Effects of the pH Value on the Electrodeposition of Fe–P Alloy as a Magnetic Film Material

Abstract

The effects of the pH value on the electrodeposition process of Fe–P alloys from a ferrous sulfate bath with sodium hypophosphite as the phosphorus source are investigated. The composition, structure, and magnetic properties are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy-dispersive spectroscopy, and vibrating sample magnetometry. As the pH value is varied from 1.2 to 2.4, the P content of the alloys gradually decreases from 11.42 to 3.34 wt % while the structure changes from amorphous to a polycrystalline structure. Electrochemical analyses indicate that the electrochemical reduction of Fe2+ ions and H2PO2– ions proceeds independently in solution with a pH value lower than 1.5, while it simultaneously proceeds in solution with a pH value higher than 2. Density functional theory calculations and molecular dynamics simulations reveal that Fe2+ ions and H2PO2– ions coordinate together to form a contact ion pair [nFe·m(H2PO2)]2n−m in weak acidic solution and thereafter a coelectrodeposition process of Fe atoms and P atoms takes place on the surface of the cathode. On the contrary, Fe2+ ions inside the solvation shell of H2PO2– ions in strong acidic solution are replaced by H3O+ ions, resulting in Fe2+ ions and H2PO2– ions being independently electrodepositing on the surface of the cathode. The diffusion of [nFe·m(H2PO2)]2n−m ion pairs is much difficult than that of H2PO2– ions coordinated with H3O+. The adsorption of [nFe·m(H2PO2)]2n−m ion pairs on the surface of the electrode is also weaker than that of H2PO2– ions. All of these make the P content in electrodeposited Fe–P alloys higher in strong acidic solutions than that in weak acidic solutions. The increase in the P content obviously inhibits the grain growth, resulting in the structure gradually transforming from polycrystalline to amorphous. A mechanism about the effect of pH values on the electrodeposition process of Fe–P alloys is proposed.