Cobalt alloy codeposition

Recent research on more coercive media with a low noise ratio involved addition of Zn to the Co alloy system [76-79]. Addition of Zn to the cobalt alloy very effectively produces a film with a fine particle structure, which results from codeposition of elements which are hardly soluble in the matrix. Such codeposition causes segregation and hence produces a film microstructure consisting of fine particles. The fine particulate structure lowers the noise ratio and increases the coercivity of the medium. 

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

A hydroxide suppression model first proposed by Dahms and Croll (2) explains anomalous codeposition behavior of zinc-iron group alloys. This explanation was later supported by a number of workers (3) who measured a rise in pH near the cathode surface during the deposition of Zn-Co alloy. In this model it was assumed that the Zn(OH)2 was formed during deposition as a consequence of hydrogen evolution, thus raising pH in the vicinity of the cathode. Zinc would deposit via the Zn(OH)2 layer, while cobalt deposition took place by discharge of Co2+ ions... 

Not only pure aluminum, but also aluminum alloys can be electrodeposited from organic electrolyte systems. Thus, codeposition of iron, nickel, and cobalt from systems composed of AlBrj, LiBr, and toluene have been reported. However, only a small content up to a maximum of 1.4% (for instance, iron) is reached [175]. 

Gomez et al. " electrodeposited Co-Mo magnetic alloys from a sulfate-citrate bath on carbon electrodes. Although the focus of their paper was not on elucidating the mechanism of induced codeposition, it was suggested that hydrogen could not be responsible for the deposition of Mo in the Co-Mo system, because its concentration was fairly low and because another mechanism should explain the need for citrate or polycarboxylate anions in solution. The deposition process was foimd to be favored when molybdate was present in solution, even at very low concentrations. Hence, the authors adopted the model of Podlaha and Landolt, according to which a mixed-metal complex of cobalt(II), citrate and molybdenum dioxide is adsorbed on the surface and promotes Mo reduction. 

Podlaha EJ, Landolt D (1997) Induced codeposition HI. Molybdenum alloys with nickel, cobalt, and iron. J Electrochem Soc 144 1672-1680... 

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