Effect of an external magnetic field orthogonal to the electrode surface on the electrocrystallization mechanism of Co-Fe films under pulsed applied potential

Abstract

Co100−xFex (x = 0; 50; 30; 20) films are prepared by pulse-reverse plating onto Cu foils, in the presence of a magnetic field of 100 mT applied orthogonally to the substrate surface (B100) and in the condition B ~ 0 (B0). Energy-dispersive X-ray spectroscopy studies indicate that the Fe/Co ratios in the films are similar to those in the feed solution. The electrochemical nucleation and growth of Co and Co-Fe films are analyzed for individual, consecutive cathodic pulses, and also considering them as a continuous transient to achieve a steady stationary state. Current-time transient are analyzed applying the generalized Scharifker-Mostany model, assuming that parameters take effective values, describing the non-single-ion diffusion involved. All Co and Co-Fe films exhibit a 3D nucleation regime and diffusion-controlled growth. Films of pure cobalt prepared at B0 exhibit progressive nucleation (3DP) while bimetallic ones an instantaneous regime (3DI) predominates. Current-time transients obtained at B100 undergo changes from 3DI nucleation, for pure Co, turning more progressive 3DP regime for bimetallic, as the iron content increases. Film morphology and microstructure largely depend on Fe/Co composition and the applied magnetic field. For low iron content, films are granular and change to dendritic-like when iron content increases. Besides, for Co70Fe30 and Co50Fe50 compositions, nanowall-like structures at the surface change into whisker-like prisms when the magnetic field is applied. Iron containing films are thinner when deposited at B100 for all compositions. The key factors explaining the influence of the magnetic field and composition on the film deposition mechanisms and microstructure are: micro-magneto-hydrodynamic convection nearby the interface, the H2 desorption and the competence/interplay of Co(II) and Fe(II) ions during the alloy nucleation and growth.