bluuebunny/crossref_metadata_2025 huggingface dataset:
Exchange magnetic coupling through nonmagnetic insulator spacers (abstract)
Authors: Zhang Shufeng
Published: July 2002

Interlayer magnetic coupling has been found in magnetic multilayers with nonmetallic spacers. In metallic spacers the coupling is provided by electrons at the Fermi sphere; in nonmetallic spacers the coupling is either due to localized states or to thermally activated conduction electrons (or by other external means, e.g., photon exposure). We calculate this coupling by extending the RKKY-like interaction to insulator bands at finite temperatures. Since the length scale 1/kF (where kF is the Fermi wave vector) that governs the oscillatory behavior of coupling for metallic spacers does not appear in the present case, the form of the coupling with respect to the thickness and temperature is quite different for metallic and for nonmetallic spacers. We study the interlayer coupling for two types of spacers, crystalline, and noncrystalline (amorphous) semiconductors. For the former, the conduction (or valence) bands are well-defined and the calculation of the coupling is rather straightforward. The coupling at room temperature oscillates from ferromagnetic to antiferromagnetic and back to weakly ferromagnetic as the thickness of the spacer layer increases. The antiferromagnetic coupling only exists in a narrow range of the thickness of the spacer layer. For an amorphous semiconductor spacer, the coupling can be expressed in terms of localized states. Within the variable range hopping picture, one can relate these localized states to the localization length. We discuss the range and temperature dependence of the coupling for amorphous spacers. Our calculations are in good agreement with experimental results.
Magnetization and magnetostriction curves from micromagnetics
Authors: DeSimone Antonio
Published: July 2002

The macroscopic behavior of magnetostrictive materials results from domain evolutions, often occurring at a microscopic scale. It is shown how to compute magnetization and magnetostriction curves by appraising the behavior of the underlying microstructures. The method hinges on an averaging device (Young measures), which allows one to pass from the microscopic to the macroscopic scale. The kinematical constraints on the accommodation of elastic effects are taken into account, and the role of material symmetry in the selection of energetically optimal microstructures is highlighted. Moreover, the effect of magnetoelastic coupling on the computation of magnetization curves is discussed.
Influence of Stoner-type excitations on the formation of magnetization and magnetic order in disordered metal-metalloid alloys
Authors: Arzhnikov A. K. , Dobysheva L. V. , Yelsukov E. P.
Published: July 2002

The Rhodes–Wolfarth parameter and the ratio T1/2c(x)/m̄(x) have been analyzed on the basis of experimental data for disordered alloys Fe–Al, Fe–Si, Fe–P (Tc is the Curie temperature, m̄ is the average magnetic moment, x is the concentration of non-magnetic impurity). The concentration dependences of these parameters show the possibility of considerable contribution of the Stoner excitations to the magnetic order formation. The analysis of the two-band Hubbard model shows that the Stoner excitations result in the turnover of the local magnetic moment at a certain temperature in the range of high concentration of nonmagnetic atoms.
Magnetoconductivity of Bi2Sr2Ca1−xYxCu2O8+δ in fluctuation regime
Authors: Dhard C. P. , Bhatia S. N. , Sastry P. V. P. S. S. , Yakhmi J. V. , Nigam A. K.
Published: July 2002

The magnetoconductivity of polycrystalline Bi2Sr2Ca1−xYxCu2O8+δ, x=0, 0.05 and 0.20 samples in the magnetic field of 4 T was measured. The excess conductivity has been analyzed in the light of Aronov–Hikami–Larkin and Bieri–Maki formalisms together with Thompson’s correction of the Zeeman term of both theories. The later theory was found within the clean limit to describe the data adequately and yielded the estimate for the phase braking time τφ≂1×10−13 s. The Maki–Thompson–Zeeman contribution (ΔσMTZ) in these samples is found to be negligible.
Long-time magnetic relaxation measurements on a quench melt growth YBCO superconductor
Authors: Bennett L. H. , Swartzendruber L. J. , Turchinskaya M. J. , Blendell J. E. , Habib J. M. , Seyoum H. M.
Published: July 2002

The decay of magnetization with time is recorded for a quench-melt-growth processed YBa2Cu3O7−x sample. The vortex-glass and collective pinning theories fit the data quite well over all time regimes with an exponent, μ, value of 0.78. The normalized logarithmic magnetic relaxation rate, S, attains a plateau value of 0.05, considerably higher than a variety of samples which fall within a universal plateau curve.
Accurate determination of permanent magnet motor parameters by digital torque angle measurement
Authors: Rahman M. A. , Zhou Ping
Published: July 2002

This article presents a novel test method to determine the important load dependent reactance parameters Xd, Xq, and magnet-excited voltage E0 of permanent magnet (PM) motors. The results bring forward a clear picture of the impact of the previous assumption of constant E0 on the value of Xd. As improved accuracy and fast data sampling are required for online control, the traditional method for measuring torque angle is deficient. To this end, a new microprocessor-based digital torque angle measurement system was designed and built. At the same time, a scheme for accurately positioning the zero torque angle is proposed without referring to the interior structure of a motor. The proposed techniques have been successfully employed for a 1 hp laboratory permanent magnet motor.
Spin wave dispersion in ferromagnetic nickel (abstract)
Authors: Rejcek J. M. , Fry J. L. , Fazleev N. G.
Published: July 2002

The spin wave dispersion in ferromagnetic nickel has been computed in random phase approximation using the wave vector and frequency dependent magnetic susceptibility including many body enhancement effects. The latter were included using an orbital basis to invert the susceptibility matrices that are encountered in a local density, first principles version of a Stoner-like theory of many body enhancements. The complicated computer codes employed in the calculation were tested by computing numerically the wave vector and frequency dependent spin and orbital magnetic susceptibility of the uniform electron gas and comparing with known analytic expressions. Numerical work was done using the analytic tetrahedron method. For nickel the theory was simplified by introducing a single adjustable parameter in lieu of calculation of complicated integrals involving the band structure. The parameter was adjusted to yield agreement with a long wavelength spin wave neutron scattering measurement. With the fit parameter, good agreement with the experimental dispersion of spin waves in nickel was obtained for other wavelengths as well.
Simulation of nuclear magnetic resonance spin echoes using the Bloch equation: Influence of magnetic field inhomogeneities
Authors: Nyenhuis J. A. , Yee O. P.
Published: July 2002

There are a number of low cost nuclear magnetic resonance (NMR) applications that are based on measurements of spin-spin (T2) and spin-lattice (T1) relaxation times. Pulsed rf magnetic fields are typically used to flip the nuclear spins in these measurements. The ability to perform these measurements in relatively nonuniform magnetic fields is a factor in minimizing costs. In this work, we explore the limitations of field inhomogeneity on spin echo techniques for measuring relaxation times. Using a computer model that was developed for this study, we numerically integrate the Bloch equation to simulate spin echo peaks for CPMG and other pulse sequences. We quantify how higher intensity rf pulses result in increased amplitudes of spin echo peaks in an inhomogeneous static magnetic field. An rf amplitude which is five times the maximum inhomogeneity in the static field results in less than 0.5% attenuation between even numbered peaks in a CPMG sequence.
Small-angle neutron scattering study of the flux-line lattice in a single crystal of Bi2.15Sr1.95CaCu2O8+x (invited)
Authors: Yethiraj M. , Mook H. A. , Forgan E. M. , Cubitt R. , Wylie M. T. , Paul D. M. , Lee S. L. , Ricketts J. , Kes P. H. , Mortensen K.
Published: July 2002

A flux-line lattice (FLL) was observed in a single crystal of Bi2.15Sr1.95CaCu2O8+x (BSCCO) using small-angle neutron scattering methods. The sample has a superconducting transition at 85 K. The flux-line lattice is observed to melt, evidenced by the rapid disappearance of diffracted intensity as the temperature is increased above a field-dependent melting temperature. Diffracted intensity due to the vortex lattice also falls off as the applied field is increased. It is believed that this is a manifestation of the transition of the three-dimensional flux lines into two-dimensional pancake vortices. The Bragg intensity of the FLL peak is inversely proportional to the fourth power of the London penetration depth (λL). Hence, the temperature (T) dependence of the order parameter can be measured quite accurately from the intensity of the Bragg spots at different temperatures. In BSCCO with an applied field of 50 mT, the measured T dependence appears linear. The low-T behavior is of great interest for an understanding of the underlying mechanism for superconductivity in these materials.
Evolution of structure and magnetoresistance in granular Ni(Fe,Co)/Ag multilayers: Dependence on magnetic layer thickness
Authors: Bian X. , Meng X. , Ström-Olsen J. O. , Altounian Z. , Muir W. B. , Sutton M. , Cochrane R. W.
Published: July 2002

Structural and magnetoresistance results on annealed sputtered (Ni81Fe19, Ni66Fe16Co18)/Ag granular multilayers are presented. Structural evolution has shown that highly (111) textured, discontinuous layered structures can persist on annealing up to 400 °C. The average magnetic particle size is controlled by the annealing temperature and the initial magnetic layer thickness. No giant magnetoresistance was observed in the as-deposited films, while significant MR was found after annealing between 300 °C and 400 °C. Magnetoresistance over 30%, together with a small saturation field, was found at 4.2 K for a starting magnetic thickness of 4 Å. Increasing the magnetic layer thickness to 20 Å greatly improves the magnetic thermal stability, and leads to high magnetoresistive sensitivities of up to 0.35%/Oe in a field of 10 Oe at room temperature. The magnetization hysteresis, anisotropy, and magnetic interaction in such a granular multilayer are also discussed.
A full electron LMTO-ASA study of electronic band structure and magnetic properties for RFe11TiNx (R=Y, Nd, Sm; x=0,1)
Authors: Hu W. Y. , Zhang J. Z. , Zheng Q. Q. , Pan C. Y.
Published: July 2002

The band structure and magnetic properties are studied for RFe11TiNx (R=Y, Nd, Sm; x=0,1) rare-earth iron intermetallic compounds using the linear muffin tin orbital with the atomic sphere approximation (LMTO-ASA) method. In order to elucidate the role played by the rare-earth atoms in these compounds, a full electron calculation is performed using a semirelativistic spin-polarized LMTO-ASA method in the local spin density approximation (LSDA) regime. The 4f electrons of the rare-earth atoms are considered to be valence electrons in the self-consistent calculations. For NdFe11TiN, a calculation in which the 4f electrons are treated as core-frozen states is also performed, and is compared with the full electron calculation. The effects of N atoms in these compounds are also discussed.
Influence of the dipole interaction on the direction of the magnetization in thin ferromagnetic films (abstract)
Authors: Moschel A. , Usadel K. D.
Published: July 2002

The magnetization of thin films depends in a very sensitive way on surface anisotropy fields which often favor a perpendicular orientation and on the dipole interaction which favors an in-plane magnetization. A temperature driven transition from one to the other orientation has been observed experimentally. In order to understand this behavior theoretically we performed detailed calculations of the magnetization of very thin films (thickness of up to 5 layers) within a quantum mechanical mean field approach. A surface anisotropy that favors a perpendicular orientation and a long range dipole interaction were taken into account. It is shown that these competing interactions for certain values of the parameters may result in a temperature driven switching transition from an out-of plane to an in-plane ordered state. Varying the strength of the dipole interaction we found that the switching temperature is a very sensitive function of the ratio of these two competing interactions. A perpendicular ground state magnetization of the film is only found for values of the surface anisotropy which are larger than a critical surface anisotropy value. The reorientation of the magnetization vector has its physical origin in an entropy increase of the system when going from a perpendicular to an in-plane ordered state.
Theory of the negative magnetoresistance of ferromagnetic-normal metallic multilayers (invited)
Authors: Falicov L. M. , Hood Randolph Q.
Published: July 2002

Transport properties for a system consisting of a ferromagnetic-normal metallic multilayer are theoretically examined. The in-plane conductance of the film is calculated for two configurations; the ferromagnetic layers aligned (i) parallel and (ii) antiparallel to each other. The results explain the giant negative magnetoresistance encountered in these systems when an initial antiparallel arrangement is changed into a parallel configuration by application of an external magnetic field. The calculation depends on (a) geometric parameters (the thicknesses of the layers); (b) intrinsic metal parameters (number of conduction electrons, magnetization and effective masses in the layers); (c) bulk sample properties (conductivity relaxation times); and (d) interface scattering properties (diffuse scattering versus potential scattering at the interfaces). It is found that a large negative magnetoresistance requires, in general, considerable asymmetry in the interface scattering for the two spin orientations. All qualitative features of the experiments are reproduced. Quantitative agreement can be achieved with sensible values of the parameters. The effect can be conceptually explained based on considerations of phase-space availability for an electron of a given spin orientation as it travels through the multilayer sample in the various configurations.
Magnetoresistance of the magnetically ordered icosahedral quasicrystals Al-Pd-Mn-B
Authors: Yewondwossen M. H. , Ritcey S. P. , Yang Z. J. , Dunlap R. A.
Published: July 2002

The transverse magnetoresistance has been measured for the newly discovered magnetically ordered icosahedral quasicrystals Al70−xPd15Mn15Bx (x=0,2,4,6,8,10). Experiments were conducted at 4.2 K in magnetic fields up to 5.5 T. The results showed a systematic change as a function of boron concentration. The magnetoresistance for x=0,2,4,6 showed a negative field dependence which became weak with increasing x. The samples with x=8,10 showed a negative magnetoresistance at low fields, which became positive for higher applied fields. The minimum in the magnetoresistance shifted to lower field value with increasing boron content (to about 0.5 T for 10 at. %B). The analysis of the results has been based on the theory of three-dimensional weak localization, with a strong influence of spin-orbit scattering. The magnetoresistance is negative for low boron concentration due to weak spin-orbit scattering, and is positive for high boron concentration due to strong spin-orbit scattering.
Electrical transport in amorphous Fe-Mn-Zr alloys
Authors: Srinivas V. , Nigam A. K. , Chandra G. , Lawther D. W. , Yewondwossen M. , Dunlap R. A.
Published: July 2002

The magnetic and electrical transport properties of amorphous Fe-Zr based alloys with compositions near 10 at. % Zr with various elements substituted for Fe are of particular interest. In the case of Mn substitutions the Curie temperature and the average magnetic moment decrease monotonically with increasing Mn content and the temperature dependence of the magnetization is significantly modified. The electrical transport properties of amorphous Fe90−xMnxZr10 (for x=0, 4, 8, and 12) over the temperature range of 4.2–300 K and the magnetoresistance for fields up to 4.0 T at 4.2 K are reported in the present work. A broad minimum in the resistivity is observed at around 255, 235, 200, and 180 K for the four compositions, respectively. In the case of the x=8 sample a second minimum occurs at around 50 K. The magnetoresistance of all samples shows a sharp increase for small fields and a linear field relationship for fields above about 0.1 T.
Magnetic phase diagrams of NdRu2Si2 and TbRu2Si2 compounds
Authors: Salgueiro da Silva M. , Sousa J. B. , Chevalier B. , Étourneau J.
Published: July 2002

Here we report a detailed study providing complete H-T phase diagrams for both NdRu2Si2 and TbRu2Si2 intermetallic compounds, based on high resolution SQUID magnetization measurements, in the temperature range 2–300 K and in applied magnetic fields up to 5 T, using c-axis magnetically oriented powders. For NdRu2Si2, our results are in good agreement with recent neutron diffraction work under applied magnetic fields, confirming the appearance of an intermediate (temperature and field) ferrimagnetic phase responsible for the two-step metamagnetic process observed in the temperature range 16–22.5 K. Our isofield magnetization curves provide unambiguous evidence on the decomposition of this ferrimagnetic phase into ferromagnetic and antiferromagnetic components. Furthermore, at low temperatures and low fields, we observe, in this compound, the presence of thermal irreversibility effects. For TbRu2Si2, we present an experimental account, on its H-T phase diagram, which shows striking differences with respect to that of NdRu2Si2. In particular, the ferrimagnetic phase, which sets in at 50 K, persists down to the lowest temperature. In addition, we observe reentrant ferrimagnetism in this compound (ferri-ferro-ferri) under applied fields in the range 28–31 kOe.
Influence of exchange bond disorder on the magnetic properties of (Pd1−xFex)95Mn5 near T
c
Authors: Wang Z. , Qi X. , Kunkel H. P. , Williams Gwyn
Published: July 2002

A summary of a detailed study of the field and temperature dependent ac susceptibility of a series of ternary (Pd1−xFex)95Mn5 alloys near the ferromagnetic ordering temperature Tc is presented. By following the behavior of the crossover line (a line of maxima in the field dependent susceptibility above Tc, the maxima moving upwards in temperature but decreasing in amplitude with increasing field) it is possible to catalog the influence of exchange bond disorder on aspects of the critical behavior. These ternary systems are not soft ferromagnets and thus asymptotic critical exponents are difficult to estimate using this technique. Nevertheless, effective exponents (i.e., obtained away from the critical point) clearly reflect the presence of significant variance in the distribution of exchange bonds.
Magnetic ordering in the three-dimensional site frustrated Heisenberg model
Authors: Nielsen Morten , Ryan D. H. , Guo Hong , Zuckermann Martin
Published: July 2002

We study transverse spin freezing in the site frustrated three-dimensional classical Heisenberg model using Monte Carlo simulations. For small values of the site randomness, there is no transverse spin freezing in the ferromagnetic state. As the fraction of the antiferromagnetic sites is increased beyond 16%, we observed that the transverse component of the spins freeze in random directions at temperatures below some value Txy. Similar behavior is observed in the antiferromagnetic state. We compare results of this model to those of the bond frustrated model. Finally an approximate phase diagram of this model is presented.
Incommensurate antiferromagnetic phase in UNiGe
Authors: Sechovský V. , Havela L. , Svoboda P. , Purwanto A. , Larson Allen C. , Robinson R. A. , Prokeš K. , Nakotte H. , de Boer F. R. , Maletta H.
Published: July 2002

By specific-heat, magnetization, electrical resistivity, and neutron-diffraction measurements on a single crystal we have confirmed that UNiGe orders antiferromagnetically below 50.5 K into an incommensurate phase with q=(0,1/2,−1/2)±(0,δ,δ), δ∼0.15. δ decreases continuously with decreasing temperature to ∼0.123 at 41.5 K, where the incommensurate phase vanishes in a first-order phase transition and a commensurate antiferromagnetic structure with q=(0,1/2,1/2) sets in and remains stable down to the lowest temperatures. If a magnetic field sufficient to induce a metalmagnetic transition (1–5 T) is applied along the c axis, both antiferromagnetic phases are transformed to an uncompensated AF phase with q=(0,1/3,1/3) yielding a nonzero magnetization M≊1/3×MS. The latter structure is destroyed and a complete alignment of U moments is achieved in fields above 10 T. The strikingly different B-T diagrams observed for a magnetic field applied along different crystallographic directions reflect strongly anisotropic exchange interactions.
Discovery of τ=2/9 lock-in in holmium
Authors: Tindall D. A. , Adams C. P. , Steinitz M. O. , Holden T. M.
Published: July 2002

The complex magnetic behavior of holmium is a well-established experimental fact and has been studied by a variety of experimental techniques, including magnetization, x-ray diffraction, thermal expansion, and heat capacity. Our recent studies have focused on using neutron diffraction to study the temperature and field dependence of the helimagnetic structure. The pitch of the helix is described by τ, the spiral wave vector. τ usually varies smoothly with temperature but tends to lock in at various values that are commensurate with the lattice when a field is applied. These lock-ins provide clues to the nature of the different magnetic phases and, up to now, all the lock-ins have corresponded to features of the magnetic phase diagram obtained from magnetization measurements. However, recent experiments have revealed a previously unobserved lock-in at τ=2/9 rlu in a 1.4 T b-axis field. It shares the same general features of other lock-ins except that it does not correspond to any feature of the magnetic phase diagram.
Fe-Nd-C-based ingot permanent magnets by solid-state transformation
Authors: Leonowicz M. , Davies H. A. , Wojciechowski S.
Published: July 2002

The effects of composition, annealing temperature, and annealing time on the magnetic properties and microstructure of Fe66M2Nd20C9.5B0.5 (M=Ga,Nb,Cu,Al) ingot magnets are presented. It has been found that the magnitude of the hard magnetic properties, which evolve during the solid-state transformation Fe17Nd2Cx to Fe14Nd2C, depends on the additive elements. These additions which decrease the melting temperature of the intergranular phase accelerate the transformation process and those which increase this temperature retard the transformation. Coercivities up to 880 kA/m combined with a remanence 0.58 T were produced in alloy ingots with Cu. The initial grain size was found to be crucial in producing good magnetic properties and coercivities as high as 980 kA/m were achieved for Fe-Nd-C-B alloy magnets hot pressed from microcrystalline melt spun ribbon. A modest increase of remanence (∼0.7 T) was achieved by die-upset forging of ingot alloys.
Magnetic characterization of epitaxial Y5FeeO12/Bi3Fe5O12 and Y5Fe3O12/Eu1Bi2Fe5 O12 heterostructures grown by pulsed laser deposition
Authors: Simion B. M. , Ramesh R. , Keramidas V. G. , Thomas G. , Marinero E. , Pfeffer R. L.
Published: July 2002

Epitaxial Y3Fe5O12(YIG)/Bi3Fe5O12(BIG) and YIG/Eu1Bi2Fe5O12(EBIG) heterostructures have been grown on [111] oriented single-crystalline gadolinium-gallium-garnet substrates by pulsed laser deposition. The effects of the layer thickness ratios on the composition, microstructure, and magnetic properties of the films have been studied employing x-ray diffraction, Rutherford backscattering spectroscopy, vibration sample magnetometry, and Kerr magnetometry. All films under investigation are single crystalline, in the [111] orientation. The multilayered heterostructures displayed superior magnetic properties in comparison to their single crystalline monolayer counterparts, deposited at the same conditions. The YIG/BIG heterostructures indicate increased in-plane saturation magnetic moments, approaching the maximum saturation value in bulk YIG. The YIG/EBIG heterostructures show a definite reorientation of the magnetic moment in the out-of-plane direction, a new set of increased saturation magnetization values that go even above that recorded for the bulk YIG, as well as an increase in coercivity.
Interfacial contributions to magnetic anisotropy in metal/semiconductor systems (abstract)
Authors: Jonker B. T. , Abad H. , Krebs J. J.
Published: July 2002

It has been empirically established that the net anisotropy exhibited by even a simple metal epitaxial film is strongly effected by the interactions at the metal/semiconductor interface, although the mechanisms have not been systematically addressed and remain an open issue. An Fe(001) film, for example, is expected to exhibit a four fold symmetry in the in-plane magnetic anisotropy, consistent with the crystalline surface symmetry. Such films grown on a bulk GaAs(001) surface, however, exhibit a significant uniaxial anisotropy (twofold symmetry). It has been speculated that this uniaxial anisotropy arises from the strongly directional bonds of the substrate surface, based on a model that assumes a bulk-terminated, unreconstructed zinc-blende (001) surface. This model fails to explain, however, the nearly ideal fourfold symmetry observed for similar Fe(001) films on the ZnSe(001) surface, which has an identical crystalline structure as GaAs and differs in lattice constant by only 0.25%. We propose here a conceptual model to account for these pronounced differences, which describes how contributions to the net magnetic anisotropy arise from the formation of the interface and early stages of the metal film growth. We suggest that these contributions originate in the initial metal adsorption sites and subsequent bond or site filling, and are strongly dependent on the semiconductor surface reconstruction. We compare this model with results for Fe growth on several surface reconstructions.
Calculated field cooled and zero field cooled magnetizations of the three-dimensional Ising spin glass using Monte Carlo hard-spin mean-field theory (abstract)
Authors: Ames Edwin A. , McKay Susan R.
Published: July 2002

We have studied the ±J Ising spin glass in a uniform magnetic field by using Monte Carlo hard-spin mean-field theory, implemented on a 24×24×24 lattice with periodic boundary conditions. This method modifies the conventional mean-field approach to retain frustration by placing a spin of unit magnitude at each nearest neighbor. Each spin’s direction is chosen using the hard spin condition, which preserves the site’s average magnetization. The method yields an average magnetization at each site and permits the exploration of stable and metastable states within the spin-glass phase and their overlaps. By direct calculation of the free energy of each final state, stability and metastability can be distinguished. The resulting average magnetizations display realistic history dependence. For example, the calculated field cooled magnetization remains roughly constant below the freezing temperature, Tg, whereas the zero field cooled magnetization rises to a cusp as temperature is increased below Tg, as is observed experimentally. Aging phenomena are also reproduced within this method, since varying the number of Monte Carlo steps at each temperature and field corresponds to altering the waiting time.
The relationship between the microstructure and magnetic properties of sputtered Co/Pt multilayer films (abstract)
Authors: Kim Y. H. , Petford-Long Amanda K. , Jakubovics J. P.
Published: July 2002

Co/Pd multilayer films (MLFs) are of interest because of their potential application as high-density magneto-optical recording media. Co/Pd MLFs with varying Co and Pd layer thicknesses were grown by sputter-deposition onto (100) Si wafers. X-ray diffraction and high resolution electron microscopy were used to study the microstructure of the films, and Lorentz microscopy was used to analyze their magnetic domain structure. The films show an fcc crystal structure with a compromised lattice parameter and a strong (111) crystallographic texture in the growth direction. The compromised interplanar spacing parallel to the surface increased with decreasing thickness ratio (tCo/tPd), and the columnar grain size decreased with increasing Pd layer thickness. Films with tCo=0.35 nm and tPd=2.8 nm (columnar grain diameter 20 nm) showed promising magnetic properties, namely a high perpendicular magnetic anisotropy (1.85×105 J m−3), with a perpendicular coercivity of 98.7 kA m−1, a perpendicular remanence ratio of 99%, and a perpendicular coercivity ratio of 88%. The magnetic domains were uniform and of a narrow stripe type, confirming the perpendicular easy axis of magnetization. The Curie temperature was found to be about 430 °C. Films of pure Co and Pd, grown for comparison, also showed columnar grain structure with grain-sizes of the same order as those seen in the MLFs. In addition the Pd films showed a (111) textured fcc structure.
Characteristics of the Mn and Co volume-doped acicular γ-Fe2O3 particles
Authors: Han D. H. , Wang J. P. , Luo H. L.
Published: July 2002

Mn- and Co-doped acicular γ-Fe2O3 particles with a coercivity of 923 Oe and Co content of 3.5 wt % have been prepared by a special process. This involves high-temperature ion diffusion, then step annealing. These particles exhibit a high magnetic thermal stability and low-temperature coefficient of coercivity. The latter demonstrates a striking contrast to the ordinary cobalt-body-doped γ-Fe2O3 particles. The induced uniaxial anisotropy constant Ku and the magnetocrystalline anisotropy constant K1 of the particles were determined by torque measurements. The novel properties of these particles are mainly due to the high uniaxial anisotropy, resulting from the anisotropic arrangements of cobalt ions. With the above characteristics and low Co content, these acicular particles can be one of the promising candidates for particulate high-density magnetic recording materials.
Optical characterization of Nd3+:Sr5(VO4)3F
Authors: Sardar Dhiraj K. , Bella Peter D.
Published: July 2002

A spectroscopic investigation of Nd3+ in Sr5(VO4)3F has been performed. The Judd–Ofelt (J–O) theory was applied to the polarization-averaged optical-absorption spectra of Nd3+ 4f3 transitions to determine the crystal intensity parameters: Ω2=9.14×10−20 cm2, Ω4=5.92×10−20 cm2, and Ω6=2.03×10−20 cm2. The intensity parameters were then used to determine the emission probabilities of transitions from the Nd3+ 4F3/2 metastable state to the lower lying J manifolds and their corresponding branching ratios, the radiative lifetime (248 μs), and the quantum efficiency (0.93) of the 4F3/2 state.
Self-assembled monolayer film for enhanced imaging of rough surfaces with atomic force microscopy
Authors: Alley R. L. , Komvopoulos K. , Howe R. T.
Published: July 2002

The significance of the surface chemical state and resulting forces on the imaging fidelity of atomic force microscopy (AFM) was investigated experimentally in ambient air. Adhesion tests were conducted on smooth monocrystalline (100) silicon substrates with sharp silicon tips of 5 nm nominal radius. Surface topography imaging of rough polycrystalline silicon surfaces was performed with similar sharp silicon tips. The substrates were initially subjected to different chemical treatments to obtain either hydrophilic or hydrophobic surface behaviors. The AFM tip treatment involved the deposition of an organized close-packed octadecyltrichlorosilane (OTS) hydrophobic self-assembled monolayer. Statistical results for the surface force at the tip-sample interface versus the separation distance and the various moments of the surface height distribution function demonstrated that imaging artifacts due to sticking of the tip on sidewalls of steep asperities were particularly pronounced when imaging hydrophilic surfaces with uncoated tips. The tip artifacts comprised spurious negative excursions or exaggerated asperity heights depending on the direction of scanning. Significant enhancement of the AFM imaging quality was accomplished with OTS-coated tips, as demonstrated by the markedly reduced magnitude of the adhesive force at the tip-sample interface (about 1 nN for both hydrophilic and hydrophobic smooth silicon surfaces), the consistency of the calculated surface height moments, and the negligibly small effect of the scanning direction on the imaged topographies. The results suggest that the present treatment of the AFM tip surface could provide an effective means for overcoming tip artifacts with other difficult-to-image surfaces.
Time-integrated optical emission studies of plumes generated from laser ablated germania glass
Authors: Wolf Paul J.
Published: July 2002

The optical emission from plumes induced by ArF laser irradiation of GeO2 was characterized as a function of laser fluence, distance from the target surface, and ambient O2 pressure. Dispersion of the light emitted by the plume in a vacuum revealed emission from both neutral and singly ionized Ge atoms as well as neutral O atoms. The spatial variation showed that the ion concentration decreased exponentially from the target surface while the neutral atom number density reached peak intensities at distances of ≊1.5–2.5 cm from the target surface. Interactions between the plume constituents and the ambient molecular oxygen increased the excited Ge atom and Ge ion populations in the plume and, most notably, significantly enlarged the excited O atom concentration over that produced directly from the ablation process.
Metal-insulator-semiconductor capacitors on cleaved GaAs(110)
Authors: Huang L. J. , Lau W. M. , Ingrey S. , Landheer D. , Noël J.-P.
Published: July 2002

Metal-insulator-semiconductor capacitors were fabricated on cleaved n-GaAs (110) facets using remote plasma-deposited silicon nitride as gate insulators. The interface properties of the capacitors made on this surface were analyzed by capacitance-voltage (C-V) measurements. X-ray photoemission spectroscopy was also used to investigate the chemical structure of the interface. Prior to the insulator deposition, the cleaved facets were processed with different surface treatments including HF etch of native oxide, passivation with an ammonium sulfide solution, passivation with hydrogen polysulfide, and passivation with a silicon interface control layer. It was found that while the passivation procedures with the sulfur compounds did improve the C-V data when compared with the HF oxide etch, the silicon interface control layer technique led to the best C-V results. By comparing the quasistatic and high-frequency (1 MHz) C-V data, it was found that the minimum interface state density of the fabricated capacitors was about 1012 eV−1 cm−2.
Influence of oxygen on the performance of GaAs/AlGaAs quantum well infrared photodetectors
Authors: Tsai K. L. , Lee C. P. , Chang K. H. , Chen H. R. , Tsang J. S.
Published: July 2002

The influence of oxygen on the performance of GaAs/AlGaAs quantum well infrared photodetectors (QWIP) has been studied. Photoluminescence and secondary ion mass spectroscopy were used to examine the relationship between the quality of AlGaAs and the oxygen content that were then correlated to the performance of QWIPs. It was found that oxygen is the dominant impurity in the GaAs/AlGaAs superlattice. Because oxygen atoms behave likes electron traps, which effectively reduce the carrier concentration in the material, both of the responsivity and the dark current of QWIP are reduced. The detectivities of QWIPs with lower oxygen concentrations are better than those with higher oxygen concentrations.
Operation of a K-band second harmonic coaxial gyroklystron
Authors: Flaherty M. K. E. , Lawson W. , Hogan B. , Matthews H. W. , Calame J. P.
Published: July 2002

Amplification studies of a two-cavity second harmonic gyroklystron with a coaxial input cavity and drift tube are reported. The inner conductor is supported by tungsten pins which intercept the beam, and it utilizes lossy dielectric rings to enhance mode suppression. A double-anode magnetron injection gun produces a 440 kV, 200–268 A, 1 μs beam with an average perpendicular-to-parallel velocity ratio near one. The TE011 input cavity is driven near 9.886 GHz and the TE021 output cavity resonates near 19.772 GHz. Peak powers near 30 MW have been achieved, although more easily reproducible peak powers hover closer to 20 MW. While stability is improved over previous devices, beam interception reduces the accessible range in parameter space and thus degrades amplifier performance. Pin erosion is also evident and qualitatively agrees well with theoretical predictions.
Dilatometric and thermo-optic properties of alkaline earth and cadmium fluorides between 300 and 1300 K
Authors: Aurora T. S. , Day S. M. , Pederson D. O.
Published: July 2002

Temperature variation of the coefficients of linear thermal expansion and optical length was measured with an interferometer using 632.8 nm wavelength light from a He-Ne laser over the temperature range of 300–1300 K. The experiment was carried out under a high vacuum to minimize sample oxidation. For the optical length variation, the laser beam was allowed to pass through the crystals while, for the thermal expansion work, the laser beam was reflected from a coating of platinum deposited on the faces of crystals. Temperature variation of the coefficient of refractive index was calculated from the measurements of the coefficients of linear expansion and optical length variation. The results were used to calculate temperature variation of the optically important quantities such as molecular polarizability, energy band gap, refractive index variation with density, and the strain-polarizability parameter with the help of the Lorentz–Lorenz relation. The optical quantities exhibit a small variation over the temperature range of 300–∼800 K. The results were compared to the values in available literature.
Thickness dependence of the magnetic and electrical properties of Fe:SiO2 nanocomposite films
Authors: Malhotra S. S. , Liu Y. , Shen J. X. , Liou S. H. , Sellmyer D. J.
Published: July 2002

Nanocomposite Fe80(SiO2)20 films with thickness from 150 to 5000 Å have been prepared by rf magnetron sputtering from a composite target. The crystallites in the Fe80(SiO2)20 films have a bcc structure with the average size of 46–66 Å which was determined by transmission electron microscopy. As indicated by the thickness dependence of resistivity, the stacking and connectivity of the crystallites depend on the thickness of the films. The magnetic properties also depend on the microstructure which changes with the thickness of the films. The magnetic coercivity of the films increases with the thickness of the film, reaches a maximum, and then decreases. The maximum coercivity of 400 Oe at 300 K and 1200 Oe at 5 K was observed for a film with a thickness of about 700 Å.
Structural response of KTiOPO4 to electric fields: Effects due to low frequency modulation
Authors: Hamichi M. , Cousins C. S. G. , Meads R. E. , Sheldon B. J. , Teat S. J. , Thomas P. A.
Published: July 2002

A 290 μm slice of z-cut, flux-grown, KTiOPO4 was subjected to square-wave electric field modulation at 1 Hz and examined using synchrotron radiation and a differential energy-dispersive x-ray spectroscopy system. Asymmetric Laue reflections (h0h and h̄0h) revealed the development of normally forbidden reflections (h odd) whose intensities grew until a threshold field of about 70 V/mm was reached, above which they disappeared. This effect was reversible and is interpreted as due to field-induced K+ ion disorder followed by domain reversal above a critical field with subsequent ordering. Symmetric Bragg reflections (00l) showed a dramatic effect: after a period of modulation, also at about 70 V/mm, the intensities began to fall and, at the same time, the peak widths began to narrow. This behavior resembles the transition from an ideally imperfect to an ideally perfect crystal and suggests that the crystal has been annealed electrically.
Observation of localized domain reversal of iron-doped potassium niobate (Fe: KNbO3) single crystal
Authors: Lu Zheng , Xiao Min
Published: July 2002

Localized reversal of patterns formed by ferroelectric domains on the naturally grown surface in an iron-doped potassium niobate (Fe: KNbO3) single crystal has been observed. Surface-profile measurement, scanning electronic microscopy, and the metalomicroscopy methods were used to verify the observation. The mechanism of forming this domain structure is discussed.
Electric-field dependence of the photocarrier hopping mobility in bismuth silicon oxide
Authors: Tayag Tristan J. , Batchman Theodore E. , Sluss James J.
Published: July 2002

The photocarrier mobility of bismuth silicon oxide was measured through a time-of-flight technique. To determine the photocarrier transit time, a method of data analysis was applied that has been developed for transit time dispersion in amorphous solids. The universality of current shape typical of low-mobility amorphous solids was observed and an electric-field dependence of the photocarrier mobility in bismuth silicon oxide is now reported. In accordance with hopping conduction among localized band-gap sites, the photocarrier mobility in bismuth silicon oxide decreases with increasing electric field.
Numerical modeling of transient characteristics of photovoltage in Schottky contacts
Authors: Chen T. P. , Ho S. M. , Fung S. , Beling C. D.
Published: July 2002

Numerical modeling of the transient characteristics of the photovoltage at metal-semiconductor interfaces has been carried out with a simple model in which the contributions of different current transport processes including thermionic emission, tunneling, carrier recombination, and leakage current have been taken into account. The simulation gives the detailed dependence of the transient characteristics on temperature, doping concentration, Schottky barrier height, and leakage resistance.
Deep center scattering potential in InGaP
Authors: Zhu Q. S. , Hiramatsu K. , Sawaki N. , Akasaki I. , Liu X. N.
Published: July 2002

Mobility limited by the scattering of a phosphorous vacancy-induced deep center in In0.5Ga0.5P alloy grown by liquid-phase epitaxy on a (100) GaAs substrate has been investigated by means of Hall mobility and deep-level transient spectroscopy measurements. Two kinds of scattering potentials, Gaussian well type and square-well type, were considered. It was found that the scattering potential can be better described by the Gaussian-type potential than the square-well one. The mobility limited by deep center scattering has been fully calculated and analyzed. As a result, the mobility is characterized by a temperature dependence of T−1/2 in the temperature range from 77 to 400 K. The point defect scattering severely reduces the total mobility as its concentration increases. In addition, the scattering case when there is an electron trapped in the Gaussian potential well was also quantitatively investigated.
Er-related deep centers in GaAs doped with Er by ion implantation and molecular beam epitaxy
Authors: Elsaesser D. W. , Yeo Y. K. , Hengehold R. L. , Evans K. R. , Pedrotti F. L.
Published: July 2002

Deep level transient spectroscopy (DLTS) and temperature-dependent Hall effect measurements were performed on undoped, n-, and p-type GaAs doped with Er either by ion implantation or during molecular beam epitaxial (MBE) crystal growth. For light Er doping (≲1017 cm−3), a hole trap was observed at 35 meV above the valence band, whereas for heavier doping (≳5×1017 cm−3), a hole trap was observed at 360 meV. Both traps were found in both ion implanted and MBE materials. The 35 meV center is attributed to the Er atom occupying the Ga site and acting as an isovalent impurity, whereas the 360 meV center is attributed to Er in an interstitial position. Furthermore, photoluminescence and DLTS measurements of Er-implanted GaAs revealed that both the Er3+ intra-4f-shell emission intensity and concentration of the deep 360 meV centers were maximized at an annealing temperature of 750 °C, and they decreased at higher temperature anneals, while the concentration of the shallow 35 meV centers increased.
Bulk and contact effects in p+-SI-n+ semi-insulating GaAs structures
Authors: Manifacier J. C. , Ardebili R.
Published: July 2002

Current–voltage characteristics, free- and trapped-carrier concentrations, electric field, and excess carrier lifetime contours are given for a p+-SI-n+ GaAs structure. This analysis is done by numerical simulation of the two carrier drift-diffusion model of conduction. The dependence of the deep trap parameters and of the semi-insulating (SI) thickness on the J-V characteristic is presented. Particular emphasis will be put on the study of the equivalent resistivity and electric-field overshoots inside the structure. It is shown that, except for very thick samples, contact and near contact effects on the low-voltage electrical conduction mechanism are all important. It is then necessary to take into account these effects for a precise and exact interpretation of transport in a p+-SI-n+ structure. When the deep trap density Nt is high (Nt≫ni), as is the case for compensated (SI) GaAs, the equivalent low current resistivity ρ is higher than the equilibrium (unperturbed bulk) resistivity ρe. The nature, electron trap or hole trap, of the deep level affects the equivalent resistivity at intermediate applied voltage. When the density of deep traps is low, ρ decreases from values higher than ρe to values lower than ρe as the sample thickness increases. Only in the limit of very thick samples, and then regardless of the trap concentration, does the equivalent resistivity tend asymptotically towards its equilibrium value.
Phases and structures of nanocrystalline TiN films
Authors: Wang Dazhi , Wang Xi , Yang Genqin , Liu Xianghui , Jia Yunbuo , Zhou Guien , Li Guofeng
Published: July 2002

Nanocrystalline TiN thin films were synthesized by Xe+ ion-beam-enhanced deposition at room temperature. X-ray diffraction was used to study different samples prepared by different energy Xe+ ions. The results indicated that different ion energies can introduce different phases. Two metastable phases (Ti2N) can be formed at higher Xe+ energies than 40 keV. The crystalline phases are dependent on the temperatures in the collision cascades, which are induced by Xe+ ion bombardment with different energies. The films were always composed of nanocrystallites. Therefore a lot of surface and interface structures are in the film. The grain sizes grow with the increase of the bombarding ion energy. The effect is caused by the increase of cascade size and local temperature. The lattice parameters are found to increase with the rise of defect concentration and to decrease with the increase of surface structure. These films often have preferential orientation. The relationship between preferential orientation and process parameters is discussed.