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The effect of bath temperature on rotation of c-axis of hcp cobalt vis-a-vis the nanowire axis of 100 nm diameter Co nanowires electrodeposited in polycarbonate membranes is studied. While the NWs deposited at 25 • C are of the polycrystalline hcp structure, the NWs deposited at 50 • C showed preferred orientation (PO) for (1010) planes along the nanowire axis. In the nanowires deposited at 60 • C, the PO changes from (1010) to (0002) along the nanowire axis. The changes in orientation are associated with the hydrogen co-deposition induced alterations in nucleation and growth mechanism at different temperatures.Magnetic nanowires (NWs) are ideal candidates for diverse technological applications in high density data storage devices, microwave devices and reprogrammable magnonic crystals. 1-3 Cobalt NWs have recently drawn considerable attraction because of their interesting structure property relationship. Cobalt exists in two allotropic forms, i.e., face centered cubic (α-Co) phase and hexagonal close packed (ε-Co) phase. The magnetic properties of ε-Co nanostructures can be tailored through variation of structural properties, e.g., crystallinity and orientation of its c-axis in the NWs. The strong uniaxial magnetocrystalline anisotropy along c-axis, in conjunction with shape anisotropy, yields a partially controllable effective anisotropy of the NWs array. 4 A simple, efficient and low cost electrodeposition process employing the commercially available nanoporous membranes can fabricate the cobalt NWs. There are reports about dependence of phase and orientation of electrodeposited cobalt NWs on deposition parameters like current density, pH, bath concentration and magnetic field applied during deposition 5-10 but only a few reports are found on the dependence of crystal orientation of cobalt NWs with bath temperature. 11 The electrocrystallisation process of depositing Co atoms at the cathode from the bath containing Co 2+ ions involves the transfer of Co 2+ ions from bulk solution to the solution-cathode interface which subsequently gets adsorbed on the cathode surface. After complete charge transfer from adsorbed cobalt ion, a cobalt ad-atom (Co ad ) forms on the cathode surface that diffuses to a vacant lattice site via nucleation and growth mechanism. 12 In this mechanism, the first step is a mass-transport process and hence it is controlled by the diameter and length of the pores in the polycarbonate template. The mass transport within the nanopores is limited by diffusion. 13 In case of less noble metal, the metal deposition is accompanied by simultaneous hydrogen evolution 2H + + 2e − → H 2 at the cathode. The standard reduction potential for cobalt is −0.227 V and for hydrogen is 0 V. The co-processes of hydrogen adsorption on the electrode surface and hydrogen gas evolution, along with the depositing cobalt ions, affect the nucleation and growth of cobalt atoms. Hence, they should play a major role in determining the texture and phase of cobalt nanowires. The aim of this work is to study the effect of t...

Effect of interdependence of aspect ratio (AR) and c-axis orientation of the hcp-cobalt nanowires (NWs) on their magnetization behavior is reported in 40 and 100 nm diameter NWs. Experimental evidence of periodically modulated magnetic state viz. large transverse-susceptibility arising due to orientation of c-axis normal to NW-axis in 40 nm NWs and magnetic domain imaging is demonstrated, which disappears at low AR owing to randomly oriented c-axes. The 100 nm NWs exhibit a crossover in the easy-axis direction from longitudinal at high AR to transverse at low AR and are explained on the basis of competition between different anisotropic contributions.

The growth and deposition mechanism of electrodeposited cobalt nanowires (NWs) at different bath temperatures (25°C–60°C) in strongly acidic bath (pH of 2.0) is studied. CV scans show that hydrogen co-deposition is most pronounced in 50°C bath. X-ray diffraction patterns of Co NWs show that a mixture of hcp and fcc phases grows at 25°C, (200) textured fcc-Co phase grows at 50°C, and (10true1̄0) and (11true2̄0) textured hcp-Co phase at 60°C, with these planes normal to the NW-axis. The magnetic properties studied by vibrating sample magnetometer show the change in coercivity, saturation field and magnetization reversal behavior is consistent with the phase of NW. Nucleation mechanism is explained on the basis of intermediates formation induced by the extent of hydrogen co-deposition leading to the change in nucleation overpotential and linked selection of the growth plane as per bath pH and temperature. Chronoamperometry study has confirmed the instantaneous and progressive growth mechanisms operative at 50°C and 60°C respectively and hydrogen co-deposition controlled grain size growth model is also put forth to understand the growth kinetics of fcc and hcp phase Co NWs. The suggested growth mechanism also consistently explains the growth in less acidic bath of pH 4.5.