The effect of solution pH and applied magnetic field on the electrodeposition of thin single-crystal films of cobalt

Electrodeposition of cobalt from a cobalt sulphate bath on to {001} and {110} copper single crystal surfaces has been carried out for a range of solution pH (1.5 to 4.0). Electron diffraction patterns from the deposits indicate that for solutions with pH <2.4 face-centred β-Co (normally stable above 425°C) was obtained in parallel growth with the face-centred cubic copper substrate. For solutions with pH >2.9 epitaxial growth of the close-packed hexagonal α-Co was observed. In the pH range 2.4 to 2.9 the deposits consisted of varying proportions of hexagonal and cubic phases. Deposition in a strong magnetic field (up to 17 kOe) reduced this pH range for `mixed' phase deposition to an almost critical pH value, viz. 2.65 ± 0.05. With the solution pH set to this value the β-Co → α-Co transition could be induced at a definite deposit thickness by the applied magnetic field. This critical deposit thickness t c was dependent upon the strength of the field such that the greater the field the smaller the value of t c . The results show, therefore, that magnetic fields can influence the crystallographic structure of an electrodeposit and these observations are discussed qualitatively in terms of the energy available at the deposit surface.