Mn 2+ Mediated Electrodeposition of FeCoNi Films with Enhanced Magnetic Properties

Abstract

The fabrication of high-permeability soft magnetic thin films via electrodeposition is often hampered by the parasitic hydrogen evolution reaction (HER), which induces porosity, surface roughness, and degraded magnetic properties. Herein, we demonstrate an effective strategy to overcome this limitation by introducing Mn 2+ ions, a non-magnetic and redox-stable species that does not incorporate into the deposit, as a process regulator in a FeCoNi electroplating bath. Systematic investigations reveal that an optimal concentration of Mn 2+ (5 g L -1 as MnCl2•4H2O) significantly suppresses HER through competitive adsorption at the cathode interface and complexation with citrate ligands, without alloying into the magnetic lattice. This leads to a remarkably denser and smoother film morphology, with surface roughness reduced from 43.66 nm to 4.07 nm and grain size refined from 9.02 nm to 7.92 nm. Concurrently, Mn 2+ promotes the preferential deposition of Ni, optimizing the alloy composition while preserving the intrinsic magnetic moment. As a direct consequence of the improved microstructure and unperturbed magnetic lattice, the magnetic performance is substantially enhanced. The saturation magnetization (Ms) increases by 28.6% to 1036.4 emu cm -3 , and the real and imaginary parts of the high-frequency permeability are dramatically boosted to 1693 and 933, respectively, which correspond to improvements of 84.8% and 61.7% over the Mn 2+ -free counterpart. Excessive Mn 2+ addition, however, deteriorates the film quality and magnetic response due to hindered charge transfer. This work provides a simple, scalable, and impurity-free electrodeposition route for engineering high-performance FeCoNi magnetic films with exceptional high-frequency permeability, bridging the performance gap between electrochemical and vacuum-based methods.