Magnetic properties of evaporated CoCr films

Perpendicular magnetic CoCr films were prepared on glass substrates using electron beam evaporation. The magnetic properties depend strongly on the substrate temperature, Ts. The anisotropy field,H_{k}^{eff}, and the perpendicular coercivity, Hc(perp) show maximum values at Tsaround 250°C. In this case, the alignment of c-axis is optimal and the grain size is smallest. The lattice spacing of the c-planes increases wlth Tsuntil 300°C. When the films prepared below 200°C were annealed around 300°C in high vacuum, the saturation magnetization, Ms, increased. The value of Ms, however, decrease by annealing above 400°C. In spite of the decrease of Msdue to the annealing above 400°C,H_{k}^{eff}and Hc(perp) increase by annealing above 400°C. If the films were bombarded by argon ions during film growth, Msdecreased and the internal stress changed from tensile to compressive. For low substrate temperature (below 150°C),H_{k}^{eff}decreased due to ion bombardment.     

Composition control for evaporated CoCr thin films

Composition control was achieved in fabrication of CoCr thin films by evaporation onto tape-shaped polymer substrates. The vapor composition during deposition was determined by an atomic absorption type of detectors and a composition calculator. The gradual change of composition during deposition due to the difference in vapor pressure between Co and Cr was compensated by supplying Cr into the evaporator. As a result, the desirable composition was obtained to within an accuracy of 0.4 wt.%. This composition control system was shown to be suitable for the preparation of CoCr perpendicular magnetic recording media     

Characteristics of RF-sputtered CoCr films

The magnetization of the CoCr recording medium has been investigated by several methods. First the perpendicular hysteresis loops are analysed in the thickness range from 500 to 20,000 Å. This provided evidence that the magnetization process is typefied by domain wall motion. Second the dependence of the coercivity on the film thickness has been determined. The dependence found can be explained if it is assumed, that the coercivity is caused by domain walls, impeded by the crystallite boundaries. Finally stand-still recording experiments have been performed, which confirm that magnetization takes place by the displacement of domain walls. The switching criterion in the writing process is best met by taking the field averaged over the film thickness.     

Composition dependence of electrical properties of simultaneously evaporated Sb-Se thin films

Amorphous thin films of Sb-Se are prepared using the three-temperature method. The films are prepared with atomic compositions from 5–90 at.% Sb. The electrical resistivity, Hall voltage and thermoelectric power of annealed samples have been measured in the temperature range 25 to 250°C. On heat treatment the sharp fall of resistance of the annealed films is attributed to radical structural transformation from amorphous to crystalline. Electrical resistivity, Hall constant and thermoelectric power are found to vary with thickness and composition of the film. Paper presented at the 7th International Conference on Thin Films, New Delhi, December 7–11, 1987     

High rate magnetron sputtering of ferromagnetic SmCo, Fe and CoCr thin films

Ferromagnetic thin films have been sputtered from a modified conventional magnetron cathode (3" diameter). The targets (5 to 6 mm thick) were stainless steel, demagnetized SmCo5and CoCr(15-18 at% Cr). The deposition rate was 6 nm/s at a power density of 15 W/cm². Increasing the power rates up to 12 nm/s could be achieved. The saturation magnetization ranged from 0.4 to 1,5 Tesla. The films sputtered from stainless steel had a magnetization only 30 % smaller than the magnetization of Fe. The coercive force varied from 0.8 to 75 kA/m.     

Stripe and bubble domains in CoCr films

Using the colloid-scanning-electron-microscope (SEM) method, an investigation was conducted of the domain structure of CoCr films (400-1740 nm thick) at different points of the hysteresis loop. In some cases, an AC field exceeding the value of the coercive force was applied in addition to the DC field. For low-coercivity samples (H_c/H_k~0.02) a bubble structure, which is energetically more favorable above a certain critical field, was observed. From the dependence of the bubble density on the DC field the value of the anhysteretic collapse field was estimated and compared with the theory. Attempts were made to observe the field dependence of the domain structure of high-coercivity samples (H_c/ H_k~0.08) with different thicknesses. For one sample the results were compared with those determined using the Kerr effect. For thicker films the colloid-SEM method permits the simultaneous observation of the magnetic domains and the columnar structure at the surface of the sample     

Corrosion behavior of CoCr films in Sulfuric acid

The corrosion behavior of CoCr films (∼500 to 1000 nm thick) deposited on glass by RF sputtering has been examined by classical electrochemical methods which provide a simple and rapid means for comparing one material with another. The development of a specially designed holder to allow electrical contact to the film and exposure to a sulfuric acid electrolyte was required. A comparison of potentiodynamic polarization curves for films containing various amounts of Cr with similar data from the literature for bulk CoCr alloys showed analogous behavior in the shape of the curve and in the level of Cr required to produce passivity, about 15-17%. Results indicate the formation of a passive layer containing a chromium oxide. In addition, comparison of an as-received sample with an electrochemically treated sample using Auger electron spectroscopy along with polarization results showed that Co oxides are apparently stabilized by the presence of Cr. Auger results also suggest that corrosion and passivation occur along the spaces or crevices between columnar grains during electrochemical treatment as indicated by the presence of a significant oxygen content.     

Post-annealing temperature dependence of infrared absorption enhancement of polymer on evaporated silver films

Infrared absorption spectra were measured at normal incidence of radiation for polymers on post-annealed silver films of 4-10 nm mass thicknesses that had been deposited in ultra high vacuum onto Si substrate surfaces. Results show that the polymer absorption intensity depends on the annealing temperature and the silver mass thickness.Results clarified that no simple relation existed, such as a negative correlation between reflectivity and the infrared absorption enhancement, even when the mass thickness was made constant and the silver particles' size and shape were changed. For infrared absorption intensity to become large, particles must be positioned discretely, but if the distance between particles is too large, absorption intensity decreases. Moreover, results verified that optimal thin-film morphology was different according to the wavelength region.     

Geometry dependence of coercivity in Ni nanowire arrays

Magnetic properties of arrays of nanowires produced inside the pores of anodic alumina membranes have been studied by means of vibrating sample magnetometer techniques. In these systems the length of the wires strongly influences the coercivity of the array. A simple model for the coercivity as a function of the geometry is presented which exhibits good agreement with experimental results. Magnetostatic interactions between the wires are responsible for a decrease of the coercive field.     

Characterization of evaporated gold-tin films

Evaporated Au and Sn films with total thicknesses in the range 1000–5000 Å were studied using backscattering of 2.0 MeV ⁴He ions, scanning and transmission electron microscopy and X-ray diffraction. Si and oxidized Si were used as substrates. In addition, some films were made self-supporting. The atomic percentage of Sn in the films ranged from about 30 to 90. Diffusion was observed within the unannealed films and resulted in the formation of the phases AuSn, AuSn₂ and AuSn₄. The backscattering spectra showed that the films form a layered structure before heat treatment. In most cases annealing for 10 min at about 200°C is sufficient to homogenize the films. The annealed films are made up of a single phase whenever this is consistent with the overall composition. Otherwise the annealed films contain a mixture of two of the Au phases or AuSn₄ and Sn depending on the atomic percentage of Sn in the films.     

High coercivity in nanostructured Co-ferrite thin films

Three methods including sol-gel, rf sputtering and pulsed laser deposition (PLD) have been used for the fabrication of high coercivity Co-ferrite thin films with a nanocrystalline structure. The PLD method is demonstrated to be a possible tool to achieve Co-ferrite films with high coercivity and small grain size at low deposition temperature. High coercivity, over 10 kOe, has been successfully achieved in Co-ferrite films with a thickness of ∼ 100 nm deposited using PLD with a substrate temperature at 550°C. The Co-ferrite films prepared by PLD at over 300°C on different substrates including amorphous glass, quartz and silicon exhibits an obvious (111) textured structure and possesses perpendicular anisotropy. Our study has also shown that the high coercivity is related with a large residual strain, which may induce an additional magnetic anisotropy (stress anisotropy) and at the same time serve as pinning centres, which can restrict the domain wall movement and therefore, increase the coercivity.     

Grain size in evaporated nickel-aluminium films

Nickel-aluminium films with compositions ranging from pure nickel to 50 at.% Al were prepared by co-evaporation from two electron-beam-heated crucibles at a substrate temperature of 570 K. The grain size and the microstructure were investigated in a transmission electron microscope. The grain size decreases rapidly when small amounts of aluminium are added. For higher concentrations the presence of intermetallic phases determines the growth and the grain size. In the range between 33 and 50 at.% Al the films are covered with surface outgrowths similar to those found on films deposited at low substrate temperatures. The formation of these outgrowths is suggested to be due to the difference in the heat of formation between the nickel and the NiAl phase.     

Deposition of HWCVD poly-Si films at a high growth rate

The process parameters for high growth rate poly-silicon films by hot-wire chemical vapour deposition have been explored. A four-wire hot wire assembly has been employed for this purpose. High silane to hydrogen flow ratios and high wire temperatures are the key process parameters to achieve high growth rate and growth rates higher than 5 nm/s can be achieved. The process conditions to incorporate high hydrogen content into the material for passivation of defects and donor states have been identified as high hydrogen dilution and lower wire temperature. With these procedures poly-Si films deposited at 1.3 nm/s showed high ambipolar diffusion length of 132 nm. Incorporating such poly-Si films as i-layer, n–i–p solar cell on stainless steel substrate without back reflector showed an efficiency of 4.4%.     

Mobility studies of evaporated tellurium films

The Hall mobilities of evaporated Te films with various grain sizes were measured as a function of temperature over the range 77–330 K and were compared with theoretical values based on lattice and ionized impurity scattering. A large deviation of the Hall mobility from the theoretical values was found in Te films with a small grain size and this deviation was attributed to grain boundary scattering. The linear correlation observed between the Hall mobility at room temperature and the average grain size supports this interpretation.     

Characterization of evaporated tellurium oxide films

The evaporation of tellurium dioxide was carried out by resistive heating and electron-beam evaporation, and the resulting films were subsequently characterized using X-ray diffraction, infrared absorption spectroscopy and electron diffraction techniques. The infrared results of these films showed a marked deviation from the initial stoichiometry, indicating decomposition of the compound. From X-ray and electron diffraction data, it was found that the films had Te₂O₅ stoichiometry. Electron-beam evaporated films showed single crystal growth on a sodium chloride substrate, while those films obtained by resistive heating were wholly amorphous. Furthermore, when fabricated into capacitors, these films showed high capacitance and high dielectric strength (4.6×10⁶V cm^–1) suitable for passive devices.