Structure of Electrodeposited Cobalt

Abstract

The structure of cobalt formed by electrodeposition and the influence of the pH of the plating solution and the cathode potential was studied by potentiodynamic measurements and X-ray diffraction. It was found that the level of overpotential significantly affects the structure of the formed cobalt. When electrodeposition is performed far from equilibrium conditions, i.e., at a high overpotential, face-centered cubic ͑fcc͒ cobalt is deposited while at low overpotential hexagonal close packed Co is formed with a lower rate of hydrogen evolution. A higher overpotential is needed in a neutral compared to acidic solution in order to enhance the evolution of hydrogen that is required for the formation of fcc cobalt.There is a growing interest in magnetic thin films, especially in cobalt films, due to their wide range of application as magnetic data storage devices. 1-5 These films are usually prepared by physical deposition methods which require ultrahigh vacuum techniques. [6][7] An alternative processing technique is electrodeposition. Electrodeposition has several advantages over dry processes. 8 Electrodeposition does not require vacuum technology and consequently is less expensive. It can easily be upscaled for use in large size areas, and it is capable of depositing uniform films on complex surfaces without the shadowing effects often encountered in other deposition methods. The experimental systems used are much simpler than evaporation or sputtering apparatus, and electrodeposition can be a room-temperature technology. However, there are some drawbacks associated with the process, such as the need for a conducting/ semiconducting substrate, the limited number of elements that can be deposited, and the large number of variables that influence this process ͑composition, pH, concentration, current density, temperature, agitation, etc.͒.Hexagonal close packing ͑hcp͒ is the usual structure for electroplated cobalt. It is also known, 9 that face-centered cubic ͑fcc͒ cobalt, which is stable at temperatures above 422°C, can sometimes be obtained from electrodeposition at ambient temperatures. Gelchinski et al. 10 have electroplated, at room temperature, cobalt-chromium alloys containing the cP8 structure type, 11,12 that is stable only at high temperatures, according to the equilibrium phase diagram.Several researchers 9,13-22 have reported that the pH of the bath solution significantly affects the structure of cobalt formed by electrodeposition. A solution with a low pH (Ͻ2.5) was reported to favor fcc cobalt, 9,13-22 whereas a high pH (Ͼ2.5) or a high temperature system favored the formation of hcp cobalt in a sulfate bath with no organic additives. 14 Agitation of a sulfate bath also favored hcp cobalt. A high deposition temperature in a chloride bath also induced the formation of hcp cobalt, 17,18 whereas high current densities in sulfamate solutions were found to favor the cubic structure. 19 In addition, a high degree of preferred orientation has been observed by Sard et al. 18 for cobalt formed in sulfate baths....

A study of the electrodeposition of Co–Cu alloys thin films on FTO substrate

“…Under equilibrium conditions at room temperature, the Co metal exists in the hcp phase though the fcc phase is stable above 420°C only [30]. It is known that electrochemical deposition from a bath at pH more than 2.9 results mainly in hcp-Co [31,32]. However, in the literature, the formation of fcc-Co phase by electrodeposition at room temperature is due to the presence of codischarged and dissolved hydrogen distorting the hcp lattice to an extent that it finally transformed to the fcc structure [33].…”

Arrays of Electroplated Multilayered Co/Cu Nanowires with Controlled Magnetic Anisotropy

“…The hcp phase, having hexagonal symmetry axis nearly perpendicular to the wire axis, favours a transverse easy magnetization axis. [8] Bulk hcp cobalt presents crystalline anisotropy constant K 1 » 5 10 5 J/m 3 at room temperature, which nearly balances the shape anisotropy, K s » p l 0 M s 2 = 6 10 5 J/m 3 squareness ratio that converts Co into an unsuitable element for arrays of magnetic nanowires. In order to overcome this situation, the growing of Co with fcc crystalline phase is highly desirable since in that case the easy magnetization axis lies nearly parallel to the wire axis, [9] which added to the shape anisotropy strongly favours a resulting longitudinal magnetic anisotropy so, enhancing the squareness of hysteresis loops.…”

“…On the other hand, Cohen-Hyams et al have found that Co can be electrodeposited into both hcp and fcc structures from a neutral bath (pH 7), depending on the cathode potential. [8] The level of overpotential significantly affects the structure of the formed cobalt. When the electrodepostion is performed far from the equilibrium conditions, fcc cobalt is deposited while at equilibrium conditions hexagonal close packed Co is formed.…”

“…but transforms into hcp as the time extended to 2 h. The reason why there is a residual hcp peak in the spectra of Figure 2 presence of also the hcp phase of the cobalt for shorter electrodeposition times. [8] The hysteresis loops when magnetic field is applied along the nanowires orientation (perpendicular to the membrane plane) are shown in Figure 3 for both nanowire arrays with different crystalline structures. First of all, arrays of long single phase hcp Co nanowires exhibit higher saturation magnetic moment than arrays of multilayer Co/Cu nanowires by a factor of 5 instead of just a factor of 2 since the total time of electrodeposition for Co and Cu was the same (60 min).…”

Electrochemical synthesis of magnetic nanowires with controlled geometry and magnetic anisotropy

“…The low pH of the electrolyte enhances the evolution of hydrogen gas and the Co ion mobility during the electroplating process. The enhanced atomic mobility may be equivalent to a high temperature condition so that the FCC phase can be obtained (Cohen-Hyams et al, 2002). Nevertheless, the coexistence of both Co crystal phases has been reported in different works (Ye et al, 2009;Sánchez-Barriga et al, 2009).…”