K. Barmak

On the relationship of high coercivity and L10 ordered phase in CoPt and FePt thin films

The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. A transformation from an atomically disordered, face-centered-cubic structure to the L10 ordered structure occurred during postdeposition annealing and was characterized using digital analysis of dark-field TEM images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from the SQUID magnetometer. In contrast to the relationship most commonly described in the literature, that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study wa...

On the relationship of magnetocrystalline anisotropy and stoichiometry in epitaxial L10 CoPt (001) and FePt (001) thin films

Two series of epitaxial CoPt and FePt films, with nominal thicknesses of 42 or 50 nm, were prepared by sputtering onto single-crystal MgO(001) substrates in order to investigate the chemical ordering and the resultant magnetic properties as a function of alloy composition. In the first series, the film composition was kept constant, while the substrate temperature was increased from 144 to 704 °C. In the second series the substrate temperature was kept constant at 704 °C for CoPt and 620 °C for FePt, while the alloy stoichiometry was varied in the nominal range of 40–60-at. % Co(Fe). Film compositions and thicknesses were measured via Rutherford backscattering spectrometry. The lattice and long-range order parameter for the L10 phase were obtained for both sets of films using x-ray diffraction. The room-temperature magnetocrystalline anisotropy constants were determined for a subset of the films using torque magnetometry. The order parameter was found to increase with increasing temperature, with ordering...

The relationship between deposition conditions, the beta to alpha phase transformation, and stress relaxation in tantalum thin films

We demonstrate that the high temperature polymorphic tantalum phase transition from the tetragonal beta phase to the cubic alpha phase causes a large decrease in the resistance of thin films and a complete stress relaxation in films that were intrinsically compressively stressed. 100 nm beta tantalum thin films with intrinsic stresses of 2.0×1010 dynes/cm2 (tensile) to −2.3×1010 dynes/cm2 (compressive) were deposited onto thermally oxidized (100) silicon wafers by evaporation or dc magnetron sputtering with argon. In situ stress and resistance at temperature were measured at 10 °C/min up to 850 °C in purified helium. Upon heating, the main stress mechanisms were elastic deformation at low temperature, plastic deformation at moderate temperatures and stress relief because of the beta‐to‐alpha phase transition at high temperatures. The temperature ranges over which the elastic and plastic deformation and the beta‐to‐alpha phase transition occurred varied with deposition pressure and substrate biasing. Incom...

Experimental evidence for nucleation during thin-film reactions

The reaction between solid layers to form a product phase has been studied using scanning calorimetry of multilayer Nb/Al and Ni/amorphous‐Si thin films. The most striking feature for both materials systems is the occurrence of two maxima in the reaction rate during the formation of a single product phase, suggesting a two step growth process. A model has been developed in which the first step is taken to be the nucleation and two‐dimensional growth to coalescence of the product phase, in the plane of the initial interface. The second step is taken to be the thickening of the product layer by growth perpendicular to the interface plane. The success of this simple model in describing the principal features of the experimental results on two different materials systems suggests that nucleation is an important aspect of phase formation and selection in these thin‐film reactions.

The early stages of solid‐state reactions in Ni/Al multilayer films

Ni/Al multilayer films with pair thicknesses of 10 and 20 nm and with overall compositions in the range 48–88 at. % Al were prepared by sputtering. For comparison, Ni‐Al alloy films in the same concentration range were prepared by co‐deposition of the elements. The films were studied by x‐ray diffraction, electron diffraction, and differential scanning calorimetry. It was found that the B2 NiAl phase with a metastable concentration of approximately 63 at. % Al was the first phase to grow upon annealing of the multilayer films. The growth of this phase could be described by Johnson–Mehl–Avrami kinetics with an activation energy of 0.8 eV and an Avrami exponent of 0.5. This low activation energy was consistent with the observation that the phase had formed during deposition and continued to grow upon annealing at low temperatures to thicknesses of a few nanometers. If the reactant phases were not fully consumed by the B2 phase growth, the subsequent reaction was the formation of NiAl3, previously thought to...

Annealing behavior of Cu and dilute Cu-alloy films: Precipitation, grain growth, and resistivity

The impact of 11 alloying elements, namely, Mg, Ti, In, Sn, Al, Ag, Co, Nb, and B, at two nominal concentrations of 1 and 3 at. %, and Ir and W, at only a nominal concentration of 3 at. %, on the resistivity and grain structure of copper was investigated. The films were electron beam evaporated onto thermally oxidized Si wafers and had thicknesses in the range of 420–560 nm. Pure evaporated Cu films were used as controls. Isothermal anneals were carried out at 400 °C for 5 h; constant-heating rate treatments, with no hold at the temperature, were done at 3 °C to 650 and 950 °C. In all cases, annealing resulted in the lowering of resistivity compared with the as-deposited state. Furthermore, annealing to a higher temperature resulted in lower, postannealing, room-temperature resistivity, unless the film agglomerated or showed evidence of solute redissolution. Annealing also resulted in significant growth of grains, except for the Nb- and W-containing films. In addition, the grain sizes for the nominally 3 ...

On the use of alloying elements for Cu interconnect applications

To address the future use of alloying elements for Cu interconnect applications in integrated circuits, first, available bulk experimental data such as residual resistivity per at. % solute and binary phase diagrams are used to arrive at a set of 24 potential elements. Next, experimental results in thin films and lines allow the authors to arrive at a smaller set that includes ten elements, namely, Pd, Au, Al, Ag, Nb, Cr, B, Ti, In, and Mn, with higher priority and six, namely, Zn, V, C, Mg, P, and Sn with lower priority for further studies. These additional studies are needed before a strong case for or against alloying additions to Cu can be made. The available thin film and line data are summarized in a series of tables that should prove useful for the readers. In particular, the thin film data allow the authors to obtain an effective average residual resistivity (EARR) per at. % solute that combines the effects of impurity scattering, second phase precipitates, and grain size refinement resulting from...

Calorimetric studies of the A1 to L10 transformation in FePt and CoPt thin films

Using differential scanning calorimetry, the enthalpy (activation energy) for the A1 to L10 transformation in FePt and CoPt thin films have been measured as −10.2±2.1 kJ/g -atom (1.7±0.1 eV) and −3.1±0.2 kJ/g-atom (2.8±0.2 eV), respectively. FePt is further seen to transform at temperatures that are approximately 120 °C lower than those for CoPt. The enthalpy of grain growth that accompanies the transformation is shown to be negligibly small by comparison to the transformation enthalpy in both films. The impact of our findings on alloy development for ultrahigh density magnetic storage media is discussed.

Calorimetric studies of the A1 to L10 transformation in binary FePt thin films with compositions in the range of 47.5–54.4at.% Fe

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.

Electrodeposited NiAl particle composite coatings

Abstract Metal matrix/metal particle composite coatings were deposited, heat treated and characterized. A nickel matrix/aluminum particle coating deposited from a sulfamate bath was chosen as a model system. By varying the process parameters, up to approximately 20 vol. % Al particles were incorporated into the coatings. Based on light optical and scanning electron microscopy, a schematic model is presented for coating morphology development during codeposition of small, electrically conductive particles. For high deposition current densities, the hardness of the as-plated Ni-Al coating was higher than that of pure Ni coatings due to the nucleation and growth of small Ni grains on the surface of the Al particles. This structural refinement leads to Vickers hardness of about 350–400 HVN (25 g load) for Ni Al coatings compared to about 200–250 HVN (25 g load) for pure Ni coatings deposited under similar conditions.