bluuebunny/crossref_metadata_2025 huggingface dataset:
Origins of 1/f noise in indium antimonide photodiodes
Authors: Su Y. K. , Liaw U. H. , Sun Tai-Ping , Chen Gin-Shiang
Published: July 2002

1/f noise in indium antimonide (InSb) photodiodes has been measured as function of temperature, gate bias, diode voltage, junction area, and photon flux in order to identify the dominant noise mechanisms. Since these parameters varied the relative contributions of the various current mechanisms, the diode current mechanism responsible for 1/f noise was isolated. It was found that 1/f noise is independent of photocurrent and diffusion current but is linearly related to surface generation current. It is proposed that 1/f noise in reverse-biased InSb photodiodes is a result of modulation of the surface generation current by fluctuations in the surface potential.
Combination of Maker fringe and total reflection technique: Nonlinear optical properties of 4-nitro-2-ethoxyamide-4′-benzyloxytolane
Authors: Kato Midori , Kiguchi Masashi
Published: July 2002

A new approach to measure the optical second-order nonlinear coefficient tensor of material which is difficult to prepare as a large single crystal is demonstrated. In such a case, due to the limitation of the crystal direction the Maker fringe measurement cannot provide each tensor component but instead yields an effective value. A technique using the second-harmonic generation with evanescent wave (SHEW) makes up for the Maker fringe result. We have applied the measurements to the organic material 4-nitro-2-ethoxyamide-4′benzyloxytolane (NEBT). The combined results from different measurements clarify the nonlinear optical character of NEBT molecules and the largest component of the optical second-order coefficient tensor was obtained to be 39±8 pm/V. The combination of the Maker fringe and the SHEW technique can be a powerful tool.
Tetragonal WSi2 formation by 0.5–5 MeV Xe-ion-beam irradiation at 250 °C and 450 °C
Authors: Yamaguchi Toru , Nakata Jyoji
Published: July 2002

We studied two-step tungsten-silicidation processes, which consist of low-energy W implantation followed by high-energy Xe irradiation. The formation of silicides was studied by Rutherford backscattering spectroscopy, x-ray diffraction and transmission electron microscopy. The formed silicide layer is richer in Si than that formed by thermal annealing. The transformation from the hexagonal to tetragonal (usually formed by thermal annealing above 600 °C) WSi2 phase occurred and a tetragonal WSi2 layer was successfully formed by 1-MeV Xe+ and 5-MeV Xe++ ion irradiation at under irradiation temperatures of 410 and 450 °C. The transformation did not occur by 0.5-MeV Xe+ ions at the same substrate temperature. The tetragonal phase was also observed after irradiation by 1-MeV Xe+ at 250 °C. The phase transformation rate normalized to the nuclear energy deposition density En increases with the electronic energy deposition density Ee. This fact indicates that the phase transformation is enhanced by the inelastic electronic scattering of high-energy ion irradiation. The irradiation temperature dependence of the phase transformation was also studied. The mechanism of the silicidation by elastic nuclear scattering and that of the phase transformation by inelastic electronic scattering of high-energy heavy-ion-beam irradiation are qualitatively discussed.
Inductive instability in electrodynamic systems with rapidly varying parameters
Authors: Dolinsky Yu. , Elperin T.
Published: July 2002

In this study we demonstrate the feasibility of generation of the electric charge and electric current in systems with rapidly varying parameters. It is shown that the electric current is excited spontaneously in the systems where electric resistance of their elements changes. Remarkably, effective damping resistance of the layered conductors may become negative, although ohmic resistance of its layers grows. Similar effects occur in electric circuits with different conductors connected in parallel. The effective resistance of such a system can decrease in spite of the increase of the ohmic resistance of the conductors.
A simple model for low energy ion-solid interactions
Authors: Mohajerzadeh S. , Selvakumar C. R.
Published: July 2002

A simple analytical model for ion-solid interactions, suitable for low energy beam depositions, is reported. An approximation for the nuclear stopping power is used to obtain the analytic solution for the deposited energy in the solid. The ratio of the deposited energy in the bulk to the energy deposited in the surface yields a ceiling for the beam energy above which more defects are generated in the bulk resulting in defective films. The numerical evaluations agree with the existing results in the literature.
Boltzmann equation analysis of spatiotemporal electron swarm development
Authors: Ould Mohamed Mahmoud M. , Yousfi M.
Published: July 2002

A powerful and a stable numerical method is developed to solve the Boltzmann equation of electrons moving under the action of an electric field in weakly ionized gases involving space and time gradients. It is based on the classical two term development of the distribution function and on a strongly implicit procedure following position and energy axis and an explicit approach along the time axis. This numerical algorithm is successfully applied to determine the spatiotemporal variation of the electron distribution function and the associated swarm parameters (mean energy, drift velocity, ionization and attachment coefficients, etc.) in the case of nonthermal electrical discharges in different gases (He, Ar and O2) under different applied electric fields and initial and boundary conditions. The transient phase, the following steady state phase and also the electrode effects are clearly emphasized and analyzed for each gas discharge studied.
Mechanism of anomalous photoinduced transient current peak in amorphous silicon thin-film transistor
Authors: Chu M. H. , Wu C. H.
Published: July 2002

The photoinduced transient current from an amorphous silicon thin-film transistor is computed and the mechanism described in terms of trap-state filling dynamics. The direction of the current flow and the location of the transient peak depends strongly on the distributions of donorlike and acceptorlike trap states in the neighborhood of the dark Fermi level. We show that the transient current can flow in the same direction as in the crystalline transistor, as well as in the opposite direction. There is also an interesting cross-over behavior in which the transient current flows out of the drain terminal as a pulse of positive charge, and then immediately reverses its direction. There is a broadening effect of the transient peak by a simultaneous switch on of the gate voltage. The transient peak typically occurs at 10−4 s and an example is provided. The transient current can be greatly diminished by switching the gate voltage long before illumination, or by doping the channel either partially or completely.
Fullerene-based polymer grid triodes
Authors: McElvain J. , Keshavarz M. , Wang H. , Wudl F. , Heeger A. J.
Published: July 2002

We report the fabrication of polymer grid triodes (PGTs) using C60 as the semiconducting medium. The structure, a self-assembling porous conducting polyaniline network placed between two semiconducting layers of C60, and sandwiched between two metal electrodes, demonstrates the I–V characteristics of a three terminal device in which the current is controlled by the grid potential. As a result of the higher mobilities of the fullerenes compared to those of conjugated polymers, the fullerene devices offer some performance advantages; the operating voltages are less than 5 V with current densities exceeding 1 mA/cm2. By fabricating devices with different grid densities, it is shown that the grid exhibits a strong influence on the I–V characteristics for high grid densities, and a correspondingly low influence for low densities, in agreement with theory. The I–V characteristics of the C60 PGTs are well described by an effective diode model, similar to that used for vacuum triodes.
Estimation of the interface states density of a Si/C60 heterojunction by frequency-dependent capacitance–voltage characteristics
Authors: Kita K. , Ihara M. , Sakaki K. , Yamada K.
Published: July 2002

Capacitance–voltage (C–V) characteristics of a Si/C60 heterojunction, i.e., a HF solution (7.3% HF+30% NH4F) treated Si surface interfaced solid crystal of C60 molecules, were measured while applying various ac frequencies. The entire C60 thin film and near-interface region of the Si wafer behaved as a depletion region, with the C–V curve showing two distinct regions: one above and one below a threshold bias voltage. Below the threshold, C–V characteristics were dependent on applied frequency, which suggests the presence of interface states that only affect capacitance at lower frequencies. This frequency dependence was analyzed by assuming a suitable equivalent circuit, and based on derived curve-fitting circuit parameters the interface states density was accordingly estimated to have a value as low as ∼1011/cm2 eV. Such a small density indicates that only a few lattice defects occur within the interface of the HF-treated Si surface and C60 crystal. Although no frequency dependence was observed above the threshold, the C–V characteristics were found to be dependent on the width of the depletion region in Si.
Interaction of turbulent plasma flow with a hypersonic shock wave
Authors: Belay K. , Valentine J. M. , Williams R. L. , Johnson J. A.
Published: July 2002

A transient increase is observed in both the spectral energy decay rate and the degree of chaotic complexity at the interface of a shock wave and a turbulent ionized gas. Even though the gas is apparently brought to rest by the shock wave, no evidence is found either of prompt relaminarization or of any systematic influence of end-wall material thermal conductivities on the turbulence parameters.
Electrical resistivity behavior of Fe/Cr multilayers deposited by different techniques (molecular-beam epitaxy, sputtering), on different substrates (MgO,Si)
Authors: Almeida B. G. , Amaral V. S. , Sousa J. B. , Colino J. , Schuller I. K. , Schad R. , Moshchalkov V. V. , Bruynseraede Y.
Published: July 2002

High-resolution electrical resistivity measurements (ρ,dρ/dT) were performed in three series of [Fe30ÅCrtÅ] multilayers in the temperature range 15–300 K, both at zero and under saturation magnetic field. The different series were prepared by MBE on MgO (100) substrates, by sputtering on MgO (100) substrates, and by sputtering on Si (100) substrates. In the temperature range 15 K≲T<50 K we always observe ρ=βT3 where β is a sample-dependent constant, indicating the dominance of phonon-assisted interband (s-d) electron scattering (ρ∝T3 when T≪ΘDebye). For the samples grown on MgO we observe that β decreases with t(Cr) whereas for the samples grown on Si, the coefficient β increases with t. For T>150 K the resistivity attains the classical dependence with ρ∝T also predicted by this s-d model. In spite of these differences our results show that ρ=βf(T) where f(T) is the same function of temperature for all the different samples studied.
Mössbauer study of the interfacial environment of ferromagnetic atoms exchange coupled to CoO (abstract)
Authors: Takano Kentaro , Parker F. T. , Berkowitz A. E.
Published: July 2002

Although unidirectional anisotropy was first observed over 40 years ago in surface oxidized Co particles, the basic physics of the mechanism remains elusive. Unidirectional and exchange anisotropies are observed when ferromagnetic (FM) and antiferromagnetic (AFM) materials are coupled to each other. Because of the interfacial nature of the coupling, the exchange strength is sensitive to the interfacial environment. The simple model of an epitaxial atomically smooth interface predicts exchange field strengths two orders of magnitude larger than the observed values. Cross-sectional pictures of NixCo(1−x)O/permalloy exchange couples by transmission electron microscopy (TEM) reveal interfacial complexities such as roughness and strain gradients that could account for the reduction of the interfacial exchange strength. In this work, we studied the interfacial environment of the first few atomic layers on the FM side. We deposited the Mössbauer isotope Fe57 as an interlayer of variable thickness (4⩽t⩽16 Å) between a 300 Å CoO base layer and a 150 Å Ni capping layer. Using room temperature conversion electron Mössbauer spectroscopy, we measured the changes in the chemical and magnetic environment of the Fe atoms as a function of their distances from the interface. These measurements revealed that all the Fe atoms were magnetically ordered. The resulting complex spectra indicated a mix of Fe2+, Fe3+, and multiple metallic Fe subspectra within the first few monolayers of the interface. A particular metal fraction appears for tFe>8 Å and dominates the thicker Fe films. The existence of several metal subspectra indicates that Fe and Ni alloy at the interface. The Mössbauer measurements show that the Fe moments, metal and ionic, lie in the plane of the film. Measurements of two different 8 Å Fe57 interlayer films verified the reproducibility of the complex spectra. The origin of oxidation of the interfacial atoms is being studied by comparing the spectra of films prepared under various deposition conditions.
Surface-induced clustering in vapor deposited Co1−xPtx and Ni1−xPtx films (abstract)
Authors: Shapiro A. L. , Rooney P. W. , Tran M. Q. , Hellman F.
Published: July 2002

We have depostied (100), (110), and (111) oriented single-crystal and polycrystalline Co1−xPtx (x=0.65, 0.75, 0.80) and polycrystalline Ni1−xPtx (x=0.25, 0.50) films over a range of growth temperatures from −50 to 800 °C. Previous work on CoPt3 films had demonstrated the correlation between Co clustering and perpendicular magnetic anisotropy.1 These observations motivated a model in which Co atoms cluster on the growing surface; this clustering is trapped by succeeding layers for growth temperatures below 400 °C, the lower limit of bulk atomic mobility. The (110) oriented samples show clustering and anisotropy identical to the (100) and (11) oriented samples despite the sign change of the surface segregation in the (110) orientation. The Ni–Pt alloy system is similar to Co–Pt in structure, lattice constant, and the polarizability of Pt. We have deposited Ni3Pt films; these films have significant clustering of the magnetic species which drops off rapidly after the onset of bulk mobility at 400 °C. No perpendicular magnetic anisotropy is found in these films, even though interface magnetic anisotropy values in Ni/Pt multilayers have been reported as having up to 1/4 the value of Co/Pt interfaces. Finally, in order to explore whether surface clustering is a kinetic or equilibrium effect, we have varied the deposition rate of CoPt3 over three orders of magnitude at temperatures below 400 °C. We found no dependence of clustering or anisotropy on the deposition rate, suggesting that the clustering is not kinetically limited but is a surface equilibrium effect.
Multiple-scale derivation of the nonlinear Schrödinger equation for solitons in magnetic thin films (abstract)
Authors: Mock S. E. , Ablowitz M. J. , Patton C. E.
Published: July 2002

Although the nonlinear Schrödinger (NLS) equation is the theoretical starting point for the analysis of magnetostatic wave (MSW) soliton experiments in thin films,12 no first principles theory to justify its use has yet been advanced. In this work, multiple-scale asymptotics have been used in combination with Maxwell’s equations in the magnetostatic approximation and the torque equation of motion to obtain the NLS equation for a quasimonochromatic plane propagating wave in a perpendicularly magnetized thin film. A nonlinear differential equation for the magnetization amplitude envelope response with the same form as the NLS equation has been obtained when the analysis is carried out to the third-order terms in the expansion. Specific results include (1) linear terms that agree with well-established magnetostatic wave theory3 and (2) an explicit wave-number k-dependent nonlinear response parameter N. In previous work,4 N has been approximated as the MSW frequency shift with the square of the dynamic magnetization amplitude. The present results indicate that this approximation leads to errors of 20%–40%, even in the usual 100–200 rad/cm low-k range of the experiments.
Magnetic properties of YVO3 from susceptibility and 89Y NMR measurements
Authors: Corti M. , Cintolesi F. , Lascialfari A. , Rigamonti A. , Rossetti G.
Published: July 2002

The magnetic properties of YVO3, a prototype of highly correlated d-electron Mott–Hubbard antiferromagnetic (AF) orthovanadates, are studied by means of magnetization and susceptibility measurements, coupled to 89Y NMR spectra and relaxation. Both field-cooled and zero-field-cooled conditions are considered. The history and field-dependent properties recently reported for the AF phases below TN≃113 K and TN′≃77 K are confirmed. The behavior in the paramagnetic phase and around TN is studied and it is shown how the field-induced short range order and canted-spin weak ferromagnetism is quenched in strong field, yielding (M/H)ZFC≃(M/H)FC for H≃6 T. At variance with the susceptibility in strong field, the 89Y NMR line width shows a divergent behavior for T→TN+. The 89Y relaxation rates do not display, for T→TN+, the behavior expected for spin fluctuations of AF-ferromagnetic (FE) symmetry as the one in the ordered state, possibly in view of the quasistatic character of the short range order.
Ruderman–Kittel–Kasuya–Yosida interaction between magnetic impurities in GaAs/AlAs quantum wells under strong electric field (abstract)
Authors: da Cunha Lima I. C. , Ferreira Filho L. G. , Troper A.
Published: July 2002

Magnetic impurities in semiconductor heterostructures interact indirectly via an electron gas generated by sources which may be far away from the region of interest, where the magnetic impurities are located. A system particularly important is the quantum well (QW) of GaAlAs/GaAs where the mismatch of the conduction band established a barrier of the order of 1 eV and the well width can be in a wide range starting from a few atomic layers. The electron gas in such structures behaves dynamically as quasibidimensional and its concentration can vary in a wide range. Indirect exchange via a Ruderman–Kittel–Kasuya–Yosida (RKKY) mechanism has been widely explored in the past. Here we have studied the RKKY interaction between two localized magnetic moments lying anywhere in a GaAs/AlAs symmetric double well when an electric field is applied parallel to the growth direction. The effect of the electric field is to break the symmetry along the growth direction at the same time that it changes the charge density inside each of the QWs. Therefore, by a change of the electric field intensity we change the interaction between impurities inside the same QW (intrawell exchange) and impurities locate in different QWs (interwell exchange). As a consequence we have obtained magnetic properties which are controlled by the intensity of the applied electric field. Our results include the effect of the barrier thickness and the concentration of the two-dimensional electron gas for QW widths in the range of 30–200 Å and electric fields in the range of 107 V/m.
Dynamic Ruderman–Kittel–Kasuya–Yosida indirect interaction (abstract)
Authors: Genkin G. M.
Published: July 2002

The spin polarization of a degenerate electron gas has been used to describe the coupling [Ruderman–Kittel–Kasuya–Yosida (RKKY) indirect interaction] between two spins in metals. Nowadays such the RKKY interaction is investigated very much in particular with the problems of coupling in magnetic superlattices. However, in well-known RKKY interaction between two spins it is supposed that the spins are not subject to the external alternating magnetic field (static case). We consider the RKKY interaction between two spins, when they are subject to the external alternating magnetic field. We shown, that the RKKY indirect interaction in free-electron gas for transversal component of the spins (value of the coupling, its spatial dependence, the periods of spatial oscillations) depends on a frequency of the alternating magnetic field. This behavior is caused by the frequency dependence of the dynamic magnetic susceptibility of an electron, which determines the indirect coupling. The largest value of the coupling is in the frequency region hω∼εF, where εF is the energy Fermi. For high frequency region hω⩽εF: In contrast to the static case, where there is only one period 2kF for free-electron, in high frequency hω∼εF dynamical case the spatial dependence of the RKKY interaction have complicated form and there are three main uncommensurable periods of oscillations kF, (1+2)kF, (2−1)kF; the spatial dependence of coupling for large kFr contains terms r−1 and r−2; value of coupling is (kFr)2 as much then in a static case. For the frequency region hω≪εF: the dynamic RKKY interaction contains terms, value of which is determined by the frequency of the magnetic field; there are the oscillations of coupling with a period 2kF, which are amplitude-modulated by a oscillation with a large spatial period; these oscillations have amplitude, which is determined also by a frequency.
Antiferromagnetic interlayer coupling in epitaxial Fe/EuS (100) bilayers (abstract)
Authors: Demokritov S. O. , Rücker U. , Arons R. R. , Grünberg P.
Published: July 2002

We present the observation of antiferromagnetic interlayer coupling between two ferromagnetic layers of Fe(100) and EuS(100), which are in direct contact. The coupling strength J is about 0.2 mJ/m2 at 5 K, it decreases with increasing tempeature and becomes negligibly small near the Curie temperature of the EuS layer. The samples were molecular-beam epitaxy-grown on the GaAs(100) single crystal substrates using a silver buffer layer. The structural and chemical properties of the layers were characterized by in situ low-energy electron diffraction and Auger electron spectroscopy and by ex situ Rutherford backscattering. Superconducting quantum interference device (SQUID) magnetometry was used for measurements of the absolute magnetic moments as a function of temperature or magnetic field. Hysteresis measurements were performed by means of the magneto-optical Kerr effect (MOKE). We found evidence of antiferromagnetic coupling between the two different ferromagnetic layers both from zero field cooling experiments through Tc(EuS) as well as directly from the shape of the hysteresis loops at low temperatures. The temperature dependence of the interlayer coupling strength shows that J is proportional to the EuS magnetization. The observed onset temperature of ferromagnetic order in EuS is up to three times higher than the Curie temperature of bulk EuS. We discuss a mechanism in which this enhancement is connected to the existence of structural defects in the EuS film, caused by the growth. The defects produce additional band carriers, which in turn enhance the Curie temperature of the layer due to a very large indirect s(d)–f exchange.
Ellipsometric measurement of solid fluorocarbon film thickness on magnetic recording media
Authors: Karis T. E. , Tyndall G. W. , Fenzel-Alexander D. , Crowder M. S.
Published: July 2002

Solid fluorocarbon films were deposited on carbon overcoated magnetic recording media. The films were made by direct ion beam deposition of fluorocarbon monomers. This paper presents ellipsometric measurements on films over a range of thickness from 0.5 to 12.5 nm. Film thickness was measured using ellipsometry, and the chemical composition was studied using x-ray photoelectron spectroscopy (XPS). A high-speed ellipsometer was set up to map thickness over the whole media surface. The ellipsometric angle Δ was found to be linearly related with the film thickness. Since the escape depth of the XPS is comparable to the film thickness, the XPS senses the underlying carbon as well as the carbon in the film. As a result, the apparent ratio of fluorine to carbon depends on the film thickness. The ion beam deposition technique can be used to produce films with controlled thickness and uniform composition.
Correlation length in La1−xCaxMnO3 (abstract)
Authors: Erwin R. W. , Lynn J. W. , Borchers J. A. , Peng J. L. , Greene R. L.
Published: July 2002

Small-angle neutron scattering (SANS) has been used to measure the ferromagnetic correlation length in La0.67Ca0.33MnO3. This is of interest in these materials because the transport mechanism responsible for colossal magnetoresistivity involves valence fluctuation on the Mn sites, and thus, spin fluctuations. There should then be a direct relationship between the magnetoresistivity and the spin–fluctuation correlation length. The magnetic SANS was determined by substracting 15 K data from the data at and above TC (250 K). When the quasistatic approximation is valid, the magnetic SANS measures the static equal-time two-spin correlation function. In an ideal isotropic ferromagnet, the fluctuations that drive the phase transition are the transverse (below TC) spin waves, which have a divergent correlation length at and below TC. Our measurements show that the magnetic scattering function for temperatures above TC is approximately Lorentzian. However, the extracted correlation length is anomalously small, starting at about 6 Å at 400 K and increasing only to about 11 Å at the nominal TC=250 K. The correlation length continues to increase below TC to about 14 Å at 225 K. Inelastic scattering measurements suggest that there are significant diffusive fluctuations near TC in addition to the spin waves. We are performing SANS measurements in a magnetic field to try to separate the correlation lengths of these two types of fluctuations.
Magnetic structures in colossal magnetoresistance perovskites (abstract)
Authors: Röder H. , Zang J. , Bishop A. R. , Trugman S. A.
Published: July 2002

Ferromagnetism in colossal magnetoresistance perovskites such as LaxB1−xMnO3 (B=Ba, Sr, Ca) is typically explained using the double exchange (DE) mechanism proposed by Zener. To explain the local structure above Tc one needs to introduce a coupling to lattice degrees of freedom. Here, we show the structure of the resulting magnetoelastic polarons and the temperature dependence of their pair potential. To investigate the “ferro”-magnetic state below Tc we study the ferromagnetic Kondo lattice model in the strong coupling limit. It turns out that the DE mechanism has unexpected properties, when there is more than one itinerant electron. We find that in general, the many-body ground state of the DE model is not globally ferromagnetically ordered. Also, the low-energy excitations of this model are distinct from spin wave excitations in usual Heisenberg ferromagnets, which will result in unusual dynamic magnetic properties.
Magnetic and magnetotransport properties of intermetallic SmMn2Si2
Authors: Zhao M. , Sun C. , Wang L. , Li W. , Su Q. , Zhao W. , Tang J.
Published: July 2002

The magnetic properties of naturally layered intermetallic compound SmMn2Si2 with textured structure have been studied. There exist a ferromagnetic transition at 35 K and two antiferromagnetic transitions at 120 and 230 K. The antiferromagnetic state below 230 K exhibits different magnetoresistance, with a negative magnetoresistance of 3%–4% for current I applied perpendicular to the c axis and with a positive magnetoresistance effect of about 4%–6% for current I parallel to the c axis. The observed magnetoresistance is likely to be related to magnetovolume effects. In the ferromagnetic state, a positive magnetoresistance with a maximum increase of 22% under an applied field of 5 T is observed at 4 K, and both H ⊥ I and H ∥ I configurations show positive magnetoresistance.
Magnetic exchange coupling through a superconductor
Authors: Sá de Melo C. A. R.
Published: July 2002

The magnetic exchange coupling between two ferromagnets separated by a superconducting spacer is analyzed. An indirect exchange coupling between the ferromagnets is assumed to exist when the superconductor is in its normal state. As the superconductor is cooled off below its critical temperature, the magnetic coupling changes. The appearance of the superconducting gap causes a reduction of the indirect exchange coupling existent in the normal state. This reduction occurs within a length scale controlled by the coherence length of the superconductor. The reduction of the coupling is temperature dependent, being weaker near the critical temperature and stronger at zero temperature. In addition, the change in critical temperature of the superconductor due to proximity to ferromagnets is discussed.
Ab initio calculations of the Curie temperature of complex permanent-magnet materials: Sm2Fe16A (A=Ga, Si)
Authors: Sabiryanov R. F. , Jaswal S. S.
Published: July 2002

A low concentration of substitutional impurities such as Ga or Si in R2Fe17 (R=rare earth) raises its Curie temperature. Self-consistent spin-polarized electronic-structure calculations are performed and exchange parameters (Jij) are derived from first-principles for pure Sm2Fe17 and Sm2Fe16A (A=Ga, Si). The changes in exchange parameters with doping are related to the magnetovolume effect and the lowering of hybridization of the iron atoms. Monte Carlo simulations have been performed using the calculated Jij’s to derive the Curie temperature of phases containing Ga and Si impurities. The results are in very good agreement with the experimental data.
Localization and hopping of magnetoelastic waves in highly magnetostrictive strings with random chain structures (abstract)
Authors: Inoue M. , Nishimura K. , Fujii T.
Published: July 2002

Analogous to the localization of the wave function of an electron in a random potential (Anderson localization), in the macroscopic system of magnetoelastic waves (MEWs) propagating in a highly magnetostrictive string exhibiting the giant ΔE effect, the piling-up of MEWs into a wave-packet in a restricted small region of the string is expected. This wave-packet is subject to hopping in a discontinuous wave by applying a magnetic field due to the change in the local elastic states caused by the ΔE effect. From this expectation, a theoretical analysis was performed focusing on the derivation of localization conditions of MEWs. The analysis was carried out using a one-dimensional string model having high magnetostriction. The string is assumed to be composed of random chains with (1) alloy-type disorder (random weights of masses with equal spacings) and (2) liquid-type disorder (random spacings of masses with equal weights). For the elastic and magnetoelastic constants of the string, the value of Fe78Si10B12 amorphous wire (Unitika) were used in the calculations. No substantial changes in the localization states were not recognized in both modes. The analysis revealed that, when the change of the apparent Young’s modulus with magnetic field ΔE is 28%, the localized MEWs are subject to hopping conserving their wave identities (eigenfrequencies and eigenstates). This result in considered to originate from the changes in the disorder conditions to support the localization of MEWs. To confirm the above theoretical prediction experimentally, MEW properties have been measured by using Fe78Si10B12 amorphous wires connecting weights made of leads to form the random chain structure. The localized MEWs are, indeed, observed by detecting the local vibrations of the wires. Theoretical and experimental results will be presented in detail at the conference.
Nondestructive evaluation of Terfenol-D for transducer applications
Authors: Greenough R. D. , Pollard D. J. , Schulze M. P.
Published: July 2002

A method and techniques have been developed for the nondestructive evaluation of grain oriented highly magnetostrictive rare-earth compounds, such as Terfenol-D, for use in actuator construction. Detection of an asymmetric flux distribution and localized fluctuations in ac permeability have been correlated with gradients in chemical composition and departures from the ideal grain structure produced by free float or contained zone preparation techniques. The data provide valuable information to gauge material quality, the response of individual samples to applied stress, and the reproducibility of transducer properties from one sample to another.
Mesoscopic effects in magnetism: Submicron to nanometer size single particle measurements
Authors: Wernsdorfer W. , Orozco E. Bonet , Barbara B. , Hasselbach K. , Benoit A. , Mailly D. , Doudin B. , Meier J. , Wegrowe J. E. , Ansermet J.-Ph. , Demoncy N. , Pascard H. , Demoncy N. , Loiseau A. , Francois L. , Duxin N. , Pileni M. P.
Published: July 2002

Low temperatures magnetization measurements of individual ferromagnetic particles and wires are presented (0.1<T(K)<6). The detector was a Nb micro-bridge-dc-SQUID, fabricated using electron-beam lithography. The angular dependence of the magnetization reversal could be explained approximately by simple classical micromagnetic concepts. However, our measurement evidenced nucleation and propagation of domain walls except for the smallest particles of about 20 nm. The switching field distributions as a function of temperature and field sweeping rate and the probabilities of switching showed that the magnetization reversal was thermally activated. These measurements allowed us to estimate the “activation volume” which triggered the magnetization reversal. Our measurements showed for the first time that the magnetization reversal of a ferromagnetic nanoparticle of good quality can be described by thermal activation over a single-energy barrier as originally proposed by Néel and Brown.
Thermally activated magnetization reversal through multichannels
Authors: Chang Ching-Ray , Yang Jyh-Shinn , Klik Ivo
Published: July 2002

Magnetization reversal of an isolated single-domain particle through multichannels was analytically studied. Results for a dichotomic channel of barriers demonstrate that the dynamical reversal process shows a multiexponential relaxation. At low attempting rate, the magnetization encounters a nearly static barrier, and at high attempting rate, it relaxes through an average of barrier heights. For the intermediate attempting rate, the magnetization usually relaxes multiexponentially, but only one relaxation time remains dominated at the low temperature.
A combined magneto-optic magnetic force microscope study of Co/Pd multilayer films
Authors: Pokhil Taras G. , Proksch Roger B.
Published: July 2002

We have combined a magnetic force/atomic force microscope (MFM/AFM) with a magneto-optic (MO) microscope. This instrument combines the high spatial resolution of the MFM/AFM and its capability to correlate magnetic structure with the structure of the sample surface with the real-time imaging capabilities and large field of view of the MO microscope. Our MO/MFM setup is based on the Nanoscope III Multimode™ MFM/AFM (Digital Instruments, Santa Barbara, CA). Currently, the spatial resolution of the MO microscope is about 3 μm and polarization sensitivity is on the order of 0.5°. Using this instrument, we observed domain structures in Co/Pd multilayer films. We found that in a film with 20 Co/Pd layer pairs and 16 nm total thickness, nucleation of domains during sample remagnetizations occurs repeatedly in the same points, and that displacement of domain walls is unidirectional. The high topographic resolution of the AFM allowed us to show that domains nucleate at small defects on the sample surface. The depth of the defects is 1–2 nm, they are 20–30 nm wide and up to 500 nm long. The unidirectional displacement of the domain walls was found to correlate with the anisotropic structure of the sample surface.
Effect of grain size on intergranular magnetostatic coupling in thin-film media
Authors: Mitsuya Haruhito , Endo Atsushi , Kikuchi Akira , Nakai Junichi , Shoji Hiroki , Takahashi Migaku
Published: July 2002

In order to characterize the intergranular coupling and to clarify the effect of grain size on intergranular magnetostatic coupling in thin-film media, the temperature dependences of magnetic properties Hc, Hkgrain, and Ms for ultraclean-(UC)-processed CoCrTa media with various grain sizes have been examined in order to discuss the effect of grain size on intergranular magnetostatic coupling. The results are that (1) in the UC-processed media, the magnetostatic coupling mode is found to be dominant as the intergranular coupling and in the normal processed media, the exchange coupling mode is dominant, and (2) the decrease in 4πMs/Hkgrain below 1.0 is the most effective to suppress the increment in the magnetostatic coupling due to the reduction in the grain size, were clarified.
Analysis from a stimulation perspective of the field patterns of magnetic resonance imaging gradient coils
Authors: Nyenhuis J. A. , Bourland J. D. , Schaefer D. J.
Published: July 2002

Gradient coils in a magnetic resonance imager are used to produce spatial variations of magnetic field. The gradients are switched at audio frequencies and for long ramp times dB/dt ≃ 60 T/s may be sufficient to induce peripheral nerve stimulation. The magnetic field pattern from the gradient coils is non-uniform, especially near the windings, and the question arises where the peak dB/dt should be specified. We propose methods for specifying the fields so that the potential for peripheral nerve stimulation for different coil geometries can be compared. The coils considered are the z gradient coil, which produces the gradient ∂Bz/∂z and the transverse gradient coils which produce ∂Bz/∂x and ∂Bz/∂y. Our analysis for the z coil shows that taking the maximal value of the magnetic field on the axis of the bore provides a reasonable representation of the potential for stimulation. The field distribution is more complex for the transverse coils. We conclude that the maximal radial magnetic field on the axis of the bore, which occurs near the center of an opposing coil pair, provides a good estimate of the potential for stimulation. Making use of the field measurements presented here and the knowledge of coil current required to achieve the desired gradient intensity will allow accurate comparison of the potential for stimulation for different types of whole body gradient coils.
Field-induced smectic ordering in model magnetic inks
Authors: Visscher P. B. , Günal Y.
Published: July 2002

We have performed Brownian dynamics simulations of model colloids of acicular (cigar-shaped) magnetic particles with aspect ratio 6.5, similar to Toda F ceramic-coated metal particles. In the absence of a magnetic field, these have a gel-like structure. We have investigated the effect of a magnetic field on this structure. At very low volume fractions the particles (after orienting along the field) form chains in the field direction. At more realistic volume fractions (3%–9%), we find a unique filamentary structure, in which the filaments themselves are smectically ordered. That is, the particles are stacked side-by-side with their poles in a common plane. The net effect of the orienting field is to promote aggregation into filaments.
The spin-dependent electronic structure of amorphous magnetic alloys
Authors: Xu Y. B. , Greig D. , Mitchell A. L. , Seddon E. A. , Matthew J. A. D.
Published: July 2002

Spin-resolved photoemission from FeB and CoB binary amorphous alloys has been measured using, for the first time, a synchrotron radiation source. A comparison of the experimental results with several ab initio calculations on two model systems Fe80B20 and Co77B23 shows that the spin-resolved photoemission provided a critical test of theoretical models, and gives insight into the spin-dependent electronic structures of these materials.
Effect of disorder on the linear and nonlinear magnetic susceptibilities of two manganeseporphyrin-based magnets
Authors: Gı̂rƫu M. A. , Wynn C. M. , Sugiura K.-I. , Miller Joel S. , Epstein A. J.
Published: July 2002

We analyze the effects of disorder on the magnetic properties of two manganeseporphyrins, [MnTPP][TCNE]⋅x(o-DCB) and [MnTPP][TCNE]⋅y(o-Xy) and (TPP=meso-tetraphenylporphyrinato, TCNE=tetracyanoethylene, o-Xy=o-xylene, o-DCB=o-dichlorobenzene, x⩽3, y⩽1). We present data for the in- and out-of-phase ac susceptibility and obtain the distribution of relaxation times using Cole–Cole analysis. The frequency dependence of the linear susceptibility suggests cluster glasslike behavior. The peak in the second harmonic of the ac susceptibility indicates the presence of a spontaneous magnetic moment. We propose that the unusual magnetic properties of these compounds reflect one-dimensional ferrimagnetic clusters interacting weakly through dipole-dipole interactions to form three-dimensional domains.
Theoretical analysis of chemical bonding and solute systematics in RE2TM17 and RE2TM4B compounds (abstract)
Authors: Welch D. O.
Published: July 2002

Recent advances in the theory of bonding and the structural properties of solids have shown that simplified many-atom bonding theories utilizing various tight-binding-based approximations to the electronic density-of-states distributions are capable of systematizing experimental data as well as the results of ab initio theoretical calculations for a wide variety of solids including metallic compounds.123 Such theories are also capable of making useful predictions of the energetics of point defects and disorder, solution energetics, and deviations from stoichiometry.4 In this article, I will describe a simple second-moment-approximation-based many-atom bonding theory, including magnetic contributions to cohesive energy, for the families of permanent magnet compounds RE2TM17 and RE2TM14B (where RE is a rare-earth metal and TM a transition metal). The use of the theory to describe the systematics of energies and volumes of formation, solute locations, and energetics, as well as the energetics of point-defect formation and nonstoichiometry, will be discussed.
PW
50
,
D50, and playback voltage formulas for shielded magnetoresistive heads
Authors: Zhang Yun , Neal Bertram H.
Published: July 2002

Simple, approximate formulas for PW50, D50, and peak playback voltage for shielded magnetoresistive (MR) heads are developed using the Potter approximation for the MR surface potential and the tanh model for the medium transition. The results can be utilized to analyze the role of parameter variation on signal voltage. The PW50 expression agrees well, for all practical magnetic spacings, with the Fourier series method which solves for the accurate MR surface potential. The product PW50D50 is nearly a constant 1.45. The calculated curve of peak voltage versus PW50 is compared with experimental data and good agreement is obtained.
Re-initialization of age imposed by magnetic field perturbation in the ferromagnetic phase of a re-entrant ferromagnet
Authors: Jonason K. , Nordblad P.
Published: July 2002

Inherent dynamic properties of the ferromagnetic phase of the standard re-entrant ferromagnet (Fe0.20Ni0.80)75P16B6Al3 have been experimentally investigated by ac-susceptibility measurements. A pronounced aging behavior and a fatal fragility to magnetic field perturbations is found—properties which also characterize a spin glass phase.
Parameter identification for a micromagnetic recording medium model (abstract)
Authors: Porter D. G. , O’Sullivan J. A. , Indeck R. S. , Muller M. W.
Published: July 2002

Many recent studies of recording and playback in magnetic recording systems have been based on the computer simulation of micromagnetic models of magnetic recording media. Medium models are capable of representing a large class of recording media by selection of appropriate values of model parameters representing the intrinisic properties of the recording medium material. Such parameters include grain size, anisotropy energy density, and exchange energy constant. Largely missing in these studies, however, has been a discussion of how to identify a set of parameter values to represent a particular medium of interest. In this presentation, we describe an identification procedure for selecting the values of the five intrinsic parameters of a micromagnetic medium model to represent a recording medium for which the following hysteresis properties are known: remanent magnetization, coercivity, remanence squareness, and coercivity squareness. Calculation of hysteresis loops verifies that the particular instance of the medium model identified by this procedure reproduces the hysteresis behavior of the medium of interest. Comparison of computer simulations of recording operations to measured recording performance of a medium on a laboratory tester may then be meaningfully made.
Analytical solution for the side-fringing fields of narrow beveled heads
Authors: Mayergoyz I. D. , Madabhushi R. , Burke E. R. , Gomez R. D.
Published: July 2002

By using conical coordinates, exact analytical solutions for three-dimensional side-fringing fields of recording heads that are beveled in the down-track direction are found. These solutions are derived under the assumption of zero gap length. The side-fringing fields for the two limiting cases of infinitesimally narrow heads and semi-infinitely wide heads are presented and compared.
Magnetic measurements on HgMnTe epitaxial layers (abstract)
Authors: Horsfall A. B. , Terry I. , Jackson W. M. , Brinkman A. W. , Oktik S.
Published: July 2002

Magnetization measurements have been performed on epitaxial layers of Hg1−xMnxTe (MMT) using an alternating gradient field magnetometer (AGFM). The layers, which have a Mn concentration in the range 0.05<x<0.20, and thicknesses between 1 and 6 μm, are deposited by metalorganic vapor phase epitaxy (MOVPE) techniques. The MMT films are grown on GaAs substrates with CdTe and ZnTe buffer layers. Growth is achieved by the interdiffused multilayer process (IMP), where alternate layers of HgTe and MnTe are deposited, and then heated, to produce diffusion and, hence, Hg1−xMnxTe. The MnTe layers that are grown are roughly 1 nm thick, and so grow in the cubic structure, rather than the nickel arsenide structure observed in the bulk material.1 The magnetization of the majority of the samples is the expected paramagnetic behavior with values of χ close to those observed in the bulk material.2 However, a number of the samples have shown evidence for magnetic order at room temperature, and this is also observed in MnTe layers grown by MOVPE. The samples showing saturation of the magnetization also shown an anomalous behavior of the Hall voltage below 100 K and at fields up to 0.3 T. The anomalies in the HgMnTe samples are thought to be due to MnTe inclusions left over from the MOVPE growth process.
Nonlinear spectroscopy of antiferromagnetic Cr2O3
Authors: Fiebig M. , Fröhlich D. , Pisarev R. V.
Published: July 2002

Antiferromagnetic Cr2O3 is investigated by nonlinear spectroscopy. A pronounced polarization dependence of the second harmonic spectrum was observed which is due to the interference of time-invariant and time-noninvariant contributions to the nonlinear signal. It can be used to visualize antiferromagnetic 180° domains with high resolution in a few minutes. Both spectroscopy and topography show that in the spin-flop phase the spins are oriented in the basal plane along a twofold axis.
Stability of soft-adjacent-layer magnetoresistive heads with patterned exchange longitudinal bias
Authors: Zhu Jian-Gang , O’Connor Daniel J.
Published: July 2002

Domain stability in patterned exchange stabilized MR/SAL heads is studied via micromagnetic modeling. It is found that the domain instability can be eliminated by making the stripe short and setting the initial magnetization direction in the SAL layer in the opposite direction of the exchange pinning field in the MR layer.