Dc magnetron sputtered Ni-Mn and Ni-Mn-Cr films are demonstrated to exhibit strong and thermally stable antiferromagnetism, as well as high corrosion resistance. For a 25.2 nm thick 53.3 Ni-46.7 Mn (in atomic percent) film deposited on top of a 28.5 nm thick 81 Ni-19 Fe film, a unidirectional anisotropy field (HUA) of 120.6 Oe is obtained at room temperature after annealing in vacuum. The equivalent interfacial exchange coupling energy (JK) is 0.27 erg/cm2, three times higher than that of bilayer Ni-Fe/50Fe-50Mn films. This strong exchange coupling appears correlated with the presence of an antiferromagnetic θ (NiMn) phase with a CuAu-I-type ordered face-centered-tetragonal structure. The blocking temperature, at which the exchange coupling disappears, is higher than 400 °C. The Cr addition to the Ni-Mn film dilutes the exchange coupling, but the JK for the Cr content ≤10.7 at. %, is still higher than that of the Ni-Fe/Fe-Mn films. Both Ni-Mn and Ni-Mn-Cr films exhibit corrosion behaviors much better than the Fe-Mn film and comparable to the Ni-Fe film. The films are proposed as longitudinal bias layers for the stabilization of magnetoresistive read sensors.
The effect of five different metal oxides on the corrosion rate of acetylene black during oxygen evolution in alkaline electrolyte was determined using the analytical methods described in Part I. All five metal oxides (FelOn, Cr20~, NiO, C0:~O4, and Ru) catalyzed the corrosion processes of the acetylene black, resulting in corrosion rates accelerated by factors of 3-14. The mechanism of this catalytic effect remains unclear. Some results with C0304 have indicated direct contact with the oxide, and the carbon is not required to accelerate the corrosion, suggesting a redox mechanism involving dissolved species. However, the effects of Fe20:~ and Cr~O~, which were significantly greater than with C0:~O4, are difficult to rationalize on this basis, as their redox potentials lie well below the oxygen evolution potential range.
A quantitative method for determining the number of oxygenated carbon atoms in a carbon black sample was developed and applied to the study of oxygen chemisorption on graphitized furnace blacks. It was found that chemisorption of oxygen atoms produced in a low‐pressure microwave discharge of argon/oxygen results in the formation of an oxygen complex on every carbon atom exposed on the edge plane surface of these carbons. These combined procedures were then used to titrate the surfaces of a variety of graphitized furnace blacks for the number of edge atoms. The corrosion rate of these carbons was found to be directly proportional to the number of sites titrated by oxygen atom chemisorption, i.e., the number of edge atoms. The number of edge atoms exposed on the surface of graphitized furnace blacks depends on the microstructure of the precursor carbon black and the conditions of graphitization. Catalyzation of these graphitized carbons by
normalNiO
has little or no effect on their corrosion rate, which results in current efficiencies for oxygen evolution greater than 98%. The absence of an effect of
normalNiO
on the corrosion rate is attributed to the specificity of the corrosion reaction to edge plane sites and an apparent necessity for contact between
normalNiO
particles with edge planes for catalyzation of the corrosion reaction by
normalNiO
.
normalNiO
particles sitting on basal planes, which compose at least 90% of the surface area in these carbons, catalyze only the oxygen evolution reaction.
Corrosion of thin film cobalt-based magnetic recording media was studied with surface analysis and electrochemical techniques. Corrosion currents of bare cobalt-chromium alloys are inversely proportional to the integral of chromium oxide concentration taken over the near-surface region of the film. For overcoated alloys, the corrosion process involves water adsorption in porous overcoat, preferential cobalt dissolution from the metal alloy, its migration through the overcoat and cobalt hydroxide and/or oxide formation on the overcoat surface. Cobalt concentrations appearing on overcoat surfaces after environmental exposures correlate well with corrosion currents.
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