Jiaxing Liu

Interdiffusion in nanometric Fe/Ni multilayer films

[Fe (3.1 nm)/Ni (3.3 nm)]20 multilayer films were prepared by DC magnetron sputtering onto oxidized Si(100) substrates. The Fe and Ni layers were shown to both be face-centered cubic by x-ray diffraction. Interdiffusion of the Fe and Ni layers in the temperature range of 300–430 °C was studied by x-ray reflectivity. From the decay of the integral intensity of the superlattice peak, the activation energy and the pre-exponential term for the effective interdiffusion coefficient were determined as to 1.06 ± 0.07 eV and 5 × 10−10 cm2/s, respectively. The relevance of the measured interdiffusion coefficient to the laboratory timescale synthesis of L10 ordered FeNi as a rare-earth free permanent magnet is discussed.

Transformation of topologically close-packed β-W to body-centered cubic α-W: Comparison of experiments and computations

The enthalpy and activation energy for the transformation of the metastable form of tungsten, β-W, which has the topologically close-packed A15 structure (space group Pm3¯n), to equilibrium α-W, which is body-centered cubic (A2, space group Im3¯m), was measured using differential scanning calorimetry. The β-W films were 1 μm-thick and were prepared by sputter deposition in argon with a small amount of nitrogen. The transformation enthalpy was measured as -8.3 ± 0.4 kJ/mol (-86 ± 4 meV/atom) and the transformation activation energy as 2.2 ± 0.1 eV. The measured enthalpy was found to agree well with the difference in energies of α and β tungsten computed using density functional theory, which gave a value of -82 meV/atom for the transformation enthalpy. A calculated concerted transformation mechanism with a barrier of 0.4 eV/atom, in which all the atoms in an A15 unit cell transform into A2, was found to be inconsistent with the experimentally measured activation energy for any critical nucleus larger than two A2 unit cells. Larger calculations of eight A15 unit cells spontaneously relax to a mechanism in which part of the supercell first transforms from A15 to A2, creating a phase boundary, before the remaining A15 transforms into the A2 phase. Both calculations indicate that a nucleation and growth mechanism is favored over a concerted transformation. More consistent with the experimental activation energy was that of a calculated local transformation mechanism at the A15-A2 phase boundary, computed as 1.7 eV using molecular dynamics simulations. This calculated phase transformation mechanism involves collective rearrangements of W atoms in the disordered interface separating the A15 and A2 phases.

Impact of deposition rate, underlayers, and substrates on β-tungsten formation in sputter deposited films

The metastable phase of tungsten, β-W, which is a topologically close packed phase with the A15 ( P m 3 ¯ n ) structure is of interest for application in spintronic devices based on the spin Hall effect. The deposition of β-W on glass substrates was studied as a function of deposition rate and the pressure of gaseous N2 introduced into the chamber along with the Ar sputtering gas. As the deposition rate is increased by increasing the deposition power, the N2 pressure required to form a given fraction of β-W increased. The variation of fraction of β-W with pressure of N2 for a given deposition rate followed the Langmuir–Freundlich isotherm, in agreement with prior work. The impact of underlayers and substrates on the formation of β-W was examined using 23 underlayers and two single crystal substrates in addition to the glass substrate. The underlayers were B, C, Mg, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Hf, Ta, W, Fe19Ni81 (permalloy) Co40Fe40B20, Al2O3, and SiO2. The two single crystal substr...