Electrodeposition of Fe, Co, Mn and Their Alloys from Choline Chloride/Urea Based Deep Eutectic Solvents
Authors: Sides William D. , Kassouf Nikolas , Huang Qiang 
Published: February 2020

Despite their industrial use for many decades, there is still desire and opportunity to improve the properties of soft magnetic materials. Applications such as hard drive write heads must attain higher saturation magnetization and low coercivity in order to maintain the ability to write to increasingly dense storage media[1]. Electrodeposition is often the favored fabrication method for such soft magnetic films due to its several advantages such as fast deposition rate, ease of growth on arbitrary shaped or recessed surfaces, and simplicity of orienting film anisotropy[2]. However, it is fairly limited in the number of different elements that can be deposited in high quality. In the past several years, room temperature ionic liquids, the current generation also being known as deep eutectic solvents (DES), have become an attractive alternative to traditional aqueous deposition baths. These solvents avoid the hydrogen generation that often reduces deposit quality and efficiency in aqueous baths, and enable the incorporation into the deposit of metals with more negative deposition potentials than are viable in aqueous baths. Despite significant progress in literature on the electrodeposition of various material from DESs, their suitability for improving characteristics of magnetic films is relatively little-studied[3]. This work studies the deposition behavior of Fe, Co, and FeCo ferromagnetic alloys from the DES formed of a 1:2 mole ratio of choline chloride and urea (ChCl-urea). Deposited films are then characterized to determine their morphology, crystal structure, and magnetic properties. Mn, which has been predicted to increase the magnetization of FeCo materials[4], has been incorporated to form a tertiary FeCoMn ferromagnetic alloy, and its effects of the film properties are determined.
Initial voltammetric studies (Fig. 1) and depositions show that deposition of Fe-Co from ChCl-urea proceeds by induced codeposition, wherein Fe deposition is promoted by Co, allowing the alloy film to be obtained at potentials less negative than required for pure Fe. Increasing the deposition overpotential leads to higher Fe content in the film, and a mass transfer limit is quickly reached for both depositing species at which the film composition no longer depends on overpotential. Composition of the deposit is easily controlled by changing the metal ion concentration in the bath. Mn is also found to deposit by an induced codeposition mechanism with Fe-Co, wherein it can be incorporated into FeCo films in small amounts at an underpotential to pure Mn electrodeposition. The incorporation of Mn in alloy films is constant at around 3 wt. % when depositing in underpotential conditions. The effects of several deposition conditions including temperature, metal ion concentrations, and bath agitation, were studied and will be discussed in the presentation.

References

[1] O. Heinonen, K.Z. Gao, Extensions of perpendicular recording, Journal of Magnetism and Magnetic Materials, 320 (2008) 2885-2888.
[2] X. Liu, G. Zangari, L. Shen, Electrodeposition of soft, high moment Co–Fe–Ni thin films, Journal of Applied Physics, 87 (2000) 5410-5412.
[3] G. Panzeri, M. Tresoldi, C. Rinaldi, L. Magagnin, Electrodeposition of Magnetic SmCo Films from Deep Eutectic Solvents and Choline Chloride-Ethylene Glycol Mixtures, Journal of The Electrochemical Society, 164 (2017) D930-D933.
[4] B.V. Reddy, S.C. Deevi, S.N. Khanna, Magnetic properties of Al, V, Mn, and Ru impurities in Fe-Co alloys, Journal of Applied Physics, 93 (2003) 2823-2827.

Figure 1. Cyclic voltammograms of Fe, Co, Fe-Co-Mn in a ChCl-urea bath on a Pt rotating disk electrode rotating at 1000 RPM, scan rate 5 mV/s, temperature 80 °C.

Figure 1