Patterning metallic electrodeposits with magnet arrays

Abstract

The influence of a pattern of a magnetic field on the structure of metal deposits at the cathode of a small electrochemical cell is investigated for cobalt, nickel, copper, and zinc. The different magnetic properties of the ions in their oxidized and reduced states, together with the influence on the patterned electrodeposits of variables, including the structure of the array of small magnets used to generate the field pattern, applied magnetic field, ion concentration, cell orientation, and deposition time lead to an understanding of the physical processes involved. The results for direct deposits from paramagnetic cations such as Cu${}^{2+}$ when convection is minimized are largely explained in terms of magnetic pressure, which modifies the thickness of the diffusion layer that governs mass transport. Patterning is governed by the susceptibility of the electroactive species relative to the nonelectroactive background. No patterning is observed until the diffusion layer begins to form, as it requires orthogonal concentration and magnetic field gradients. An inverse effect, whereby deposits are structured in complementary patterns, such as antidot arrays, is observed when a strongly paramagnetic but nonelectroactive cation such as Dy${}^{3+}$ is present in the electrolyte, together with an electroactive cation such as Cu${}^{2+}$or Zn${}^{2}$. Inverse patterning is related to magnetically induced convection produced by the inhomogeneous magnetic field. Blocking of sites in the double layer by the rare-earth ions may also be involved. The inverse deposits are concentrated in regions where the magnitude of the field is lowest; they can also be produced directly by superposing a uniform magnetic field on that of the magnet array.