permeability with a longitudinal easy axis, while samples with 3:1 composition show lower permeability and a rotated easy axis. By fitting the resistivity and magnetoresistance thickness variation with a semiclassical model, we have determined an interface contribution to the resistivity and a possible contribution to the magnetoresistance.
First principles calculations of the Kerr effect in MnBi, MnSb, and Mn2BiSb (abstract)
Authors: Huang W. , MacLaren J. M. , Victora R. H.
Published: July 2002

First principles electronic structure calculations are used to predict the energy dependence of the polar Kerr effect in MnBi, MnSb, and a Mn2BiSb alloy. The compounds all have respectable Kerr angles and thus are potential data storage media. The technique used in this work is a full-potential linearized augmented Slater orbital approach. The Kerr angle is obtained from the conductivity tensor that is calculated, within linear response theory, using the Kubo formula. There are several desirable features in this approach. First, unlike methods based upon muffin-tin potentials, open crystal structures present no problems. Second, since the electronic wave functions are defined over the crystal unit cell rather than over space filling overlapping spheres, difficulties in handling the interstitial part of the required matrix elements are avoided. Finally, the small basis set (25 orbitals per atom) leads to reasonable computational times. Results of these calculations show that the Kerr angle in MnBi is above 1° at 1.6 eV and that its variation over 2–4 eV is linear and decreasing. MnSb shows a smaller rotation, about 0.5°, that is almost constant up to 3 eV and then also decreases linearly. The Kerr rotation of Mn2BiSb is approximately the average of MnBi and MnSb. This work has been supported in part by the Louisiana Quality Education Support Fund under Grant No. LEQSF (1991–1994)-RD-A-30.
Low-temperature specific heat of the degenerate supersymmetric t-J model in one dimension
Authors: Lee Kong-Ju-Bock , Schlottmann P.
Published: July 2002

We consider the one-dimensional SU(N)-invariant t-J model, which consists of electrons with N spin components on a lattice with nearest-neighbor hopping t constrained by the excluded multiple occupancy of the sites and spin-exchange J between neighboring lattice sites. The model is integrable and has been diagonalized in terms of nested Bethe ansatze at the supersymmetric point t=J. The low-T specific heat is proportional to T. The γ-coefficient is extracted from the thermodynamic Bethe-ansatz equations and is expressed in terms of the spin wave velocities and the group velocity of the charges for arbitrary N, band filling, and splitting of the levels (magnetic and crystalline fields). Our results contain the following special cases: (i) For N=2 the traditional spin-1/2 supersymmetric t-J model, (ii) for exactly one electron per site the SU(N)-Heisenberg chain, and (iii) for N=4 the two-band supersymmetric t-J model with crystalline field splitting.
Susceptibility and 89Y nuclear magnetic resonance in Y1−xCaxVO3
Authors: Cintolesi F. , Corti M. , Rigamonti A. , Rossetti G. , Ghigna P. , Lascialfari A.
Published: July 2002

Temperature-dependent (10–300 K) field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities, and 89Y NMR spectra and relaxation in Y1−xCaxVO3 with 0≤x≤0.9 are reported. In the paramagnetic phase (PA) of YVO3 and for T≥120 K the susceptibility χ is of antiferromagnetic (AF) type, with no field dependence. Around the Neel temperature T(1)N=113±1 K a field-dependent ferromagnetic behavior is observed under FC condition. Below T(2)N≂77 K (AF phase with AF interplane coupling) strong difference is present between FC and ZFC χ’s. For T→T(1)+N the 89Y NMR line shows marked broadening and no measurable shift, consistent with a dominant dipolar contribution in the nucleus–electrons interaction. The 89Y relaxation rate T−11, related to the correlated spin dynamics of V3+ S(t), seems to indicate that the interplane ferromagnetic symmetry of the spin fluctuations is quenched in strong fields. The peculiar findings for χ and T1 could result from a field-dependent weak ferromagnetism of Dzyaloshinsky–Moriya character. In Y1−xCaxVO3, χ and T−11 decrease on increasing x, still preserving the qualitative behavior as for x=0. For x≂0.6 (Ca concentration which drives the transition to the metallic phase) the marks of the PA–AF transition are practically suppressed and behaviors characteristic of correlated metals are observed.
Fundamental studies of stripe-domain chopping in the presence of magnetic fields
Authors: Fogarty K. J. , Matsuyama K. , Asada H. , Clegg W. W.
Published: July 2002

The effects of externally uniform applied orthogonal and parallel in-plane magnetic fields on the stripe-chopping characteristics and negative vertical Bloch line replication probability have been studied experimentally and by numerical simulation on a 5 μm bubble film. The orthogonal and parallel in-plane fields on the stripe domain were varied from −50 to 50 Oe and −9 to 9 Oe, respectively, to represent actual working device parameters. The numerical simulation results agree with the experimental data, confirming the suppression of horizontal Bloch line nucleation by the external-uniform in-plane fields and the in-plane field components of the chopping conductor.
Micromagnetic study on write operation in submicron magnetic random access memory cell
Authors: Asada H. , Matsuyama K. , Taniguchi K.
Published: July 2002

In a spin-valve random access memory, the binary bit states of the storage cell are determined by the magnetization direction in the free magnetic layer. The write operation of a submicron memory cell element has been studied by a micromagnetic computation based on an energy minimization scheme, which aids in the chip design. The magnetization of the binary bit states in the element was found to take a single domain structure having the opposite direction of the long-axis component. The mean long-axis component of magnetization of each binary state was ±0.97 without external fields. The selective switching of the bit state in the element was performed by the write currents applied into the two level conductors overlying the element for various conditions. The influence of the write currents to the neighboring element on a two-dimensional memory array with a 1×1 μm pitch was also simulated, in order to confirm the selective switching of the memory element. It was found that the selective write current amplitude decreased with an increasing assist current amplitude and the range was extended by the large difference of the transverse magnetic field between the selected and neighboring element. The effect of the exchange interaction from the pinned magnetic layer on the write operation was also discussed.