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Near the Curie temperature, the anisotropy field of magnetically uniaxial L10 FePt is expected to follow the scaling law (1 – T/Tc)β, where T is the temperature and Tc is the Curie temperature. In the literature, β values between 0.36 and 0.65 have been reported. Based on recording measurements and micromagnetic analysis, we show that only the values of β near the low end of the reported range are compatible with the data. We also conclude that thermally activated magnetization reversal at temperatures near Tc cannot be ignored, even at time scales smaller than 1 ns. We demonstrate that thermally activated magnetization reversal at temperatures close to Tc is well described by conventional theory with a frequency factor f0 of the order of 1012 Hz. It is reasoned that the unusually high value for f0 is a consequence of the temperature-induced reduction of the degree of alignment of the micro-spins within the grains. [ABSTRACT FROM AUTHOR]

Atomistic spin model simulations using Langevin dynamics are performed to study the factors that determine the thermomagnetic recording time window in FePt media. The onset of thermomagnetic writing occurs at a temperature To larger than the Curie temperature Tc as a result of the finite time of relaxation of the magnetization by the linear reversal mode. The Bloch relaxation rate of magnetization growth during cooling below Tc is independent on the write field, provided the field is stronger than some threshold value. Application of a strong write field reduces switching time through better spin alignment in the paramagnetic regime. Finite size effects on the probability distribution of freezing temperatures Tf and the free energy provide insight on the thermomagnetic reversal mechanism. Constraints on the "pulse-mode" of recording when the head field reverses direction during cooling are also considered. [ABSTRACT FROM AUTHOR]

Finite size effects on the temperature dependence of the uniaxial magnetic anisotropy, longitudinal and transverse susceptibilities and specific heat are examined for L1o-ordered FePt nanoparticles using an atomistic model based on an effective classical spin Hamiltonian. At low temperatures below criticality, we study the intrinsic uniaxial magnetic anisotropy energy (MAE) K1 and its scaling with magnetization K1(T)∼Ms(T)δ and using Langevin dynamics simulations we show that the dependence of the exponent δ on the size L and aspect ratio of the grain arises from decomposition of the MAE into bulk and surface dependent terms. Monte Carlo simulations in the critical regime near the Curie temperature Tc, show that the temperature variation of the specific heat and longitudinal susceptibility is given by finite size scaling relations c=Lα/νc(L1/νε) and χ=Lγ/νχ(L1/νε), respectively, where ε=(T-Tc)/Tc is the reduced temperature, and the susceptibility scaling function χ can be approximated by a Lorentzian. Our estimates of the critical exponents α,γ, and ν appear to be in agreement with the universality class of the 3D Ising model. [ABSTRACT FROM AUTHOR]

Chemically ordered and highly textured L10 FePt-C granular films are potential media for future heat-assisted magnetic recording. Vibrating sample magnetometry of such films in fields up to 14 T reveals a perpendicular coercivity of up to μ0HC=4.92 T and an anisotropy field of μ0HA=9.2 T, which translates to a (uni-axial) anisotropy constant as high as KU=5.3 MJ/m3. An analysis of the remanent magnetization and demagnetization curves shows that the spatially separated FePt nanoparticles act as a Stoner-Wohlfarth ensemble of uni-axial nanomagnets with negligible dipolar inter-particle coupling. The magnetic texture spread of 23° as determined from an analysis of the hard axis magnetization curve is found to be clearly larger than the structural texture width of roughly 3°. Aberration corrected high-resolution transmission electron microscopy reveals that the latter is due to the remaining roughness of the seed layer that causes the particle growth to nucleate at step edges of this layer. [ABSTRACT FROM AUTHOR]

We introduce a novel solution of the coupled-channel Schrödinger equation. This new procedure dramatically improves on our previous paper on this subject. The method uses a truly adiabatic internal basis and combines a smooth variable discretization (SVD) with an enhanced renormalized Numerov (ERN) propagator. Although the basis is truly adiabatic, this method does not require derivative coupling terms, and it involves less numerical work than previous SVD approaches. Boundary conditions are applied using Jacobi coordinates for bound states and using hyperspherical coordinates for continuum states; that allows application of the boundary conditions at smaller distances. We apply this new algorithm to the model collision-induced dissociation process Ne[sub 2]+H→Ne+Ne+H for zero total angular momentum. We study the convergence of the probabilities as a function of the number of channels, distance propagated, and step size in the propagation. The method is fast, reliable, and provides considerable savings over previous propagators. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A global potential energy surface (PES) for the 1[sup 4]A' spin-aligned state of Li[sub 3] is presented. The surface is constructed as a many body expansion of the potential which is the sum of pairwise additive two-body potentials plus a three-body term. The two-body potential is that for the a ³&Zigma;[sup +, sub u] state of the lithium dimer. It combines the most recent Rydberg-Klein-Rees potential available [A. Ross (private communication)] with well-known short and long range expansions and accurately reproduces all known experimental data. To obtain the three-body contributions, an ab initio PES was computed at 1122 points using full configuration interaction for the three valence electrons with an augmented Gaussian basis and the effective core potentials of Stevens, Basch, and Krauss [W. J. Stevens et al., J. Chem. Phys. 81, 6026 (1984)] for the other electrons. The two-body interactions are also calculated using the same basis and then subtracted from the full interaction to give the three-body term. To construct the three-body potential at arbitrary configurations we use interpolation for small perimeters of the triangle formed by the triatomic system and an analytic fitting function for large perimeters. A switching function guarantees the smoothness of the potential function everywhere. The equilibrium position occurs at D[sub 3h] symmetry with a bond distance of 5.861a[sub 0], nearly 2a[sub 0] smaller than the equilibrium value of 7.886a[sub 0] of the lithium dimer. The well depth at the equilibrium is 4112.64 cm[sup -1]. This is considerably deeper than the well depth of 1001.22 cm[sup -1] for the pairwise additive potential at its equilibrium. Three-body effects are even more important for Li[sub 3] than in the recently reported Na[sub 3] case [J. Higgins et al., J. Chem. Phys. 112, 5751 (2000)], and the nonadditive three-body term cannot be neglected in any calculation on this system. [ABSTRACT FROM AUTHOR]

A comparison is made of the electrical effects of carbon in n- and p-type in situ doped polycrystalline Si[sub 1 - y]C[sub y] and Si[sub 0.82 - y]Ge]sub 0.18]C[sub y] layers. Values of resistivity as a function of temperature, effective carrier concentration and Hall mobility are reported. The n-type polycrystalline Si[sub 1 - y]C[sub y] and Si[sub 0.82 - y]Ge[sub 0.18]C[sub y] films show dramatic increases in resistivity with carbon content, rising from 0.044 Ω cm to 450 Ω cm (0 and 0.8% C) and 0.01 Ω cm to 2.4 Ω cm (0 and 0.6% C), respectively. In contrast, the increase in B-doped films is much less severe, rising from 0.001 Ω cm to 0.939 Ω cm (0 and 7.9% C) and 0.003 Ω cm to 0.015 Ω cm (0 and 4% C) for the Si[sub 1 - y]C[sub y] and Si[sub 0.82 - y]Ge[sub 0.18]C[sub y] layers, respectively. The grain boundary energy barrier, determined from the temperature dependence of the resistivity, is found to vary as the square of the C content in the n-type polycrystalline Si[sub 1 - y]C[sub y] and Si[sub 0.82 - y]Ge[sub 0.18]C[sub y] layers, but linearly in the p-type Si[sub 1 - y]C[sub y] layers. The square law dependence seen in the n-type layers for C contents up to 0.9% is explained by an increase in the grain boundary trap density due to the presence of carbon, whereas the linear relationship seen in the p-type layers for C contents between 2% and 8% is explained by a shift in the grain boundary trap energy toward the valence band. Finally, lower values of grain boundary energy barrier are obtained in p-type Si[sub 0.82 - y]Ge[sub 0.18]C[sub y] layers with a C content of 4% than in equivalent Si[sub 1 - y]C[sub y] layers, which could be explained by a larger shift in trap energy toward the valence band. [ABSTRACT FROM AUTHOR]

Accurate quantum reactive scattering calculations in the full three-dimensional physical space have been carried out for the Li+FH reaction at zero total angular momentum using the adiabatically adjusting principal axis of inertia hyperspherical coordinate formalism. The procedures for fitting the potential energy surface, calculating the surface functions, and propagating the solutions in a coupled channel treatment are given and discussed. Features of the resulting reactive probability plots are analyzed, and simple explanations of a number of the quantum resonance and oscillatory features are found. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a study which compared the electron-neutral collision rates for a particle-in-cell code, a Monte Carlo code, a convected scheme Boltzman code and a spherical harmonic expansion Boltzman code. Outline of computational techniques; Results of the study.

The validity of transition state theory is examined for the bimolecular reaction of Li+HF→LiF+H. Accurate three-dimensional quantum scattering theory calculations of the cumulative reaction probability are reported for energies ranging from threshold (0.255 eV) up to 0.600 eV and a total angular momentum J of 0. Transition state theory estimates of the effect of both the entrance and exit channels on the cumulative reaction probability are reported for the same energy range and J value. The transition state theory results are found to provide an accurate description of the smoothed energy dependence of the cumulative reaction probabilities with a maximum disagreement between the two calculations of about 25% arising at the highest energy considered of 0.6 eV. [ABSTRACT FROM AUTHOR]

The Darakjian–Hayes direct method for determining quantum lifetimes for three atoms scattering in three physical dimensions is used to determine accurate state-to-state time delays for the reaction of helium with H+2 for total angular momentum J=0. These results are compared with the time delays obtained by numerical differentiation of the S-matrix elements generated using the APH (adiabatically adjusting principal-axis hyperspherical) formulation of Pack and Parker. The direct method was found to be accurate and efficient for calculating the energy derivatives of the S matrix. The calculated eigenvalues of Smith’s collision lifetime matrix (eigen lifetimes) for this reaction predict numerous long-lived metastable states, many with lifetimes over 0.5 ps. The extent of the coupling of metastable states to specific scattering states provides an indication of the nature and magnitude of the time delays associated with particular state-to-state scattering processes. The direct method for calculating the energy derivatives of the S matrix is also found to be accurate and efficient for determining the energy derivative of the cumulative reaction probability. [ABSTRACT FROM AUTHOR]

Accurate 3D coupled channel calculations for total angular momentum J=0 for the reaction F+H2→HF+H using a realistic potential energy surface are analyzed. The reactive scattering is formulated using the hyperspherical (APH) coordinates of Pack and Parker. The adiabatic basis functions are generated quite efficiently using the discrete variable representation method. Reaction probabilities for relative collision energies of up to 17.4 kcal/mol are presented. To aid in the interpretation of the resonances and quantum structure observed in the calculated reaction probabilities, we analyze the phases of the S matrix transition elements, Argand diagrams, time delays and eigenlifetimes of the collision lifetime matrix. Collinear (1D) and reduced dimensional 3D bending corrected rotating linear model (BCRLM) calculations are presented and compared with the accurate 3D calculations. [ABSTRACT FROM AUTHOR]

Presents a study that developed a long mean-free-path model of transport of neutral particles. Application of the model to the chemistry of a low neutral-pressure plasma etching system; Creation of a transition matrix; Functions of the matrix.

In this study, we present time-resolved Kerr microscopy of the magnetization dynamics in response to the reversal of the write current in the soft magnetic underlayers (SULs) films used in dual layer perpendicular recording media. 20-nm-thick FeCoB films with a 5 nm Pd cap layer are positioned in contact with a single-pole-type high-speed perpendicular magnetic write head. The temporal responses of both in-plane and out-of-plane magnetization components of the SUL were simultaneously measured using the time-resolved Kerr effect. Precessional effects were observed in both magnetization components during the SUL switching, using a driving pulse current with rise-time of about 0.8 ns. The precessional frequencies range from 3 to 7 GHz, depending on the distance (χ) away from the main pole. The present results highlight the importance of soft magnetic underlayer dynamics for high data rate perpendicular magnetic recording technology. [ABSTRACT FROM AUTHOR]

A three-dimensional (3D) reciprocity based model of the perpendicular read process with a soft underlayer is presented. In 3D with finite track width, the read flux does not decay to zero because flux is always entering the reader from the field caused by a nonvanishing curl of the magnetization. As the reader is moved off track, the signal amplitude and resolution decrease, and at some off track position there is a reversal in the sign. A comparison of the reciprocity based reader model with a micromagnetic CPP reader model shows excellent agreement both on and off track. [ABSTRACT FROM AUTHOR]

Numerical simulations of submicron Co extruded elliptical dots were performed to illustrate the relative importance of different physical parameters on the switching behavior in the easy direction. Shape, size, magnetic moment magnitude, and the magnitude and distribution of the crystalline anisotropicity were varied. The simulation represents magnetostatic, exchange, and crystalline anisotropicity fields on a structured mesh using finite difference techniques. The smooth boundary of the dots is accurately represented by use of the embedded curve boundary method. Agreement with experimental hysteresis measurements of submicron dot arrays is obtained when an appropriate angular distribution of the grain anisotropicity axes is invoked. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Focuses on a study which examined the mean-field phase diagram for an XY model on a square lattice. Calculations of the critical ratio for the appearance of noncollinearity; Information on the phase scenarios likely to occur in a square lattice; Schematic diagrams for square lattice.

Examines how spin disorder is produced by individual bond defects. Purpose of the mean-field theory; Information on the single-bond problem.

Heat assisted magnetic recording (HAMR) has been recognized as a leading technology to increase the data storage density of hard disk drives. Dispersions in the properties of the grains comprising the magnetic medium can lead to grain-to-grain Curie temperature variations, which drastically affect the jitter in the recorded magnetic transitions and limit the data storage density capabilities in HAMR. Here we present a method to measure the switching probability of an ensemble of exchange-decoupled grains with perpendicular anisotropy subject to nanosecond heating pulses. The short heat exposure time ensures that the grains switch by reaching the Curie temperature rather than through thermal activation. The switching probability can be directly interpreted as representing the Curie temperature distribution. The method is applied to two sets of samples to reveal the sensitivity of the Curie temperature distribution to FePt HAMR media fabrication parameters. This technique is of importance to engineer suitable HAMR media capable of high density magnetic recording and for fundamental studies on sources of magnetic disorder in granular magnetic media. [ABSTRACT FROM AUTHOR]

An accurate and comprehensive self-consistent kinetic model is applied to a helium radio frequency (rf) discharge which in experiments exhibits an ‘‘α to γ’’ transition. The main conclusions are (i) there is good agreement (factor of 2) between the experiment and simulation in plasma density, power density, and current density across two orders of magnitude of the applied voltage. (ii) The bulk electron temperature exhibits a transition from high at low driving voltages to low at high voltages. The low-voltage regime exhibits a larger bulk electric field in the simulation. (iii) Multistep ionization is important under all conditions studied. [ABSTRACT FROM AUTHOR]

The construction and performance of a single stage nuclear demagnetization apparatus utilizing 0.267 mole of praseodymium nickel material is described. An optimization procedure to minimize entropy production during the demagnetization of the hyperfine enhanced material has been developed which enabled the cryostat to cool a liquid-3He sample to a temperature of 350 μK. The effects of measured heat leaks and inferred eddy-current heating are accounted for satisfactorily in a model of this procedure. [ABSTRACT FROM AUTHOR]

Accurate three dimensional quantum calculations have been carried out for the Li+HF reaction at zero total angular momentum J on a potential energy surface fitted to ab initio points. By adopting a J shifting approximation reactive cross sections are estimated and compared with the experiment. The agreement is very good. [ABSTRACT FROM AUTHOR]

L10 FePt has been proposed as a magnetic medium suitable for heat assisted magnetic recording, owing to its high magnetocrystalline anisotropy and moderate Curie temperature. Carbon has previously been used as a segregant creating small magnetically decoupled grains by sputter deposition at high temperature. Here, we deposit granular, segregated FePt films having a high degree of chemical order, coercive fields of 4.8 T, high remanence, and average grain size of 7.2 nm with 17% size distribution. Magnetic characterization reveals a large intrinsic switching field distribution of 15 kOe. We incorporate the results obtained by transmission electron microscopy, magnetometry, and x-ray diffraction into a micromagnetic model to better understand the origin of the intrinsic switching field distribution. We find that the dominant contributions to the observed magnetic properties are (i) a grain-size dependent distribution in anisotropy field and (ii) small angle variations in perpendicular anisotropy axis orientation. The presence of small superparamagnetic grains, contributions originating from the grain size distribution, as well as from grains with longitudinal or randomly oriented anisotropy axis are found to play a relatively small role. The results help understand the observed magnetization loops used to evaluate FePt media. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations using the single histogram method are performed to study the effects of finite size and Cu doping on the Curie temperature of a single FePt nanoparticle. An atomistic model is used with an effective classical spin Hamiltonian constructed on the basis of first-principles calculations for L1o FePt. The Curie temperature, evaluated from the peak in the temperature variation of the magnetic susceptibility, is consistent with the finite-size scaling relation Tc(L)=Tc(∞)(1-xoL-1/ν), where L is the in-plane grain size, xo is found to depend on the grain aspect ratio, and the critical exponent of the correlation length is ν=1.06±0.06. A theoretical relation is obtained for the Tc dispersion arising from a lognormal grain size distribution. Addition of Cu results in reduction of the Curie temperature proportional to the dopant content. [ABSTRACT FROM AUTHOR]

We present experimental and numerical micromagnetic data on the effect of spin momentum transfer in current perpendicular to the plane spin valves. Starting from a configuration with orthogonal free- and pinned-layer magnetizations, the free-layer magnetization exhibits abrupt current-induced switching that is qualitatively consistent with the spin torque model. When operating the spin valve as a field sensor, spin transfer can produce a change in resistance that mimics an effective magnetic field and induce magnetic instability that requires a larger bias field in order to stabilize the device. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report ultrabroadband measurements on waveguides of photoluminescent silicon-rich silicon dioxide produced by plasma enhanced chemical vapor deposition. Material absorption below 700 nm and waveguide loss above 1300 nm leave a broad spectral region of good transmission properties, which overlaps with the photoluminescence spectrum of the core material. Proposed mechanisms for the material absorption and photoluminescence are discussed based on our findings. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We demonstrate that ultrabroadband ultrashort-pulse white light supercontinua can be used to track the group velocity of photons in optical waveguides using a Kerr gate technique. Results on silicon nitride slab waveguides show both polarization birefringence and multimode dispersion, which vanish at critical wavelengths. When photonic crystals are embedded in the waveguides, the higher order modes are excited within the band-gap region, demonstrating the need to control their dispersion to make effective use of photonic crystal waveguide devices. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We discuss the design and performance of a radiant heating assembly for limited reaction-rate processing applications. The heating assembly is fabricated in stainless steel on a 300 mm bore flange for incorporation with an ultrahigh vacuum compatible low-pressure chemical vapor deposition system. The heating source is an array of 18 tungsten-halogen lamps; each lamp is rated at 360 W and has its own integral parabolic reflector. A 55-mm-diam "O" ring sealed quartz window is mounted in front of each lamp allowing the transmission of infrared radiation into a water-cooled stainless-steel deposition chamber. [ABSTRACT FROM AUTHOR]

Discusses the results of a study that first considered square lattice spins with nearest neighbor ferromagnetic interactions, except for one frustrated spin with two antiferromagnetic bonds.

Presents an erratum for an article on direct calculation of collisional properties that require energy derivatives.

The sheet resistance, effective carrier concentration, and Hall mobility of in situ boron- and phosphorus-doped polycrystalline Si[sub 0.82-y]Ge[sub 0.18]C[sub y] films are presented for carbon contents between 0% and 4%. Phosphorus and boron doping levels of 4x10[sup 19] and 2x10[sup 20] cm[sup -3] were achieved for the n- and p-type layers, respectively, and remained largely unaffected by carbon content. The phosphorus-doped films showed a dramatic increase in sheet resistivity and a corresponding drop in effective carrier concentration and Hall mobility. In contrast, the boron-doped films showed only a minor increase in resistivity. This is attributed to interstitial carbon increasing the defect density and also shifting the defect energy levels at the grain boundaries towards the valence band. This causes an increase in the grain-boundary energy barrier in n-type layers, but leaves the p-type layers largely unaffected. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We demonstrate experimentally the tuning of complete photonic band gaps in patterned silicon nitride waveguides. Transmission measurements were performed using an ultrabroadband high-brightness white light laser continuum, extracting extinction ratios as low as 10[sup -4] in the gap regions. Angle-resolved measurements show the perfect alignment of the gap around the Γ-J direction. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]