The early stages of Co deposition on a silver electrode in ammonia medium were studied using cyclic voltammetry and chronoamperometry coupled with quartz crystal microbalance (EQCM) in ammonia solution. The results obtained by means of EQCM showed that during the initial stages of cobalt deposition a monolayer is formed on the substrate both in the underpotential and overpotential region, and this monolayer is formed at ？600 mV and ？980 mV. Once the cobalt deposition process starts, the growth is very fast making the investigation of the initial stages rather difficult. During this process, cobalt atoms transfer their two electrons through free species and not through cobalt hydroxide species adsorbed on the electrode as CoOH+ or Co(OH)2. In addition, it has been found that at potentials more positive than ？600 mV, ammonia adsorption takes place on the substrate surface, and theses species are replaced when the cobalt atoms arrive at potentials more negative than ？600 mV. 1. Introduction Cobalt is an element that involves a great interest due to its physical and chemical properties and technological applications. Some of its uses are as a catalyst in different reactions of technological interest or as a pigment for strong dark blue color. Also, this element is used in permanent magnet production as well as for preparation of super alloys of great hardness and resistant to high temperatures [1]. One of the properties of this element, when combined with another metal, is the giant magnetoresistance (GMR) whose study has drawn the attention of diverse research teams [2–12]. The importance of this effect lies in that applications of these systems include domains such as informatics, for preparation of reading devices, or technology, for magnetic sensor production. On the basis of these studies it has been possible to show [3] that GMR occurs in materials formed by one magnetic element (Ni, Fe, or Co) and another nonmagnetic material (Au, Ag, Pd, Cu, etc.). Until now, the commercial preparation of these materials has been preferably performed by metal evaporation techniques [4–6], in which to avoid undesirable contaminations the whole system must be subject to high-vacuum conditions that generate high production costs. Because of this, electrodeposition of these materials using a mixed bath is explored with the aim of making their elaboration cheaper. Unlike the techniques of metallic evaporation, electrochemical methods allow us to work in conditions that are similar to those of the environment as well as to reduce the risks of contamination under inert and
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