Influence of Ni2+/Co2+ ratio in electrolyte on morphology, structure and magnetic properties of electrolytically produced Ni-Co alloy powders

Nickel-cobalt (Ni-Co) alloy powders were produced galvanostatically by using sulphate electrolytes with various ratios of Ni2+/Co2+ (mole ratios). The morphology, phase structure, chemical composition and magnetic properties were examined by scanning electron microscope (SEM), X-ray diffractometer (XRD), atomic emission spectrometer (AES), and SQUID-based magnetometer, respectively. Morphology of the particles changed from cauliflower-like and dendritic to coral-like and spongy-like ones with increasing Ni2+/Co2+ ratio from 0.25 to 4.0. XRD analysis of the Ni-Co powders revealed that the decrease of Ni2+/Co2+ ratios (the increase of Co content) caused a change of structure from face centered cubic (FCC) obtained for the ratios of 4.0, 1.5 and 0.67 to a mixture of FCC and hexagonal closed-packed (HCP) phases for the ratio of 0.25. The increasing content of nickel led to change of mechanism of electrolysis from irregular (up to similar to 40 wt.% Ni in the electrolytes) to close to equilibrium (between similar to 40 and 60 wt.% Ni in the electrolytes) and anomalous co-deposition (over 60 wt.% Ni in the electrolytes) type. All of the obtained Ni-Co alloy samples behaved as soft magnetic materials while their magnetic parameters showed immediate composition dependence since both coercivity and saturation magnetization almost linearly increased with increase of the Co content.