The crystal structure and surface morphology of hydride vapour phase epitaxy grown thick (12-105 μm) GaN layers have been investigated as a function of growth rate using several structure sensitive techniques like atomic force microscopy (AFM), x-ray diffraction (XRD) in ω-2θ and ω-rocking curve measurements as well as low temperature photoluminescence (PL). PL and XRD measurements reveal rather narrow lines: full width at half maximum (FWHM) values of the strongest donor-bound exciton line are in the range from 6.0 to1.8 meV and ω-2θ FWHM values are between 80 and 23 arcsec indicating good structural quality of the films. The ω-rocking curves show a single peak for the thinnest films with a FWHM of 250 arcsec and multiple peaks with FWHM of about 250-350 arcsec in thicker films indicating the formation of several high-quality domains when increasing either thickness or growth rate. Optical microscopy and AFM images reveal a domain type of morphology and also show an appearance of spiral hillocks in layers grown at growth rates exceeding a critical value. We interpret these results as dominating 2D multilayer growth at low growth rates, and competing 2D multilayer and spiral growth mechanisms at high growth rates.

In-situ fluid-cell AFM has been used to investigate the corrosion mechanisms of TaZrO and BCN thin films as protective coatings for magnetic media. This technique allows for real-time, in-situ monitoring of corrosion with nanometer resolution. The technique proved valuable in revealing peculiarities that cannot be detected using conventional techniques. In contrast to the nitride films, the results point to defect-based diffusion type corrosion mechanism in the oxide films.

Electrochemical codeposition of a series of face-centered cubic NixPt1-x alloys is demonstrated (0.1 < x < 0.95). The alloy composition is a monotonic function of potential. The Pt-rich NixPt1-x alloys are formed at potentials positive to that required to deposit elemental Ni. Codeposition is ascribed to the negative enthalpy of NixPt1-x alloy formation that proceeds via a Ni underpotential deposition reaction in concert with Pt deposition. Interestingly, this process occurs at higher Ni underpotentials than anticipated based on extrapolated literature data from thermochemical measurements and ab initio calculations of alloy formation. In contrast, Ni-rich NixPt1-x alloys are produced at Ni overpotentials although the films are formed under conditions where pure Ni deposition is otherwise kinetically hindered. The alloy composition corresponding to the transition from underpotential to overpotential deposition is a function of the PtCl4/NiCl2 electrolyte composition. The films were found to be bright and specular over the full range of compositions studied (grain size < 10 nm ). Atomic force microscopy yielded root-mean-square roughness values on the order of 5 nm for Ni-rich deposits up to 2.5 mu m thick. (c) 2007 The Electrochemical Society.

The electrodeposition of epitaxial cube-on-cube Fe100-xCox films onto (100)-oriented n-GaAs from ferrous ammonium sulfate solutions containing various concentrations of cobalt sulfate is described. The cobalt composition in a series of 400-nm-thick films was found to vary in proportion to the concentration of cobalt sulfate in the electrolyte and six compositions in the range from 0 to 74% were deposited. Epitaxial (100)-oriented bcc films were produced throughout this range, as suggested by symmetrical x-ray diffraction and confirmed by x-ray pole figure measurements. Rocking curve measurements yielded data that could be fitted to the sum of a Lorentzian and a Gaussian, corresponding to the (200) Fe100-xCox planes and to a residual contribution from GaAs (400), respectively. The full width at half maximum obtained for the film Lorentzians varied from 0.44 degrees to 0.90 degrees and was apparently uncorrelated with composition. X-ray (211) pole figure measurements revealed a high degree of twinning of the (100) bcc Fe100-xCox consistent with a recent study of Fe films electrodeposited on n-GaAs.

Surfactant coated FePt nanoparticles were characterized using Fourier transform infrared spectroscopy (FTIR). The FTIR spectra indicate that there is a conversion of the alkyl chain of the surfactant from the oleyl form (cis-9-octadecenyl) to the elaidyl form (trans-9-octadecenyl) during the synthesis of the FePt nanoparticles. This is revealed by the presence of several vibrational absorption bands in the region of the olefinic CH stretching modes. The appearance of infrared absorption bands due to olefinic CH stretching is due to the presence of Fe in the nanoparticles. The FTIR spectrum of platinum nanoparticles does not reveal the presence of olefinic CH modes. The concentration of Fe in the FePt nanoparticles influences the intensity of the olefinic CH stretching modes. The high intensity of these olefinic CH modes in nanoparticles with high Fe concentration indicates that Fe acts as a catalyst for cis to trans conversion and may lead to dehydrogentation of the alkyl chains.

In this work, a combinatorial approach to the synthesis of magnetic multilayers is explored. Combinatorial libraries of Co/Pd multilayer thin films were prepared using off-axis magnetron sputtering to enable thickness gradients across the wafer-magnetic properties of the multilayers are controlled by the thicknesses of Co and Pd layers in the repeated bi-layer stack. Polar magneto-optical Kerr effect (MOKE) was used to map magnetic properties of the combinatorial libraries. Multivariate regression analysis and back-propagation neural network (neural networks are known to better handle nonlinear approximations) were used to analyze the combinatorial data and to enable predictive capabilities. In the multivariate analysis, the relationship between the descriptive and output variables was approximated by a second order polynomial of Co and Pd thickness.The neural network model was utilized inversely to design a multilayer with pre-determined magnetic properties.

The microstructure of Fe films electrodeposited onto n-GaAs(001) from FeCl2 and FeSO4-(NH4)(2)SO4 electrolytes was examined by X-ray and electron diffraction. Symmetrical theta-2 theta X-Ray diffraction from films deposited from chloride solutions indicates a dominant (001) texture with the presence of additional minority orientations. In contrast, deposition from an ammonium sulfate electrolyte yields films that only exhibit (001) scattering in the theta-2 theta geometry. Transmission electron microscopy (TEM) cross-sectional images of films grown in chloride solutions reveal intermittent cube-on-cube epitaxy that is also evident in plan view TEM dark field images. Interestingly, X-ray pole figure measurements for all specimens examined reveal an additional (221) orientation that is undetectable by symmetric theta-2 theta diffraction. The (221) oriented material most likely derives from twinning of (001) oriented grains during growth. (c) 2006 The Electrochemical Society.

Ordered self-assembled monolayers of FePt nanoparticles have been studied for the first time on fluorinated carbon thin film substrates. High resolution scanning electron microscopy of the resulting films showed homogenous hexagonally closed packed monolayers of FePt ordered over length scales of several millimeters. The annealing of self-assembled monolayers of FePt nanoparticles at 600 degrees C for 30 min showed significantly less sintering than on other types of substrates and the hexagonal ordering of the self-assembled monolayers was preserved during the annealing step. This is a significant step towards development of a viable patterned media for high density data storage. (c) 2006 Elsevier B.V. All rights reserved.

The effect of surfactant coatings on the shapes and sizes of cobalt nanoparticles has been investigated. Cobalt nanoparticles were synthesized by thermal decomposition of a cobalt containing precursor in the presence of various surfactants. At room temperature the cobalt nanoparticles have a β-manganese structure also known as the ε-cobalt structure. The shapes of the cobalt nanoparticles include: spherical, triangular, rod-like, and hexagonal. The shapes of cobalt nanoparticles depend on the type of the surfactant used in the synthesis and the temperature at which the cobalt precursor was added to the reaction mixture.

Chemically modified patterns were used to achieve selective adsorption of magnetic FePt nanoparticles on substrates. In this work, thiol chemistry was used to create chemical patterns on silicon surfaces and gold patterned silicon surfaces. The adsorption of FePt nanoparticles on the silicon and gold patterned surface was studied using FTIR and HRSEM (high-resolution scanning electron microscopy). We have demonstrated in this work that chemical patterning can be used to achieve selective adsorption of magnetic nanoparticles on surfaces. Our results indicate that FePt nanoparticles coated with a 2 nm thick surfactant layer will adsorb when spin-coated onto gold surfaces modified with 11-mercapto-1-undecanol {HS(CH2)(11)OH, MDOL}. The adsorption of nanoparticles was confirmed by FTIR spectra and HRSENI. FTIR spectra revealed the presence of peaks at 3004 cm(-1) and 1709 cm(-1) on MDOL modified gold surfaces, which are indicative of the presence of the FePt nanoparticles on the surface. Adsorption of FePt nanoparticles was not observed on I-octadecanethiol {CH3(CH2)(17)SH, ODT} modified gold surfaces. (c) 2006 Elsevier B.V. All rights reserved.

We have investigated the circumstances underlying recent reports of very large values of ballistic magnetoresistance (BMR) in nanocontacts between magnetic wires. We find that the geometries used are subject to artifacts due to motion of the wires that distort the nanocontact thereby changing its electrical resistance. Since these nanocontacts are often of atomic scale, reliable experiments would require stability on the atomic scale. No method for achieving such stability in macroscopic wires is apparent. We conclude that macroscopic magnetic wires cannot be used to establish the validity of the BMR effect.

Ternary FeCuPt alloys with 1–5at.% Cu produced no advantage over equivalent binary FePt alloys in terms of lowering the kinetic ordering temperature. The Curie temperature decreased with increasing Cu content. The transformation enthalpy and activation energy were not affected.

Differential scanning calorimetry, in conjunction with x-ray and electron diffraction, is used to investigate the A1 to L10 ordering transformation in binary FePt films with compositions in the range of 47.5–54.4at.% Fe. The kinetic ordering temperature, taken as the calorimetric peak temperature at a heating rate of 40°C∕min, decreases from 447to357°C in this composition range. In contrast with the kinetic ordering temperature, the Curie temperature of the L10 ordered phase increases from 384to455°C as the Fe content is increased. The activation energies of ordering lie between 1.4 and 2.0eV, and the transformation enthalpies are in the range of −8.2to−13.6kJ∕g-at. The Avrami exponent for the transformation is lower than expected and lies in the range of 1.1–1.8. The lattice parameter of the A1 phase and the c∕a ratio of the L10 phase decrease with increasing Fe content.

We have achieved excellent exchange decoupling of grains in CoPd multilayers by annealing in air at 300°C. Samples exhibit a slope in the hysteresis loop close to 1.0, nucleation fields as large as 11kOe, and coercivities as large as 16kOe. These are among the best properties ever achieved for CoPd multilayers. In samples of the general type seed layer\(0.3nmCo\1nmPd)×15\capping layer, it appears that atoms diffuse rapidly up and down the grain boundaries at 300°C. From x-ray photoelectron and Auger depth profiling, we have found that when Co atoms arrive at the surface they become oxidized and remain at the surface, leaving the grain boundaries depleted in Co and apparently nonmagnetic. If the annealing is carried out in vacuum, exchange decoupling of the grains does not occur. This result supports our conclusion that the presence of oxygen plays a crucial role. When Au or Pt atoms are present in the seed layer or capping layer, they diffuse into the CoPd grain boundaries and apparently enhance the exchange decoupling of grains.

Electrochemical co-deposition of CoxPt1-x alloys is demonstrated at potentials positive to that required to deposit elemental cobalt (underpotential, or "induced," co-deposition). Co-deposition is attributed to the negative enthalpy of CoxPt1-x alloy formation. A regular solid-solution model is used to describe the dependence of the film composition on the deposition potential. X-ray diffraction of films grown on Cu(100)parallel to Si(100), as well as poly- Au parallel to SiO2 parallel to Si, reveal the formation of a randomly oriented CoxPt1-x face-centered cubic alloy for x > 0.3. A transition to the hexagonal close-packed phase is evident at higher cobalt concentrations (x > 0.9). (C) 2004 The Electrochemical Society.