Thick Film Limit Research Articles

Magnetic properties of pseudomorphic ferromagnetic alloy films on Cu(100)

We report the variation of the thickness dependence of the Curie temperature for a range of ultrathin ferromagnetic alloy films. By simultaneously depositing Fe or Co and Ni on Cu(100) we produce pseudomorphic alloys with controlled stoichiometry. An analysis of the Curie temperature versus thickness curves with an empirical finite size scaling formula reveals that the Curie temperature in the thick film limit increases monotonically with Co or Fe concentration. We compare the bulk alloy Curie temperatures to those of the alloy films extrapolated from the few monolayers limit.

Scanning Tunneling Microscopy Study of the Thick Film Limit of Kinetic Roughening.

The spatial and temporal scaling behaviors of vapor-deposited silver films have been investigated by means of scanning tunneling microscopy for the film thickness range \ensuremath{\cong}10-1000 nm. The roughness, growth, and dynamic scaling exponents have been independently measured ($\ensuremath{\alpha}=0.82\ifmmode\pm\else\textpm\fi{}0.05,\ensuremath{\beta}=0.29\ifmmode\pm\else\textpm\fi{}0.06, \mathrm{and} z=2.5\ifmmode\pm\else\textpm\fi{}0.5$), and they exhibit no evolution with film thickness.

Phase separation during co-deposition of Al–Ge thin films

We present the results of a combined experimental and theoretical investigation of phase separation and microstructure development in co-deposited Al–Ge thin films. For small film thicknesses and deposition temperatures above 150 °C the phase-separated films consist of an array of domains of the Al- and Ge-rich terminal phases (lateral phase separation). Films deposited at 100 °C or less contained one or both of the terminal phases plus a metastable phase. We show that the domain structure evolves during deposition in a manner consistent with a surface interdiffusion controlled process. As film thickness increases we observe a transition from the laterally phase-separated microstructure to a layered microstructure exhibiting phase separation perpendicular to the film/substrate interface (transverse phase separation), with Al segregating to the film surface. We present a thermodynamic argument based on the competition between surface and interfacial free energies to explain this transition. Finally, we discuss the stability of the transverse phase-separated microstructure in the thick-film limit in terms of the transport of Ge through the Al-rich surface layer.

Transition from lateral to transverse phase separation during film co-deposition

We report observations of two distinct types of phase-separated microstructures in co-deposited Al-Ge films. In the initial stages of growth, lateral phase separation is observed, with a temperature dependence consistent with surface diffusion. As the film grows thicker, the Ge-rich phase becomes increasingly buried, and a transverse phase-separated microstructure results, consisting of an Al-rich layer covering a Ge-rich layer. This observation is explained in terms of the competition between surface and interfacial free energies. We discuss the kinetic aspects of the phase separation process, and the resulting behavior in the thick-film limit.

Thermomolecular Pressure in Surface Melting: Motivation for Frost Heave

A thermomolecular pressure is associated with surface melting, and it can drive mass flow along an interface under a lateral temperature gradient. The pressure is a universal thermodynamic function in the limit of thick films. It may be responsible for frost heave in frozen ground.

Wetting and melting in a methane/graphite system

We report a new thermodynamic analysis of melting in thick adsorbed films and experimental results of an NMR survey of melting in methane films adsorbed on graphite. We find that solid-liquid stratification, related to surface melting in the bulk, may occur in a system that also exhibits triple point dewetting. Each phenomenon has distinctive asymptotic thermodynamic behavior in the thick film limit. We predict the existence of surface tension induced surface freezing, symmetric with surface melting, and new first-order phase transitions at coexistence between stratified and homogeneous films. The NMR data are used to show that solid methane wets graphite to at least 50 layers and that capillary condensation is not nucleated in our system (methane on Grafoil). We confirm that solid-liquid stratification does not occur in this system. Instead, melting is a first-order phase transition between homogeneous phases of solid and liquid, which we trace from a triple point dewetting transition at bulk coexistence down to 1.8 layers, where the latent heat of melting is known to have vanished. We conclude that melting in this system has a tricritical point at about 4 layers. Finally, we observe an increase in molecular mobility associated with the surface layers of the roughened solid phase.

Magneto-optic rotation and ellipticity of ultrathin ferromagnetic films.

Magneto-optic rotation \ensuremath{\varphi}' and ellipticity \ensuremath{\varphi}'' values are reported as a function of thickness for 0--400 A\r{} of bcc Fe(100) epitaxially deposited on Au(100) for both s- and p-polarized He-Ne laser light. The values are derived from a formula that connects \ensuremath{\varphi}' and \ensuremath{\varphi}'' with our recently reported longitudinal Kerr-effect measurements of ${\ensuremath{\varphi}}_{m}$=(\ensuremath{\varphi}${\mathcal{'}}^{2}$+\ensuremath{\varphi}'${\mathcal{'}}^{2}$${)}^{1/2}$ and the parameters of two optical compensators that are used to convert ellipticity to rotation. The measurements were made in situ through an ultrahigh-vacuum window; our approach eliminates the effect of the window birefringence. The dominant contribution to ${\ensuremath{\varphi}}_{m}$ reverses from being \ensuremath{\varphi}'' in the ultrathin limit to being \ensuremath{\varphi}' in the thick-film limit. Also, in the 0--30 A\r{} range, the rotation remains near zero while the ellipticity increases linearly.

Magneto-Optical Properties of Ultratitn Ferromagnetic Films

The surface magneto-optic Kerr effect (SMOKE) has been used to explore the properties of ultrathin ferromagnetic films. The ultrathin regime corresponds to thicknesses less than the depth penetration of light and includes the monolayer range. The ultrathin regime possesses unique magneto-optic properties: the Kerr rotation and ellipticity, in general, behave differently than in the thick film limit. Measurements and simulation in the longitudinal geometry for bcc Fe on Au(100) provide a dramatic example of the metallic reflector enhancement effect due to the nonmagnetic Au underlayer. The rotation enhancement is accompanied by a high reflectivity, as opposed to being at the expense of the reflectivity. Measurements in both polar and longitudinal geometries for epitaxially-stabilized fcc Fe films grown on Cu(100) and Pd(100) indicate the presence of perpendicular surface anisotropy, which suggests new approaches to realizing vertical data-storage media.

Theory of thin-film orientation by ion bombardment during deposition

We study the development of orientational order in thin films grown with off-normal incidence ion bombardment during deposition. The overall orientational order in our model results from the dependence of the sputtering yield on grain orientation. We demonstrate that the degree of orientational order at the surface of a thick film grows slowly with increasing ion flux until, at a critical value of the flux, it begins to rise more steeply and then saturates at its maximum value. The time needed to approach the thick-film limit displays a peak as the ion flux is varied. We compare our work with the experimental results of Yu et al. [Appl. Phys. Lett. 47, 932 (1985)] and use our results to show how the deposition technique can be optimized.

Optical constants of absorbing materials by transmission interferometry on medium-thickness films

We describe a new method providing the complex index of an absorbing material from interferometric measurements of the transmittance and the corresponding phase shift for a series of unbacked films, of medium to large thickness. In fact, the usual method, giving the optical constants independently from the slope of the line obtained by a least-squares fit to the experimental points, may be used only in the thick-film limit, although it has the advantage of being insensitive to the surface conditions. The new method suggested here retains this advantage and allows the thickness range to be extended to more transparent specimens. Furthermore, it provides information on the surface quality and the thin-film model validity. Application to unbacked gold films is shown in the visible and near ultraviolet.

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a3He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,Tc, of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedTc. The ratios between energy gaps andTc's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.

Parabolic oxidation of metals

A new parabolic law for the kinetics of growth of coherent oxide films on metal crystals is presented which is applicable in the thick-film limit where approximate space-charge neutrality becomes a reasonable assumption.

Theory of Magnetization Ripple in Ferromagnetic Films

In a polycrystalline ferromagnetic film, the magnetization direction is not uniform but exhibits small wave-like fluctuations known as magnetization ripple. In this paper, a general theory of ripple is developed, extending previous treatments by the present author, Hoffmann, and others. The theory is applicable to almost any magnetic film without gross domain structure, containing randomly oriented local anisotropies arising from inhomogeneities on any scale and of any physical origin. The magnetization direction is assumed to fluctuate in the plane of the film only. In addition to local anisotropy fields, the theory includes the effects of magnetostatic and exchange interactions, and a uniform field that consists of uniform uniaxial anisotropy and external fields. Nonlinear magnetostatic and uniform fields, which in previous treatments have been either neglected or treated as small perturbations, are taken fully into account through third-order torque terms. The ripple spectrum is derived, and from it a physical picture of the ripple is obtained in terms of a coupled region which (except for very large-scale inhomogeneities) is an elongated rhombus with long axis perpendicular to the mean magnetization. The spectrum contains a nonlinear effective field, which is evaluated in several limiting cases determined by the scale of inhomogeneity, the film thickness, an exchange length, and a magnetostatic length. The magnetization dispersion (mean ripple amplitude) is also obtained for these limiting cases, but no attempt is made to calculate other measurable magnetic-film properties. The results show: a weakening of first exchange and then magnetostatic forces as the scale of inhomogeneity increases, with the consequence that the dispersion shows an increasing dependence on the uniform field; a weakened dependence of the dispersion on both the magnitude of the local anisotropy and the uniform field for large-amplitude, nonlinear ripple; and a thickness-independent ripple in the thick-film limit, where the magnetostatic field is given by its bulk value.

Thermodynamical properties of superconducting contacts

We investigate the thermodynamical properties of superconducting contacts, and concentrate on the calculation of the specific heat in the vicinity of the superconducting transition point. By considering the thick film limit, it is found that the reduction in the specific heat is a much more delicate measure of the proximity effect than is the change in transition temperature, for example. For the case that the normal film consists of a paramagnetic metal, it can be shown that for superconducting films even as thick as 100-times the coherence distance the reduction in the specific heat is still approximately 15%. Our computations apply to arbitrary mean free path. Finally we discuss experiments byShiffman et al. on eutectic Pb-Sn alloys and show that reasonable good agreement between theory and experiment can be obtained.