Y. Austin Chang

Self-assembled growth of epitaxial erbium disilicide nanowires on silicon (001)

By choosing a material that has an appropriate asymmetric lattice mismatch to the host substrate, in this case ErSi2 on Si(001), it is possible to grow one-dimensional epitaxial crystals. ErSi2 nanowires are less than one nanometer high, a few nanometers wide, close to a micron long, crystallographically aligned to 〈110〉Si directions, straight, and atomically regular.

Thermodynamic assessment of the Co-Ta system

Abstract The phase equilibria and thermodynamic properties of the binary Co-Ta system were analyzed and a complete thermodynamic description of the binary system was obtained with the CALPHAD technique using a computerized optimization procedure. The thermodynamic descriptions of pure Co and Ta elements were taken from the SGTE database. Based on the experimental data, six binary intermetallic compounds were considered. They are three Laves phases, μ phase, θ phase and Co 7 Ta 2 phase. The last two compounds are treated as stoichiometric compounds. Good agreement was obtained between calculated results and experimental data in the binary system.

Calculation of the equilibrium phase diagrams and the spinodally decomposed structures of the FeCuNi system

Abstract Thermodynamic stability of ternary systems with miscibility gaps is discussed and applied to the f.c.c. phase of Feue5f8Cuue5f8Ni alloys. The stability of the system with respect to infinitesimal composition fluctuation is described. The direction most likely for initial composition fluctuation is defined as the one with the largest negative curvature on the Gibbs energy surface. Along this direction, zero-time wavelength at initial stage of spinodally decomposed alloy structure may be calculated. The thermodynamic values of the f.c.c. Feue5f8Cuue5f8Ni alloys are obtained from limited amount of thermochemical and phase equilibrium data. It is found that the ternary interaction parameters for both the f.c.c. and liquid phases of Feue5f8Cuue5f8Ni may be approximated from the binary interaction parameters of the constituent binaries.

A thermodynamic analysis of the PbSn system and the calculation of the PbSn phase diagram

Abstract Thermodynamic data of the liquid phase, the (Pb) phase and the (Sn) phases are analyzed in terms of the Marqules type of equations for the excess Gibbs energies. For the liquid phase, a quasi-sub-regular solution model is used. For the fcc (Pb) phase and the bct (Sn) phase, a quasi-regular model is used. The calculated phase diagram of the Pb Sn binary using these data agrees well with the experimental diagram. The present analysis yields a set of internally consistent thermodynamic data and the lattice stabilities of Pb and Sn, i.e. the Gibbs energy differences between the fcc and bct forms of Pb and Sn.

Evaluation of the thermodynamic properties of the Re–Ta and Re–W systems

Abstract The phase equilibria and thermodynamic properties of the Re–Ta and Re–W systems were analyzed and complete thermodynamic descriptions of the two binary systems were obtained with the CALPHAD technique using a computerized optimization procedure. The thermodynamic descriptions of pure Re, Ta and W elements were taken from the SGTE database. Two binary intermetallic compounds were considered in both binary systems, namely σ phase and χ phase based on available experimental data. Good agreement was obtained between calculated results and experimental data in the two binary systems.

Growth of intermediate phases in Co/Si diffusion couples: Bulk versus thin‐film studies

Bulk diffusion couples of Co/Si were annealed at 800, 900, 1000, and 1050u2009°C for times ranging from 24 h to 1 month. The growth rates of the three intermediate phases Co2Si, CoSi, and CoSi2 and the concentration profiles across the couples were determined by optical microscopy and electron probe microanalysis, respectively. Using these data and the data reported in the literature at lower temperatures, the interdiffusion coefficients of Co2Si, CoSi, and CoSi2 were obtained as a function of temperature. The activation energies obtained were 140, 160, and 190 kJ/mol (or 1.45, 1.66, and 1.97 eV) for Co2Si, CoSi, and CoSi2, respectively. The generally small interdiffusion coefficient of CoSi2 and its high activation energy cause the growth rate of CoSi2 to be extremely small at low temperatures. Using the interdiffusion coefficients of Co2Si, CoSi, and CoSi2 extrapolated to low temperatures, the growth rates of Co2Si, CoSi, and CoSi2 in thin‐film Co/Si couples were predicted. The predictions were made by nume...

Over 70% tunneling magnetoresistance at room temperature for a CoFe and AlOx based magnetic tunnel junction

More than 70% tunneling magnetoresistance (TMR) ratio has been observed at room temperature for a CoFe and AlOx based magnetic tunnel junction. The annealing of the epitaxial bottom electrode, Si (001)/Ag fcc (200)∕Co84Fe16 bcc (200), at 400°C prior to fabricating the tunnel barrier and the upper electrode is crucial for achieving this high TMR ratio. Moreover a high output voltage could be obtained for this magnetic tunnel junction due to its high V1∕2, the bias voltage at which the TMR ratio is reduced to half of that near the zero bias. The rationale for obtaining this high TMR ratio is discussed, and there are reasons to believe that the TMR ratio for this junction could be further improved.

Paramagnetic to antiferromagnetic phase transformation in sputter deposited Pt-Mn thin films

Sputter-deposited, equiatomic Ni-Mn thin films were observed to possess a metastable, nanocrystalline, chemically disordered, fcc (A1) structure. Grain growth and a phase change to a chemically ordered, antiferromagnetic L10 structure were identified by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) experiments revealed exothermic signals that correspond to the grain growth and phase transformation reactions. The enthalpy of transformation for the A1 to L10 phase change was calculated as −3.5 kJ/mol, which agress with thermodynamic modeling. An activation energy of 139 kJ/mol was calculated for the phase transformation by the Kissinger method.

The formation of amorphous alloy oxides as barriers used in magnetic tunnel junctions

The quality of a tunnel barrier is of critical importance for the success of a magnetic tunnel junction. An amorphous state of the precursor metal films is beneficial for the formation of an ultrasmooth and likely defect-free oxide tunnel barrier layer, thus leading to an improved performance of a magnetic tunnel junction. However, prior to oxidation, most sputter-deposited thin films of pure metals are polycrystalline instead of amorphous. In contrast, a sputter-deposited alloy thin film could have a better amorphous-forming ability than a pure metal. Using an Al–Zr binary as an example, the amorphous-forming ability of alloy thin films was investigated thermodynamically and experimentally. A thermodynamic model was formulated to predict the compositions of a binary alloy exhibiting favorable amorphous-forming abilities when sputter-deposited alloy thin films are made. The thermodynamically calculated compositions of Al–Zr alloys with great tendencies to form amorphous metallic thin films were confirmed ...

Thermodynamic stability and diffusivity of near-equiatomic Ni–Mn alloys

The equilibrium phase diagram of the nickel-manganese system is determined between 500 and 850u200a°C, in the composition range between 25 and 70 at.u200a% Ni. A combination of electron probe microanalysis, x-ray diffraction, and optical microscopy was employed to analyze 47 samples that were annealed anywhere from three to seven months. The equiatomic, antiferromagnetic, L10-NiMn phase that is of considerable technological interest was found to exist continuously between 500 and 700u200a°C. No other intermediate phases were found in this study at low temperatures. These results are in contrast to the currently accepted phase diagram published in most handbooks. A hot-isobaric-pressing method was used to initially bond samples that were subsequently used to determine interdiffusion coefficients in the Ni–Mn system at 650u200a°C. The Boltzmann–Matano method [T. Heumann, Z. Phys. Chem. 201, 168 (1952)] allowed the calculation of these interdiffusion coefficients across the α-Mn, β-Mn, γ-Mn, L10-NiMn, and γ-Ni phases.