Magnetic Structuring of Electrodeposits

Abstract

Metal electrodeposition reflects the pattern of the magnetic field at the cathode surface created by a magnet array. For deposits from paramagnetic cations such as Co 2þ or Cu 2þ , the effect is explained in terms of magnetic pressure which modifies the thickness of the diffusion layer, that governs their mass transport. An inverse effect allows deposits to be structured in complementary patterns when a strongly paramagnetic but nonelectroactive cation such as Dy 3þ is present in the electrolyte, and is related to inhibition of convection of water liberated at the cathode, in the inhomogeneous magnetic field. The magnetic structuring depends on the susceptibility of the electroactive species relative to that of the nonelectroactive background. ( 1) where j is the current density in the cell and B is the applied magnetic field. Another force is operative when the field is nonuniform and induces a magnetization, M ¼ H, in the electrolyte. Here is the susceptibility, which is expressed as m c where m is the molar susceptibility and c is the concentration of ions in the electrolyte in mol m À3 . Since ( 1 for the electrolytes used in electrochemistry, the difference between B and 0 H can be neglected. The Kelvin force then takes the form F K ¼ 0 MrH. Provided the cell current is also negligible as a source of H, the field gradient force can be written as F rH ¼ 0 HrH orThis may exceed the Lorentz force when the electrolyte is paramagnetic [15]. The ratio of the magnitudes of the two forces isThe magnetic field gradient does not drag ions in solution into the vicinity of a magnet, because the energy of a single ion with spin S at temperature T in the field B, g 2 2 B SðS þ 1ÞB 2 =6 kT, is about 5 orders of magnitude less than kT, the thermal energy driving diffusion. ( B =kT ¼ 0:67 K T À1 , where B is the Bohr magneton and k is Boltzmann's constant). Taking the curl of F rB it is possible to distinguish two regions where the influence of the field gradient force is different [16],In the bulk solution, where there is no concentration gradient, the force is conservative, meaning it cannot induce convection, but it can modify or bifurcate existing convective flows. However, close to an electrode interface where a concentration gradient exists, the force is able to induce local convective flows. There is growing interest in the influence of nonuniform magnetic fields on electrochemical reactions, ranging from confinement of organic species at disc microelectrodes [17], remapping of MHD flows [18], electrodeposit patterning by magnetized iron wires [19], and spatially correlated suppression of corrosion of iron wires [20]. Here we show how electrodeposits of both magnetic and nonmagnetic cations can be structured using nonuniform fields. The deposits of magnetic ions are governed by the magnitude of the field at the cathode surface, and on the difference Á between the susceptibility of the electrolyte with and without electroactive cations. The ratio R is ) 1 in our experiments, so the field gradient force dominates.Elect...

Magnetic structuring of linear copper electrodeposits

“…2 The application of a magnetic field during deposition of these ferromagnetic metals or alloys may introduce crystallographic or atomic-scale texture, which can influence the anisotropy and magnetic reversal in the electrodeposited material. [3][4][5] More recently, there have been reports of structuring of electrodeposits from paramagnetic solutions such as Cu 2þ using nonuniform magnetic fields produced by permanent magnets, 6,7 or soft iron, which is magnetized in an external field. 8,9 These studies establish that it is the magnetic susceptibility of the electroactive species, not the susceptibility of the electrolyte itself, which determines the pattern.…”

“…Inverse patterns where the electrodeposit builds up in regions where the magnitude of the field at the surface is smallest, have been observed when a strongly paramagnetic but non-electroactive species, such as a trivalent rare earth ion, is added to the electrolyte. 7,11 We have recently given a detailed account of how the normal and inverse electrodeposits are influenced by a variety of different arrays of small, cylindrical permanent magnets. 12 Here we report on the normal and inverse electrodeposition of copper when permanent magnet line arrays are used to generate the nonuniform stray fields at the cathode surface.…”

“…8,9 These studies establish that it is the magnetic susceptibility of the electroactive species, not the susceptibility of the electrolyte itself, which determines the pattern. 7,10 The deposits are controlled by the magnitude of the field at the cathode and its gradient across the surface. Permanent magnets are particularly suitable for generating magnetic fields, which vary over a short length scale.…”

“…7,10 An estimate of the forces involved in the structuring is obtained by changing the orientation of the cathode relative to gravity. When the cathode is downward-facing, as it is in Fig.…”

The magnetoelectrochemical switch

“…There is general agreement in the literature (6,22,26,27) that the diffusive forces dominate magnetic forces by orders of magnitude. Our scaling reasoning, explaining the numerical enhancement of the magnetic force field for our case, also applies to the diffusive forces.…”