Phase analysis of cobalt-iron films electrodeposited from ammonium citrate stabilized electrolytes

Co-Fe alloys are soft magnetic materials, which exhibit high saturation magnetizations and low coercivities, making them particularly useful for magnetic storage applications. Co-electrodeposition of Co and Fe, from a single electrolyte, represents a cost effective and reproducible means of fabricating the CoFe alloy films. Conventional Co-Fe electrolytes have stability issues; however, these can be alleviated by the addition of ammonium citrate as a complexing agent. In this work, we report on a series of electrodeposition experiments, where Co and Fe are codeposited from citrate-stabilized electrolytes, under various process conditions. The [Co
²⁺
] to [Fe
²⁺
] ratio in the electrolyte is varied, as is the electrolyte temperature (20-60 degC). The Co/Fe ratio in the electrodeposited films increases with increasing [Co
²⁺
] to [Fe
²⁺
] ratio in the electrolyte, as one might expect, and with increasing temperature. Deposition behavior depends on the electrolyte temperature, however. At lower temperatures (e.g., 23degC), anomalous deposition occurs, i.e., deposits are Fe-rich relative to the electrolyte composition. Thermodynamic calculation indicates that cobalt is more noble than Fe in the tested electrolyte, and would be expected to deposit preferentially. This anomalous behavior increases as the [Co
²⁺
] to [Fe
²⁺
] ratio in the electrolyte increases. At higher temperatures (e.g., 60 degC), the behavior is as normally expected, i.e., the more noble metal (Co) is deposited preferentially; deposits contain more Co than the electrolyte composition. Microstructural analysis (using X-ray diffraction and electron microscopy) shows that a complex bee phase (alpha-Mn type), containing Co and Fe, is favored at higher temperatures and higher Co contents, while a bee alpha-(Fe,Co) solid solution forms at lower temperature and lower Co contents. A two phase structure, i.e., the alpha-Mn phase plus the bee solid solution phase, forms at intermediate conditions. These results are in contrast to those reported in the literature for electrodeposited Co-Fe, where some combination of the bee and fee solid solution phases form and the formation of the alpha-Mn phase has not been reported, to our knowledge.